Lecture 7 Flashcards
3 types of selection
positive (directional) selection
negative (purifying) selection
selection to maintain variation (balancing selection)
describe the distribution of polygenic traits
- continuous distribution
- typical of size/number traits
- example of quantitative inheritance
draw and describe 3 different modes of selection on quantitative traits
stabilising selection favours average traits
directional selection favours one extreme
disruptive selection favours both extremes
what may be the effects of disruptive selection?
- trait divergence
- speciation, if trait divergence causes a reduction in gene flow
how can we study adaptation?
- test for correlation of alleles or traits with environment over space and time and analyse genomic diversity (genes targeted by selection ought to show distinctive patterns)
- experimental manipulations in field/lab
describe the struggle to determine the agents of selection
- research through today shows thousands of measurements of selection
- demonstrates fitness differences & evolutionary change in traits
and yet: - many fewer convincing cases document the mechanisms (agents) of selection in natural populations
- linking evolution to ecology is difficult
describe the peppered moth (biston betularia) and industrial melanism
- light and dark forms of species that rest on trees
- in the UK, before 1850, dark moths were rare
- dark form caused by a single dominant allele of Cortex gene
- industrial pollution blackened tree trunks near cities and resulted in an increase in the dark form
- dark variant replaced light form in polluted areas, light form predominated in rural unpolluted areas
- mechanism of selection due to predation by birds
- differences in moth crypsis (camouflage) depend on trunk coloration
what happened to the darker melanic variant of the peppered moth after the introduction of the UK ‘clean air act’ in 1956?
there was a decline,
what does the lag in evolutionary response to changes in air pollution levels reflect for peppered moths?
the time required for forests to return to a more natural (unpolluted) state as well as a low initial frequency of the recessive allele for a typical coloration
describe the evolution of heavy metal tolerance in plants
- mine waste is heavily polluted with heavy chemicals such as lead, copper, nickel, cadmium
- heavy metal tolerant genotypes can occur at very low frequency in nearby uncontaminated pastures
- tolerant genotypes invade mine tailings from nearby pastures
- gene flow between pasture and mine is restricted because of flowering time differences between them
- alleles enabling tolerance maintained on mines but not pastures
define a selective sweep
when selection causes a new mutation to increase in frequency so quickly that nearby alleles ‘hitchhike’ and also increase in frequency
describe DNA Variation at glucose-6-phosphate dehydrogenase deficiency in humans
recent natural selection has caused
- low diversity
- high frequency of derived (new) allele
- G6PD gene shows evidence of the recent, rapid spread of resistance allele (consistent with hypothesis of selection for malaria resistance)
describe the long term experimental evolution study of adaptation by E.Coli
- 36 years if propagating 12 population flasks
- > 75000 generations of evolution
- propagated in minimal glucose/citrate medium
results of long term experimental evolution study of adaptation by E.Coli
- all populations rapidly increased in fitness
- similar adaptations across populations (eg larger cell sizes, higher max growth rates on glucose)
- parallel mutations in same genes
- some unique adaptations and distinct genetic changes
- only one strain evolved ability to grow on citrate
