Literally speaking, every author has a proud mom. She probably would be the one reading each and every writing composed by the author. And my proud mom who happened to read my blog posts asked me why most of the topics I wrote happened to be SEX. I answered, “Mom. Reproduction is very very important. We typically spend about 2/3 of our life, finding and keeping mate. I would not be here if there is no sex between you and dad. And you would not be there, if there were no sex between grandma and grandpa. It could go all the way back.” This article is not about my mom or my compliant on why she asked me about the nature my blog posts. This is about the evolutionary origin of sex and why organisms have sex.
Humans, among many species, reproduce sexually via gametes and sperms. Others do not. In some species, individuals called hermaphrodites can produce both male and female gametes. For example, in Indo-Pacific reef-dwelling cleaner fish Labroides dimidiatus exist as a group of females with only one male. The male is the largest fish in the group. If the male is removed from the group, the largest female turns into the male fish (Halliday, 1980).
Others, unlike humans and many other species, reproduce asexually. Many form of asexual reproduction in protozoan organisms simply divide themselves into two. Learn more about the examples here: http://www.biology-pages.info/A/AsexualReproduction.html
In many plants and animals, the organisms reproduce asexually, resulting in identical individual with whom organisms share 100% of the genes. If evolution is about passing on the genes, then why don’t all living organisms reproduce asexually? Sexual reproduction in humans, for example, only result in offspring with whom two parents share only 50% of their genes (half from your mom and half from your dad). Only half of the genes from dad and half of the genes from mom are passed on. Plus, There are so many disadvantages of sexual reproduction. One has to find so much time finding, attracting, retaining and copulating with mates. There are also higher risks of predation and intense species competition while finding mates. So why SEX?
The Origin of Sex
Before we go deeper into different theories on what sexual reproduction can confer, compared to asexual reproduction, let’s think about the evolutionary history of how sexual organisms came to exist as two distinct gametes (sperms and eggs). What is the origin of Adamic and Evian gametes?
Eukaryotic multicellular organisms seemed to have arisen from unicellular gametes working together as a unit. A possible hypothetical model in explaining the origin of sex would start with marine ancestors that release gametes (Lehtonen and Parker, 2014). Gametes could have various degree of sizes. Larger gametes have a hard time finding partner to fuse with other gametes in water because the size slows down the swimming rate. Therefore smaller gametes could be more effective at swimming and finding other gametes more quickly. However, the larger zygotes via fusion of larger gametes, have other advantages (Bell, 1997) because larger gametes provide the fertilized zygote with much more nutrients. It is important to realize that there is no fossil or eye witness that this situation did indeed happen; however, the science in explanation of origin relies heavily on mathematical models and stimulations (Lessells et al., 2009). One model by Parker et al. (1972) suggested a situation in which the two distinct gametes (eggs and sperms) could be stable. This would lead to distinct males and females. We will also explore further into evolutionarily benefits of sexual reproduction.
Evolutionary Advantages of SEX
Why Sex, rather than Asex? Halliday (1980) explained that sexual reproduction could give huge advantages for the organisms living in the unpredictable harsh environment. The unstable environment could be more prone to strong Darwinian hostile forces of nature such as diseases, predations, competition for resources, etc. In these situations, sex could be really useful in passing on the genes. In the article, I have briefly mentioned three theories, which I think have been widely advocated by researches.
The lottery principle
Imagine you are in the lottery program to win money. One of the odd rules is that there could be multiple tickets with repeating numbers. Again imagine that you know the winning lottery number ahead (predictable environment), how would you increase your chances of winning? Of course, by having multiple identical winning tickets. However, if you imagine a situation in which you do not know the winning lottery number ahead, what tickets would you buy to increase your winning chances? The likelihood is statistically increased by having multiple tickets with all different numbers.
The lottery principle is first suggested by Williams (1975) http://press.princeton.edu/titles/838.htmlthat sex led to variety in order for the genes to survive in unpredictable novel environment. Variety of genes in offspring increase the chances of passing on the genes, especially in the harsh environment. I read some pretty interesting animal examples of lottery principle in Cartwright (2002).
Some creatures such as asphids breed both sexually and asexually. In stable environment and time such as summer, they reproduce asexually passing 100% of the genes. In harsh time like winter, they often reproduce sexually. Typically parasites act this way; first the parasite invade the environment via asexual reproduction. After the environment is filled with identical replicates, parasites switched into sexual reproduction, increasing variety and enhancing fitness.
Red Queen Hypothesis
The Red Queen Hypothesis is referred to the Red Queen in Alice in Wonderland, whom ran fast with Alice. When both of them stopped running, they are still in the same position. Alice commented on this and the Red Queen answered, “ Now, here, you see, it talks all the running you can do to keep in the same place.” Looks here: http://www.indiana.edu/~curtweb/Research/Red_Queen%20hyp.html
In comparison to the story, you can imagine that organisms that reproduce sexually shuffle genes generations after generations to keep up with parasitic invasions.
Based on lottery principle suggestion that the predictability of the environment is an important factor in sexual reproduction, evidence finds that the global population of sexual reproduction is mostly practiced in environments that are stable. Evidence of sexual reproduction seems to be more common when the environments are stable but have higher chances of parasitic interactions in the communities (Bell, 1992).
Plucking Rubies Hypothesis
Organisms that reproduce asexually pass on 100% of their genes, including harmful and deleterious ones. This is bad news. Natural selections would weed them out from the population. On the other hand, sexually reproduced organisms pass on the genes and reshuffle the genes. It decreases the chances that the parental generations pass on their harmful mutations or deleterious genes to the next generations. And the Plucking Rubies hypothesis is the idea that sexual reproduction increases the “good” mutations that are beneficial to organisms and weed out “bad” mutations more quickly than asexual reproduction. A recent experimental research published in Nature demonstrated how sexual reproduction picked rubies from the trash, using Brewer’s yeasts. Here is the secondary article with the primary research cited within. http://www.economist.com/news/science-and-technology/21693542-it-not-just-emotional-mystery-its-very-existence-poses-deep-question-sex
There are tons of scientific studies and experiments that deeply investigated the materials I briefly mentioned here. I have always been amazed by the explanatory power that evolutionarily informed studies can provide. To me, it is much more informative, meaningful and fascinating of a story than Adam and Eve in the book of Genesis.