18.4 - Effects Of Different Forms Of Selection On Evolution Flashcards
What are the 3 types of selection
- Directional selection
- stabilising selection
- disruptive selection
Define stabilising selection
stabilising selection preserves the average phenotype (phenotypes around the mean) of a population by favouring average individuals, in other words, selection against the extreme phenotypes
Define directional selection
directional selection changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population, in other words, selection for one extreme phenotype
Define disruptive selection
disruptive selection favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population.
Describe how stabilising selection works
- Stabilising selection tends to eliminate the extremes of the phenotype range within a population and with it the capacity for evolutionary change.
- It tends to occur where the environmental conditions are constant over long periods of time.
- e.g. human birth weights
Describe how directional selection works
- Within a population there will be a range of genetically different individuals in respect of any one phenotype.
- The continuous variation amongst these individuals forms a normal distribution curve.
- This curve has a mean that represents the optimum value for the phenotypic character under the existing conditions. If the environmental conditions change, so will the optimum value for survival.
- Some individuals, either to the left or the right of the mean, will possess a combination of alleles with the new optimum for the phenotypic character.
- As a result there will be a selection pressure favouring the combination of alleles that results in the mean moving to either the left or the right of its original position.
- Directional selection therefore results in one extreme of a range of variation being selected against in favour of the other extreme or even the average
Describe how disruptive selection works
- Disruptive selection is the opposite of stabilising selection.
- It favours extreme phenotypes at the expense of the intermediate phenotypes.
- Although the least common form of selection, it is the most important in bringing about evolutionary change.
- Disruptive selection occurs when an environmental factor, such as temperature, takes two or more distinct forms.
What is polymorphism
- Some species of organisms have two or more distinct forms.
- These different forms are genetically distinct but exist within the same interbreeding population.
Consider each of the following statements and suggest which form of selection it best relates to.
A) A baby with a birth weight greater than 4.0 kg or less than 2.5kg has an increased risk of dying.
B) Some species of insects have changed very little over millions of years.
C) Elephants have evolved longer trunks enabling them to reach leaves higher up in trees.
D) Small mammals can escape from predators by hiding in small spaces while large ones can resist attack by predators.
E) it is the most important type of selection in bringing about evolutionary change.
F) The mean ear length in arctic foxes has reduced over time. gt preserves the characteristics of a population.
A) stabilising
B) stabilising
C) directional
D) disruptive
E) disruptive
F) directional
G) stabilising
Suggest which form of the peppered moth, Biston betularia, is now most common in cities like Manchester and explain why in terms of selection pressure.k
- The light coloured (non-melanic) 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 melanic form
