Genetic Change

DARWIN'S THEORY OF NATURAL SELECTION

General Points Populations can produce large numbers of offspring with inherited variation among them. Those with phenotypes best suited to the conditions of the environment are more likely to survive and breed. These 'fitter' individuals have more offspring (than those without this particular variation) and pass on these features to their offspring through their genes. The population's gene pool changes over time due to the build up of beneficial mutations.

KEY Points To Remember Although individuals are selected for and against by natural selection, it is populations that actually evolve (NOT individuals!) The key to natural selection occuring at all is the variation that is naturally present in any sexually reproducing population.

NATURAL SELECTION

DEFINITION: The reproductive success of an individual with favourable features. Process whereby certain individuals best suited to the environment are selected for, as they provide a survival advantage. Individuals with these adaptations will leave the most offspring.

Stabilising SelectionIndividuals that vary from the mean are selected against (i.e. the extremes of the range of variation). This actually decreases variation in a population.e.g. Coelacanth fish have retained an almost constant form 

^ The mean is selected for and the two extremes are selected against ^

^ The frequency of individuals with the mean trait increases ^

Directional SelectionIndividuals at one side of the mean (towards one extreme of the range of variation) are selected for and will increase in number. Individuals at the other side of the mean are selected against and their numbers decreasee.g. Peppered moths. The dark mutant was selected for during the Industrial Revolution when it camouflaged against the dirty black bark of the trees

Disruptive SelectionIndividuals with variation on either side of the mean (both extremes of the range of variation) are selected for and those closest to the mean are selected against. The mean moves in two different directions. This can be the start of speciation as two different variants will develop in the population.E.g. African black-bellied finch. Those with large beaks can crack hard seeds, those with small beaks can feed on soft seeds, those with intermediate beaks are not well adapted to do either.

MIGRATION

DEFINITION: Movement of individuals of a species into or out of a population.Individuals entering or leaving a population can bring (usually beneficial) alleles with them that SPREAD through the population OR they can leave, potentially taking unique alleles with them. This will change the allele frequencies and alter the gene pool causing GENETIC CHANGE.

MUTATIONS

DEFINITION: Sudden change of makeup of organism.If a beneficial mutation occurs in a gamete that is fertilised, new alleles may be introduced  and spread through the population, increasing the frequency of a beneficial allele and increases the size of the gene pool causing GENETIC CHANGE. This results in an increase in variation with the population.Mutations can also be harmful. They are more likely to remain within a population and spread throughout it if they are recessive (as heterozygotes can carry the allele without expressing it).Mutations can also be neutral (have no effect on the phenotype of the organism) and therefore spread throughout the population.

GENETIC DRIFT

DEFINITION: Random changes in allele frequencies in a population due to chance.This is the result of random events and it has a greater impact on the allele frequencies of smaller populations. In a small population it is possible that alleles may become lost or fixed (decreasing the gene pool, and reducing genetic variation in the population).

BOTTLENECK EFFECT

The Bottleneck effect occurs when there is a dramatic reduction in the population size As a result there is often a loss of some of the alleles from the population. The gene pool is limited to the alleles possessed by the few remaining individuals and the genetic diversity (variation) is significantly reduced Even once the population size recovers much of the genetic diversity present in the original population is lost. (The genetic diversity can only be increased through mutations or through outbreeding (if possible) and this takes time)

FOUNDER EFFECT

e.g. Cheetahs underwent a population bottleneck during the last ice age. Today's cheetahs may all descend from just one surviving litter. They have problems breeding as defective genes have become more common.

Occasionally a small number of individuals from a large population may migrate away or become isolated from their original population. This is called a founder population The isolated founder population is likely to have different allele frequencies to the original population causing GENETIC CHANGE There will be no gene flow between the original population and the founder population as they are geographically isolated Selected pressures (e.g. environmental conditions) acting on the two populations may differ significantly and this will cause the populations to diverge This can lead to speciation (once the two populations become reproductively isolated)

e.g. New Zealand hares are all descendants of six individuals that were introduced from Australia. Although they multiplied greatly, their gene pool contained fewer alleles than the parent population.

Darwin's Theory - General Notes

Natural Selection

Migration, Mutations & Genetic Drift

Effects