Stupid Evolution Question: Only Two Genders?

[Akatsukami]

*Originally posted by Hari Seldon *
**Finally, a comment on the XY method of sexual determination. It is universal or nearly so in mammals. My daughter once told me that bird have what is called WZ method. Here there are two chromosomes named W and Z and males are WW, while females are WZ […] There remains the question of sex determination, but that is not insoluble since at least one organism has solved it. **

Far more than one. In reptiles, sex determination is due to external factors (incubation tempertature of the egg in every case that I know of, although there could easily be different cases that I don’t know of). This is how the evolution of the two post-reptilian lines of mammals and birds could use opposite methods of genetic sex determination.

In the honeybee (and, I believe, many other Hymenoptera), haploid (one set of chromosomes) zygotes are male, whilst diploid (two sets of chromosomes) zygotes are female. This lends some plausibility to the SciAm contention that the Y chromosome may disappear in future mammals or in post-mammalian (or post-avian) evolution. There are also “genetic parasites” in Hymenoptera, ranging from intracellular bacteria to supernumerary chromosomes (once a virus?), that skew sex ratios.

Bisexualism does have many reproductive advantages, although as has been pointed out in this thread, there are costs associated with it; among multicellular organisms, bisexualism usually, but not always, has the minimal net cost. Trisexualism has even greater costs, but with no corresponding advantages. However, the mammalian X-Y chromosome determination is clearly not the only way to go.

[Chicago_Faucet]

Okay, so if we take Xiphos’ post above, and bounce it off of Hari Seldon’s, we start to see a trend here. We definitely are not done evolving, and most of the evolving is happening at the chromosome (midichlorian ) level. It appears that sex is winding its’ way out of the human genes? If this is true, then it cannot be good news. Or does it matter at all?

I get very little sex as it is now. I jest.

What does it actually mean that chromosomes are changing, and in some instances, disappearing?

I remember from Biology Class that the actual number of chromosomes doesn’t mean anything. Lobsters have far more chromosomes, in number, than humans, and elephants have far less. But our basic chemistry is changing. What does this mean, if anything at all?

[Sergio]

For an interesting story about three sexes read The Gods Themselves by Isaac Asimov

[Chicago_Faucet]

But we digress. I still don’t understand how the two genders came to be, from one gender.

Please have patience with me. I’m only a layperson.