Evolution Patterns of Selection

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

Sickle-cell anemiaThis allele is different from the normal gene for hemoglobin by having a single base-pair mutationHomozygous individuals are severely affectedHeterozygous “ “ mildly affected but are much more resistant to malaria than those with the normal hemoglobinCarriers of the allele are at a disadvantage in areas where malaria does not occur, while they are favoured in areas where malaria is commonTherefore they are much more likely to survive and pass on their genes to the next generationIn this case the environment provides the selection pressure

Sickle-cell anemiaThe allele is only common where it provides an overall advantage to the individualThis 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

Stabilizing selectionOccurs when the most common phenotypes within a population are the most favoured by the environment==conversely any trait that deviates from this is selected againstExample: human birth weights are variable and partially inheritable3 kg is the norm for a human babyBabies that weigh less are often premature, and those that weigh more often have complications affecting theirs and their mother’s survivalNatural selection has eliminated the extremes, so that today most babies are near the ideal weight

Stabilizing selection By far the most common form of selection

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

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

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

Black-bellied seedcracker finchThey depend on soft seeds and hard seeds for foodFinches with small bills are efficient feeding on the soft seeds, and those with large bills are able to crack the hard seedsImportant because it forms distinctive forms within a population that may become isolated breeding populations with separate gene pools

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

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

Sexual selectionSome features are a compromise between mating and remaining conspicuous to predators==bright colours and songSometimes leads to extremes==runaway selection==stalk eyed fly

% favouring long

% favouring short

Female Line of Origin

What about plants?Sexual diversity is not limited to just animalsPlants do not select mates but they do need to attract suitors to assist in pollinationFlowers and scents are the most obvious examples of sexual features that have evolved==maximize pollination

References Pgs 556-564

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