Goals: learn why the original hypothesis for the evolution of sex is based on group selection, and therefore unlikely to be correct. Learn other hypotheses for the evolution of sexual reproduction. Learn how sexual reproduction, once it has evolved, may be subject to species selection.
Related Textbook Material: Freeman and Herron (2001) Chapter 7
Lab Manual Questions over this material are in Lab Manual Chapter XX
The Lecture:
Today we're going to ask "why do we have sex?" But before you get too excited, note that what I mean by this question is "why has sexual reproduction evolved and why is it present in so many species?"
First let's note that what we mean by sexual reproduction is reproduction in which different individuals combine genetic material to produce offspring so that offspring are not genetically identical to their parents, and not all of a parent's genes get passed on to an offspring. Sexual reproduction occurs in many different species, including most animals, plants, and fungi, and also including single-celled organisms which can combine genetic material with each other before undergoing cell division to reproduce. In some groups, like most plants, both sexual and asexual reproduction can occur. There are also many groups in which only sexual reproduction is possible (until we learned to clone them, this was true of mammals, and it's still true that unless we cause asexual reproduction to occur through cloning, mammals reproduce sexually.) So there are a lot of different groups with sexual reproduction and it is something whose evolution we need to explain.
Sexual reproduction turns out to be another trait, like some we discussed last week, whose evolution was initially hypothesized to have occurred through group selection. The main original group selection hypothesis for the evolution of sex states that since sexual reproduction causes recombination of genes and therefore creates more combinations of genetic traits in a population -- more genetic variation -- a population of sexually reproducing organisms can go through more natural selection, and adapt better, than an asexual population. Note that while this is a plausible advantage for a population of sexual organisms, that it is an advantage to the population not the individual. Since we saw last week that group selection is unlikely, this hypothesis is NOT likely to explain the evolution of sexual reproduction.
The only way the group selection hypothesis for sexual reproduction could explain sex is if there are no individual level costs to sexual reproduction. If there were no individual fitness costs to sexual reproduction, then even though group selection takes a long time it could eventually result in sexual reproduction evolving since the reason group selection is generally unlikely is that if there is individual selection for other traits, they will evolve much faster and we won't see the effects of group selection. So we can ask whether there are individual level fitness costs to sexual reproduction and the answer is YES. So the group selection hypothesis will not work as an explanation. Now let's consider the fitness costs to individuals from sexual reproduction.
The MAIN fitness cost to sexual reproduction in diploid species is called the two-fold cost to sex. The two-fold cost to sex refers to the fact that when sexual reproduction occurs, each reproducing individual passes on just one of the two alleles it has for each trait, but when asexual reproduction occurs, each reproducing individual passes on both alleles it has for each trait. So since sexual individuals have the alleles that code for sexual reproduction and asexual individuals have the alleles that code for asexual reproduction, the allele for sex gets passed from generation to generation at half the rate of the allele for asexual reproduction. The asexual individuals pass on two alleles for asexual reproduction for every one allele for sexual reproduction that the sexual individuals pass on. This means that sexual reproduction has half the fitness of asexual reproduction..
Some other costs to sexual reproduction may apply in some species. It may take time and energy to find and attract a mate. Sex may by risky -- while mating one may be vulnerable to predation, and exposed to sexually transmitted diseases. Whether or not these costs occur depends on the environment of the species.
Since sexual reproduction is generally subject to the two-fold cost to sex, and is sometimes subject to additional costs, we would predict asexual reproduction to evolve through individual selection too fast for group selection to cause sexual reproduction to evolve. So the group selection explanation won't work. Since there are so many sexually reproducing species, there must be individual level natural selection benefits to sexual reproduction that are strong enough to outweigh the individual level costs we've discussed.
For sexual reproduction to be an advantage on the individual level, it must be an advantage for parents to produce genetically variable offspring. For this to occur, some aspect of the environment must be changing fast enough so that there is a good chance that offspring will be in a different environment from the parents -- otherwise more offspring would survive if parents adapted to an environment simply produces offspring like themselves, asexually.
We need to ask what aspects of the environment would change rapidly enough so that there is a good chance that the environment of offspring would be different from that of the parents. Two kinds of change are: change in the physical environment and change in the living environment.
Change in the physical environment: For some small species in which there are several generations in a year, changes in the physical environment that occur from season to season might be enough to give sexual reproduction high fitness. In some of these species, both sexual and asexual reproduction occur, and the timing of sexual reproduction supports the hypothesis that sexual reproduction is an adaptation to environmental change. For example, in water fleas (crustaceans in the genus Daphnia), which live in ponds, reproduction is asexual -- females produce females asexually -- throughout the spring and summer, but when they are getting ready to produce the forms that will overwinter and hatch out the next spring (possibly in a very different environment, since it will be a different year), males are produced and then they reproduce sexually. So sexual reproduction is timed to occur when the environment is about to change.
Change in the living environment: For many species, it appears that the physical environment does not change rapidly enough so that offspring will be in a different environment from the parents. For such species, it is thought that change in the species around them, especially in parasites and pathogens that infect them, may be what causes sexual reproduction to have high fitness. Remember that parasites evolve rapidly to overcome host defenses. As a result, the parasites in the environment keep changing -- they keep evolving. Offspring are likely to be in an environment with parasites that have adapted to be able to attack their parents, so producing offspring that are genetically variable and not identical to either parent may result in offspring having higher resistance to parasites, and therefore better survival.
Now that we have seen hypotheses that can explain how sex can evolve within a species, let's see what effects it has once it does evolve. One thing sexual reproduction may do is to affect the rate of speciation. Since sexual reproduction causes recombination of genes, it produces new combinations of alleles at different genes much more quickly than does asexual reproduction. Based on this increase in genetic variability in sexual species, evolution can occur more rapidly, so speciation should also occur more rapidly. Sexual reproduction is thus subject to species selection. One reason there are so many species with sexual reproduction may be that sexual reproduction increases the rate of speciation, and so many species with sexual reproduction are produced. As with other cases of species selection, note that this does not explain why sexual reproduction evolves within a species. It may, however, help to explain why there are so many more sexual species than there are asexual species.