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A Discussion on Sexual Selection
I. Introduction
In human social interactions, there is a general pattern for sexual selection in
reproductive success – that is, an attractive or powerful individual will be more likely to
be selected for than the average person in our short-term human evolution. Darwin was
first to apply this notion of reproductive struggle to the behavior of animals, in his 1871
tome The Descent of Man and Selection in Relation to Sex. He writes that:
Males have acquired their present structure.. not from having better fitted to
survive in the struggle for existence, but from having gained an advantage over
other males [in reproductive access to females], and from having transmitted this
advantage to their male offspring…it was the importance of this distinction which
led me to designate this form of selection as Sexual Selection (Darwin 569).
The distinction from natural selection is apparent when one sees that the secondary
sexual characteristics that tend to increase reproductive success are not necessary. At
times, they can even be detrimental to the function of the primary sexual characteristics
that are directly involved with reproduction. The long train of the great bird of paradise
is a prime example: it is fairly inhibitory to survival, but it is quite important in its
reproductive success among female birds of paradise (Figure 1). Initially, Darwin
received little approval or interest from other evolutionary thinkers, notably Alfred
Russel Wallace and Theodosius Dobzhansky (Wallace, Dobzhanksy). The general
scientific community tended to disregard sexual selection because individual variation
was not very highly thought of, so long as reproduction continued. Others retained the
peculiar Victorian notion of female disinterest in sex and thus utilized a moral criticism
of Darwin’s postulates. However, the redoubtable logic of such a position began to
ingratiate itself on the scientific mind, leading to the general theories of Zahavi and
Fisher and the explosion in sexual selection studies through to the modern day.
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II. Male Competition
Darwin posited that sexual selection would come in either of two forms. In one
form, usually known as intrasexual selection or male competition, sexual selection would
come about as a result of fighting with other males for the chance to mate with a female.
He attributed the ‘useless’ presence of horns and spangles to their use as weapons for
advantage during these male competitions (Darwin). Even in the modern era, those
sexual modifications involving better adapted combat traits are quite prevalent in nature
and receive much attention from biologists. The implication is that the so-called winners
should be more vigorous than his competitors, and thus future offspring should also be as
vigorous. In a study of courtship among Drosophila, John Maynard Smith found that
females tended to reject those males who could not complete the intricate mating dance
(Maynard Smith 1956). These unfortunate few were usually the result of high mutational
loads or poor adaptations to novel environments, all signaling poor survival traits. His
results addressed two issues which had been weaknesses in Darwin’s original
formulation: false advertising of genetic quality could skew selection, and there would
be no selection if advantageous genes could be so quickly fixated (Bell). These female
‘tests’ of competition would ensure that there could be no false advertising. Also, the
assessment of mutational loads and adaptations by female behavior would allow for
preferential mating without losing genetic variance, through the fluctuations of mutationselection balance and frequency-dependent selection traits such as infection resistance.
Heritability, the proportion of differences in traits due to genetic differences,
usually plays a large role in evolution through sexual selection. However, it is also
possible for non-heritability to be a vital contribution during male competition. Amotz
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Zahavi’s handicap theory conceived that survivors of competition with some nonheritable handicap must have extraordinary survival and adaptation skills despite the
obvious maladaptation (Zahavi 1975). He argues that these males thus have superior
genes that can be passed on with higher selective advantage to their offspring, without the
negative effect of inheriting the accidental handicap. The fact of this test’s costliness
tends to promote its reliability as a measure of sexual advantage. This theory does
encounter some difficulty with selection. Selection automatically reduces the inherent
heritability of fitness in Zahavi’s theory and hence leads to fixation. This problem is
adequately addressed by the fact that variance can be maintained by mutation-selection
balance and frequency-dependent selection as mentioned above (Heino et al. 1997).
As a residual effect of sexual biases in human social institutions, it is mostly
assumed that males are necessarily the more aggressive creature in the relationship, and
females the more ‘coy’ type. Though male dominance is present in most cases, there are
significant examples of a reversal of these stereotypical roles through female competition
in nature, especially among female-dominated systems. Bengt Hansson and colleagues
observed the interaction of primary and secondary great reed warbler females and found
that the eggs of the primary females were being targeted by the secondary females
(Hansson et al. 1997). Warbler males allocate fatherly aid to the females in accordance
with the appearance and continued existence of their clutch – thus, infanticide would
destroy the primary female’s clutch and allow the male warbler’s attention to move on to
someone new, increasing the chance that the secondary female would be next in line.
Patty Gowaty found female eastern bluebirds to be murderously aggressive in claiming
and delineating their territory (Gowaty 1997). Even more brutal are the social insects –
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the honeybees, the wasps, the hornets – whose entire social structure revolves around
support for the queen. The queen supports her own survival by killing her competitors,
while the workers ensure their own by planting their own eggs in the hive or
manipulating brood numbers. Mary Jane West-Eberhard, when working with paper
wasps, found that females worked better in groups, but the dominant wasp would be
rewarded most in terms of reproduction (West-Eberhard 1979). This ensured a
continuous and violent struggle for domination within the hive. These conclusions help
to correct some of the masculine characterizations common in evolutionary research, but
there are still significant traces of human bias and sociological generalizations that can
prove misleading in a study of evolution.
III. Female Choice
The other form of sexual selection, epigamic selection or female choice, has been
passionately debated since its debut on the scientific scene. Malte Andersson was able to
show that the existence of long tails – natural or artificial – attracted more female
attention than did normal tails in the case of the African widow-bird (Andersson 1982).
In a study of peafowl, Marion Petrie discovered similar results using the manipulation in
numbers of eyespots on their tail feathers (Petrie 2002). Interestingly enough, the
offspring of the more successful birds were heavier and more likely to evade predators,
both traits of higher survival. Female choice in these studies tended to be open ended
rather than absolute; that is, the preference was only for more exaggerated characters
rather than specific detailed qualities.
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Darwin originally envisioned this female choice as a pre-existing bias that was
generally unselective, or at most, could be explained by some other part of the creature’s
evolution. However, Sir Ronald Fisher was able to come up with a more graceful
explanation of the existence of female choice – the runaway sexual selection model
(Fisher 1915, Genetical). He conjectured that female preference began with heritable
traits that were already favored by natural selection. The ability to choose fit mates
through these traits would end with fitter offspring and enhance discrimination for that
trait by increased viability and sexual success. In the next generation, sexual selection
would ensure both increased female discrimination and exaggeration of the male trait
without any positive adaptation for the trait’s original function, leading to heightened
conflict with natural selection. Only when natural selection pressures rose too high
would sexual adaptations stop (Figure 2).
Critics of female choice tend to disagree with the mechanism by which female
choice arises. They concede that preference for certain traits might exist, but this does
not dictate how animals themselves view the trait. A study of cricket songs found that
females indubitably chose the males with the ‘longest’ and flashiest songs, even if they
were merely recorded and played back (Hedrick 2000). In a study of cichlids, the female
colored fish preferred males of the same color (known as positive assortative mating)
unless the water was cloudy, in which case males of either color had no advantage
(Seehausen and van Alphen 1998). In the first study, there is significant proof for some
sort of artificial sexual selection, but it does not dictate an ingrained preference in the
natural state. The second study shows the marketability of the color trait, but at the same
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time, it gives no reasons. Are familiar colors more adaptive to certain conditions? Are
they more attractive? It isn’t quite clear what makes them selected for.
Other issues come up once one views human situations of female choice. For
example, it is a general trend that the more attractive mate in a relationship can get away
with doing nothing, while the less attractive individuals in the pool must compensate by
showing that they can work harder with the offspring. This doesn’t mean that female
choice in animals can possibly work the same way – for example, there are no similar
decision-making components in animal brains, and even among humans, female choice
often does not apply, especially in older cultures where marriages were more strictly
controlled. Science does not know enough about the intricacies of animal culture to be
able to refute the possibility of prohibited female choice in animals. However,
evolutionary biologists only need to contend with proving the existence of sexual
selection and in this case, of female choice. They need not be too concerned with the
various thoughts running through the female mind before selecting a mate, only with the
evolutionary outcome of her decision.
IV. Parental Investment and Genetic Survival
As biologists study evolutionary models, there is a tendency to humanize the
animals used in each simulation of nature. However, selection in nature has less to do
with the individual motivations for reproduction than with the passage of genes by
maximizing reproductive success. It is the genes that are key, as they are the basic units
of evolution. The question does arise: are genes inherently selfish? The answer should
be a resounding yes, as the essence of reproduction is passing along one generation’s
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genetic heritage to the next. Within sexual selection, this makes for a more elegant,
logical and neutral explanation for the existence of such anomalies as high promiscuity,
differing clutch sizes and variations in parental investments.
Robert Trivers was first to propose the idea that males and females differ because
of distinctions in parental investment (Trivers). Females would tend to leave the most
genes in the next generation by having the best children she can have, as there are
frequent limitations on copulation on account of the one-year gestation period. This
limits the amount of offspring she can and should have. Males are not limited in this
way. The impetus for sex is returned to the male, but only because the male must only
ensure he fertilizes as many females as possible, in order to have the most kids. It is also
almost a rule that sexually active males outnumber sexually receptive females, leading to
high competition for female attention. This means that females are expected to be
choosy. Trivers was especially persuaded by a study done by Angus Bateman using
Drosophila (Bateman 1948). Bateman was able to conclude that males are more
successful the more they have sex, but this promiscuous behavior does little or nothing
for female flies (Figure 3). Trivers suggests that male sexual behavior is not limited by
gametic formation. Sperm are easy to produce while eggs are not, and so only males
have this seemingly inexhaustible supply.
Interestingly enough, the Coolidge effect – which was observed in the lab during
animal studies in which males got ‘bored’ with females and could only be rearoused by
novel females – was seen by Dewsbury in promiscuous grasshopper mice, which had
been thought to be monogamous (Bermant, Dewsbury 1981). In certain cases, females
are no better. A study by Olin Bray found that a majority of the clutches of red-wing
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females from the territories of vasectomized males’ territories were fertile, meaning that
the females were copulating with some other male red-wing (Bray et al. 1975). Lisle
Gibbs, who studied red-wings in Canada, found similar results when DNA-typing chicks
across the territorial males’ domains (Gibbs 1990). The gels revealed that most of the
chicks in any territory were fathered by a different male than the territorial one – and this
without any vasectomy. Thus, the tendency to cheat or be promiscuous is supported by
both evolutionary theory and practical observation. It is not proven, however, why
females are sometimes promiscuous or simply polyandrous – it is hypothesized that it
may be fertility insurance, or like humans, merely an artifact of more productive females
attracting more males (Birkhead, Figure 3)
As has been shown, it is a mistake to assume that the mother is more ‘coy’ or
even the more parental types; in some cases, roles can be reversed with the male. The
male magpie, in addition to his copulation role, must also feed his mate and his clutch,
thus limiting his promiscuity (Birkhead). This is not in opposition to any part of Trivers’
theory – it is just an instance of role reversal, where higher male parental investment
ensures higher quality young and thus, his genes into the next generation (Figure 4). As
yet, there is no strong logical explanation as to why some species should have the father
be the more parental type. Some would hold that the ability to produce milk should
dictate the parental unit, delineating one sex over the other. However, there are instances
of creatures in which either sex is eminently capable of doing all the important tasks of
childrearing. Also, male mammals have the apparatus to produce milk and can
sometimes lactate, but they aren’t the main parental unit (Zuk). These differences are
mainly assumed to be continuations in evolutionary quirks by selection.
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It must be again stressed that the major goal of sexual selection is the
maximization of reproductive success or fitness. Courtship, prior to reproduction, is “the
advertisement, by a male of how fit he is…genetically. In submitting only to a male with
such signs of fitness a female would probably be aiding the survival of her own genes”
(Williams 34). While it may be possible, rampant aimless fecundity does nothing for
species survival. This is why there are instances of abortions and adoptive societies
throughout nature. The kangaroo will expel its joey from its pouch if living conditions
deteriorate, just as other mammals can resorb their embryos, birds desert their nests and
human parents put their kids up for adoption. Worker bees and ants can give up their
reproduction in order to take care of the queen’s offspring. This seems contrary to the
genetic preservation inherent in sexual selection; however, it is not. In order to maximize
fitness, it may make more sense to discontinue rearing children in harsh conditions and
attempting reproduction again later. As for ‘adoption’, it is not quite clear why so many
insects would pass up their own chance at reproductive success. It has been hypothesized
that reproducing on one’s own is too unstable and risky such that indirect reproduction
through relatives is a much better alternative.
Other species pass up parental care all together in the quest for genetic
preservation. Brood parasites, as they are often known, leave their young in other birds’
clutches to be raised (Zuk). Cuckoos are especially notorious for this – they are the basis
for the term “cuckold” (Birkhead). Another aspect of the vicious struggle present in
sexual selection is visible here. As long as the cuckolded parents are raising children not
their own, they are lowering their own reproductive success at the same time they are
heightening that of the cuckold himself. This form of behavior is very expensive for the
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cuckolded – but life is tough. Nature isn’t fair, and one should not attempt to apply
human visions of justice and morality to these systems.
Geoff Parker elaborated on the presence of the cuckold with his theory of sperm
competition (Parker 1970). In this variant on intrasexual selection, sexual selection
favors males who can fertilize females who have been previously inseminated by other
males. This means that a settled female will raise children with the cuckold’s genes
without requiring any other contribution from him. Sperm competition also includes
defensive strategies by the paired male and his innate mechanisms to ensure that he will
not be cuckolded, or should he be cuckolded, that his sperm should lose out. These
include mate guarding, in which a male will essentially follow the female to prevent
extra-pair mating attempts; frequent mating attempts; increases in ejaculate and testis
size; increasing accuracy of the male reproductive organ; and cooperation among similar
sperm, against the activity of alien sperm. The role of the female in all of this is basically
unknown. It has been hypothesized that mate guarding is merely a female prompt to
ensure the male’s manliness, or find a new mate in somebody else who is willing to stand
up to her current mate. Nancy Burley, in studying the zebra finch, found that 80% of all
extra pair matings were aggressive, and none of these matings resulted in offspring
(Burley et al. 1996). This meant that the female was doing something to avoid bearing
the forced embryo – in some species, the female has the ability to eject sperm after a
copulation, but the mechanism for this function is unknown.
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V. Further Questions and Conclusion
With such variance in mating strategies and fidelity, a major question being
addressed in current research is the method by which male and female parents can adjust
the amount of energy they expend on raising their offspring when the proportion of
offspring from extra-pair males within one clutch can vary from 0 to 90% (Bell). In
human contact, the motivations are varied: fear of social reprisal, love or loss of love,
socio-economic benefits. With animals, these answers are more obscure. It can be
judged how much survival and genetic benefit a female can derive from being with one
male or certain males over others, but it is not clear why and how females choose their
extra-pair partners. Also, it is not clear why animals should even choose monogamy in
the first place. Along with these questions are the more general unanswered questions
plaguing evolutionary biology. What are the mechanisms of female choice – are there
motivations? Was sex really an adaptation? How did the environment play a role in
creating sexual selection? Did most of these adaptations come about as adaptations or as
a neutral by-product of selection (Gould and Lewontin)? Are our explanations, however,
as neutral as we try to create them, or are we still merely imprinting our life experience
on animal society? As science further explores the limitless bounds of evolution, these
questions, caveats and exceptions will continually arise in our search for truth.
What science does know about sexual selection is very persuasive. Most scientists
are firm with Darwin’s belief in sexual selection and its distinction from natural selection,
as are they with his belief in intrasexual and epigamic competition. It is visible in all
arenas that there exists a level of same-sex competition for mates as well as a certain
amount of choice from the opposite sex.. Furthermore, it is understood that the basic
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premise on which all selection stands is the passing on of genes – so the ultimate goal of
reproduction is the extension of genetic heritage into future generations. Observations of
social behavior tend to prove the existence of mating strategies for specific characteristics
that have tended to be over-promoted in species, over survival selection pressures. The
theories of Zahavi, Fisher, Parker and Trivers are very reasonable and make sense within
the context of what science knows about sexual evolution, but, as in all sciences, one
must be very careful of wholly accepting what is still theory and not entirely proven fact.
Hopefully, with further research, science will be able to answer all that is necessary to
know to understand why animals – and us as human in general – have developed this
way, and how.
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Figure 1 Taken from Charles Darwin’s The Descent of Man and Selection in Relation to
Sex, pg. 718. Features the decorative tail feathers of the Great Bird of Paradise
(Paradisea papuana). These ornaments appear prominently on related species and seem
to function only as an attractant to the female, as it makes these birds easier to be caught
by predators.
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Figure 2 Taken from Stevan J. Arnold’s chapter on “Sexual selection: the interface of
theory and empiricism,” page 74, in Mate Choice, ed. Bateson. A visual description of
Lande’s model for evolution by natural and sexual selection. A sex-limited character will
be distributed with a mean and certain standard deviation. If the sexually selected
character is exaggerated beyond survival means, it will be selected against by natural
selection.
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Figure 3 Taken from Tim Birkhead’s Promiscuity, page 9 and 208. The first shows
results from Bateman’s experiment, showing that male reproductive success in
Drosophila increases with the number of copulation partners, whereas female
reproductive success does not increase by copulating with more than one male
With the prairie dog, litter size is bigger as number of copulation partners increase. This
is unusual, and occurs possibly because of direct fertility benefit or a genetic
compatibility benefit.
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Figure 4 Taken from Robert Trivers’ “Parental Investment and Reproductive Success” in
his book, Natural Selection and Social Theory: Selected Papers of Robert Trivers, page
76. A male’s parental investment starts only after copulation, and increases as the young
grow older. However, the female’s main contribution happens directly after copulation
and as they are born. As there are birds, the male can typically incubate the eggs while
the female does nothing relevant. When feeding her young, both parents are able, but the
female can do so at a more rapid rate (the male time span shortens at this point).
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