lecture 3: natural selection, genetic drift, gene flow Flashcards
directional selection
the distribution of alleles shifts to one side;
shifts the population mean; ex. over time, the size of finch beaks increases; larger beak size = advantageous
diversifying selection
the distribution of alleles shifts to the extremes; selects against the mean phenotype
ex. flies feeding on apple and hawthorn fruits: apple and hawthorn trees fruit at different times; flies whose life cycles are coordinated with the fruiting of specific tree species would have greater fitness than flies whose life cycles place them between the fruiting of either tree species
stabilizing selection
the distribution of alleles converges toward a central value; maintains the status quo and acts against extremes; ex. birth weight for babies: if a baby is too small, its chances of survival after birth are low; if it’s too large, there can be complications for mother and baby during delivery; the optimal birth weight is between these 2 extremes
frequency-dependent selection
when the fitness value of a particular trait depends on how common or rare it is in a population
positive frequency-dependent selection
more common phenotypes have a higher fitness; ex. there are several color patterns (morphs) in the Heliconius butterfly genus. all the morphs are poisonous. when a morph is common, it will be more likely that birds will have already learned to avoid them, whereas birds will not yet have learned to avoid a rare morph. an individual of a rare morph is therefore more likely to be the unlucky prey that educates the bird and gets killed in the process.
negative frequency-dependent selection
more rare phenotype have a higher fitness; negative selection can maintain genetic diversity for phenotype within a population; ex. grove snails have a wide variety of color and banding patterns on their shells. song thrushes find their prey by sight and develop a search image for the most common types of their prey, such as snails. snails with common shell tyes are eaten more often giving a selective advantage to those with rare shell types. the fitness of a particular shell type will vary overtime depending on how common or rare it is. this maintains variation in the population.
genetic drift
a process in which allele frequencies within a population change by chance alone as a result of random sampling from generation to generation; as genetic drift occurs due to random factors, it is an evolutionary force that does not lead to adaptation; sampling error is higher with a smaller sample; can also result from random reproduction of certain individual populations; chance events can affect whether an individual survives or reproduces regardless of its phenotype or genotype
consequences of genetic drift
harmful alleles may increase in frequency by drift & advantageous alleles may be lost by drift; genetic drift has a large effect in small populations; can by chance lead to the spread of detrimental alleles, causing rare genetic diseases
why is a loss of genetic variation important?
harmful alleles may increase in frequency by drift & advantageous alleles ma be lost by drift; can by chance lead to the spread of detrimental alleles, causing rare genetic diseases
genetic drift: the bottleneck effect
a bottleneck occurs when the size of a population is decreased significantly for at least 1 generation; the resulting small population is likely to show the effects of genetic drift
genetic drift: the founder effect
the loss of genetic variation when a new colony is formed by a very small number of individuals from a larger population
gene flow
the transfer of genetic material, in the form of alleles, from one population to another; gene flow can add new alleles to a population or change the frequencies of existing alleles; often constrains local adaptation; prevents populations from genetically diverging
factors affecting gene flow
habitat fragmentation
- species mobility
- birds can fly long distances
- plants must remain in place, although pollen and seeds can move
- fish in lakes/ponds can’t easily move from place to place
- location
- populations on islands don’t easily exchange genetic material
