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Sexual selection essay
Sexual selection is about how well adapted an organism is for finding a mate. Typically males have a
greater potential reproductive and rate and females invest more in the offspring. This is due to the
aisogamous nature of sexual reproduction. One theory suggests that originally sperm and egg cells were
the same size but due to selection for productivity and for zygote fitness the smallest and largest
gametes were selected for. This led to disruptive selection as smaller gametes were selected as this
allows for larger numbers to be made per unit time and larger gametes allowed for increased zygote
fitness. Due to the greater potential reproductive rate in males they are, in most cases, the ones
competing for the females. However, there is more than one force at work when it comes to sexual
selection. We have to consider the competition between males but also the preferences of the female,
or the female choice.
Competition between males, sometimes referred to as intra-sexual selection, can be direct or indirect. A
prime example of direct selection can be seen by looking at elephant seals. The male elephant seals are
huge in size when compared with the females. They arrive at the breeding grounds before the females
and fight for the best breeding sites. In this case the bigger and stronger the elephant seal, the more
likely it is he will be able to pass on his genes to the next generation.
Sperm competition is competition after the sperm had been transferred to prevent the sperm of rival
males from fertilizing the egg. There are many examples of this, some more extreme than others. Sperm
removal is when the male scoops out previous sperm or pushes it to the side with horn like appendages
to ensure that his sperm is the one most likely to fertilize the egg. Some species had evolved the ability
to cement closed the female’s genitalia after copulation with a ‘copulatory plug’ to prevent competition
from other males’ sperm. Males of a parasitic worm found in the intestines of rats, Moniliformes dubilis,
is also able to cement the genitalia region of another male to remove competition. One more example
of sperm competition is that some invertebrates produce two forms of sperm: eusperm and parasperm.
Paraspem is sterile and is used to fill up the female storage organs after mating to delay the female from
mating again.
When discussing sexual selection the main area of focus is normally how the female chooses her mate.
Fisher’s runaway theory states that there is no direct benefit to the female and that the reason females
choose mates with a certain trait is because they will have sons with this trait and they in turn have a
high mating success and therefore pass on more of her genes. Males will have genes for the trait and for
preferring males with the trait and vice versa but the preference gene will not be expressed in males
and the trait gene will not be expressed in females. This theory assumes that there is a genetic
correlation between the trait and the female preference for that trait. The runaway process will only be
stopped when natural selection starts to act against it and eventually equilibrium is reached between
the sexual advantages of having the trait and the survival advantages of not having the trait. This
explanation is not complete because it assumes that there will be a difference in female preference.
There is no explanation as to the origin of this female preference; it just exists.
Zahavi’s Handicap theory is that the males have such large ornaments (such as heavy antlers) because
they can. They are fit enough to survive even with such a large handicap. The ornament will also be
expensive to grow and so it may prove to the female that he has the ability to provide enough food for
himself, and therefore perhaps for his future offspring. This theory also assumes a genetic correlation.
One argument against this theory is that if ornaments are a direct result of ‘good genes’ then why does
genetic variability not decline? This is not an issue because it is nowhere near as straight forward as
good and bad genes. So many genes will affect the health of the male and it is not a case of one form or
another. Another reason is that the males will constantly be evolving due to other forms of selection, for
example, due to arms races between host and parasite or between predator and prey. This means that
the gene pool will not stay constant as the organisms are constantly evolving. Also mutations will still be
occurring, whether advantageous or disadvantageous.
An experiment carried out in the 1990s (Petrie, 1994) suggested that there was a correlation between
male attractiveness (mean area of eyespots) in peacocks and the health of their offspring. Petrie
randomly paired up males and females and then raised the offspring of all the pairs under the same
controlled conditions and then released them into a park. After 84 days there was a correlation between
male attractiveness and the size of his offspring and after 2 years there was a considerable correlation
between male attractiveness and the survival rate of his offspring. This provides good evidence that
there is a link between the sexual ornaments displayed by a male and the health of his offspring
suggesting that females select males as those with bigger ornaments will give her offspring a genuine
genetic advantage.
Male house finches display carotenoid pigmentation that varies from pale yellow to red. Colouration is
deposited at the time of the annual moult. Feathers grow in daily cycles of dark feathers deposited in
the day and light at night. This produces bars of growth. The carotenoids cannot be ingested and so have
to be taken in by eating foods containing carotenoids. The width of the bars can be used to infer the
condition of the individual, because the feathers would grow more slowly under nutritional stress. Hill
and montgomerie found a positive correlation between male plumage colour and average bar width.
Males that had intensely pigmented plumage also grew faster. Females prefer redder mates. Benefits
offspring – better at foraging. Feed mates and offspring at rate correlated to pigmentation. Females use
colour as an assessment of male superiority.
A slight variation of this is the gene-parasite theory. The ornament or colour is a direct representation of
his level of parasites. In other words: an advertisement to females that the male is parasite free. To
prove that there is a correlation between health and ornaments (Manfred Milinski and Theo Bakker,
1990) did an experiment involving sticklebacks. When placed under white light the females preferred
the reddest sticklebacks. When put under green light the females can’t tell the difference and show no
preference even though the males still perform courtship dancing, showing that the main factor is the
colour. Redder males had a greater mass per unit length and also lost their redness when infected with a
parasite. This suggests that females may prefer redder males as it is an outward sign that they are not
infected with a parasite.
The final theory that I am going to discuss is called sensory exploitation, (Ryan, 1990). This is the theory
that the preference for a particular trait evolved in a separate way and is therefore not directly
genetically linked to the trait. For example, when the sensory system in an organism evolves this could
change the female preference due to her increased ability to sense, for example, vibrations in the water.
The males can then exploit the pre-existing bias, although in many cases have not yet done so. This
theory assumes that the preference for the trait evolves before the trait itself.
An example of this can be found by looking at water mites. These water mites detect prey though the
small vibrations in the water. This implies they have evolved the ability to detect vibrations to improve
their ability to find prey. Males mimic these vibrations and so females will be attracted towards the
males due to their pre-existing sensory bias for vibrations in the water (Proctor, 1992).
I think it is important to realise that there theories do not have to be looked at in isolation. For example,
the male behaviour or ornament could have evolved due to sensory exploitation but then a positive
feedback system could have been set up where Fisher’s theory might explain the upkeep of the
ornament. I believe that there must be some benefit to the female’s offspring because when looking
from the genes point of view this would mean more genetic material would be likely to be passed on
and so there is a genetic advantage. There are also proven cases of links between health and sexual
ornaments.
Another area that has often been overlooked is female variation in preference. Most of the studies
carried out are male focused and yet the influence of females is vast. Female preference will not be a
constant parameter when looking at the co-evolution of male ornaments. One example of this is context
dependent variation in mate preference, for example the presence of predators. An experiment on two
populations of guppies from rivers with different intensities of predations was carried out to test this
hypothesis of context dependent preference. Females from both populations preferred brightly
coloured males in the absence of predators. When predators were introduced, guppies from the area of
high predation decreased their level of sexual activity and preference for brightly coloured males. The
behaviour of the female guppies from the area of normally low predation was unaffected by presence of
a predator. This, the context dependent female preference has evolved in response to the level of threat
of predation in their natural habitat, due to how costly the preference is in terms of reduced survival.
Another theory is condition dependent variation in preference of females. This means that the
preferences of the females will vary depending on the condition that they are in. An experiment carried
out on stalk-eyed flies showed that females with large eyespans mated more frequently with males with
large eyespans. However, large eyespans are positively correlated with high visual acuity and so it could
be argued that this may not be due to variation in preference but rather that females with larger
eyespans could distinguish between males more effectively. In a further experiment females were
standardized for eyespan and maintained on diets of high quality and low quality. Those on a high
quality diet showed a preference for large eyespan males, but those on a low quality diet did not, and
also had reduced fecundity. This highlights a strong correlation between female fitness and mating
decisions.
Males will also benefit from variation in females. If mating choices are condition dependent, then males
with ornaments, for example larger eyespans, will not only benefit from attracting more females to pass
on genes to, but also higher quality females, as they will have a preference for large eyespans. By
attracting high condition females, there will be direct benefits such as increased offspring production, if
the condition has an effect on fecundity. Also there will be genetic benefits to the offspring, as it is likely
that at least some of the female condition is genetically determined, and so the offspring will be more
likely to survive to reproduce.
We cannot treat sexual selection as different to natural selection as it doesn’t make sense unless it is in
the context of natural selection. Natural selection is the key force that drives evolution and adaption
and sexual selection is a manmade concept that has arisen to help us explain certain aspects of nature
that at a first glance do not appear to be following the ‘rules’ of natural selection.