lec19: natural selection and adaptation Flashcards
fitness
genetic contribution of individuals to next generation RELATIVE TO OTHERS as a result of differences in viability and fertility
= Darwinian fitness
higher fitness = has a selective advantage = better adapted but also depends on the environment
is fitness absolute to survival or offspring number?
it is a relative quantity, not absolute survival or offspring number.
the fit fitness of a genotype of an individual represents the relative probability of that individual’s survival and reproduction relative to others as a consequence of its genotype
(an organism’s success in passing on its genes is not solely determined by the number of offspring it produces or its absolute survival rate. Instead, fitness is relative to the organism’s ability to adapt and compete within its specific environment compared to others in its population)
selective advantage
the amount by which some individuals of a given genotype are better adapted to a given environment
- reflects relative differences in fitness
adaptation (2)
a TRAIT that contributes to fitness by making an organism better able to survive or reproduce in a given environment
- compared to the prior ancestral state
- link between trait and environment makes it adaptive
the evolutionary PROCESS that leads to the ORIGIN and MAINTENANCE of such traits
- natural selection (the only way in which you can get adaptation is if by evolution by natural selection - adaptive evolution is the product of natural selection)
artificial selection
selection by humans toward a goal
- domesticated plants and animals
- selection experiments in genetics
- evolution of attenuated vaccines
can think adaptation as a ___ (noun) or as a ___ (verb)
trait or a process
natural selection
selection by abiotic and biotic environment. it affects all organisms (including humans)
does natural selection have a goal
no “goal”
Natural selection removes organisms whose genetic information is not well adapted to the present environment. Natural selection does not “create” better adapted organisms. Organisms that have genetic information that is better suited to the environment do not get that genetic information from natural selection.
how to study adaptation? (considering , is the variation adaptive? what athe agent of natural selection is?…” (3)
- monitor correlations of alleles or traits with environment over space and time
- analyze genomic diversity
- genes targeted by selection ought to show distinctive patterns across the genome - experimental manipulations in field and lab (connect differences in the environment with evolutionary change over time)
alleles are the different types of genetic variant at a site or at a locus. based on a population of alleles, we’re seeing if natural selection is favouring those alleles or not (adaptive?). so what are the 3 selections based on the frequency of allele. list the fitness of AA, aa, Aa in order
- positive directional selection (adaptation)
- reach fixation as frequency approaches 1 - negative purifying selection
- variation maintaining selection (balancing)
AA>Aa>aa fitness
why is it that in a positive directional selection, even tiny selective advantages (1% or less) can spread through populations with enough time
positive directional selection: heterozygotes has a higher probability of surviving and producing than aa. AA has a higher fitness. aa has the lowest fitness. The A allele will spread rapidly, so by 50 generations after the introduction of A mutation, we expect say 99.9% of the population to be AA. this illustrates the awesome power of natural selection to drive evolutionary change. genetic drift is a random force. we would not expect it to drive really rapid evolution like this because it’s just random fluctuations every generation. but with natural selection we loaded the dice, it’s no longer a random chance. A has an advantage, a much higher probability of survival and reproduction if you’re a genotype with A and this leads to extremely strong rapid selection. The strength of natural selection, even a small 1% advantage of fitness should lead to the spread of that allele over time and lead to evolutionary change.
draw a graph for each of discrete vs continuous effect on fitness on the allele/ phenotype
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many phenotypic traits are polygenic/genotypic and show discrete/continuous distributions
polygenic and continuous
identify and draw the 3 types of natural selection on quantitative/continuous traits (eg. phenotype) —-to—-> after selection graphs
stabilizing selection
directional selection
disruptive selection
give an example on the stabilizing selection
on human birth weight
small and large birth weights cause higher infant mortality while the lowest infant mortality is in the middle. so at the extremes there’s higher mortality. there’s selection against the extremes
give an example on directional selection
on beak size in Galapagos Finches
a change in environment (drought) led to a reduction in seed supply and hardened the seed causing a change in the size of the beak
give an example on disruptive selection
on beak size in African Finches
(LS)beak adapted for soft seed
(middle) you are not good at either alternatives
(RS) break adapted for hard seeds
what does disruptive selection require
requires spatial heterogeneity or discrete resources
what does disruptive selection lead to?
disruptive selection leads to trait divergence and in some cases, may lead to speciation
can maintain the genetic variation in a population because there can be different strategies maintained in the population. in some cases, as you start to get this divergent between phenotypes, this might lead to speciation
- evolution by pollution of industrial melanism in peppered moths –> after introduction of the Uk “clean air act”
- mouse coat colour polymorphism in contrasting habitats maintained by avian predators –> mice experiments: studying in light vs dark habitats –> strong evidence of natural selection on coat colour —> natural selection at one single gene of major effect
- G6PD deficiency in humans –> measuring genetic diversity in populations: allele frequencies for G6PD –> natural selecton on G6PD –> DNA variation at G6PD
- evidence of natural selection on vitamin D deficiency from ancient to modern genomes in the UK
so to fully understand natural selection we need to document that there’s heritable variation, there’s change through time, and connect that heritable variation to some sort of ecological
these are examples of how evolution and ecology are linked. factors like environment and predation are causing/influencing rapid natural selection and rapid evolutionary change in populations; there’s a correlation between genotype and the environment that suggests an ongoing action of natural selection
can also connect ancient DNA with natural selection
we can connect the genetics to the phenotype, to ecology, to natural selection. an example of our understanding of natural selection from genotype all the way to evolutionary change
what is an example where natural selection was being studied in the lab (experimental evolution in the lab)?
long-term experimental evolution study of adaptation by E.coli throughout 75,000 generations of evolution of 12 population flasks
explain their set up of the E.coli experiment and the results.
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