Patterns of Selection

Last day we looked at how mutations provide a continuous supply of new genetic variations, which can be inherited and expressed as different phenotypes Today: how do these factors relate to natural selection: genetic variation, competition within populations and diverse environments

This allele is different from the normal gene for hemoglobin by having a single base-pair mutation Homozygous individuals are severely affected Heterozygous “ “ mildly affected but are much more resistant to malaria than those with the normal hemoglobin Carriers of the allele are at a disadvantage in areas where malaria does not occur, while they are favoured in areas where malaria is common Therefore they are much more likely to survive and pass on their genes to the next generation In this case the environment provides the selection pressure

The allele is only common where it provides an overall advantage to the individual This establishes a relationship between mutations and evolution: I. Harmful mutations are frequent, but are selected against and thus these mutant alleles remain rare II. Beneficial mutations though rare, when selected for, accumulate over time Genes provide the source of variation but not the selective forces

Occurs when the most common phenotypes within a population are the most favoured by the environment==conversely any trait that deviates from this is selected against Example: human birth weights are variable and partially inheritable 3 kg is the norm for a human baby Babies that weigh less are often premature, and those that weigh more often have complications affecting theirs and their mother’s survival Natural selection has eliminated the extremes, so that today most babies are near the ideal weight

By far the most common form of selection

Occurs when the env’t favours individuals with more extreme variations of the trait, which could result in an observable change in the population

Humans can cause directional selection as well=salmon have large populations and short generation times

Occurs when the env’t favours individuals at the extreme ends of the trait, as opposed to the intermediate variations

They depend on soft seeds and hard seeds for food Finches with small bills are efficient feeding on the soft seeds, and those with large bills are able to crack the hard seeds Important because it forms distinctive forms within a population that may become isolated breeding populations with separate gene pools

Favours the selection of any trait that confers an advantage in terms of the mating success of the individual This is associated with sexual dimorphism: which is the physical (often extreme) differences in the appearance of males and females The most common forms of sexual selection are the results of female mate choice and male to male competition

Females can chose based on physical traits, colouration, or behavioural traits such as courtship displays and songs Sometimes males develop features that enable them to establish and defend a territory from other males=sometimes detaining the females How would you be able to tell these are not env’tal selective pressures? Both sexes would possess the features MHC=sweaty shirts

Some features are a compromise between mating and remaining conspicuous to predators==bright colours and song Sometimes leads to extremes==runaway selection==stalk eyed fly % favouring long % favouring short Female Line of Origin

Sexual diversity is not limited to just animals Plants do not select mates but they do need to attract suitors to assist in pollination Flowers and scents are the most obvious examples of sexual features that have evolved==maximize pollination

Pgs Finish up lab HW questions