Topic 6: Natural Selection

Question 1

What is absolute fitness?

Answer 1

• a measurement of fitness comparing the actual number of individuals by genotype at two different times, such as generation to generation, or birth and adulthood

Question 2

What is relative fitness?

Answer 2

• A measurement of fitness once the genotype is assigned a value of 1.0 and the fitness values of other genotypes are defined relative to that genotype
• gives us more information than absolute fitness.

Question 3

How do you calculate absolute fitness genotypes?

Answer 3

present genotype/ past genotype

Question 4

How do you calculate the relative fitness values of genotypes?

Answer 4

divide the absolute fitness genotype/the highest absolute fitness

Question 5

What is the probability that someone with a given genotype will contribute genetically to the next generation?

Answer 5

HW genotype frequency X relative fitness values

Question 6

What is mean fitness?

Answer 6

• W-bar
• this tells us how much selection has taken place relative to the case where no selection has occurred
• it is a survival expectation

Question 7

What are the three types of Natural selection?

Answer 7

1. selection against the recessive homozygote
2. selection against dominant alleles
3. selection with codominant alleles

Question 8

What is selection coefficient (s)?

Answer 8

the probability of not surviving and reproducing

Question 9

What is the major difference between selection against recessive and dominant homozygotes, if they are lethal alleles?

Answer 9

• when a recessive allele is lethal, it takes time to remove it from the population, because it is not ‘visible’
• with selection against dominant alleles, we can remove it from a population in a single generation, because it is so ‘visible’, and visible in heterozygotes too

Question 10

What is selective sweep?

Answer 10

• a consequence of selection, which has ripple effects to genes on the same chromosomes
• a gene on a chromosome is physically linked to other genes. if there is selection for or against that allele, this selection will influence other genes on the same chromosome.
• will effect a large chunk of the chromosome.

Question 11

How does positive selection for a mutation result?

Answer 11

Selection for dominant: frequency will increase very fast (it is visible as homozygous and heterozygous)

Selection for recessive: frequency will increase very slowly

Question 12

What is Muller’s rachet?

Answer 12

• the idea that asexual populations will have an increasing mutational-load over time
• reproduction via cloning means you can’t get rid of mutations by recombination, so mutations continuously increase
• it is an argument for why sexual reproduction may be favored over asexual reproduction

Question 13

How does selection interact with genetic drift? How does population size influence the relative strengths of these two forces?

Answer 13

• drift ISN’T strong in large populations, but IS strong in small populations
• Selection IS strong in large populations, but INS’T in small populations

Question 14

How can genetic drift cause an ‘evolutionary rescue’?

Answer 14

• if an environmental change causes a species to drop in frequency (because previous selection does not suit the new environment), and approach extinction, once the population hits a small size, drift may be able to influence random allele changes, rescuing the population
• drift is most strong in small populations

Question 15

How can selection and inbreeding effect one another?

Answer 15

• if inbreeding depression occurs, there is a drop in fitness as there are an increase in homozygotes
• if there is then selection against homozygotes, alleles are more exposed, and changes in the population will occur more rapidly