|Chapter 4 - Evolution
|“Nothing in biology makes sense except in the light of evolution."
|Geneticist Theodosius Dobzhansky
Although about half of all Americans 1 and Britons do not believe in evolution and, in particular, that man and the great apes living today evolved from an ape common ancestor who probably lived between about 4.5 and 8 mya, 2 all of the scientific theories of the origin of man postulate that beginning. It is not the purpose of this book to dispute Creationism or Intelligent Design, but simply to present evolution as scientists understand it.
Since that epic separation, the human and ape lineages have diverged genetically, culturally, and intellectually to such an extent that the chasm between us has grown so vast that one could question whether we were ever once the same species. But we were. There are about 3 billion genetic units (base pairs) in the genetic blueprints for chimps and for man and, when they are matched up, only 40 million of them are different. We are therefore genetically 1.3% “not-chimpanzee,” but 98.7% “chimpanzee,” 3 and men and women differ by more than that. 4 Small genetic differences in genetic blueprints (the “genotype”), however, can result in huge differences in the traits (the “phenotype”) of living creatures made using those blueprints, as we shall see. 5
Biologists apply the word “evolution” to two different questions: (1) “Have species changed over time?” and (2) “If they have changed, what caused them to change?” The first question is a question of fact. There is so much evidence that species have changed over time, that scientists say the answer to that question is “Yes, evolution has occurred,” without any doubt. 6 The second question asks for an explanation, a theory that describes the mechanisms that caused those changes. The only theory that scientists believe is valid, however, is Darwin’s theory of evolution, which is today called “neo-Darwinism” because it is confirmed and supported by genetics.
As the Creationists love to point out, theories can always be disproved, and certainly neo-Darwinism can be disproved. Indeed, there are all kinds of potential evidence that could refute neo-Darwinism, e.g., dinosaur bones that are only a few thousand years old or fossils organisms in an older rock stratum than their progenitors. But, so far, there is no evidence that refutes the theory and mountains of evidence that is consistent with it.
Darwin’s theory can be expressed as a syllogism:
Premises: If an individuals in a population have traits that
(1) are heritable;
Conclusion: the frequency of the traits that result in greater reproductive success will increase in that population.
(2) and are different;
(3) and result in a difference in reproductive success between individuals who have them and individuals who do not have them, then:
There are only two ways that the syllogism can be “wrong”: (1) by showing that it is not relevant because the premises do not apply to a particular population, i.e., in that population all individuals have the same traits or, if their traits are different, the traits are not heritable or, if they are different and heritable, possessing them does not result in differences in reproductive success, or (2) by showing that the conclusion does not follow from the premises. But, given that individuals in a population have such traits, which all populations do, except possibly laboratory organisms (e.g., clones, and animals with medical conditions), the conclusion must follow. 7
Traits that increase reproductive success pass on the alleles that code for those traits. Reproductive success alone determines whose lineage continues and whose becomes extinct.
Note that the syllogism requires a population from which individuals who have heritable traits that differ in their contribution to reproductive success can be selected, 8 which means that evolution cannot occur if all the individuals in the population have the same heritable traits. 9 In other words genetic equality, egalitarianism, makes evolution impossible. And, without the possibility of evolving, a species can only go extinct when its environment changes, as it inevitably does.
Generalized Versus Specialized
In this book, generalized and specialized survival strategies play a critical role in deciphering human evolution. A species, individual, or portion of an individual is more generalized if it can perform more functions, and is more specialized if it is limited to a smaller number of functions. A species is more specialized if it has evolved the anatomy (and/or physiology) needed to better exploit a particular niche, e.g., a food source, territory, or reproductive strategy.
A generalist is an opportunist, ready to exploit any niche that it happens upon before the specialists find it. Raccoons, rats, and cockroaches are generalized species; the koala eats only eucalyptus leaves and many parasites live off only a single host species, so they are specialized.
Humans, omnivores eating a variety of plants and animals and living everywhere on the planet, including under the water, in the air, at the poles, and even in spaceships and on the moon, are by far the most generalized species. Our feet, however, have become specialized, since they have lost the ability to grasp things (though I have an ex wife who picks things up with her big toe), but are excellent for bipedal walking, unlike the feet of the great apes, which can also grasp branches, but are poorly constructed for bipedal walking.(Fig. 4-1). 10
The human hand, however, is so generalized that it can thread a needle, swing a bat, or play a piano concerto. Compare your hands to the specialized hands of the baby aye-aye in Figure 4-2. Aye-ayes, an early primate, stick the middle finger of their hand into termite mounds, then withdraw it and eat the disgusting termites clinging to it. 11
Like so much else in biology, there are tradeoffs between generalizing and specializing. A generalized species is like a Swiss army knife – it can do a lot of things, but none of them as well as a tool made to do just one thing. A species that is anatomically more generalized is less vulnerable to changes in its environment because it can function in a variety of environments. Specialized species, on the other hand, can exploit a particular environment to the fullest, but when that environment goes, it goes with it. Should a disease kill off the termites, the aye-aye in Figure 4-2 will be hampered by his long, weak fingers. A specialized species bets all its resources on one niche; a generalized species diversifies its investments.
Humans are not exempt from the same tradeoffs that other animals face – we, too, could not be both specialized and generalized and, for the most part, we stayed generalized. But unlike all other animals, we discovered a way to nevertheless become much more effective at performing almost any task. We lack the anatomy (and physiology) for running as fast as a cheetah, swimming as efficiently as a dolphin, jumping as high as a grasshopper, or flying as acrobatically as a hummingbird, but we can nevertheless out-perform almost any animal at almost any task by means of our technology – we are anatomically generalized, but can be technologically highly specialized. Perhaps counterintuitively, the more adept we become at using technology to enhance our natural abilities, the more “human” we become, as that is a major difference between us and all other species. And, unlike anatomically more specialized animals, our technological specializations have made us less vulnerable to extinction when our environment changes.
Rules of Evolution
Unraveling the story of man’s evolution is like trying to put together a thousand piece puzzle with only 10 of the pieces. But because certain rules apply as to where the pieces can or cannot be placed, it is still possible to position them, by their straight edges and colors, even when there are no contiguous pieces. Similarly, there are rules that constrain evolution, including the evolution of man.
Evolution, because it occurs over great periods of time and large numbers of individuals, is less of a hit-and-miss or random process (“genetic drift”) than it is usually portrayed. 12 Accidents and good and bad luck do happen, of course, but as the amount of time and the number of individuals increase, their importance diminishes. The result is that evolution follows rules as logical as the evolution syllogism itself, not in every instance, of course, but often enough that the rules can usually be relied upon. Here are few rules that will be used to explain the evolution of humans:
(1) Evolution is cumulative. The genome of a population, altered by mutations, deaths, and individual differences in reproductive success, is passed on to the next generation, where it is then subjected to additional changes, and so on. (Barkow, 1991, p. 83). Thus, evolution proceeds by changing what is already there; evolution is not God and does not, and cannot, re-design species from scratch. If the environment changes, individuals can evolve only by changing what they already have; if that cannot be done to meet the demands of a new environment, they go extinct. For that reason, genomes will more and more come to resemble Rube Goldberg inventions rather than masterpieces of intelligent design. That is one reason why biochemistry is so complicated.
MacLean’s triune theory of the human brain is a good example of the additive nature of evolution. To a 500 million year old reptilian brain (midbrain – the interior of the cerebellum), was added the 200 million year old limbic system of lower mammals (amygdala, and hippocampus), then the 500 thousand year old neocortex (outer portion of cerebrum) of higher mammals. (Fig. 4-3). 13
Another good example of this rule is the Biogenetic Law, originally stated as “ontogeny [fetal stages] recapitulates [repeats] phylogeny [evolutionary stages],” but more accurately stated as “fetal stages repeat evolutionary fetal stages.” 14 In other words, later fetal stages are the result of adding additional stages to earlier fetal stages.
The additive nature of evolution implies that organisms will almost always become more complex, and that is indeed the case. (Adamowicz, 2008). It also implies that organisms at each step of the way must have traits that enable them to be reproductively successful. In other words A cannot evolve into B unless organisms at all the stages in between A and B survive and reproduce. 15 It also means, to paraphrase the “Law of Storage,” that useless genetic material accumulates to fill space in the genome and is cleaned out only when those who have it die without issue; no icon has been discovered in the genome that is labeled “Empty Spam Folder.”
(2) Addition is easier than subtraction. Like a government bureaucracy, the evolution of new traits is more likely to occur by adding alleles, copies, and regulations to an existing genome than by removing them. A new trait can arise either when a new allele is expressed, copied, or gene regulators change the expression of alleles. If the new trait increases reproductive success, it spreads through the population.
Losing a trait, on the other hand, implies that a trait that was an asset has become a liability, i.e., the niche made more exploitable by having that trait has disappeared. Fish that get trapped in a cave can no longer exploit a sun-lit niche, so eyes become an unnecessary cost and fish that invest fewer resources in their eyes now have the advantage; eventually cave fish become blind.
New traits arise by tinkering with an organism’s alleles, e.g., a DNA mutation or adjusting regulators bit by bit, with each tiny change usually making only a small improvement, if any. But getting rid of that trait means undoing all that tinkering and each step back must also make a small improvement in order to be selected, and it may not. Turning off a key allele may end the trait it coded for, but other alleles and regulators probably changed and were selected because they facilitated the expression of the key allele, and they will be left unchanged, perhaps producing unnecessary, and now deleterious, polypeptides.
When a daughter population splits from its parent population to exploits a new niche it will usually acquire new traits that facilitates that exploitation of that new niche. Meanwhile, the parent population does not acquire those new traits, but instead acquires other traits useful in the old niche that the daughter population does not acquire. If the new niche disappears, the new traits become liabilities and the daughter population cannot successfully compete with its parent population in the old niche. Once a fish becomes a land-walker, it cannot again become the fish it evolved from if the land disappears.
(3) Generalized → specialized → extinction. Generalized populations tend to evolve into specialized populations, not the reverse. 16 A population becomes more specialized if its traits evolve anatomically (or physiologically) to better perform a function they already perform. Thus, specialization requires changing what is already present, not returning to a previous state and, by Rule 2, it is easier to add an allele or the regulation of an allele, which could produce a new phenotype (the expression of a gene), than it is to lose or change the regulation of an allele to re-acquire a previous phenotype. 17 This rule implies that evolution goes mostly in one direction and ends in extinction when the environment changes and the specializations become liabilities. While specialized populations can evolve from specialized populations and generalized populations can evolve from generalized populations, the dominant generalized-to-specialized directionality of evolution suggests that generalized populations will be the source of most evolutionary changes.
If the environment changes, and it always does sooner or later, one of the many functions that the traits of a generalized species can perform, but the specialized species cannot perform as well, is likely to be useful in the new environment; the specialized species, however, is stuck with traits that enable it to perform only one or a few functions well. If the niche the species became specialized to exploit becomes less available, the species can become more generalized only by becoming less efficient at exploiting that niche, which only brings about its extinction sooner.
There are several ways a population can avoid this rule and become more generalized. A fetus has less structure than an adult so, if the adults in a species retain their juvenile traits (“neoteny,” Chapter 6), the species can become more generalized. 18 Neoteny played an important role in making man more generalized and thereby more capable of migrating out of the warmer climates. Also, a population could acquire more generalized traits by interbreeding with a more generalized population, thereby becoming more generalized than one of its parent populations.
A specialized species can become more generalized by partially changing its behavior and use its existing structure for a different purpose (“exaptation”), e.g., a fish can walk on its fins and still use them to swim, and evolve to walk better on its fins while still retaining the usefulness of the fins for swimming, though it will do neither as well as a fish that can only walk or only swim.
Similarly, a portion of an existing structure may remain unchanged, performing its usual function, while another portion of the same structure evolves to perform a different function, e.g., a retina that has only rods for seeing in black and white retains some of those rods while other rods evolve into cones that see in color. Fewer rods mean less definition in black and white, but that was the price for seeing in color; now the retina is more generalized than it was initially. 19
(4) Specialized populations evolve in a stable environment; generalized populations evolve in a changing environment. If the environment is stable, then a population that specializes to exploit a niche in that environment has an advantage over a population that remains more generalized, at least as to that niche, because individuals will be selected for traits that make the exploitation of that niche more efficient. The individuals in any population will vary in their degree of specialization and a plot of degree of specialization versus number of individuals will approximate a normal curve. The average of that curve will be higher for a more specialized population and its standard deviation will be less (Rule 5).
The longer an environment is stable (and the more time populations have had to evolve towards equilibrium, Rule 10), the greater will be the ratio of specialized populations to generalized populations in that environment. Conversely, in a changing environment, e.g., a seasonal climate, generalized species will be more likely to evolve. (New Scientist, Apr. 21, 2007, p. 21). Since tropical and polar climates are more stable than seasonal climates, populations that live in the tropics and at the poles will be more specialized than populations that live in a seasonal climate. 20 A species whose territory encompasses both a changing environment and a stable environment may split, with the more generalized individuals living in the changing environment and the more specialized individuals living in the stable environment, so that two species evolve.
In accordance with Rule 3, it is more likely that a generalized population will evolve from another generalized population in a temperate zone than that a specialized population will evolve into generalized population in the tropics or in a polar region, then migrate into a temperate zone and become generalized; and the greater the evolutionary change is, the truer that statement is.
(5) Specialized populations have less genetic variation than generalized populations. Individuals who deviate from the most efficient traits in a specialized population are more likely to be selected against than individuals who deviate from the most efficient traits in a generalized population because the specialized population lives in a more stable and less variable environment (Rule 4). 21 Thus, the evolution of a more generalized species, such as man, is more likely to occur in a more variable temperate zone than in the tropics. Although humans are often described as a tropical species because, for example, they sweat to keep cool and cannot survive (naked) in cold weather, the fact that they are so generalized compared to other species suggests that although their lineage began in a warm climate, they either were generalized or became more generalized at some stage in their evolution. 22
(6) Specialized populations evolve less and more slowly than generalized populations. Since a specialized population has less genetic variation than a generalized population (Rule 5), there are fewer alleles and traits that can be selected. Thus, when the environment changes, a specialized population cannot evolve quickly through the selection of alleles that are already present in its gene pool, but must wait until mutations occur. As a result, populations will change more slowly in a stable environment, though a stable environment may still end up with more species (Rule 8). 23 Since man is a relatively generalized species, and generalized species are more likely to arise in a changing climate (Rule 4), man is more likely to have evolved, at least in his later stages, in a temperate zone, not in the tropics. This is especially true of Caucasians, who are more generalized than Africans and Asians.
(7) Specialization increases carrying capacity. The carrying capacity (maximum possible biomass or numbers) in a stable environment is greater when populations specialize to exploit slightly different niches, because specialized individuals are more efficient at extracting useable energy; a more generalized population is less efficient at exploiting a niche in a stable environment. Thus, by specializing, a population can increase its numbers and therefore the rate at which mutations enter the population, which may enable it to evolve faster.
Here, a caveat is needed. Man, unlike almost all other forms of life, can specialize by using technology instead of by evolving (except the extent needed to create and use the technology). Thus, by creating technology to perform special tasks instead of evolving specialized traits to perform them, e.g., building a sailboat or an airplane instead of evolving flippers or wings, he can increase the carrying capacity of his territory even though he physically remains generalized. Although there is a physical limit to the amount of useful energy that can be extracted from a territory, the carrying capacity of a territory will increase as evolves the traits needed to create and use it; the carrying capacity of a given territory will then depend upon the population living there, and will be greater for some populations than for others.
(8) More useable energy → more biomass and more species. The greater the amount of energy available for life per unit area (or volume), the greater will be the biomass 24 and (usually) the number of species in that area. 25 There is a minimum number of individuals needed to sustain a population (175 to 475 individuals for modern hunter-gatherers; Hoffecker, 2002, p. 10) and, when more individuals can live in the same territory, more populations having that minimum number are possible and, if niches are different so that specialization can occur, those populations will evolve into more species. The tropics receive the most energy as sunlight, so the tropics have the most biomass and, because the tropics are more stable, the greatest number of species (again, per unit area or volume). Although specialization, which evolves in a stable environment (Rule 4), increases the population size of a species by extracting more energy (Rule 7), that effect may be overwhelmed by the splitting of populations into more species (Rule 8), which reduces population size. The number of individuals within northern species tends to be greater than the number within tropical species, probably because they are less concentrated (i.e., their numbers are less per unit area) and they spend less time in any one niche because they migrate more, and therefore specialization is less selected.
Note that Rules 7 and 8 somewhat mitigate against Rule 6. That is, specialization reduces evolution due to less variation (Rule 6), but increased carrying capacity (Rule 7) and more useable energy (Rule 8) increase variation, due to the extraction of more energy and the availability of more energy, respectively, and all three are more likely in a stable environment, e.g., the tropics.
(9) More biomass → a more “r” reproductive strategy. A population that lives in the tropics has more offspring and cares for them less (a more “r” reproductive strategy, Chap. 11) than a population of the same species that lives in a colder climate. The reason is that, due to greater energy and biomass per unit area in the tropics, less care is required in order to raise the young to maturity, so individuals who expend their resources having more offspring with less care on each have greater reproductive success than individuals who expend their resources on extra care for fewer offspring. This would suggest, for example, that mammoth calves received more parental resources than elephant calves, though both receive lots of care compared to other species.
(10) A trait evolves until it reaches its optimum, and a population evolves until it reaches equilibrium. The amount of each trait a population has gradually (i.e., asymptotically, because, on average, the additional benefit from each succeeding genetic change decreases) optimizes for that population in that environment. 26 Of course, as a population evolves or its environment changes, the optimums for its traits can also change. All the traits an individual has must work together to ensure its reproductive success, and too much or too little of any one trait will reduce its reproductive success, i.e., plotting reproductive success against amount of a trait will produce a bell-shaped curve. A change in one trait has subtle effects on other traits, as the change may free up or use up resources needed for other traits, facilitate or interfere with reactions, etc. (That is another reason why biochemistry is so complicated.) Thus, the optimum for each trait will change as other traits move towards their optimums; when each trait in each individual is at its optimum, the population is in equilibrium with that environment, a condition that will hardly ever exist.
A first important corollary is that the farther a species is away from its optimum, the faster it evolves or the sooner it goes extinct. This is, of course, an approximation as the desperate need for a genetic change does not produce one, but it does spread it around much faster. This corollary suggests that the magnitude of the gap between the traits a species’ genome codes for before the environmental change and the amount the genome must change is achieve equilibrium once again will be somewhat proportional to the rate at which the species evolves. Thus, after an environmental change, evolution will be rapid, then will gradually slow down as equilibrium is approached.
A second important corollary is that the amount of a trait that a population has, especially if the environment has been the same for a long time (stable or constantly seasonal), is likely to be close to optimum for that population in that environment. 27
(11) The origin of a trait is where it is found. Unless a population has migrated away from the source of a trait, 28 that trait is most likely to have originated in the population that has the highest percentage of it. Over time, the same mutation may occur in individuals living in many different territories, but it is likely to become established only in that territory where it confers a significant reproductive advantage, e.g., if traits adaptive in the tropics arise in the Eskimos, they simply disappear. Interbreeding can, and does, transfer traits, but a population is more likely to acquire a trait by mutation than by interbreeding. 29
(12) Behavior changes before the genome changes. Behavior changes to take advantage of changes in the environment, then individuals who have or acquire the traits that best facilitate that behavior have more reproductive success and the genome changes. First, apes struggled to walk on two feet, then they evolved to walk more facilely. 30
Since reproductive success occurs only when an individual acquires resources and breeds, 31 evolution is driven by changes in the environment and changes in the behavior of individuals in response to those environmental changes. Similarly, individuals can change their behavior to better acquire resources and more and better mates then, if those individuals are more reproductively successful, a sub-set of them who have the anatomy and physiology that best facilitates the new behavior will be selected.
(13) Time and population size increases the genetic variability of a population and disasters decrease it. Because mutations occur constantly, the longer a species is around, the more variation, i.e., non-lethal new alleles, it accumulates. Also, populations tend to increase their numbers with time and the larger a population is, the greater is the number of mutations that occur and accumulate.
On the other hand, disasters, e.g., accidents, disease, predators, bad luck, etc., remove alleles from the gene pool and reduce variation. Thus, a population with less variability may actually be older, if disasters have reduced its numbers.
(14) The longer a population has not interbred with other populations, the more homozygous (inbred) it becomes and the percentage of its alleles that are recessive increases. The more closely two persons are related, the more alleles they share, so the likelihood that they each have a copy of a recessive allele increases with relatedness. Thus, increased inbreeding increases the expression of recessive alleles, whether the recessive alleles are advantageous, disadvantageous, or neutral. If they are advantageous, they spread throughout the population. If they are disadvantageous, they are lost when the individual in whom they are expressed dies before he can breed. Thus, the longer a population has been isolated, the more it will be free of disadvantageous recessive alleles and the greater will be the percentage of its expressed alleles that are recessive; also, the percentage of those expressed recessive alleles that are advantageous or neutral, and not disadvantageous, will be greater. (See Chap. 30). As a corollary, the greater the percentage of a population’s expressed genes that are recessive, the longer a population has been isolated. (And Caucasians may win the prize for having the most expressed recessive alleles.)
Note that Rules (13) and (14) work against each other in isolated populations. Over time, mutations occur and an isolated population picks up and retains alleles that do not reduce its reproductive success, adding to the variability of the population (Rule 13). On the other hand, the longer a population is isolated, the more likely it is that less advantageous alleles will be lost; even beneficial alleles will be lost if still more beneficial alleles arise (Rule 14). The net result of these two effects is that any increase in variation due to Rule (13) will not be random, but will be an increase in beneficial alleles.
There are (at least) six ways that the genome of individuals in a population can be altered (i.e., so that the genome of their descendants is different than it otherwise would have been): mutation, epigenetics, isolation, hybridization, recombination, and selection, but nature has made only one of them fun.
Populations change genetically when their DNA changes. A heritable change occurs only if the DNA in a germline cell (an egg or sperm, or a cell that makes eggs or sperm) changes. 32 Genetic material in sperm and eggs can be changed by, e.g., cosmic rays, high temperatures, misreading the DNA code when sperm and eggs are made, and mutagens, such as certain pollutants.
It has recently been discovered that non-coding nuclear DNA (“junk” DNA), which can itself be mutated, can become coding DNA, thus changing the traits of the next generation if it occurs in a germline cell. 33 Additionally, DNA can be altered when a germline cell is invaded by a virus or bacteria and its genetic material is incorporated into the nuclear DNA of that cell. The occasional movement of sections of DNA within a gene, or even between genes, also alters the DNA code. (Patterson, 1999, Chap. 6). The DNA code can also be changed if germline DNA is duplicated not once, but multiple times; it has been estimated that at least 12% of the human genome (about 20,500 genes) differs in the number of copies that people have. (Redon, 2006).
Over time, DNA that is least vital accumulates the most mutations, as one would expect. This includes some non-coding DNA (“introns”), 34 genes that have been silenced (“pseudogenes”), and often DNA that codes for the same amino acid (“synonymous DNA”).
Since access to the DNA blueprint is controlled by means of gene regulators, if the environment changes the regulators in germ cells (“epigenetic changes”), those changes can be passed on the next generation (Wikipedia, “Epigenetics”), 35 though most are not and epigenetic changes may be lost after a few generations. Regulators determine whether or not DNA is read, what portion of a string of DNA is read, when it is read, how many times it is read, and which sections are spliced together to be read. 36 There are quite a few gene regulators and more are being discovered all the time. Best known are the histones, the proteins that entwine the DNA strands in chromosomes and uncoil to permit DNA to be read. Various chemical groups, such as methyl, phosphate, and acetyl, can be attached to a DNA strand to prevent it from being read. When DNA is being copied, the number of copies made is regulated and differences in copy number can affect susceptibility to disease as well as racial differences.
Gene regulators are inherited along with the DNA they are attached to. 37 Regulators are estimated to evolve about 10 times as fast as DNA, so most evolution results from changes in the regulators rather than from changes in DNA itself, 38 though changes in DNA are more fundamental. Changes in the regulators occur more easily because there are no error repair mechanisms for regulators, as there are for DNA, and environmental influences change regulators more readily than they change DNA. 39
The gene regulators of the races are likely to differ by a far greater percentage than the DNA of the races. However, this is a new area, and the study of racial differences in gene regulators is still in its infancy.
Isolation changes the genome of populations by increasing inbreeding (Rule 14), which makes it easier for advantageous, but rare, combinations of alleles, especially recessive alleles, to spread through a population when they arise. Since inbreeding enhances the likelihood that an individual will inherit two copies of the same allele, inbreeding can also more quickly eliminate from the gene pool alleles that code for traits that are lethal prior to maturity or that otherwise impair reproductive success. Isolation requires only no interbreeding, not physical separation. People on different Melanesian islands have become genetically different because, despite the closeness of their islands, they were reproductively isolated from each other. (Friedlaender, 2007).
Hybridization occurs whenever (genetically different) populations interbred. After a population has become isolated from its parent population and genetically different from it, its males, females, or both can interbreed with another population, even its parent population, thereby infusing different alleles into the resulting hybrid population. This can occur when an isolated population simply increases in numbers and expands into the territory of another population or is driven there by climate changes or other factors. Caucasian men were explorers and typically bred with women in the other lands they went to. Africans captured as slaves were brought to other territories in Africa, as well as to India, the Middle East, southern Europe, and the Americas, 40 where they interbred with the populations already there. Early man lived in groups of about 150 people (Arsuaga, 2001, p. 295) and the males in these groups would raid the territory of other groups, killing off the males and taking the women, 41 thus hybridizing their own group.
The individuals in the hybrid population will have various combinations of the alleles they received from the two parent populations, with some individuals being better adapted, and others worse adapted, than either parent population. If there is natural selection of the hybrid population (there is little natural selection in the welfare state, where even the poorly adapted can survive and reproduce), the best adapted hybrid individuals form a new population. This is called “adaptive introgression” because new alleles are introduced into the two parent populations and the individuals having the most adaptive combination of alleles in the hybrids are more reproductively successful. Chapter 30 covers hybridization in more detail.
Sex, which has been enjoyed for 1.2 billion years, 42 changes populations genetically in two ways. First, when an egg is made, some of the nuclear DNA in each of a woman’s 23 chromosomes that came from her mother (other than the X chromosome) is exchanged with the corresponding nuclear DNA in each of the 23 chromosomes that came from her father. (Ditto for making sperm, except for the Y chromosome.) This means that the DNA in each chromosome is no longer all from the women’s father or all from her mother, but contains a mixture of DNA from each of her parents; this is called “crossover.”
Each egg and each sperm then receives 23 of these mixed chromosomes, not 23 pairs of unmixed chromosomes, as other cells do. When a sperm fertilizes an egg, its unpaired 23 mixed chromosomes pair up with the egg’s corresponding unpaired 23 mixed chromosomes, resulting in 23 pairs once again, a process called “recombination.” Because of crossover, the fertilized egg has DNA from each of the 4 grandparents, rather than from only two of them. Recombination and crossover ensure that the mixture of DNA is different, not only between generations, but also between siblings. 43 Sexual reproduction scrambles alleles so much that everyone except identical twins and clones has a different DNA blueprint, and very likely a unique combination of traits. If the new mixture results in greater reproductive success, the population changes genetically with each birth. 44
Why did this elaborate scheme to mix up DNA, and thereby make siblings genetically different, evolve? Because it avoids putting all the parents’ fertilized eggs in one basket. If all their offspring were genetically identical they would all have the same vulnerabilities and none might survive. If the environment changes, e.g., a different climate, different predator, different food source, different parasites, etc., that would be the end of their lineage, but if their progeny are different, some might survive. (Zuk, 2007).
A trait may not be controlled by a single gene, but by the interactions of several different genes. Many traits, including high intelligence, require the presence in a single individual of particular alleles of a number of different genes. (Lykken,1992). Thus, each time alleles are mixed there is a different collection of alleles for that trait, which can result in more or less of the trait or even in an entirely new trait.
Traits that are helpful in achieving reproductive success are “positively selected” 45) or “selected for,” 46 traits that reduce reproductive success are “negatively selected” or “selected against,” and some traits may do neither and be neutral. 47 Traits that are positively selected in one population, or in one environment, may be more or less positively selected, or even negatively selected or neutral, in another population or environment. When the sun is almost directly overhead, dark skin is a life saver as it protects the body from receiving too much ultraviolet light but, if there is little sunlight, it prevents the absorption of enough ultraviolet light to make enough foliate and vitamin D. 48 As selection works its magic, a population becomes more and more adapted to the environment it finds itself in, whether it migrated to that environment or it stayed put while its environment changed. Thus, over time, selection pushes the individuals towards optimal mixes of alleles and traits for their particular environment (Rule 10). If a costly trait (a trait that requires the expenditure of extra resources, e.g., high intelligence) has been present (or absent) in a population for a considerable time, that trait is very likely an advantage (or disadvantage) for that population in that environment (Rule 10 second corollary).
And, because traits are not “free,” but must be “paid for” with the body’s resources, more of one trait means less of others, and the others that will be sacrificed are those whose loss reduces reproductive success the least. Some tradeoffs are obvious, e.g., more speed (fast twitch muscles) means less endurance (slow twitch muscles), and other tradeoffs are obscure, e.g., larger testicles means a smaller brain (Note 4 of Table 12-1, p. 90). As in economics, where no voluntary exchange occurs unless both parties believe they will gain from it, so in evolution, sacrificing some of one trait to acquire more of another does not occur unless it increases reproductive success, and trades and tradeoffs will be made until values and reproductive success, respectively, are maximized. More of every desirable trait is not an option.
Nor is it true that it is always better to have more of even the most desirable traits – even for those traits, there is an optimal amount at which reproductive success is maximized. Too much brain and too little brain will both bring less reproductive success than somewhere in between. Nor is the optimal amount of a trait the same in every environment. A small brain may be optimal when one is living in technologically simple times, but may not be optimal once the technology becomes complex.
Traits need not become more and more complex – they can become simpler and simpler, as a bird, such as the ostrich, that still has wings, but can no longer fly or a snake that still has (vestigial) legs, but can no longer walk. Traits are “lost” when they are no longer positively selected – individuals who lack them reproduce at least as successfully as those who have them – the traits are no longer “reproductively profitable,” i.e., they contribute less to reproductive success than do other traits that could be “bought” with those resources.
Nietzsche said, “That which does not kill me makes me stronger.” That may or may not be true, but evolution’s version, “Selection that does not kill off an entire population, accelerates its evolution,” is true. And the greater the number of individuals that don’t reproduce, the faster the population will evolve (provided at least the minimum number of individuals required to sustain the population are left). 49 The more that having a particular trait increases the chances of an individual successfully reproducing (or not having it decreases the chances), the faster that trait will spread through the population (or the faster that trait will disappear). Nature has no soft feelings, no empathy for the weak and helpless, and is not trying to make any particular type of individual. The end product is whatever succeeded in reproducing, regardless of how despicable, degrading, or degenerate we find it to be. Reproduce more than others and you stay in the game; otherwise, you’re out. Permanently.
Another way to more rapidly evolve is to increase the rate of “turnover,” the replacement of one generation by the next. Aging is a waste of breeding adults and is not a biological necessity as some species live for hundreds or even thousands of years (e.g., bristlecone pines – 5000 yrs). 50 But if individuals do not age and die, freeing up territory and resources for the next generation, there will be less turnover and the species will not be able to evolve quickly should its environment change; that problem is avoided if there is a genetic clock that causes individuals to age. 51
Faster evolution leads to the concept of “selection pressure,” an indication of the magnitude of the “gap” between how successful a population is in its environment and how successful it would be if it could evolve a new trait or traits. A population can be said to have been under great selection pressure when, after acquiring a new trait, the number of its members having that trait increases rapidly.
An important consequence of selection pressure is that if an environment is stable and the population has reached, or nearly reached, equilibrium in that environment, it will be under little or no selection pressure and is unlikely to evolve (Rule 10). On the other hand, if the environment changes, the population will be farther away from equilibrium and will be more likely to evolve. Compared to a population that stays put, a population that moves from one climate zone to another, as man’s predecessors did when they migrated north (Section IV), enters a new environment and faces stronger selection pressures, which accelerate its evolution. 52
Selection pressure therefore helps determine where evolution is most likely to occur. Except for occasional drastic changes in the amount of precipitation in Africa, 53 the African and Asian tropics and the Arctic and Antarctic polar regions have a more stable environment than the temperate zones in between, which not only have wide yearly changes in seasons, but have also suffered through several ice ages that lasted thousands of years. As a consequence, selection pressures are greater in the temperate zones, and species, including man’s predecessors, were more likely to have evolved there than in the tropics or the polar regions. 54
Table of Contents
1. Only about 40% of US adults accept the basic idea of evolution, lower than any European country and second only to Turkey. (Michigan State University Press Release, Feb. 15, 2007). About half: (“Who Believes in Evolution,” Half Sigma, Jan. 25, 2008). Back
2. “It is even harder for the average ape to believe that he has descended from man.” (H. L. Mencken). A recent article says the split occurred 4.1 mya ± 400,000 ya. (Hobolth, 2007).
3. (Curnoe, 2003) would even classify chimpanzees in the same genus as man, Homo. A more recent study, however, found only 86.7% genetic similarity, when indels (insertions/deletions), in addition to substitutions, were counted. (Anzai, 2003). Another recent study showed 96% consistency (Mikkelsen, 2005; Redon, 2006) and the most recent “at least 6%” difference” (Demuth, 2006), when the number of copies of genes are included. Also see (Watanabe, 2004). Chimpanzees are genetically closer to humans than they are to gorillas. www.bonobo.org Back
4. Because the male Y chromosome is much smaller than the X chromosome, men and women differ in their DNA by about 1.5%, but one cannot conclude that men and women are more closely related to chimps than they are to each other. Differences in how strings of DNA are read and assembled have a greater effect than differences in the DNA itself. (Schwartz, 2005, p. 241-242). Back
5. “Genetic blueprint” means any inherited information and “DNA blueprint” means just DNA. Back
6. One can actually watch evolution occurring in a Petri dish as mutant bacteria with favorable traits increase in numbers. (Hittinger, 2007; Griffin, 2004; Losos, 2006; Holmes, B. "Bacteria make major evolutionary shift in the lab," New Scientist, June 9, 2008; and Ariza, L.M, "Evolution in a Petri Dish," Scientific American," Nov., 2007, for worms.). Particularly convincing evidence for evolution is that way that single-celled organisms can cooperate, suggesting how even the great leap from single-celled to multi-celled organisms, 600 mya, could have been bridged. (Wingreen, 2006). Also see (Herring, 2006) and the behavior of slime molds. (Ardrey, 1966, p. 202; Navas, 2007). Back
7. “In this sense, natural selection is not a scientific theory but a truism, something that is proven to be true, like one of Euclid’s theorems.” (Patterson, 1999, p. 118). Back
8. “Mutation provides the raw material, but selection will propagate a new mutation only if it is favoured by the environment, and this is most likely in a changed or changing environment.” (Patterson, 1999, p. 78). Back
9. Evolution has been aptly described as “blind variation and selective retention.” (Campbell, cited in Barkow, 1991, pp. 23, 112). In other words, mindlessly create and try a multitude of different solutions, keep whichever one works and throw the rest away. Evolution can also be applied to ideas. A “meme” (Dawkins, 1976) is an idea that is like a germ, e.g., a cold virus that makes a person sneeze and cough to propagate itself, except that a meme is not a physical thing but an idea that gets into people’s minds, then alters their thinking and behavior to make them try to put that idea into the minds of others. The meme evolves because it is modified from time to time, with the more “reproductively successful” memes controlling more minds. Successful religious memes, e.g., Islam, require keeping women subservient and pregnant, justify the forced conversion or death of non-believers (i.e., those not infected with the meme), and make promises of rewards for adhering to the meme and punishment for not doing so, to be redeemed only after death. The free market is also analogous to evolution, with old firms (species) that do not change with the times (evolve) dying (going extinct), releasing their resources (territories, energy sources) to new firms (species), who may grow (achieve reproductive success), change (evolve) according to selections made by their customers (the environment), while competing with other firms (species) for profits (stores of energy). Back
10. The human foot has only an arch to remind us that it was once good for something other than walking on. (Howells, 1959, p. 94). Back
11. Picture from National Geographic News, Apr. 20, 2005. Man, no doubt, would find other uses for such a finger. Back
12. There is probably too much reliance upon genetic drift (random changes) to explain evolution. (Kiontke, 2007). Although mutations cannot be made to occur as needed, they do not occur randomly because some are far more likely to occur than others. And, once they do occur, the number of mutations that are truly neutral (and therefore cannot be selected, but proliferate randomly) is likely to be very small. Only a few mutations have a dramatic effect, and those that appear to have no effect may have such a small effect that it is concealed by “noise,” chance events in the environment. A “noiseless” laboratory environment may be required to measure the effect. Even then a great deal of time may be needed before the effect becomes statistically significant. Moreover, in a natural environment there will be infrequent events (e.g., floods, drought) that only then cause selection. There are very few “clean” chemical reactions, where only a single product, and no byproducts, is made; that may be especially true inside a living organism, which would explain why virtually all drugs have side effects. Thus, many seemingly neutral mutations will have subtle effects that are difficult to detect. In math, it is very difficult to generate numbers that are truly random; it is probably even more difficult to generate random or neutral mutations in biology. The egalitarians have exaggerated the role of drift and neutral alleles because those concepts suggest that racial differences are accidental and of little importance, instead of having been selected because they made the difference between reproductive success and failure. Back
13. Illustration from “The Reptilian Brain” by David Icke. Back
14. (Schwartz, 2005, pp. 55-56). A bit of the earlier evolutionary stages can be seen not in the fetus, but in the still-developing infant. "... the newborn infant concords very well with 20 million years ago in the Miocene epoch, when our ancestors were apes of some sort. Newborn infants can often grasp and suspend themselves and even swing enough to suggest brachiation. Their hallux or big toe is often highly movable and the rest of their feet (showing a slope of their curled toes that is virtually tranverse) are apelike." (Swan, 1990). Back
15. It is possible, however, for an organism at a particular stage to do rather poorly, but to still hang on until another mutation occurs that enables it to do better. Back
16. (Howells, 1948, pp. 11-15). Rule 3 is intended to apply to changes in the alleles present in the population’s gene pool, not to their frequency. That is, a population will include both individuals who are more generalized and are more specialized than the average for that population and, depending upon which individuals have more reproductive success, the ratio of more generalized to more specialized individuals can change, thereby changing the average amount of specialization in that population without changing any alleles. Back
17. Even with the selection being made by man instead of by nature, it is doubtful that one could breed a (generalized) wolf from a (specialized) Chihuahua in the same amount of time it took to breed a Chihuahua from a wolf. Another reason for the rule may be “environmental heterogeneity.” In a seasonally-changing environment, a (specialized) population who has traits advantageous in only one season may be at a disadvantage relative to a (generalized) population who has traits less advantageous in that season, but more advantageous over the entire year; to become generalized, the specialized population has to acquire the allele(s) of the generalized population but, to become specialized, the generalized population only has to turn off one or more alleles. Back
18. A fetus has some specializations for survival as a baby, e.g., short limbs, subcutaneous fat, epicanthic folds, and round heads, which are lost in Caucasian and African babies when they become adults, but are not lost in East Asians. Thus, neoteny can generalize an adult if the adult remains at a stage after fetal specializations have been lost, but prior to a stage where later specializations were acquired.
19. Similarly, a monkey’s tail, used for balance, can evolve to become prehensile, becoming heavier and sluggish, and therefore less useful for balance. Going from specialized to generalized may seem similar to going from a more ordered state to a less ordered state, which should occur spontaneously according to the Second Law of Thermodynamics. However, the generalized state is not necessarily less ordered and may actually be more ordered. Back
20. A good example is the bear. The tropical giant panda bear’s diet is 99% bamboo shoots, the polar bear eats almost entirely marine mammals and, although the American black bear prefers picnic baskets, it will eat a wide variety of foods. However, although polar regions are stable, they support less life and that may limit the niches for specialized species. Back
21. This is not true of Africans, who have more variation, but that will be explained in subsequent chapters. Back
22. That change is believed to have occurred when man became more neotenic. Man’s neoteny can be seen in the loss of primitive features in fossil skulls (Chap. 2), which began slowly with the first Homo species, then gradually accelerated. Back
23. Until recently, biologists have believed that most evolution occurred in the tropics because the tropics had the most species. Now there is support for the idea that not only did man evolve at higher latitudes, so did most other animals. (Weir, 2007). The New Zealand Tuatara is the fastest evolving animal. (Hay, 2008). Back
24. There is more biomass in the tropics (tropical rain forest = 2299 g/m2yr, temperate deciduous forest and grassland = 600 – 1200 g/m2yr.; Hoffecker, 2002, p. 6). Back
25. The amount of energy needed to create a new species is 1023 joules. (Discover, Sept., 2006, p. 14). Back
26. There may be multiple optimums for a species, each for a different combination of traits, even in a single environment. Individuals in a species may even have different optimums for a particular trait, depending upon the other traits they possess. There can also be an optimal percentage of individuals in the population that have a trait. Since catching and repairing all DNA errors would not only be very costly, but would also reduce variability, there will even be an optimal amount of DNA repairing, with the optimum being lower in a more variable environment. (Sniegowski, 2000). Back
27. “… any adaptation exists because it increases the reproduction of the genes encoding it, relative to that of the alleles for alternative characters.” (Ridley, 1996, p. 334). Back
28. Some migrants to the Americas were more successful than those they left behind in Asia. (Green in Fig. 21-1). Back
29. Individuals in a population who do not or can not interbreed with individuals in other populations preserve their collection of alleles, which have been selected to work well together in that environment. On the other hand, by not interbreeding they forego the possibility of picking up beneficial alleles that may have arisen in other populations. Thus, even the amount of interbreeding will optimize. But, since beneficial alleles arise rarely, the optimal amount of interbreeding will be low. Back
30. “A bird does not fly because it has wings; it has wings because it flies.” (Ardrey, 1966, pp. 7, 9). Back
31. Up to the Industrial Revolution, the rich had more surviving children than the poor, as one would expect. (Clark, 2007). Also see (Wikipedia, “Baldwin Effect”). Back
32. (Sykes, 2001, p. 55). Even if a mutation occurs in the DNA of a germline cell that makes an egg or sperm, none of the eggs or sperm produced may be fertilized and produce breeding offspring. And, even if a mutation occurs in the mitochondria of a germline cell that makes an egg, the mutated mtDNA may not be part of the mtDNA that ends up in the egg or, if it does, that egg may not be fertilized. On the other hand, the germline divides 24 times between generations. (id., p. 157), increasing the chances that a mutated mitochondria will end up in an egg that is fertilized. Back
33. (Cheng, 2006). “Junk” DNA also performs other useful functions. (Lowe, 2007). Back
34. “We now know that more than 98 per cent of our DNA is of the non-coding variety.” Only 1.2% of our DNA codes for proteins. (New Scientist, July 14-20, 2007, pp. 43, 3). Back
35. (Pray, 2004; Carroll, S.B., “Regulating Evolution,” Scientific American, May, 2008). Here is an excellent four-part video on epigenetics. Note that epigenetic change, i.e., changing regulators, is not the same thing as the inheritance of acquired characteristics, “Lamarckism,” because acquired characteristics do not necessarily change the regulators, i.e., there is no mechanism for an acquired characteristic to change an individual’s genome. “Imprinting” is due to a regulator that silences either the allele from the mother or the allele from the father, so that the sex of the parent determines whether or not a gene is read. (Montgomery, 2005; Goos, 2006; Bereczkei, 2004). A genetic defect inherited from the father causes Prader-Willi syndrome, where the infant eats litte, then becomes voracious when a few years old; the same genetic defect inherited from the mother causes Angelman syndrome, where the child perpetually smiles and laughs, but also has symptoms found in severe autism. (Zimmer, C., "The Brain," Discover, Dec., 2008). Back
36. That is why even though the same DNA is in all the cells, the cells can nevertheless grow into brain cells, liver cells, and so on – the regulators cause different genes to be read; different portions of a gene are read, depending upon the tissue that gene is in at the time. (Wang, 2008). The DNA code for the polypeptides that are assembled into proteins can be in different locations, even on different chromosomes. Back
37. We inherit chromosomes from our parents, not naked DNA. The DNA is only 50% of the chromosomes. Back
38. (Choi, “Regulators Evolve Faster Than Genes,” The Scientist, Aug. 9, 2007). Back
39. That is why our DNA can be so similar to chimp DNA, yet we are so different from chimps. (Schwartz, 2005, p. 242). Back
40. “[The] Arabs are known to have taken slaves from Africa to south Arabia, Persia, the Far East, China, and Japan …” Some were even found in Russia. (Eribo, F., In Search of Greatness, 2001, Chapter 1). Back
41. “How could Moses prohibit murder and then, in Numbers 31, fly into a rage because a returning Israelite war party has slaughtered only the adult male Midianites? ‘Now kill all the boys,’ he tells them when he calms down. ‘And kill every woman who has slept with a man, but save for yourselves every girl who has never slept with a man.’ [Numbers 31:17]” (Lazare, 2002). A study of 500 skeletons massacred in North and South Dakota about 1325 A.D. showed “a striking absence of young women.” (Buss, 2005, p. 10). Most murders are by men in their years of reproductive competition. (Buss, 2005, p. 23). Back
42. It’s hard to believe that anyone would give up sex, but some entire species have. (Patterson, 1999, pp. 136-137; "...bdeloid rotifers abandoned sex about 100 million years ago...," Zimmer, C., "What Is A Species?," Scientific American, June, 2008 ). Back
43. Although the progeny have some of the same alleles as each of their parents, crossover may alter traits. Alleles can also move to a different chromosome which may affect traits so much that the species splits. (Masly, 2006). Back
44. On the other hand, “The cost of sex, in terms of fitness, is enormous.” (Patterson, 1999, p. 136). In asexual reproduction 100% of the alleles are passed on; in sexual reproduction, each parent passes on only half of his alleles. Sexual reproduction requires two individuals to produce one offspring; asexual requires only a single individual. Sexual displays also make males more vulnerable, and both sexes are more vulnerable during sex.
45. Alleles are inherited in large blocks (“haplogroups,” Chap. 20). If an advantage allele arises, those who have it will have more progeny. Many years later, as mutations accumulate, there will be more variation in other blocks than in the block with the new allele because that block has not been around as long as the other blocks. So, less variation in a block means that the block contains an allele that was positively selected. Back
46. Culture, although it is not inherited behavior, is also subject to selection and can lead to the selection of alleles that accommodate it. (Rogers, 2008; Chap. 4, Rule 12). Anything that can be affected by the genome can be selected and anything that changes the genome can select. Lawnmowers have selected dandelions for low leaves and fast-growing stalks. Back
47. (FN 88, p. 19). Note that traits are selected, not the alleles responsible for the traits. Even synonymous alleles can affect the function of the encoded protein by altering its structure (Goymer, 2007) and “neutral” DNA strings may be lumped with non-neutral strings during cross-over, making the combination non-neutral. Back
48. Polar bears’ fur appears white but consists of transparent hollow hairs that conduct light to their heat-absorbing black skin; they also obtain sufficient vitamin D from their food. Back
49. “ … selection at the rate of .01 can increase a gene’s frequency from 1% to 99% in 1000 generations …” (Levin, 1997, p. 123). Back
50. There is some evidence that women do not die soon after menopause because they help care for their grandchildren, thus increasing the number of them who survive. (Wikipedia, “Grandmother Hypothesis”). Back
51. (Fuerle, 1986, p. 133). This can be accomplished by losing telomeres at the end of chromosomes; when all the telomeres are gone, the chromosome can no longer replicate. Dietary restriction extends life (Bishop, 2007), which reduces the likelihood of extinction during scarcity; this suggests that aging and death are programmed. Back
52. Environmental change, and the resulting increase in selection pressure, can result in “bursts” of evolution separated by periods of little genetic change. “Although each species must have passed through numerous transitional stages, it is probable that the periods, during which each underwent modification, though many and long as measured by years, have been short in comparison with the periods during which each remained in an unchanged condition.” (Darwin, 1859). Back
53. (Lippsett, 1998). The longer the time in between the recurrence of an event, and the faster its effects dissipate, the less alleles for traits that are advantageous during the event will be selected. Back
54. There is evidence that people living in different geographical locations, and therefore usually in different climates, are under different selection pressures, as one would expect. (Voight, 2006). Alleles selected in one racial group were therefore quite different from those selected in other racial groups. Back