Effects Of Different Forms Of Selection On Evolution Flashcards
What is stabilising selection
Preserves the average phenotype of a population by favouring average individuals, in other words, selection against the extreme phenotypes
What is directional selection
Changes the phenotype of a population by favouring the phenotypes that very in one direction from the mean population, in others words, selection for one extreme phenotype
What is disruptive selection
Favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population
When does stabilising selection tend to occur
Where the environmental conditions are constant over long periods of time
Given an example of stabilising selection
Body mass of human children at birth
Babies born with a body greater or less than the optimum have a higher risk of dying in few months after birth
When does directional selection take place
If the environment conditions change, so will the optimum value for survival. They will move either left or right or the mean,and will posses a combination of alleles with new optimum for the the phenotypic character
When would disruptive selection take place
When an environmental factor such as temperature takes two distinct forms
E.g. 5 degrees in winter -long fur
15 degrees in summer - shirt fur length
Suggest which form of the peppered moth, is now most common in cities like Manchester and explain why in terms of Selection pressure
The light coloured (non-Melanie) form because pollution control means buildings are no longer black. The melanic form is therefore more conspicuous than the light form and so preferentially eaten by predators. The light form is more likely to survive and reproduce to give more light coloured offspring. There is a selection pressure favouring the light form that has led to it outnumbering the melanix form
