Mutation as an evolutionary force Alleles may be kept in a population through a balance between mutation (creating deleterious alleles) and selection (removing them) - in mutation-selection balance, the frequency with which new alleles are created by mutation equals the rate at which they are eliminated by selection When the frequency of a harmful allele (say, cystic fibrosis) is higher in a population than you’d expect from the mutation rate of that gene, then you have reason to suspect some other force (i.e., selection) may be keeping that allele around

Mutation as an evolutionary force Why does the  F508 allele, which causes cystic fibrosis, occur at a high frequency (0.02) in populations of European descent? - selection against homozygotes is strong - mutation rate is too low to explain high allele frequency - protects against infection by typhoid fever bacterium?

Mutation as an evolutionary force More importantly, mutation promotes evolutionary change by genetic innovation - once a rare beneficial allele is created by mutation, it can rapidly become fixed in the population through selective sweeps bacteria evolved in a series of jumps: new mutations that resulted in larger cell size appeared, rapidly spread through the population

Migration Migration is the movement of alleles between populations Migration can rapidly change allele frequencies, especially for small populations - individuals leaving a continent make little difference to the allele frequencies on that continent 

Migration Example: banded vs unbanded water snakes

Migration Example: banded vs unbanded water snakes - one gene w/ 2 alleles determines banded, unbanded or intermediate morph - natural selection favors banded snakes on mainland, where they are cryptic (hidden from predators) - selection favors unbanded snakes on islands, where bands stand out when snakes sun themselves on rocks to warm up

distribution of banded vs unbanded snakes proportion of unbanded snakes increases as you move further out into middle of lake there’s always some banded snakes present on every island, though

Why doesn’t selection fix the unbanded allele on islands? (drive it to a frequency of 100%) - migrants from mainland continually introduce banded allele into island population - about 13 snakes per year move to islands, which have ~1300 snakes (roughly 1% migration per year) Migration acts as a homogenizing force: - equalizes allele frequencies among populations; makes them more similar than they would otherwise be  Migration

What is a population? Turns out to be a difficult thing to describe... may be defined by:

What is a population? Turns out to be a difficult thing to describe... may be defined by: - geography - potential for dispersal (movement) - genetic similarity (equal allele frequencies)

What is a population? Turns out to be a difficult thing to describe... may be defined by: - geography - potential for dispersal (movement) - birds can fly over a barrier other animals and plants can’t 2 populations 1 population

What is a population? Turns out to be a difficult thing to describe... may be defined by: - geography - potential for dispersal (movement) - genetic similarity (equal allele frequencies) AA Aa AA Aa AA Aa aa Aa aa

Genetic Drift A sampling process (flipping a coin, drawing beans from a bag) may produce results different from theoretical expectations - flip a coin four times, and you may get 4 heads When the actual results differ from theory, this is sampling error Sampling error depends largely on the number of samples drawn - flip a coin 40 times, and you are very unlikely to get 40 heads - will probably get ~20 heads, give or take a few 

Genetic Drift Sampling error in production of offspring in a population is genetic drift Initial frequencies are always heavily skewed during random sampling - ie, drawing alleles one at a time from a big “batch” (= gene pool)

Genetic Drift Sampling error is very sensitive to population size - as population increases, effects of genetic drift diminish - odds of getting the expected allele frequencies when you make 10 zygotes by drawing alleles at random

Random fixation of alleles Given enough time, any allele will eventually become fixed or disappear if genetic drift is the only mechanism at work - when one allele is fixed, all others have a frequency of zero - the odds that any given allele will be the one that goes to fixation is the initial frequency of that allele  pop. size =

Genetic Drift (1) Every population follows a unique evolutionary trajectory, because sampling error affects allele frequencies at random - if selection were at work, different populations would evolve along similar trajectories (2) drift works faster and stronger in small populations - allele frequencies change more dramatically if population size is small (3) even in large populations, drift can cause substantial evolution over long times - geographic isolation results in differentiated populations eventually, different species