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Define and distinguish clearly between Natural Selection, Sexual Selection and
Kin Selection.
These three types of selection aim to explain the evolutionary behaviour of organisms in three distinct
ways. The theory of Natural selection was first put forward by Charles Darwin and Alfred Russell Wallace
in their 1859 publication, ‘On the origin of species’, it aims to explain how certain biological traits change
in frequency over time due to the struggle for existence of organisms. Natural selection relies on three
factors, firstly variation must be able to occur in organism. This can occur from random mutation or the
recombination of genes during meiosis in sexually reproducing organisms. Secondly, there is a
relationship between some of the biological traits occurring from variation and the probability that the
organism can survive longer and therefore reproduce more, passing this advantageous trait on to many
offspring, this forms the third condition, heredity, an ability to pass on traits to offspring so that they
resemble their parents.
Natural selection is now combined with modern synthesis which allows an even greater understanding
of the process. Genes from the parent are replicated and pass on intact to offspring, allowing a stable
process of heredity. Each individual gene will occur in several different forms called alleles and the type
of allele relates to a physical trait of the organism, this shows clearly how natural selection is able to
increase the abundance of a certain trait. Mutation and/or recombination can create new alleles of a
genes which, if they prove to be advantageous, will enable to organism to survive longer and reproduce
more, producing more offspring with the advantageous allele, eventually the allele frequency will
increase to cover the entire population if it is advantageous enough.
Charles Darwin carried out In vivo experiments on the Finches in the Galapagos Islands which
demonstrate natural selection well. In 1977 there was very little rainfall on Daphne Island and as a result
of this, the abundance of seeds decreased and those remaining were larger and harder as these couldn’t
be as readily eaten by the medium ground finch (Geospiza fortis). This resulted in the decrease in
numbers of the birds, by the end of 1977 only those individuals which had the alleles making them
larger in body size and a harder, rounded beak were able to survive. These randomly occurring traits
were selected for by natural selection as they provided an advantage to the finches over other birds as
they were larger and so able to out compete the smaller birds for food and also had beaks better suited
to eating the larger, harder seeds. In just one year, natural selection was able to change the
characteristic of an entire population.
Contrastingly, sexual selection selects traits that are solely able to increase the mating success of the
individual and therefore passing on an individual's genes to as many offspring as possible which are also
able to go on to reproduce and hence continually increase the frequency of the original parent's alleles.
This theory explains the differences in mating behaviour between males and females, as in general,
males tend to mate with many different females, whereas females are more careful when selecting their
mate. This is due to the higher investment the females give, both in the gestation and care of the
offspring and in the production of the gametes.
The competition between males in sexual selection takes on two forms, Darwin's 'Law of battle' is simply
when two males fight each other over the possession of a female, the struggle here is not for existence,
but for the opportunity to mate. The other form of male-male fighting is sperm competition, whereby
males try to prevent the sperm of another male fertilising the eggs of a female they mate with. This is
shown in the behaviour of the red flour beetle (Tribolium castaneum), the males have a curved
extension which they use to remove the sperm of previous males, out of the female they are about to
mate with. Sexual selection has selected for these traits (larger, more aggressive males and the
existence of the curved extension) as they lead to an increased success in reproduction for the males.
Sexual selection has led to females in general being more choosy about their mates in two ways, firstly
the female may receive some non-genetic benefits such as food or a more protective father if she
chooses the right mate. This is seen in the behaviour of Hylobittacus apicalis, a species of Mecoptera,
where the male offers food to the female which if she accepts it, is eaten by the female during
copulation. The female also carefully selects her mate as to get the best possible genetic benefits from
the male that will lead to an increase in fitness of the offspring.
A major paradox with natural and sexual selection is altruistic behaviour which is observed in many
organisms. If an individual decreases its own chance of survival and reproduction success for the gain of
another's, we would assume that the altruistic gene of this individual would be deselected for by both
natural and sexual selection and so the gene would not be passed on enough for it to be establishing in
any population. The solution to this is by a third type of selection, kin selection. This is the selection of
genes for social actions which favour the reproductive success of an individual's relatives.
Hamilton's rule shows that kin selection is a mathematical weigh up of the advantages to the offspring
of the Altruist's relative's young and the disadvantage to the altruist's own offspring. The rule also takes
into account how related the altruist is to the benefactor using 'r', the relatedness coefficient which is
calculated by how many pathways there are linking the individuals in a pedigree chart, in each
generation(or pathway) the probability of the offspring containing the parent's altruist gene is 0.5 due to
meiosis.
Taking a worker termite who possesses the altruistic gene as an example, the female has two options,
she can either leave her relatives and raise her own young or stay with her relatives and help her sisters
raise their young. If she produces her own young then she can rear four offspring, statistically two of
which with the altruist gene. However if she helps nurture her nieces and nephews (of which there will
be many more than four due to the extra help, for example twelve) then a quarter of them will have the
altruist gene (relatedness coefficient r=0.25), giving rise to three new individuals with the altruist gene.
In this way, the gene for altruism will eventually spread through the population. Kin selection can also
apply to social actions other than altruism such as cooperation (where both individual's gain), selfishness
(actor gains and recipient loses) and spite (where both individuals lose).
The three types of selection discussed can be easily distinguished because each type favours certain
genes. Natural selection favours genes advantageous for survival, sexual selection favours genes
advantageous for highest rate of successful reproduction and kin selection favours the highest
probability of being able to successfully increase the number of individuals in the next generation with
your selection of alleles.