Battle of the Sexes – Part B: Phases of sexual selection

Since the conception of sexual selection by Charles Darwin in his books, On The Origin of Species by Means of Natural Selection¹ and The Descent of Man and Selection in Relation to Sex² , more than 200 years ago, this field³ has seen substantial progress over the years through evolutionary biologists. As discussed in Part A of this article, sexual selection is conceptualized in two broad categories, intersexual selection, where one sex chooses the counterpart on the basis of certain desirable physical or behavioral traits (e.g. the evolution of exaggerated traits like peacock’s tail, size of deer’s antlers) and intrasexual selection, where selection occurs within the sexes (e.g. male-male competition to acquire a female mate). We now know that different species have various ways to get attracted to the opposite sex, suggesting that standards of ‘beauty judgment’ in most cases in the animal kingdom is distinct from what humans perceive as sexually attractive for a given trait to be sexually selected. It need not be just a visual attraction for mating display or a nuptial gift; it could be pheromones, or even the ability to manipulate fertilization of eggs. Sexual selection, therefore, is a means by which natural selection acts on the variation in reproductive success of individuals in a given population. This variation arises either due to competition for mates, preference for a particular type of mate, or fertilization success after completion of mating ritual. In this context, the current article aims to broadly discuss two phases of how sexual selection operates. Studying sexual selection has been on the research table in largely two different categories: pre-copulatory⁴, i.e., selection acting on traits (characters) prior to mating and post-copulatory^5,6, i.e., selection acting on traits after successful mating. While species undergoing selection on traits prior to mating provides adaptive significance, selection acting on traits in the post-mating stage is not entirely obvious. As we discussed in Part A, sexual selection as a direct result of female choice is fairly straightforward: if females choose a particular trait in males, those males with the preferred trait will get a relatively higher number of mates consequently siring greater number of offsprings. This implies that both the female choice and male’s preferred trait would tend to propagate in a given population as long as the correlation between these two traits exists. But why does female preference exists to begin with?

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Darwin explained the concept of sexual selection on the basis of positive assortative mating, i.e.; individuals with similar phenotypes tend to mate with one another more often than expected under an ideal random mating system. Ronald A. Fisher took this observation a notch further to understand the mechanisms behind female preference and tried to explain with single preference and preferred traits in females and males, respectively. He essentially laid down the importance of correlation between these two traits for evolution of mate choice^7,8. For e.g. if females of a given species give preference to mate with males with largest size of antlers, then their offspring will have both the genes for large antlers from father and genes for the preference for large antlers from the mother. As a result, genes for producing large antlers will get selected and inherited across generations and hence evolve as a result of female preference and genes for preferring large antlers will evolve as a correlated response to this selection. In some species, the females choose males that provide immediate ‘gain’⁹ such as a nuptial gift by male spiders^10-11, parental care in some fish species¹², or territorial defense in social animals. The reason why this trend of gifting would evolve is fairly simple since females have access to nutrition, care, or protection; however when males just offer sperm and nothing more, why females  should fall for their flashy plumage is not obvious to comprehend^2,3,13-16. Researchers have been using theoretical models, experiments and fieldwork in order to understand this intriguing method of nature.

So far our understanding is that, in cases where females of a given species do not receive a direct benefit from their partners,  do still choose to mate and reproduce because it increases her evolutionary fitness since her offspring gains higher fitness owing to the mother mating with a ‘good quality’ male.

Several theoreticians have laid down models to explain why such preference and preferred traits would evolve and to what extent. Could the peacock’s tails have been even longer than what we see today? Could the male birds of paradise (in Papua New Guinea, Indonesia, and some parts of Australia) sing and dance even more elaborately? Could the antlers of red deer be even more complicated, heavier, and stronger? Well, it has to stop being fancier to the eyes somewhere, since evolving any given trait has a ‘cost’ attached! Yes, it comes with a price.

A preferred trait can only evolve to please or acquire the counterpart gender to an extent when it is not detrimental to its own survival.

While we humans have been trying to understand this phenomenon since ages and have inferred that such traits could be reliable indicators of genetic quality and hence females would prefer those males. Consequently, female choice^2,7,14,17 evolves because the chosen males mate and reproduce offspring that tend to have greater survival and reproductive ability to carry forward this set of genes. The other approach to study this selection aims to understand levels of sexual conflict in a given species. Females tend to choose males who complement their own genome^18-20, in other words, when males and females have a ‘difference in opinion’, rather incompatibility in terms of what kind or how many offspring to reproduce, or how many times to mate, etc. This has been under both theoretical and experimental work by researchers in a variety of species aiming to understand sexual conflict and its importance in female choice evolution.

Sexual selection is not limited to just fighting for acquiring a mate and mating itself, because not all matings result in successful sperm transfer and not all sperm transfers result in successful fertilization of eggs.

This is where the 2nd phase of sexual selection comes to play. As technically termed ‘post-copulatory’ sexual selection, from an adaptive point of view why should the males bother after mating with a female? That’s because even in a simplistic situation if a female mates with more than one male and if the second male is better capable of removing the previously deposited sperm (or ‘kill’ them) from the female reproductive tract, then the second male will stand a greater chance to sire its offspring through this female. The females obviously do not lose out on such situations so easily; rather the evolutionary arms race continues to fascinate us. In species where post-copulatory sexual selection is prominent, females also evolve defensive mechanisms⁶  and ‘choose’ sperms! For e.g., females modify pH of their reproductive tract to reduce defensive mechanisms against microbial action that potentially is designed to kill sperms, some female species maintain the sperm alive in storage sites of reproductive tract, some can discard sperm from previous males while some can securely store them⁶! Some can even modify their reproductive tract morphology in a way to refrain from subsequent matings being successful or simply refrain to re-mate. The list of female defensive mechanisms studied is long and interesting. Basically, the next step then is any male being capable of overcoming these mechanisms stands a fair chance of siring its own offspring. Male-male competition at this level is essentially what researchers terms as “sperm competition”^21-24 that is defined as competition between² or more sets of sperm potentially having adverse effects on each other in order to fertilize a given set of eggs. For e.g., sperm removal has been shown to occur in damselfly where the male fly performs intercourse in a way in which first he removes the previously deposited sperm and then fills the female reproductive tract with its own sperm. Some species choose a safer mechanism for winning this competition just by sticking around the female and not let her re-mate until she lays eggs or reproduces. Some males even plug the female reproductive tract or inject a cocktail of chemicals (hormones and proteins) that makes her non-receptive for prospective males. What researchers have realized over the years is that females do counter these kind of post-copulatory mechanisms^{5,13-15,25-27} by males by evolving mechanisms to bias which sperm she wants to fertilize her eggs with. But where does this evolutionary arms race get limited and what factors limit it? Male adaptations to anyhow make their sperms better than another male is speculated to directly damage the females, including her egg laying output and lifespan. Does this mean a newer way is needed to evolve the sperm’s competitive ability since a female’s direct fitness is at stake? Several interesting experiments including selection experiments have been carried out in a laboratory setting^28-30 using fruit fly populations as the model to examine the effects of such competition on female reproductive ability in terms of physiology and direct egg output. This includes studying sperm morphology, the chemical composition of the ejaculate, sperm length, and sperm swimming speed and how this diversity affects fertilization in females; understandably a hot topic of research among evolutionary biologists interested in sexual selection.

As we can see there is no one explanation that could account for all cases of diversity with respect to male-male competition or female choice and evolution of such processes. It is definitely impressive, nevertheless, to see that a man called Charles Darwin had a vision more than 200 years ago about the phenomenon of sexual selection (among other things) through observation and sharp analytical bent of mind that still gives us enough food for thought and continues to inspire progress, power to speculate, and explain new findings in modern research!

Author/ Illustrator

Illustrator

Bhrugu is passionate about communicating the science in a creative way. He loves to create simple but effective write-ups and illustrations that present the scientific story in a fun and impactful manner. Have a look at his SciComm portfolio on his website; Art BY Scientist. Currently, Bhrugu is working as a postdoctoral fellow at Emory Vaccine Centre, Atlanta, USA and trying to cure HIV/AIDS. He gets the energy to do all these different things by meditating in his soul laboratory where he attempts to bring amalgamation of science and spirituality. Would you like to connect with him? If so, you can find him on LinkedIn and Twitter.

Editor and Blog design

Dolonchapa is a Postdoctoral Fellow at NYU Langone working on Infectious disease with a focus on cell wall metabolism to identify new targets for therapeutic attacks by Pseudomonas aeruginosa, a common opportunistic human pathogen. She also serves as the Co-Chair of the National Postdoctoral Association’s Outreach Committee. She believes in the power of technical storytelling as an effective tool for scientific outreach and looks forward to practicing this art as an editor at Club SciWri. Follow her on Twitter.  

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