2.2 - Organisms

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2.2 (a) - Drift and Selection

Evolution is the change over time in the proportion of individuals differing in one or more inherited traits

Evolution can occur either through the random process of genetic drift or the non-random processes of natural selection and sexual selection

Genetic drift is the changes in the frequencies of alleles in a population that occur by chance

This is more important in small, isolated population as alleles are more likely to be lost from the gene pool

Natural selection is the non-random increase in frequency of DNA sequences that increase survival

As organisms produce more offspring than the environment can sustain, those individuals with variations that best fit their environment are the ones most likely to survive and breed

Through inheritance, these favoured traits are therefore more likely to become frequent in subsequent generations

Sexual selection is when the increase in allele frequency is determined through the selection of mating partners from, for example, female copulation

Variation in traits arises as a result of mutation

Mutation is the original source of new sequences of DNA

These new sequences can be novel (new) alleles

Most mutations are harmful or neutral but in rare cases they can be beneficial to the fitness of an organism

Fitness is how successful a particular genotype has been in surviving from one generation to the next

Absolute fitness is the ratio of frequencies of a particular genotype from one generation to the next

Relative fitness is the ratio of surviving offspring of one genotype compared with other genotypes

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2.2 (b) - Rate of evolution

Where selection pressure are high the rate of evolution can be rapid

The rate of evolution can be increased by shorter generation times, warmer environments, the sharing of beneficial DNA sequences between different lineages through sexual reproduction and horizontal gene transfer

2.2 (c) - Co-evolution and the Red Queen Hypothesis

Co-evolution is frequently seen in pairs of species that interact frequently or closely

A change in traits of one species act as a selection pressure on the other species

Examples include pollinators and plants, predators and prey, parasites and their hosts, herbivores and plants