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Jessica Griffiths
20/10/2013
Define and distinguish clearly between Natural Selection, Sexual Selection and Kin Selection
Natural selection, the key mechanism of evolution, proposed by Charles Darwin in 1858, is the natural
gradual process whereby the frequency of alleles within a population changes over time. Individuals within
a population vary in their appearance and behaviour because they have different alleles. This variation
arises partly due to random mutations in the genome of individual organisms, which are then inherited by
offspring, and also partly due to the interaction between the individual’s genome and its environment (the
biochemistry of the cell, other populations, other species and other individuals).
Competition for resources creates a struggle for survival. Individuals with alleles that make them better
adapted to their environment are more likely to survive and successfully reproduce. These advantageous
alleles will be inherited by the organism’s offspring. This results in a greater proportion of the next
generation possessing the beneficial allele. In turn, they are more likely to survive and reproduce, and pass
on their genes. In this way, the frequency of the beneficial allele increases in each generation – this is
natural selection.
Sexual selection is a type of natural selection where the varying abilities of individuals within a population
to secure mates results in some individuals out-reproducing others. There is competition between
individuals of the same sex for sexual access to females. Females will sometimes select males on the basis
of offerings of resources such as protection, or food. This can act as an indication of future prowess as a
caring father.
In some species, males evolve rather extreme and potentially disadvantageous behavioural and physical
traits in an attempt to out-compete other males of the same species. This is referred to as the handicap
principle. These costly traits and ornaments are used to indicate vitality, and exceptional fitness and genetic
health. Females exhibit preferences for these males when selecting a mate as it increases the chances of
offspring inheriting this vitality; individuals with inferior fitness and health would not be able to afford such
costly signals. Female offspring will inherit the preference for these attractive characteristics, in addition to
the genes for these characteristics (which are not expressed). They are therefore more likely to select males
with these traits, and have genetically fit offspring themselves. Conversely, undiscriminating females are
more likely to bear unfit or defective offspring than choosey females. Her offspring will likely inherit her
tendency to be less choosey in their selection of mates, resulting in another generation of defective
offspring. This is therefore a positive-feedback system, which can lead in a process called runaway
selection. This is where advantageous male traits and the female preferences for these traits coevolve.
The behaviour of Trinidadian guppy, Poecilia reticulate, for example, is consistent with the handicap
principle. Male guppies will cautiously approach predatory fishes that have approached the school of
guppies to investigate and inspect the threat. This exhibited risky behaviour is deemed attractive to female
guppies as it is an indicator of the guppy’s superior fitness. Less-fit male guppies who attempt this are often
consumed by the predator. Investigations have revealed that when no female guppies are present, no male
guppies take this risk, providing evidence for the handicap principle. Interestingly, there is a correlation
between the boldness of individuals within a school of guppies and their colour; the most colourful males
are often those individuals who inspected the predators from a close-range. Female Trinidadian guppies
have therefore evolved a preference for more vibrant, colourful males as it indicates increased fitness and
boldness. [2]
Within a species, the sex with the greater potential reproductive rate – usually males - is under strong
selection to compete for and seek out the other sex. The sex with a greater investment in the gametes and
offspring – usually females – are more selective in their choice of mates. This is due to the fact that female
gametes are more valuable as they are produced slowly and less often – they are in limited supply.
Furthermore, females are often the sex that carries, gives birth to, and raises the offspring. Their
investment in the offspring is usually greater than males, whose gametes are produced at a faster rate and
in far greater quantities. Males often produce millions of expendable sperm and in the majority of cases do
not carry or raise the offspring. The male’s reproductive investments are therefore much smaller, resulting
Jessica Griffiths
20/10/2013
in proportionately smaller biological costs. It is therefore often in the interests of males to mate with more
females, to propagate their genes.
In some species the roles of the sexes are reversed and males are the more selective gender, such as some
species within the family Syngnathidae. In these cases, females are generally the more ardent sex, and
males are more choosey in their selection of mates.
Kin selection is defined as a form of natural selection in which “characteristics that may be disadvantageous
to an individual… can persist or increase in the population if they contribute to the survival and
reproduction of the individual’s close genetic relatives”. [1] The evolutionary success of a gene is ultimately
determined by the number of copies of it in the population - this is not necessarily achieved solely through
maximising the number of viable offspring an individual has. Closely related organisms share some identical
genes; therefore the survival and reproduction of these related individuals can contribute to the
evolutionary success of the gene. The overall genetic success of an organism can therefore be achieved
through altruistic social behaviour.
Altruism towards related individuals is not incompatible with the theory of natural selection. Assisting the
reproduction of close relatives can help to pass on an individual’s own genes. Genes that increase the
reproductive success of an individual are favoured by natural selection; however this does not mean that
every individual with that genotype has to have higher reproductive success. It is the average reproductive
success of organisms with that genotype that is important. Parental altruism can be explained by kin
selection – sexually reproducing parents share half of their genes with their offspring. Genes that promote
altruistic behaviour of parents towards their offspring will be favoured by natural selection, as it increases
the reproductive success of the parent’s genes.
Kin selection is not limited to relationships between parents and their progeny. Altruistic behaviour is
favourable when the losses of the altruist, in terms of personal reproduction, are compensated by the
increased fitness of the relative. The closer the genetic relationship between the altruist and the
beneficiary, the higher the costs that can be afforded by the altruist.
Urocitellus beldingi, commonly known as Belding’s ground squirrel, is an example of a species that
demonstrates kin selection. When the presence of a predator is detected by an individual, it will alert the
rest of the group using an alarm call. Although this compromises the safety of the individual, it helps to
protect genetically related individuals within the group. The number of genes passed on by the survival and
reproduction of related individuals exceeds that that would be passed on if the squirrel reproduced on its
own. This altruistic behaviour is therefore favourable in evolutionary terms.
Natural selection results from the struggle for survival, whereas sexual selection results from the struggle
for reproduction. Variation in the lower extremes of fitness is decreased by natural selection whilst the
diversity of organisms is increased by sexual selection. Both of these processes are mechanisms for
evolution. Natural selection increases the frequencies of genes within a population that enhances the
fitness of the individuals carrying them and decreases the frequency of those which decrease an
individual’s fitness. In kin selection, the frequency of genes that decrease the fitness of the individual whilst
improving the fitness of related individuals may increase. This is due to the fact that closely related
organisms share some identical genes; therefore the survival and reproduction of these related individuals
can contribute to the evolutionary success of the gene.
Bibliography:
1. http://www.oed.com/view/Entry/103433?redirectedFrom=kin+selection#eid218489417
2. http://www.biologycorner.com/worksheets/articles/how_females_choose.html