Evolution of sex

[Strinka]

First, I believe in evolution and don't want any replies about god or anything. If you absolutely must, go start a new thread in GD. (As if we don't have enough debates like that)

How did sex evolve? I can imagine how the organs evolved, but not how two sexes would originate.

[Captain Amazing]

I don't know how it happened, but I'll mention that sexual reproduction can be good for a species, because it increases genetic diversity. With asexual reproduction, the only way the daughter organisms can be different than the mother organism is through mutation.

I think you might find a clue as to how it started by looking at paramecia, which reproduce asexually, but sometimes engage in a process of conjugation, where two paramecia exchange genetic material.

[ultrafilter]

I don't know how it happened, but I'll mention that sexual reproduction can be good for a species, because it increases genetic diversity.

But selection doesn't occur at the species level, so does it matter if something is good for the species?

[Josh_dePlume]

Right on, ultrafilter.

The OP is mixing together two related but distinct questions: 1. Why is there sex, i.e., the reshuffling of genes from two individuals? 2. Why are there two different sexes, i.e., why the asymmetry? In many microorgansims, and presumably in the first organisms that had sex, there is sex without asymmetry (there are usually different mating types, but not different sexes). The first is what evolutionary biologists usually mean by "the evolution of sex". The second falls under "the evolution of anisogamy" and "the evolution of sexual dimorphism". I should really make further distinctions; in a population of hermaphrodites, there are not two sexes, but there are two roles and there is anisogamy. Much ink has been spilled on these topics. I think we've done this before.

[Lightnin']

But selection doesn't occur at the species level, so does it matter if something is good for the species?

Of course it does, if I'm understanding your question.

If two species are fighting over the same resources, the more prolific and/or efficient species will win, driving the other species either out of that particular niche, or into extinction.

[ultrafilter]

Of course it does, if I'm understanding your question.

If two species are fighting over the same resources, the more prolific and/or efficient species will win, driving the other species either out of that particular niche, or into extinction.

Maybe, maybe not--100% one way and 0% the other isn't the only stable situation--but the point is, natural selection os a matter of which *individuals* prosper. Traits are selected because they benefit an individual organism, not because they benefit the species.

[Colibri]

But selection doesn't occur at the species level, so does it matter if something is good for the species?

There are some species of lizard that are all female, which reproduce by parthenogenesis. These species in general seem to be much younger than the ancestral sexually-reproducing species. This implies that non-sexually reproducing species are much more prone to extinction than sexually producing ones, most likely because they are less able to adapt to environmental change. This certainly seems to constitute selection at the species level.

[Darwin's Finch]

How did sex evolve?

Good question, but not one you will likely find an answer to here - no one knows yet. There are several competing hypotheses (see this Wikipedia article (http://en.wikipedia.org/wiki/Evolution_of_sex) for a summary, as well as this article (http://www.newscientist.com/article.ns?id=dn4960)), and we might have a few insights as to how it might have happened and why, but we just don't really know yet.

[Tapioca Dextrin]

Not every spcies has two sexes. If you were a slime mold (http://en.wikipedia.org/wiki/Slime_mold), the whole notion of sex is completely alien. As best we can figure it those guys have 200+ genders (depending on how you count).

[Diogenes the Cynic]

Not every spcies has two sexes. If you were a slime mold (http://en.wikipedia.org/wiki/Slime_mold), the whole notion of sex is completely alien. As best we can figure it those guys have 200+ genders (depending on how you count).
Yikes! Imagine how complex the Kinsey scale must be for those buggers. I wonder which configurations constitute the "perverts" of the slime mold world. Like is it normal to be into 70 different genders but if you like 72 you're a "sicko?"

The mind boggles.

[SCSimmons]

I have to imagine that sex between slime molds involves a lot of alcohol ....

[ultrafilter]

This certainly seems to constitute selection at the species level.

Strictly speaking, yes. The point I'm trying to make, though, is that traits are selected for or against based on their effect on the individual, not the species. You'll agree with that, right?

[Colibri]

Strictly speaking, yes. The point I'm trying to make, though, is that traits are selected for or against based on their effect on the individual, not the species. You'll agree with that, right?

In the short term, yes. However, certain traits that may be advantageous for individuals in the short term may make the species they belong to more susceptable to extinction in the long term, so in this sense such traits (such as asexuality) may be selected against at the species level.