Natural Selection

Question 1

what do gene flow and genetic drift interact with? and what is the driving force?

Answer 1

• natural selection
• natural selection is also the driving force q

Question 2

what is the criteria for natural selection?

Answer 2

• traitt variation
• variation in survival and reproduction (fitness)
• covariation between fitness and trait
• inheritance of trait variation

Question 3

what is phenotypic selection?

Answer 3

the connection between phenotypic variation and fitness

Question 4

what must a trait be for natural selection to cause change between generations?

Answer 4

heritable

Question 5

what is directional selection?

Answer 5

when individuals homozygous for one allele have a fitness greater than that of the individuals with the other genotypes and individuals with other genotypes
* natural selection pushes the trait distribution in one direction

Question 6

what is stabilizing selection?

Answer 6

what happens to an individual trait when the extreemes of the trait are selected against

Question 7

what is birth weight in humans an example of? explain

Answer 7

• stabilizing selection
• very small or very large babies suffered increased mortality compared to infants of intermediate bith weight
• therefore, the extremes are selected against

Question 8

what is disruptive selection?

Answer 8

when the extremes of a trait experience high fitness, while the intermediate phenotypes experience low fitness. this is thought to be less common

Question 9

what is an example of distruptive selection?

Answer 9

• Darwin’s finches
• birds with bigger beaks: can crack open bigger fruits
• birds with smaller beaks: good at handling small seeded fruit
• seeds that are intermediate sized are in short supply
• there are peaks in fitness at both ends on the spectrum, and at a low point in between

Question 10

describe the scenario of the sickle cell mutation?

Answer 10

• homozygous fro the normal allele: have normal red blood cells
• homozygous for the sickle cell allele: have misshapen red blood cells, that tend to clog small blood vessels
• heterizygous: have a mix of normal and sickle shaped cells
• many parts of the world: normal allele has the highest fitness, heterozygous suffer some fitness disadvantages. this results in directional selection for the normal allele (which should eliminate the sickle cell allele from the populatin)
• where malaria is high: heterozygotes have an advantage since they can avoid some extent of infection from malaria, since the parasite has a hard time completeting its life cycle in a sickle shaped red blood cell. fitness of homozygote for the normal allele is a little depressed, which leads to heterozygote advantage.

Question 11

what is underdominance? why is it hard to maintain

Answer 11

• heterozygous have lowest fitness
• homozygotes have high, but equal, fitness
• maintains both alleles in the population
• but only in extremely restricted circumstances, which almost never occurs

Question 12

what is sexual selection?

Answer 12

• selection acts during mating (diff genotypes differ in their success at finding mates) and in the end producing offspring
• different in males compared to females
• females like to choose mates with ornaments