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Factors that influence evolution

The three key factors that drive evolution are described here.

Evolution is a process of gradual change in the traits of a population that occur over many generations. At a molecular level, it is the differences in an individual’s genes (genotype) that result in differences in their physical traits (phenotype).

There are three key factors that drive evolution in a population:

  • genetic variation between individuals
  • natural selection
  • sexual recombination.

Genetic variation between individuals

There is a large amount of variation in the genes within a population. This genetic variation arises from random mutations in the DNA sequence. Random mutations occur when DNA is replicating. These mutations can cause a nucleotide base to be inserted, deleted or substituted in the DNA sequence. Factors in our environment, such as UV light, radiation and some chemicals, can increase the number of random mutations.

Mutations in the DNA of a gene coding for a protein can cause changes in the amino acid sequence of the protein. The resulting protein may function differently.

A mutation in the DNA of a gene encoding an enzyme may make it better or worse at catalysing reactions. As enzymes are responsible for many chemical reactions in cells, this could affect cell function.

Natural selection

All populations respond to changes in their environment. Individuals will respond in different ways depending on their genes. Those individuals whose genes are best suited to the environment are more likely to survive and pass on their genes to the next generation. This is natural selection. Gradually, favourable genes will start to predominate in the population and less favourable genes will decline.

Sexual recombination

During sexual recombination, the genes from each parent are recombined and shuffled to produce new combinations in the offspring.

Sexual recombination has three steps that increase genetic variation in a population:

  • crossing over
  • independent assortment
  • fertilisation.

Crossing over occurs when gametes are formed by meiosis. Homologous chromosomes – one inherited from each parent – pair along their lengths, gene by gene. Breaks occur along the chromosomes, and they rejoin, trading some of their genes. The chromosomes now have genes from either parent in a unique combination.

Independent assortment is the process where the chromosomes segregate randomly into separate gametes during meiosis.

During fertilisation, the gametes from each parent join randomly, which produces a unique combination of genes in the resulting zygote.

These three steps reshuffle genes into unique combinations. This increases the genetic variation in a population and allows for natural selection to occur.

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