Issue 3, May 2003
Male vs. Female: Gender Conflict as an Evolutionary Force
Biochemistry, University of California - Santa Barbara
Image courtesy of Greg Campbell
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
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
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
The enforcement of monogamy versus the promotion of promiscuity
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
Image courtesy of Greg
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.
(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
A relationship between human intelligence and sexual antagonism?
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.
William. Dangerous Liaisons. Proc Nat'l Acad Sci. 2000;97:12953-5.
Miller, Gary and Scott Pitnick. Sperm-Female Coevolution in Drosophila.
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.
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.