Download evolution-and-behaviour-essay-1 1 mb evolution-and

Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 14th October 2014
Define and distinguish clearly between Natural Selection, Sexual Selection and
Kin Selection
Natural selection, sexual selection and kin selection are all mechanisms of
selection of alleles for particular traits or features. Natural selection is defined as
the differential survival of alternative alleles. Sexual selection is a form of natural
selection where alleles that specifically increase mating success are selected for.
Kin selection is another form of natural selection where alleles for traits that
increase survival and reproductive success of a group of genetic relatives, but
not necessarily individuals, are selected for.
Natural Selection is the differential survival of alternative alleles, where alleles
that improve the chances of surviving to reproduce are selected and increase in
frequency. It results in organisms in a population that are best adapted to the
environment, with more advantageous alleles, increasing in frequency relative to
less well adapted organisms, over a number of generations. It is an entirely
automatic process that explains the origin of adaptation in living organisms, and
is a mechanism underlying evolution (another is genetic drift). Natural selection
occurs because there is a “struggle for existence”(Darwin), from competition for
limited resources between and within species. Organisms produce more
offspring than are able to survive and reproduce, as the offspring may be killed
by predators or parasites, and resources are limited, resulting in competition to
survive where only those better adapted will be able to reproduce. There are
several conditions required for natural selection to occur. In the population there
must be reproduction, heredity, variation and fitness differences between
individuals. Heredity means that genes can be replicated and passed on from
parent to offspring. Variation results from genes being present in two or more
alternative alleles. This variation is generated by mutations, which occur
randomly. Fitness is defined as the average number of offspring produced by an
individual relative to the number of offspring produced by an average member of
the population. Fitness differences result from different alleles having
differential effects on the ability of an organism to survive and reproduce,
therefore some individuals are more likely to reproduce than others. The result
of these conditions being fulfilled is that there is differential survival of
alternative alleles, so natural selection
occurs. An example of natural selection is on
colour patterns in the guppy Poecilia
reticulata. They display a colour pattern that
results from a balance between avoiding
predation, and the requirement to find mates.
Natural selection results in an increase in
frequency of alleles that cause improved
camouflage of the guppies, as this increases
chances of survival, and sexual selection acts via natural selection to increase the
frequency of alleles that result in a colour pattern that attracts mates, as this
increases reproductive success. Sexual selection is a form of natural selection.
image source: http://acquariofiliaconsapevole.it/foto_guppy-hristo-hristov-1
Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 14th October 2014
Sexual Selection involves selection of alleles for traits that increase an organism’s
success in mating and ensuring that its gametes are successful in fertilisation.
They are traits solely concerned with increasing the mating success of an
individual. Sexual selection largely acts via male competition and female choice.
It selects for traits in males that make them more successful at competing to
fertilise the females’ eggs. For example, a male insect may guards his mate after
copulation in an attempt to maximise the probability of his sperm fertilising their
eggs, so their own offspring are produced. The alleles for the trait of guarding
successfully would be selected for by sexual selection. The selection of traits for
optimal competing reflects that males (or the sex with the greater reproductive
rate) generally display ardent mating behaviour, where they attempt to mate
with as many females as possible (in polygynous species), since mating is less
costly than for females who have a smaller number of larger gametes. Therefore
males seek out and compete for mates. Females however generally display
choosy mating behaviour, as they make a bigger investment in the gametes. As a
result, sexual selection also acts through female choice. Females may get direct
benefits from mating with the male, such as food from courtship feeding or
provision of good territory, which gives an indication of being a caring father and
would be a trait selected for by sexual selection as it is concerned with increasing
mating success. Traits selected for via female choice also include those that are
indicators of good genetic fitness, and so should result in alleles for good fitness
being passed on to offspring, meaning they will have a greater chance of
surviving to reproduce. These traits are selected for since selection would favour
females with preference for males with these features, and males that have these
features. Female choice also results in the selection of traits that are attractive
without being linked to greater fitness, but will increase the mating success of
their male offspring because they will be more attractive.
It can appear that sexual selection and natural selection can act in opposition to
one another, as sexual selection can result in an increase in frequency of alleles
that reduce chances of survival of individuals, a failure in the struggle for
existence. For example long colourful tails on male peacocks are costly, as it
requires energy to grow and carry the tails. One would assume that alleles for
this feature would be selected against by natural selection. However males with
longer tails have increased reproductive success from female choice, the feature
is selected for by sexual selection, and since it results in an overall increase in the
number of offspring, the frequency of the alleles that cause it increase. Despite
sexual selection and natural selection appearing to sometimes act against one
another, sexual selection is a subset of natural selection. Alleles selected for by
sexual selection do in actuality increase fitness as the number of offspring
produced relative to the average is increased.
Kin selection is also a subset of natural selection. Kin selection acts on an
individual in favour of survival not of that individual, but of its relatives, which
carry many of the same alleles. The alleles selected for increase the chances of
relatives surviving to reproduce, but not necessarily the individual. For example,
kin selection selects for genes that result in altruistic behaviour, where the
individual behaves in a way that results in a loss of its own fitness but improves
fitness of other individuals. This aids the survival of the alleles that are common
Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 14th October 2014
to a group of closely related organisms. Natural selection favours altruism among
relatives, provided that the improvements in lifetime reproductive success of the
relatives is great enough when compared to the loss in reproductive success of
the individual from their altruistic behaviour, when benefit to relative x
relatedness > cost to individual (Hamilton’s rule). The more closely related one
organism is to another, the more they will value the reproduction of the other
organism, as they will share genes with a higher than average frequency. An
example of a trait that is selected for by kin selection is termite Globitermes
sulpheureus soldiers exploding to release defensive secretions, increasing
chances of survival of the colony and the queen (who can continue to reproduce),
but clearly not of the individuals as the behaviour results in their death. Another
example is parental care; caring for offspring reduces survival chances of the
parents, but increases it of the offspring who will be more likely to survive and
reproduce. Sexual selection and kin selection are similar in that they initially
appear to work against natural selection, but in actuality are both subsets of it.
Kin selection appears to select for traits that are not favoured by natural
selection as, for example, altruistic behaviours result in a cost to the individual
and a decrease in chances of survival of that individual. However, kin selection is
a type of natural selection, but it works at the level of the group, not individual.
When considering selection of replicators, the genes, natural selection acts on
vehicles of the genes, which can be individuals (most commonly), but can also be
groups of organisms, as in kin selection.
In conclusion, both kin selection (selection of alleles that result in increased
survival and reproduction of genetic relatives) and sexual selection (selection of
alleles that result in increased reproductive success) are subsets of natural
selection (the differential survival of alternative alleles, with increased survival
of alleles that improve chances of surviving to reproduce). Sexual selection and
kin selection can appear to act in opposition to natural selection as they can
select for traits that reduce individuals’ chances of survival, but they ultimately
results in increased reproductive success or survival and reproduction of
organisms with similar alleles respectively, and so work by natural selection.
Bibliography
Lectures ‘Introduction to Evolutionary Biology’ by Dr. William Foster
Lecture notes ‘Evolutionary Genetics’ Dr. John Welch
Mark Ridley (2004) ‘Evolution’ 3rd Edition Blackwells: Oxford
John A. Endler (1980) ‘Natural Selection on Color Patterns in Poecilia reticulata’
Evolution, Vol. 34 pp. 76-91
John Alcock (1994) ‘Postinsemination associations between males and females
in insects: The Mate-Guarding Hypothesis’ Annual Review of Entomology pp. 1-21
Andersson et al (1997)‘Sexual Selection and Mate Choice’ Trends in Ecology and
Evolution Vol. 21pp. 296–302
Matt Ridley (1981) ‘How the peacock got its tail’ New Scientist pp. 398-401
Bordereau et al (1997) ‘Suicidal defensive behaviour by frontal gland dehiscence
in Globitermes sulphureus Haviland soldiers (Isoptera)’ Insectes Sociaux Vol. 44
pp 289-297
Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 14th October 2014
-