How Did Gender and Sexual Reproduction Evolve in Organisms?

[Sikz]

The simplest organisms reproduce by copying their OWN genetic code- the next step was sexual reproduction; creating a new set of genes out of two old ones. So what caused some organisms to lose their ability to choose any partner in their population, having instead to choose a partner of the opposite sex? Perhaps a mutation of an X chromosome in a single-celled creature into a Y and the eventual change of the hermaphrodite X chromosomes into female Xs? Just a guess... Does anyone know?

Additionally, does the fact that the males of many species have nipples have anything to do with this? If not, why are they there?

 

[wasteofo2]

https://www.physicsforums.com/showthread.php?s=&threadid=10055&highlight=nipples

for the nipples question.

 

[Moonbear]

Okay, I'll start with the easy part. Nipples has nothing to do with it. Only mammals have nipples, not all animals. As for why males have them, well, simply because there's no reason for them NOT to have them. That's the simplest explanation of how evolution works.

As for how sexual reproduction evolved from asexual reproduction, that's a much tougher question. The simple answer is there is no answer. The evolution of separate sexes may not even be the same question. There's one theory with which I'm familiar. There are likely others, but I haven't looked that far into this question. There's a researcher, J.J. Crews, who studies lizards. The particular species he studies is interesting because they can be either parthenogenetic (I probably spelled that wrong), which means a haploid, unfertilized egg can develop into a lizard, if there are only females and no males present, or, if there are males present, they can reproduce sexually with fertilization of the egg by the male. Crews and his lab study these lizards because they may provide clues about the evolution of sexual reproduction (though not really the evolution of two sexes). A further interesting thing about these lizards is that in order for the parthenogenetic development to occur, two females engage in the same mating ritual as the female would otherwise engage with a male. This suggests there is something more than just fertilization that is important about the behaviors involved in mating for reproduction of that species. The theory arising from this work is that sexual behavior may have actually evolved before sexual reproduction. You can look up his articles for more details (or ask your librarian for help if you're not sure how to do this). Even if you don't have a lot of scientific background, many of his papers on the behavioral experiments are a reasonably straightforward read. I don't know what his recent work involves, so you might have to dig back to things published 10 years ago to find the stuff I'm talking about (I haven't followed that field for a long time now).

Now, how two sexes evolved may be a completely different question. I haven't thought much about it before. One direction you might want to explore in seeking some answers to this would be work in species such as certain reptiles that can develop into either males or females depending on environmental conditions, such as temperature. It seems like an obvious question now that I think about it, but I've never paid much attention whether there is a genetic difference between the sexes in those species or if it's entirely at the protein expression level.

 

[selfAdjoint]

A new and interesting data point on evolution of sex: http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v427/n6972/abs/nature02228_fs.html

 

[LURCH]

Perhaps a clue can be found in the lives of hermaphroditic flat worms. These worms gather each year for mating and, as their name implies, each individual has both male and female sex organs. The female reproductive capacity of each individual is entirely internal. There is no external female sexual organ. The external portion of the male reproductive system is particularly long and quite pointed at the end.

The reproductive act pretty much amounts to a "fencing match" between two individuals. When one achieves penetration, the other is inseminated. The apparent evolutionary purpose for this competitive style of mating is that the loser now becomes the female, and is done mating for the year. She is now pregnant and must focus on the task of successfully completing the pregnancy and delivering her young. The "winner" may continue to fight/mate. This ensures that the fastest and strongest individuals spread their DNA more widely.

It is conceivable that, over time, the schism between the two types could become permanent and begin manifesting external differences. This would seem to be a good explanation for the case we see among primates, felines, certain canines, and other mammals, wherein the largest and most aggressive eventually evolved into the males while the smaller and less muscular became specialized in childbearing. However, it does seem to contradict the many species in which the female is by far the larger and stronger of the two genders.