Variation and Selection

Variation can be caused by genes, the environment, or both Variation is the differences that exist between individuals. Variation within a species means that individuals in a population can show wide range of different phenotypes. Although individuals of the same species have the same genes, they have different alleles - this causes genetic variation within a species. The main source of this variation is mutation. But genetic variation is also introduced during meiosis (through crossing over of chromatids and independent segregation) and because of the random fertilisation of gametes during sexual reproduction. Variation within a species can also be caused by differences in the environment, e.g. climate, food, lifestyle. Most variation within a species is caused by a combination of genetic and environmental factors, but only genetic variation results in evolution.

Evolution is a change in allele frequencies over timeThe frequency of an allele in a population changes over time - this is evolution.Natural selection is one method by which evolution occurs. Here's a reminder of how it works: Individuals of the same species vary because they have different alleles. Predation, disease and competition (selection pressures) create struggle for survival. Because individuals vary, some are better adapted to the selection pressures than others. This means there are differential levels of survival and reproductive success in a population. Individuals are likely to survive, reproduce and pass on their genes than individuals with a different phenotype. This means that a greater proportion of the next generation inherit the beneficial alleles. They, in turn are more likely to survive, reproduce and pass on their genes. So the frequency of the beneficial alleles in the gene pool increases from generation to generation.

Different types of natural selection lead to different frequency patternsStabilising selection and directional selection are types of natural selection that affect allele frequency in different ways. STABILISING SELECTION is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. It occurs when the environment isn't changing and it reduces the range of possible phenotypes. DIRECTIONAL SELECTION is where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce. This could be in response to environmental changes. DISRUPTIVE SELECTION is where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce. It's the opposite of stabilising selection because characteristics towards the middle of the range are lost. It occurs when the environment favours more than one phenotype.