8 Flashcards
define a population
a group of individuals of a single species occupying a given area at the same time
define migration
the movement of individuals from one population to another
define gene flow
the movement of alleles from one population to another
how can we measure gene flow?
use experimental approaches
use neutral genetic markers:
- polymorphic genetic variants that aren’t direct targets of selection
experiment to answer the question: how much gene flow occurs between geographically separated populations?
- establish two populations, fixed for alternative alleles, separated by a given distance
- score FS heterozygotes in offspring
- frequency of heterozygotes = an estimate of gene flow
formally define genetic drift
stochastic changes in allele frequency due to random variation in fecundity and mortality
what does random mean in evolution?
stochastic (unpredictable or random) evolutionary forces:
- mutation
- recombination
- genetic drift
deterministic (predictable or non-random) evolutionary forces:
- natural selection
define population bottlenecks
- a single sharp reduction in abundance, usually followed by a rebound
- causes a loss of diversity
define a founder event
- colonisation by a few individuals that start a new population
- colonising group contains only limited diversity compared to the source population
why is genetic drift more pronounced in small populations?
- more drastic fluctuations in each generation
- more rapid loss of genetic diversity (i.e. faster time to allele fixation or loss)
- less consistency across replicate populations
as distance increases, gene flow
decreases
define isolation by distance
accumulation of local genetic variation due to geographically limited dispersal
phenotypic variation may be:
- adaptive (‘local adaptation’
- due to genetic drift
- phenotypic plasticity
how do we test for local adaptation and plasticity?
reciprocal transplant studies
Genomic analyses
define phenotypic plasticity
the ability of a genotype to modify its phenotype in response to a particular environment
draw 3 graphs for no plasticity, plasticity, and highly variable plasticity
how does phenotypic plasticity occur?
through modifications to development, growth, and/or behaviours under genetic control
in what organisms is phenotypic plasticity common?
in sedentary organisms like plants and corals, but also in animal behaviour
phenotypic plasticity often is an adaptation to…
unpredictable environments
does all phenotypic plasticity result from adaptation?
no
describe reciprocal transplant studies
Growth of equivalent genotypes in contrasting environments and comparisons of their relative
performance
why are reciprocal transplant studies useful?
- Can separate phenotypic variation into genetic and environmental components
- Enables measurement of selection against non‐local genotypes
- Can provide evidence for/against local adaptation
Clausen‐Keck‐Hiesey Transplant Conclusions
- Differences between populations due to BOTH plasticity and genetics
- Evidence for widespread local adaptation
– Local populations had highest fitness
How do we test for plasticity and adaptation in species that we can’t manipulate experimentally?
genomic studies
Tradeoffs Associated With Skin Pigmentation
High UV radiation:
– Degrades folate, critical in highly dividing tissues (e.g. embryos, testes)
– May have selected for increased pigmentation
- Strong purifying selection on MC1R in equatorial
regions
Low UV radiation:
– Reduced vitamin D synthesis
- VitD critical for bone development, immunity, etc.
– May have selected for reduced pigmentation
was there a history of local adaptation in skin pigmentation?
Numerous genes known to affect skin pigmentation
* These genes show higher between‐population differentiation than most others
–> Evidence supporting a history of local adaptation
* Pigmentation genes show evidence for positive selection in regions with distinctive skin
colouration
