Journal of Young Investigators
    Undergraduate, Peer-Reviewed Science Journal
Volume Seven
Issue 3, May 2003

Male vs. Female: Gender Conflict as an Evolutionary Force

Gregory Campbell
Biochemistry, University of California - Santa Barbara

Image courtesy of Greg Campbell

Most people are at least somewhat familiar with Charles Darwin and his writings on natural selection. He wrote that those individuals who were better suited to their environments would be more likely to reproduce. Over time, this would cause a shift in the overall properties of the population. Today, we recognize the relationship between natural selection and evolution as random mutations in the genetic material of some population's members, which, in turn, will cause variety in that population. Because all individuals are different, it is only natural to expect varying levels of ability, intelligence, appearance, limitations, goals, decision-making, etc. This leaves no room to doubt that a particular organism's genetic composition will naturally make it more compatible with its environment than some other member of the same population. It's really just common sense, and, it makes a pretty solid basis for the principles of evolution and speciation.

The theory of sexual antagonism

However, this idea of "survival of the fittest" is not the only way that evolutionary forces influence populations. In fact, there are situations in which a "pseudo-equilibrium" (where the population genetics are changing, but in a cyclic fashion such that there is no apparent net gain or loss) is established between the two genders of a population. One working explanation for non-"survival of the fittest" evolution is the theory of sexual antagonism, which provides a mechanism for an evolutionary equilibrium. The fundamental principle behind this theory is the notion that under some circumstances, an organism can exhibit some heritable property, which will increase its own fitness, or sexual efficiency, while being detrimental to the fitness of its mate. In response to the spreading of this one-sided genetic factor (which could be in favor of either gender) in the population, the disfavored sex can counter-adapt to the harmful trait in such a way that the previously favored sex is now at some disadvantage, while the previously disfavored one holds the natural "lead.

Image courtesy of Greg Campbell

Dr. Bill Rice at the University of California, Santa Barbara, has done extensive research in evolutionary genetics, and has made more than his fair share of contributions to the understanding of sexual antagonism. Using Drosophila melanogaster (the fruit fly) as a model system, Dr. Rice maintains a variety of pure, genetically unique populations which can be manipulated under highly controlled conditions. One area of experimental interest is the enforcement of monogamy versus the promotion of promiscuity. It should first be explained that the seminal proteins of male D. melanogaster have a mildly toxic effect on the females. This is a result of adaptation/counter-adaptation circumstances that is an integral part of the evolutionary history of Drosophila. One function of these toxic proteins is to act as a spermicide to the sperm already present in the female from another suitor.

The enforcement of monogamy versus the promotion of promiscuity

In 2000, Dr. Rice published his findings showing that when flies in normally promiscuous habitats were experimentally forced to be monogamous, the evolution of the seminal fluids proteins ultimately promoted the extension of mate survival time, rather than the reduction of it, relative to the promiscuous controls. In addition to this observation, Dr. Rice noted that monogamous males who were placed back into a promiscuous environment displayed an overall reduction of fitness. These data are of particular interest because they show a correlation between the rate of mutation of sexually antagonistic traits, and the degree of commitment between an individual and its mate. This rate is important because of the elevated evolutionary rate of reproduction-associated proteins. A 2002 article in Science indicates that the divergence of these proteins is especially noticeable in the primate lineage leading to humans, mice and rats, marine invertebrates, and D. melanogaster. The observed relationship shown here, in the reproductive proteins of humans and D. melanogaster, gives credibility to the idea that we can learn about our own evolutionary past by observing the rapidly multiplying fruit flies (~1 generation per 2 weeks).

Image courtesy of Greg Campbell

Because sexual antagonism acts as an evolutionary catalyst, at least in terms of these reproductive proteins, it is quite possible that this mechanism is likely to influence the initiation of other evolutionary landmarks, like speciation, Dr. Rice said in a 1997 sociobiological publication. Speciation is the creation of so much genetic divergence that isolated members of what was once one species will become reproductively incompatible. Over time, they will become separate species. Because inter-sexual conflict is so intimately involved with the genetics of the reproductive biochemistry, anatomy, physiology, and behavior, its role as a catalyst for these changes certainly makes it seem like a vitally important factor in the speciation process.

Monandrous (monogamous) groups of female flies were tested against polyandrous (promiscuous females, having high inter-sexual conflict) groups with respect to speciation-causing factors. Another of Dr. Rice's observations, published in 2000, was that speciation occurred four times faster in the polyandrous groups, once again supporting the notion that gender conflict is what Dr. Rice calls an "engine of speciation."

A relationship between human intelligence and sexual antagonism?

Another possible consideration for a trait heavily influenced by sexual antagonism is the phenomenally high level of intelligence in human beings. It may be reasonable to think that there exists some relationship between human intelligence and sexual antagonism because the rapid increase in our brain size is evolutionarily marked by the development of language. As we developed a more sophisticated means of communication, we opened doors to a refined and precise system of various signals and responses. Many of these types of systems can be super-stimulated via an emotional or instinctual reaction which changes the receptive nature in favor of the gene set responsible for compensating for the over-stimulation. Basically this means that over time, what was once thought overwhelming will become the norm.

Perhaps not at this exact moment in time, but somewhere along the line, it will be of huge significance not only to better understand sexual conflict as a very relevant evolutionary force, but also to develop a better grasp on the extremely influential evolutionary forces that we have yet to consider, especially stabilizing or destabilizing forces that, on the surface, don't exactly fall under Darwin's natural selection theory of "survival of the fittest." There is a whole world of changes happening constantly: in every atom, molecule, cell, organism, social construct, planet, galaxy, etc.; so there must exist, in vastly greater numbers, causes of these changes. As we learn more about how each individual contributes to the whole, we leave ourselves exposed to tremendous insight into the nature of life and existence, so long as we remain open to them.

Further Reading

Rice, William. Dangerous Liaisons. Proc Nat'l Acad Sci. 2000;97:12953-5.

Miller, Gary and Scott Pitnick. Sperm-Female Coevolution in Drosophila. Science. 2002;298:1230-1233.

Rice, William and Brett Holland. The enemies within: intergenomic conflict interlocus contest evolution (ICE), and the interspecific Red Queen. Behav Ecol Sociobiol. 1997;41:1-10.

Special thanks to Dr. Bill Rice, Andrew Stewart and all of their colleagues

Journal of Young Investigators. 2003. Volume Seven.
Copyright © 2003 by Gregory Campbell and JYI. All rights reserved.

JYI is supported by: The National Science Foundation, The Burroughs Wellcome Fund, Glaxo Wellcome Inc., Science Magazine, Science's Next Wave, Swarthmore College, Duke University, Georgetown University, and many others.
Copyright ©1998-2003 The Journal of Young Investigators, Inc.