lec20: population structure, gene flow, and genetic drift Flashcards
population
a group of individuals of the same species occupying a given area at the same time
migration
the movement of individuals from one population to another
gene flow
any mechanism that allows genes to flow from one population to another
What are Key questions for evolutionary studies of variation within populations? (2)
How many of these variations are genetic in origin?
Does variation contribute to fitness differences among organisms?
What are Key questions for geographical differentiation? (2)
What proportion of genetic variation in a species is found within a population vs between populations?
How is diversity distributed within and between populations?
Are the differences between populations heritable and adaptable?
considering the “effects on population divergence” what is the job of gene flow
acts to homogenize to make populations more similar each other
how does selection and drift affect populations?
selection:
- natural selection will act the populations to drive apart
- if have these two populations in the same environment, natural selection might keep them more similar; but if have different environments that these populations occur in, natural selection drive them to be more differentiated
drift:
- causes the populations to be different
what are ways to measure gene flow? (2)
Experiments
most direct way: use neutral genetic markers (seeing if variant is moving between populations)
- Polymorphic genetic variants that are not direct targets of selection
If two populations have different genes, one fully F and only fully S, and we want to find out how much gene flow occurs between them, what do we do?
score FS heterozygotes in offspring
- frequency of heterozygotes = an estimate of gene flow
do most gene flow occur over a long or short distance?
short but a small amount occurs as far as 1km. so further distance away = less gene flow, but there’s evidence that these genetic markers are getting into these populations
If there is a field of crop and weed sunflowers, what happens to gene flow the further these are from each other?
Less gene flow
genetic drift
Random fluctuations in alleles
other words: the change in frequency of an existing gene variant in the population due to random chance
it’s like rolling the dice
Does genetic drift lead to particular direction of evolutionary change?
no
stochastic
unpredictable or random
What are stochastic evolutionary forces? MRG
mutation, recombination, genetic drift
deterministic
predictable or non-random
What is a deterministic evolutionary force?
natural selection
stochastic vs deterministic in terms of dice
drift is like rolling the dice (a unloaded dice) and with natural selection is like a loaded dice (like an unfair dice)
genetic drift is “by chance” or due to “genetic differences”
Unlike natural selection, genetic drift does not depend on an allele’s beneficial or harmful effects. Instead, drift changes (stochastic changes) in allele frequencies purely by chance (to random variation) in fecundity and mortality and can lead to these random losses of alleles leading to loss of diversity
What are Stochastic changes in allele frequency due to (don’t say the evolutionary forces) ?
random variation in fecundity and mortality
What type of population size do stochastic changes affect?
small population
what are the 3 kinds of stochastic processes that play a role in loss of diversity?
- genetic drift
- population bottlenecks
- founder events
population bottlenecks
an event that drastically reduces (a single sharp reduction) abundance (the size of a population) causing a loss of diversity
founder events
occurs when a new population is established from a small number of individuals drawn from a large ancestral population
other words: colonization by a few individuals that start a new population leading to loss of genetic diversity because colonizing group contains only limited diversity compared to the source population
Which type of population size is genetic drift more common in?
Genetic drift more prominent in small populations
eg. starting at 50% of allele frequency; 9 individuals, 18 gene copies vs 50 individuals, 100 gene copies
What are the effects of genetic drift in small populations? (4) FDAC
More drastic fluctuations each generation
Rapid loss of genetic diversity
Faster allele fixation or loss
Less consistency across replicate populations
draw a diagram of genetic variation + founder events + continued population bottleneck. what does it represent?
over time, if the population expands it’s going to start accumulating new genetic variation but over a long time you get a loss of genetic diversity and as you get these founder events, you get recurrent loss of this genetic variation
how does genetic drift differ in populations/countries that are closer or further apart from each other?
populations that are closer to each other like CAN-US populations has more genetic variation THAT’S SHARED.
CAN ad SWE, we see a lot less of the genetic variation is shared, a lot more of it is unique and we start to see fixed differences between the populations
so populations that are further away from each other has experienced more genetic drift that’s differential between them, less gene flow, and so they’re more differentiated than populations that are closer to each other
between countries, we can see fixed differences between the populations due to the distance. what about in human population (between humans) differentiation?
very super duper rare to find fixed differences between human populations. so we have a lot of allele sharing across populations and that’s because we’ve experienced relatively recent origins and reasonably high levels of gene flow around the globe.
certainly much greater allele sharing than the plant population
most genetic variation that you see is found both within and between populations, in other words, we share most of our alleles around the world
explain why there’s human population differentiation from East Africa with increasing distance
population that colonize further away experience more genetic drift and potentially less historical gene flow from East Africa allowing us to see differences in allele frequencies across human populations even though allele sharing is extremely high
this is because of the Neanderthals that originated from East Africa and their relation to modern humans.
draw a diagram of the gene flow between humans and neanderthals + what does it illustrate
there’s higher levels of neanderthal ancestry in Eurasia than there is in Africa
human populations out of Africa is consistent with ancient interbreeding averaging about 2% of the genome that have short stretches of neanderthal-derived DNA
founder events/bottlenecks and lower/higher gene flow with increasing distance from East Africa.
founder events and lower gene flow with increasing distance
genetic differentiation among populations is often observed across a _______ range + explain
geographic range
meaning phenotype across the geographic range differs (populations differ in what they look like across the geographical range)
- eg. the rat snakes on the first slide
phenotypic differentiation may be ____ or due to ____ or _____
adaptive (“local adaptation”): local adaptation to different environmental conditions driving these differences in phenotype between populations)
OR due to genetic drift (random chance)
OR phenotypic plasticity
phenotypic plasticity
ability of a genotype to modify its phenotype in response to environmental changes
in a phenotypic plasticity, does that mean that our genotypes are changing, changing our phenotype? if not then how does it occur?
not changes in our genotype that are happening but changes in the expression of genes in epigenetic regulation depending on the environment we’re exposed to
occurs through modifications to growth and development and behaviour
- phenotypic plasticity is under genetic control
(there are genes built in that allows us to respond to the environment but the key thing to remember is that it’s not genetic differences that are causing the difference in phenotype, it’s under genetic control, but those genes are allowing the phenotype to differ even though the genotype has stayed the same )
what factors make a phenotype?
genomic and environmental factors
do genotypes grow the same in every environment? + ex
no
can 2 species with exact same genotype, no difference in the genetic composite, look very differently due to the different environment each are in
yes such as the aerial leaves (terrestrial phenotypes) and submersed leaves (aquatic phenotypes) which are both arrowheads species
what organism is phenotypic plasticity most common in? (2)
common in sedentary sedentary organisms (eg. plants and corals)
- eg. seed may germinate in very different kinds of environment so there can be a lot of plasticity in sedentary organisms
and in animal behaviour
- eg. if we’re hot we tend to have plasticity around that we might sweat more or go to the shade as a behavioural response
do all phenotypic plasticity result from adaptation
no. phenotypic plasticity often is an adaptation to unpredictable environment BUT not all results from adaptation.
extra:
can be advantageous to have a different phenotype depending on the environment BUT when we see differences between organisms and how they look in different environments, the reason why they look different is not because of local adaptation when talking about plasticity. Also important to remember that not all phenotypic plasticity is adaptive, some might be maladaptive specially if organisms are experiencing environments that they didn’t evolve in or unexpected to evolve in. so some plasticity but not all evolved and been adaptive
what is done in reciprocal transplant studies + what is done using phenotype? + what does this tool help us distinguish + how in terms of, what does it enable?
Growing equivalent genotypes in different environments and comparing their relative performance
this tool helps us distinguish the role of phenotypic plasticity vs the role of local adaptation
- can separate phenotypic variation into genetic and environmental components
- this enables measurement of selection against non-local genotypes
Reciprocal transplant studies provide evidence of?
local adaptation
what components does Reciprocal Transplant Studies separate phenotypic variation into?
genetic and environmental components
what were the conclusions in the Clausen-Keck-Hiesey Transplant?
Differences between populations are due to BOTH plasticity and genetics
Evidence from widespread local adaptation because local populations had the highest fitness
what does high UV radiation select for in terms of skin pigmentation? what does it interfere with?
May select for increased pigmentation
Interferes with folate (the natural form of vitamin B9, water-soluble and naturally found in many foods)
what does low UV radiation select for in terms of skin pigmentation? what does it reduce the making of?
Reduced making of vitamin D
Increase selection for reduced pigmentation
is there a single best phenotype across the globe? why
No single best phenotype across the globe due to tradeoffs
what was the history of Local adaptation on skin pigmentation?(4) NADl
Numerous genes are known to affect skin pigmentation.
Alleles of these genes show rapid allele frequency change over time using ancient genomes (indicates natural selection).
Alleles of these genes show higher between population differentiation than most other genes.
Evidence supporting a history of local adaptation
