Natural selection (lec 5) Flashcards
Population structure effect on variation
-migration between populations (gene flow)
-isolation of populations makes them more distinct
size of population and amount of gene flow will determine evolutionary outcomes
Disruptive selection
- diversifying
- homozygotes fitter than heterozygotes or multiple variants favoured
Positive selection:
- eg. capsid proteins of viruses
- difficulty developing vaccine due to changing coats to avoid immune system (red queen)
- mutations in the sites that the host immune system recognise are under positive selection
- recognition sites thus change v rapidly
Purifying selection
eg capsid proteins of viruses
- some parts of virus can’t tolerate changes as they are necessary to maintain structure or functions
- these regions are under purifying selection, show few changes
In reality most genes probably experience a mix of both:
-purifying and positive selection
Balancing selection
-e.g. sickle cell anaemia gene for malarial resistance
- multiple variants are favoured
- results in maintenance of high levels of polymorphism
- often because different alleles are favoured in different circumstances or because theres an advantage to being different (frequency-dependent selection)
Frequency-dependent selection (balancing selection)
e. g. MHC
- heterozygotes have an advantage
Inbreeding vs outcrossing (reproductive advantage)
- inbreeding increases homozygosity
- recessive alleles persisting in populations brought out into the phenotype due to inbreeding
- inbreeding depression - reduced fitness
Inbreeding could be an advantage to not breaking up
co adapted gene complexes
-after a period of inbreeding, negative deleterious mutations purged so no inbreeding depression, as long as environment remains the same
Outbreeding can lead to
heterozygote advantage
-also called heterosis or hybrid vigour
eg, Soay sheep in Hebrides
-higher fitness sheep have higher levels of heterozygosity
Outbreeding depression=
- organisms well adapted to an environment mate with individuals adapted to another environment
- eg conservation genetics and captive breeding
Purging
-individuals that sirvive have gotten rid of the nastiest deleterious recessive mutations
Assortative mating
-can ensure that similarly adapted individuals mate
Dis-assortative mating
-can ensure individuals mate with genetically different individuals to increase genetic diversity
Kin recognition
-behavioural recognition of close relatives