75 The Role of Variation in Populations

Key Idea: Variation refers to the diversity of phenotypes within a population. Variation is important in evolution because it is the raw material for selecting favourable phenotypes. Populations are groups of individuals belonging to the same species that live in the same region at the same time. They are not necessarily isolated but may make contact with other populations of the same species (as in human populations). Each individual in a population is a carrier of its own unique combination of alleles (genotype). An individual's genotype largely determines its phenotype (appearance). Those individuals with phenotypes well suited to the current environment have a greater chance of surviving and reproducing to pass on their genes to the next generation, i.e. they will have higher fitness. Individuals with less favourable phenotypes are less likely to survive and reproduce and their genes will have a lower representation in the next generation.

Mutations

• Gene (point) mutations • Chromosome rearrangements Provides the source of all new genetic information (all new alleles).

Selection pressures

• Competition • Predation • Climatic factors • Disease and parasitism

Sexual reproduction

• Independent assortment • Crossing over • Recombination • Mate selection Rearrangement and shuffling of the genetic material into new combinations. Favour some phenotypes more than others

The phenotype is the product of the many complex interactions between the genotype, the environment, and the chemical tags and markers that regulate the expression of the genes (epigenetic factors).

Genotype

Determines the genetic potential of an individual. Environmental factors influence the expression of the genotype in producing the phenotype.

Environmental factors

• Diet/nutrients • pH • Temperature • Wind exposure • Sunlight

Phenotype

Each individual in the population is a 'TEST CASE' for its combination of alleles.

Favourable phenotypes

Phenotypes well-suited to the prevailing environment have better survival and greater reproductive success (i.e. higher fitness). They produce many offspring with the favourable traits.

Selection pressures on the phenotype will affect an individual's fitness. Selection pressures are those factors in the environment that determine whether an organism will be more or less successful at surviving and reproducing.

Unfavourable phenotypes

Phenotypes poorly suited to prevailing environment have lower fitness and produce few offspring with the unfavourable traits.

1. (a) What is variation?

(b) Identify the sources of variation in sexually reproducing organisms:

2. What is the importance of variation to evolutionary change?

3. (a) What is meant by fitness and why is it important?

(b) Define the term selection pressure and explain how it relates to fitness: