Variation can be due to genetic and environmental factors, which is what allows individuals in a population to have different phenotypes. Genetic variation gives rise to evolution which in turn is caused by genetic drift and natural selection. These processes can lead to the formation of new species.

Variation

Definition

Variation describes the differences that exist between individuals. It is variation within a species that means individuals within a population can have a wide range of phenotypes.

Different alleles cause genetic variation. When a mutation in DNA occurs this can lead to the production of new alleles. Equally, genetic variation can arise during meiosis and because of random fertilisation of gametes during sexual reproduction. It is genetic variation that results in evolution.

Environmental factors such as climate, food and lifestyle can causes variation in populations. However, most variation arises as a combination of both genetic and environmental factors.

Evolution

Definition

The change in allele frequencies in a population over time.

Natural selection

Evolution can occur via natural selection.

1.	Individuals of the same species have different alleles.
2.	Selection pressures such as predation, disease and competition make survival and reproduction difficult.
3.	Due to variation in the population, some individuals are better adapted to the selection pressures.
4.	There are therefore differential levels of survival and reproduction within a population. Individuals with a phenotype that increases their survival and reproductive rate are more likely to pass on these successful alleles to offspring.
5.	A greater proportion of beneficial alleles are inherited by the next generation. The next generation is more likely to survive, reproduce and pass on their genes.
6.	This means the frequency of beneficial alleles in the gene pool increases between generations.

Type of selection

There are three types of natural selection that affect alleles frequency.

Stabilising selection

This is when individuals with alleles for characteristics towards the middle of the range (around the mean) are more likely to survive and reproduce. It happens when the environment isn't changing and leads to a reduction in the range of possible phenotypes.

Example

An example is mammalian species and fur length. When temperatures are hotter than usual, individuals with shorter fur have the advantage as they can lose body heat rapidly. The opposite is true for colder temperatures. However, if the climate is stable then animals with alleles for average fur length are more likely to survive and pass on their alleles. This causes the proportion of these alleles to increase and the range of fur lengths to decrease.

Biology; Genetics, populations, evolution and ecosystems; KS5 Year 12; Evolution: variation, selection and speciation — A. Selection pressure, B. Optimum fur length. 1. There is a wide range of fur lengths, 2. In a stable climate the individuals with very short or very long hair die.

Directional selection

This is a change in the phenotypes of a population to favour those that vary in one direction

Example

If the temperature falls to $5\degree C$ , mammals with longer fur are more likely to survive and breed. This causes a shift in fur length over generations until the extreme fur length becomes the mean.

Disruptive selection

Disruptive selection favours individuals with extreme phenotypes. It is the opposite of stabilising selection.

Example

When the summer temperature is constant around $15\degree C$ and the winter temperature is constant around $5\degree C$ individuals with the two extreme fur lengths will predominate. After many generations, two distinct populations are formed.

Speciation

Definition

Speciation is the development of new species from an existing species. It occurs when populations become reproductively isolated and this changes the allele frequency which, in turn, causes changes in phenotype and means these species can no longer interbreed. There are two types of speciation.

Allopatric speciation

Allopatric speciation occurs when populations become geographically isolated and therefore experience different conditions (e.g. a different climates, on either side of the physical barrier). The two separated populations may experience different selection pressures which will cause different changes in allele frequencies.

Different alleles may be more advantageous is different populations.

Example

Longer fur length is beneficial in colder climates. So directional selection will act on the alleles for fur length and increase the frequency of long fur alleles.

Mutations may also cause allele frequencies to change and they would occur independently in each population. Genetic drift also affects allele frequencies.

These differences accumulate in the gene pools of the separate populations which causes changes in phenotype frequencies. Eventually, the isolated populations will become so different that they cannot interbreed to produce fertile offspring.

Sympatric speciation

Sympatric speciation is speciation without a physical barrier. Random mutations could occur within a population and this may inhibit interbreeding between species.

1.	Many eukaryotic organisms are diploid, meaning they have two sets of homologous chromosomes. Mutations can increase the number of chromosomes - this is known as polyploidy.
2.	Polyploid organisms normally can't reproduce sexually with diploid organisms to give fertile offspring, they are therefore reproductively isolated.
3.	Polyploid organisms can reproduce asexually, this could lead to the development of new species.
4.	This is only the case if the polyploidy isn't fatal. It is more common in plants than animals.

Reproductive isolation

Reproductive isolation happens due to changes in alleles and phenotypes of individuals in a population, preventing them from successfully breeding with other individuals that do not have these changes. There are different types of changes that can occur.

Change	Description
Seasonal	Individuals from the same population may develop different flowering or mating seasons, or become sexually active at a different time of year.
Mechanical	Changes in genitalia or size of the organism will prevent successful mating.
Behavioural	A new group of individuals may develop courtship rituals that are not attractive to the original population.

Genetic drift

Definition

The change in the frequency of an allele in the population due to random chance. This is another way that evolution can occur.

1.	In a population there is a variety of genotypes, e.g. A and B.
2.	The allele for the genotype A may be passed on to offspring more frequently than other alleles by chance.
3.	The number of individuals with the allele A increases.
4.	If the allele frequency changes in two isolated populations this could eventually lead to reproductive isolation and speciation.

Genetic drift works in conjunction with natural selection to drive evolution. The population size will impact which process drives evolution more as genetic drift normally has a greater effect in smaller populations as chance can have more influence. Whereas, in large populations any chance variations in allele frequency are more likely to even out across the whole population.

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The process of natural selection

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Evolution by natural selection

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