Evolution Evolution is the change in organisms over time. Specifically the proportion of individuals in a population that differ in one or more inherited traits. Evolution has been taking place from the moment living organisms started passing on genetic information. When did that start? Where did life come from?

Drift and Selection The Hardy – Weinberg principle Genotype proportions in a population will remain constant if: The population is very large Random mating occurs No mutation takes palce No genes are input from other sources (migration) No selection occurs

Drift and Selection But the frequency of alleles is changing in natural populations. Five factors affect the proportion of homozygotes and heterozygotes enough to move away from the constant. mutation migration (immigration/emigration) genetic drift non random mating selection

Genetic Drift Evolution can occur through the random process of genetic drift. Genetic drift has the greatest effect on small populations. It is a random change of the allele frequency due to only a few individuals gametes forming the next generation. The two causes of genetic drift are: Founder effect – a few individuals are dispersed and form an isolated population. They have limited genetic material, rare alleles will have a higher frequency than in the original population. Bottle neck effect – depletion of genetic diversity by a dramatic reduction in the original population. All subsequent members in the population arise from a few individuals.

Selection Natural selection as proposed by Darwin. Evolution can occur through the non random processes of natural selection and sexual selection. Natural selection as proposed by Darwin. Organisms reproduce more offspring than are necessary for the species to survive. Variation exists in the offspring. Selection pressures have an effect on individuals. Those best able to survive are more likely to pass on their genetic information to their offspring. The frequency of beneficial characteristics (alleles) in a species increases so the population changes over time.

Sexual Selection Sexual selection is a "special case" of natural selection. Sexual selection acts on an organism's ability to obtain or successfully copulate with a mate. Sexual selection is often powerful enough to produce features that are harmful to the individual's survival

Sexual selection Female Choice: Intersexual selection, in which females choose males based upon elaborate ornamentation or male behaviours. Male Competition: Intrasexual selection, in which males compete for territory or access to females, or areas on mating grounds where displays take place. Male-male competition can lead to intense battles for access to females where males use elaborate armaments (e.g., horns of many ungulates).

New Variation New genetic information is produced by mutations. Mutations can add, delete or otherwise alter the sequence of DNA in an individual. However most mutations are harmful or neutral in effect. So they do not provide an advantage that will be passed on. In rare cases the mutation will give benefit and improve the ‘fitness’ of an individual and be passed on.

Fitness Absolute fitness absolute versus related fitness 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.

Just to clarify if needed: Those offspring that best fit the environment because of their variation are most likely to survive and breed. Those inherited traits (alleles) are likely to become more frequent in future generations

Rate of Evolution 2 theories on how evolution progresses: punctuated equilibrium and gradualism. The rate of evolution is influenced by selection pressures. Evolution rate will be more rapid if selection pressures are high. Other factors will also speed up the rate of evolution such as shorter generation times, warmer environments, sharing of beneficial DNA sequences between different lineages through sexual reproduction and horizontal gene transfer.

Co-evolution and the Red Queen hypothesis Species that interact closely or frequently can show co-evolution. This can be seen between: predator and prey parasites and their host herbivores and plants pollinators and plants A change in a trait in one species can have an influence (selection pressure) on the other species so co-evolution occurs.

Red Queen hypothesis The “Red Queen” hypothesis in evolution is related to the co-evolution of species. It states that species must constantly adapt and evolve to pass on genes to the next generation and also to keep from going extinct when other species within a symbiotic relationship are evolving.

Red Queen hypothesis illustrated by parasite and host relationship. Hosts better able to resist and/or tolerate parasites show greater fitness. Parasites better able to feed, reproduce and find a new host show greater fitness As either host or parasite evolves each will have an effect on the evolution of the other.