L2 Variation and polymorphism Flashcards
What are the three main types of selection?
Directional, disruptive and stabilising
What is directional selection?
Shifts the overall population by forming an extreme
Give an example of directional selection
Malaria parasite, shows a trend towards drug resistance once they have been introduced, driven by directional selection in a mutation for resistance
What is divergent selection?
Favours variants of opposite extremes, and intermediates are selected against
What is disruptive selection?
Divergence within the same population
Give an example of divergent selection
In great tits, melanin based patten on neck, female preference differs with males in urban and country populations, may be due to difference in resources.
What is stabilising selection?
Acts against extreme phenotypes, intermediates are selected for
Give an example of stabilising selection?
birth size most favourable in the middle
What species can you see all three types of selection acting at once? Explain
Spadefoot toads, see two different morphs, carnivorous and omnivorous. Divergence is seen as intermediates are selected against as are less effect feeders
Directional is seen because carnivores are actually favoured over omnivores.
Stabilising is seen because although selected against under normal conditions, when occurring with spade foot bombifrons, the intermediates are favoured.
What does selection often do to variation?
Reduces it, however variation is necessary for selection
What is polymorphism?
Diversity in morphology across populations, is evidence of genetic variation
What mechanisms are present to maintain variation for selection to act on?
Diploidy
gene flow/migration
mutation
balancing selection
What is diploidy?
Alleles of low fitness can be kept in the gene pool without being expressed. Dangerous recessive mutations can hide from selection.
What is gene flow?
Random changes in unselected alleles, accounts for changes in the genetic background.
How does migration maintain variation?
Brings genetic background to new populations
How does mutation maintain variation?
Can bring new variation into the population
What is balancing selection?
Mechanism for selection that keeps allele proportions in the population at the same level, if the population is acting at max fitness
What are the two mechanisms by which balancing selection acts?
Heterozygote advantage
Frequency dependent selection
What is heterozygote advantage?
Case in which heterozygote genotype has a higher fitness, known as overdominance
What are the examples of heterozygote advantage?
Sickle cell anaemia - increases resistance to malaria
Connexin 26 - builds thicker skin, two copies is deaf
Thalassemia - one copy can give resistance to heart disease and malaria
Is heterozygote advantage likely to be a common method of balancing selection?
if an adaptation is good, should dominate the population which would wipe out heterozygotes. Suggested that every allele from mutation is fitter in heterozygote form.
May appear to be HA, but might actually be HA.
Why might HA not be what it appears to be?
Need to look at what is happening in the background, may actually be in linkage disequilibrium with a beneficial allele, so the heterozygote alleles are not what is causing the benefits seen
What is associative overdominace?
An increase in fitness of heterozygotes at a neutral locus because it is in linkage disequilibrium with a locus that is under selection
What is the selection coefficient?
the difference between the men relative fitness if individuals of a given genotype and that of a referee genotype
