Evolution: variation, selection and speciation

In a nutshell

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.