Why are there only two sexes?

[Sunstone]

For the most part, living species come with only two sexes. That is, although there are a few species that have more than two sexes (at least one species of mushroom, for instance, has over 30,000 sexes), most species on earth have only a male and a female sex. Why is that?

 

[Me Myself]

I think the answer would come from why are there two sexes instead of one to begin with, and I think (the more versed in biology her will tell and onw better than me on it) that it had something to do with not passing down sick or damaged genetic material. I dont knnow, i think it was something like that

 

[Sunstone]

I think the answer would come from why are there two sexes instead of one to begin with, and I think (the more versed in biology her will tell and onw better than me on it) that it had something to do with not passing down sick or damaged genetic material. I dont knnow, i think it was something like that

The issue of why there are sexes to begin with is separate from why there are, for the most part, only two sexes.

 

[Me Myself]

The issue of why there are sexes to begin with is separate from why there are, for the most part, only two sexes.

I would assume it is related. For there to be three sexes I would assume they shod provide some sort of advantage over two sexes or no sex. Right?

Which advatage would is be? I other words, why would it be good for a sspecies to have three sexes?

 

[mycorrhiza]

My guess is more genetic variety, but with less complexity than 3+ sexes.

 

[InformedIgnorance]

The fact that organisms with a single gender managed to eventually produce descendants with two genders is significant enough - to suggest a move from one to three would have been incredibly more complex - while the move from two to three is probably not considered as useful for most organisms.

 

[Quintessence]

Question for you, Sunstone. How are you defining the sexes? Because in some respects, the numbers you put to it is is something of a cognitive construct. For example, would you consider an intersexed human a third sex? Why or why not?

This becomes particularly relevant when you look at the plant kingdom. Although plant reproduction is still based on male-female interaction, I don't consider plants to have two sexes. Why? Because any particular plant may be either:

- Sex Type 1: male reproductive structures only
- Sex Type 2: female reproductive structures only
- Sex Type 3: both, with reproductive structures located on distinctly different parts of the plant
- Sex Type 4: both, with reproductive structures located on the same part of the plant
- Sex Type 5: both, but with male reproductive structures developing before female ones
- Sex Type 6: both, but with female reproductive structures developing before male ones
- Sex Type 7: both, but with the male reproductive structures sterile
- Sex Type 8: both, but with the female reproductive structures sterile

These are not equivalents, and represent different reproductive strategies and habits within the plant kingdom. The question "how many sexes to plants have" is, to at least some degree, a construct. It's convenient categories we stuff things in to talk about them.

 

[Penumbra]

Because there would be too much drama otherwise.

 

[Mr Spinkles]

Great question, Sunstone! Frubals.

 

[Musty]

For the most part, living species come with only two sexes. That is, although there are a few species that have more than two sexes (at least one species of mushroom, for instance, has over 30,000 sexes), most species on earth have only a male and a female sex. Why is that?

My understanding is that sexual reproduction is a necessary evil in biological terms to ensure genetic variation in a population. Even single celled organisms that reproduce asexually do eventually have to exchange genetic materially either laterally or through sexual reproduction to maintain genetic fitness.

Lateral gene transfer isn't really an option for multi-cellular organsism leaving sexual reproduction. Sexual reproduction is more costly than asexual reproduction and I would imagine that these costs increase with the addition of more sexes directing evolution towards favouring only two sexes. There will be exceptions where the reproductive strategy itself negates some of these costs.

Of the top of my head I'm not aware of any species that have more than two sexes, only species with more elaborate reproductive cycles than humans.

 

[Valjean]

Biologic parsimony, I'd imagine. Ie: the KISS principle (keep it simple, stupid).

The whole point of sex is to mix things up genetically, producing greater variation in offspring and, thus, greater adaptability. The simplest approach is for two anatomically identical individuals to simply exchange genetic material and then split by fission. Works for bacteria and protozoa.

More complex, multicellular organisms couldn't just cleave together and let their insides mix, so they split their genes, packaged them in specialized structures and then exchanged halves. Sort of like sex, but still a pretty simple strategy between anatomically identical individuals.
Works for earthworms.

The complete sexual dimorphism you see in mammals & al is harder to explain. It's a more complicated tactic, though I suppose specializing in only one half of the genetic compliment is a sort of anatomic simplification.

It's hard to imagine what advantage further sexual complexity would confer, or how it would evolve. What would be the function of a third sex? Meiosis only produces two types of gametes, after all
I'd hesitate to underestimate the extravagance of Nature, though. Some of the reproductive strategies of certain parasites would put Rube Goldberg to shame, for example.

 

[Penumbra]

Biologic parsimony, I'd imagine. Ie: the KISS principle (keep it simple, stupid).

I agree. Two sexes probably balances genetic diversity with simplicity in a way that no other method has displaced due to being better.

It's hard to imagine what advantage further sexual complexity would confer, or how it would evolve. What would be the function of a third sex? Meiosis only produces two types of gametes, after all
I'd hesitate to underestimate the extravagance of Nature, though. Some of the reproductive strategies of certain parasites would put Rube Goldberg to shame, for example.

I can imagine some advantages.

Partner Mating Percentages:
-If there is one sex that is a hermaphrodite (but that is sexual, not asexual), then any individual could mate with 100% of the other individuals of that species. Any two individuals could pair up.

-If there are three sexes, and any combination of two works for reproduction, then any individual could mate with 66.67% of other individuals of that species.

-If there are four sexes, and any combination of two works for reproduction, then any individual could mate with 75% of other individuals in that species.

-Currently with only two sexes, approximately 50% of individuals represent viable mating partners.

Specialization:
-Two sexes means two opportunities to specialize. Male and female humans are optimized for slightly different things. Male and female lions are optimized for slightly different things.

-More sexes means more opportunities to specialize. In one of the Isaac Asimov novels, 'The Gods Themselves', there is an alien race that has three sexes that has to form a 3-member bond to mate. One sex is rational, one is emotional, and one is parental.

.....

Most of those scenarios I guess don't offer enough survival benefits to offset the additional complexity. The hermaphrodite scenario where all members have both sexes and can mate with one another may actually be the simplest and most optimal but probably for a good reason or another is not how it developed for most complex species.

 

[Iti oj]

For the most part, living species come with only two sexes. That is, although there are a few species that have more than two sexes (at least one species of mushroom, for instance, has over 30,000 sexes), most species on earth have only a male and a female sex. Why is that?

I think the answer would come from why are there two sexes instead of one to begin with, and I think (the more versed in biology her will tell and onw better than me on it) that it had something to do with not passing down sick or damaged genetic material. I dont knnow, i think it was something like that

Great questions. To me myself it was a beneficial mutation that added more genetic diversity into the gene pool I think sped up adaptaion.
why two probably the starting point and optimal.

 

[BSM1]

I woke up one morning with what I think was a member of a third sex. Woof! That was all the argument I needed for two sexes.

 

[Jainarayan]

I woke up one morning with what I think was a member of a third sex. Woof! That was all the argument I needed for two sexes.

Tell me about it. A blind date I had described himself on the phone as 6', long brown hair (I like long-hairs... well, whom do I not like!? ), had a sexy voice. For whatever reason only God knows, I pictured Antonio Banderas. It was not Antonio Banderas who showed up at my door... it was more like Cher.

By the way, in my circles, "woof!" refers to a really hot guy. When I worked at Macy*s, there were two other gay guys in my department (they don't call it Gacy*s for nothing). Whenever we saw some hot guy shopping we'd be "woofing" like crazy, and tripping over ourselves to help him. A female co-worker said "geez, it's like being in a kennel with you guys".