According to the Hardy-Weinberg principle, allele frequencies stay constant from generation to generation as long as certain conditions are met Factors such as mutation, non-random mating and selection can operate in natural populations These factors may lead to changes in allele frequency and bring about adaptation and evolution The principal force in bringing about changes in the genetic composition of populations is that of Natural Selection

In 1835, Charles Darwin visited the Galapagos Islands where he studied many different species of plants and animals and the variation that existed between them Following his observations of variation within species, Darwin published his book The Origin of Species (1859) in which he put forward his views on the process of evolution by natural selection

Darwin wrote: “As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, will have a better chance of surviving, and thus be naturally selected” Darwin published his work without any knowledge of genetics or the nature of mutation, and the re-examination of his theory in the light of modern knowledge is known as Neo-Darwinism

Organisms tend to produce a far greater number of offspring than the the environment can support There is, therefore, a struggle for existence and a high mortality rate as organisms compete for limited resources Members of the same species display variation in all characteristics; the main sources of variation are mutation and the behaviour of chromosomes during the process of meiosis Individuals who display variation that is better suited to their immediate environment will compete more successfully, have a better chance of survival and be more likely to reach maturity and breed – Survival of the fittest

Organisms whose phenotypes are better suited to their immediate environment are described as being at a selective advantage Organisms whose phenotypes are less suited to their immediate environment are described as being at a selective disadvantage Over a period of time, the best suited variants will predominate in the population and allele frequencies will have changed Since environmental conditions are constantly changing, then natural selection is forever favouring the emergence of new forms which may culminate in the origin of a new species

The brown rat is classed as a pest as it forages for food intended for human consumption and spreads disease Rat populations have been controlled by a rodenticide called warfarin since the 1950’s Warfarin is an anticoagulant which acts by interfering with the way in which Vitamin K is used in the process of blood clotting When food bait containing warfarin is eaten by rats, their blood fails to clot and they suffer from fatal haemorrhages Since the introduction of warfarin in the 1950’s, populations of warfarin-resistant rats have been identified in various parts of Britain

Large populations of rats compete for the available food A random, spontaneous mutation occurs within the rat population conferring resistance to warfarin on one of the members Warfarin-resistant rat

The warfarin-resistant rat is at a selective advantage in areas where warfarin is used for pest control The resistant rat is more likely to reach maturity and breed and pass the resistance allele onto some of its offspring In this example, natural selection is exerting its effect on a single major gene with two alleles

Some of the rats in the first generation are warfarin-resistant and are more likely to breed than their susceptible brothers and sisters After many generations, a high proportion of the rat population are resistant to warfarin as the mutant allele spreads throughout the population several generations

Natural selection may affect allele frequencies within populations in several different ways Three types of selection that operate within populations are: Stabilising Selection Directional Selection Disruptive Selection For continuously varying characteristics, these selection methods modify their frequency distributions in different ways

Stabilising selection acts against the extremes within a range of phenotypic variation This type of selection leads to a reduction in the range of variation within the population without any change in the mode mode unchanged Stabilising selection operates in an unchanging environment to maintain the best adapted genotypes within the population

Stabilising selection occurs in the selection of birth mass in humans Infant mortality is greatest for babies of very high or very low birth masses

Directional selection operates in changing environments and acts for or against extremes of phenotype within the variable population; it is the main type of selection practised by man when selecting domesticated plants and animals for the improvement of stocks This type of selection leads to a reduction in the range of variation within the population, together with a progressive shift in the mode Selection pressure against these extreme phenotypes new mode

The evolution of the long neck of the giraffe is thought to have arisen by Directional Selection Ancestral giraffes would have displayed variation in neck length with random, spontaneous mutation giving rise to giraffes with slightly longer necks than the average of the population During periods when food was scarce, longer-necked giraffes would survive as they would be able to reach the available food Natural, directional selection would favour the long- necked giraffes; these variants would survive, breed and pass on their genes to the next generation Longer-necked giraffes were at a selective advantage during times of food shortage

Selection pressure operating against shorter-necked giraffes when food was scarce Directional selection has led to a reduction in the range of neck length within the population together with a progressive increase in the mode In time, longer- necked giraffes became more common within the population

Bacterial population Spontaneous mutation in growing population Bacterial population exposed to antibiotic Mutant bacterial cell is resistant to a specific antibiotic The resistant bacterial cell survives and divides to generate a population of antibiotic-resistant bacteria

The peppered moth (Biston betularia) exists in two forms or morphs in Britain This polymorphism involves a single gene locus with two alleles; the recessive allele (c) determines the 'typical' phenotype of light colour speckled with black; recessive homozygotes display this phenotype The mutant allele is dominant (C) and determines the 'carbonaria' form of the moth; the carbonaria form is a dark-coloured melanic form During the daytime, moths rest on tree trunks and are preyed upon by birds such as the blue tit In unpolluted areas, trees are covered with lichens and mosses and the light peppered morph of the moth is well-camouflaged against this background

The allele determining the carbonaria form of the moth arose by spontaneous mutation and, prior to the second part of the 19 th century, the melanic moth was a rare specimen In the largely unpolluted cities of Britain, the melanic morph was probably at a selective disadvantage with high mortality rates due to predation by birds The less conspicuous peppered morph enjoyed a selective advantage on the lichen covered trees and fewer numbers were eaten by birds By 1895, over 95% of moths sampled in the Manchester area were of the melanic type This marked increase in the frequency of melanic moths was shown to be associated with the Industrial Revolution when large numbers of factories released soot into the atmosphere

Sulphur dioxide from the atmospheric pollution killed the mosses and lichens on the trees which then became blackened by the soot particles in the smoke Within these polluted areas, the melanic morph was less conspicuous to predators and the light, peppered morph proved to be at a selective disadvantage Melanic moths suffered less predation by birds and the allele for dark colour increased in frequency The relationship between atmospheric pollution and the increase in the frequency of melanic moths is called Industrial Melanism This gradual replacement of the ‘melanic’ allele for the ‘pale’ allele in areas of high pollution is an example of transient polymorphism

The peppered morph is at a selective advantage in unpolluted areas

The melanic morph is at a selective advantage in polluted areas