Selection and drift Flashcards
Why doesn’t selection lead to uniformity give 2 reasons?
*Disruptive selection (but could still lead to speciation)
*polymorphism
What is the reason for polymorphism?
give an example of where this is common
To maintain genetic variation
*found with cuckoo gentes
What is Balancing selection and what are the 2 types?
Maintains a balance or equilibrium between different morphs or alleles
*Negative frequency-dependent selection
*Heterozygote advantage
Give an example of a species with negative frequency-dependent selection
scale eating cichlid fish (perissodus microlepis)
50/50 left and right specialisation
What is apostatic selection and give an example?
a form of negative frequency-dependent selection
Helix nemoralis (snail)- Birds tend to eat the more common looking snail as they know they are not poisonous.
There are also variable immune systems
Constant adaptation by the parasite to overcome the host’s immune defences.
Hosts and parasites (or competitors) are in an arms race (takes a lot of change from both to just have changes at the same rate)
Constant adaptation by the parasite to overcome the host’s immune defences.
Co-evolution among hosts evolving new defences.
Common genotypes are more easily overcome by the parasite.
Rare genotypes are less-frequently encountered, and eventually spread through the population.
Rarity an advantage: negative frequency-dependent selection
What is positive frequency dependence selection
There is an advantage to blend in
Explain the heterozygote advantage in rats
Resistant rats- Structurally different enzyme, not affected by Warfarin (poison used) but there is a cost to resistance such as
less efficiency in Vitamin K recycling therefore, being heterozygote is an advantage as they are resistant and don’t require as much vitamin K than a resistant homozygote.
Why doesn’t selection lead to uniformity?
*Negative frequency-dependent selection
*Heterozygote advantage
Other circumstances that can lead to the maintenance of polymorphisms:
*Spatial variation in selection e.g Rock pocket mouse- (different colour mice as different coloured rocks)
*Temporal variation in selection (moth colour changed due to pollution changing tree colour)
Describe genetic drift?
What happens if there are bottleneck events?
genetic drift can be seen as a sampling error within the population purely by chance.
Regardless of selection, not all individuals will successfully breed into the next generation just by chance.
In populations that are very large, this is unlikely to affect the frequencies of alleles significantly
In small populations genetic drift can very rapidly erode genetic diversity
Alleles can randomly reach fixation in small populations very quickly through drift
Drift can have a profound effect if populations go through bottleneck or founding events
Such events can lead to isolated populations having quite different genetic variation than others
What is genetic variation expected to be eroded by?
Directional and stabilizing selection, which leaves a puzzle as to how genetic variation is maintained in populations: polymorphisms are common.
How is genetic variation maintained?
Balancing selection (negative frequency dependence and heterozygote advantage), along with spatial and temporal variation in selection pressures acting on populations.
What is genetic drift?
Drift is the random fluctuation of alleles through generations purely due to chance. It can act to maintain diversity in species by creating differences in allele frequencies through time in different populations.
How does drift impact small vs large populations?
Genetic drift has a much more significant impact on small compared to large populations. When populations go through bottleneck or founder events, drift can result in deleterious alleles reaching high frequencies.
