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Razib’s recent entry on the erosion of the Y chromosome got me thinking: what advantages does this method of sex determination have that allow it to persist despite the ongoing problem of Y chromosome deterioration? A little research suggests that it may help us mammals make larger evolutionary jumps than would be possible for other classes of vertebrates, which use other methods of determining sex (birds also use chromosomes for sex determination, but more on them later). To begin with, see here for a discussion of a recent Y-chromosome mutation that may have led to the creation of modern humans as a separate species. The paper also goes into some detail on how mutations of the Y chromosome provide a mechanism for evolutionary saltation that overcomes some of the objections raised by the gradualists.
This has interesting consequences. It implies that mammalian and bird evolution could follow the model of punctuated equilibrium while other creatures might be (mostly) stuck with gradual evolution.
Birds, however, have homogametic males (where humans and mammals in general have heterogametic males). As a result the risks associated with mutations to the non-recombining sex chromosome fall first on the females of the species, and consequently the costs are higher. Birds would seem therefore less able to make large evolutionary leaps, and I would guess that the amount of genetic variation throughout the class of birds (10,000 species or so) is less than that found in mammals (4,000 species). Examination of the phenotypes seems to support this idea.
Another question this raises is whether creatures must be able to manifest some minimal rate of species change or else risk extinction. There are a number of features that a species can have that allow for rapid mutation and selection:
- a large number of descendants (which also implies rapid elimination of bad variations)
- a short time interval between successive generations
- imperfect DNA replication mechanisms to increase various sorts of error rates
- the Y chromosome mechanism described above

Since mammals and birds don’t do well in the first two categories, and can’t afford the third feature given their investment in their young, I would guess that the development of their method of sex determination was a necessary precursor to the high-K reproductive strategies they use. Without it, the speciation or evolution rate for a high-K species would fall too low, and we would not have seen the great bursts of adaptive radiation that the mammals have shown.

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