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 inthe genetic composition of populations is that

of Natural Selection

In 1835, Charles Darwin visited theGalapagos Islands where he studied

many different species of plantsand 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, spontaneousmutation occurs within

the rat population conferring resistance to

warfarin on one of the members

Warfarin-resistant rat

The warfarin-resistantrat is at a selectiveadvantage in areas

where warfarinis used for pest control

The resistant rat is more likely to reach maturity and breed and pass the resistance allele

onto some of its offspringIn this example, natural selection is exerting its

effect on a single major gene with two alleles

Some of the rats inthe first generation arewarfarin-resistant and

are more likely to breedthan their susceptible brothers and sisters

After many generations, a highproportion of the rat population are

resistant to warfarin as the mutantallele spreads

throughout the population

severalgenerations

Natural selection may affect allele frequencies within populations in several different ways

Three types of selection that operatewithin 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 populationwithout any change

in the modemodeunchanged

Stabilising selection operates in an unchanging environment to maintain

the best adapted genotypes within the population

Stabilising selection occurs in the selection ofbirth mass in humans

Infant mortality is greatest for babies of very highor 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 stocksThis 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 theseextreme

phenotypes

newmode

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 generationLonger-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 giraffesbecame more

common withinthe population

Bacterialpopulation

Spontaneousmutation in

growingpopulation

Bacterial populationexposed to antibiotic

Mutant bacterial cell isresistant to a specific antibiotic

The resistant bacterial cellsurvives and divides to

generate a population ofantibiotic-resistant

bacteria

The peppered moth (Biston betularia) exists intwo forms or morphs in Britain

This polymorphism involves a single gene locus with twoalleles; 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 formDuring 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 19th century, the melanic moth was a rare specimenIn the largely unpolluted cities of Britain, the melanic morph

was probably at a selective disadvantage withhigh 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 whenlarge 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