evolutionary processes

Question 1

history of population genetics and evolution

Answer 1

the modern synthesis (1900s-60s) put together Darwin and Wallace’s (1859) theory of evolution with Mendel’s (1850s-60s) principles of genetics to describe natural selection in terms of allele frequencies

Question 2

genetic equilibrium

Answer 2

• allele and genotype frequencies stay the same from one generation to another
• testing for genetic equilibrium involve calculating the statistical difference between observed and expected allele frequencies of offspring

Question 3

7 conditions required for genetic equilibrium

Answer 3

1. organisms are diploid
2. organisms only reproduce through sexual reproduction
3. no overlap between generations
4. mating is random
5. population is large
6. no migration, mutation or selection
7. allele frequencies are equal in both sexes

Question 4

hardy-weinberg equation

Answer 4

• allows for calculation of allele and genotype frequencies within a population at genetic equilibrium
• p^2 + q^2 + 2pq = 1
• p= frequency of dominant allele
• q = frequency of recessive allele

Question 5

non random mating

Answer 5

• assortative mating = similar genotypes mate e.g. pigeons with size
• dissasortative mating = opposite genotypes mate e.g. pigeons with colour

Question 6

Assumption of no net mutation

Answer 6

mutations of dominant to recessive alleles occur at the same frequency as mutations from recessive to dominant alleles e.g. mutations from wild type to albino in rabbits occur more frequently than mutations from albino to wild type, but there is a selection balance as albino rabbits are more likely to be predated

Question 7

relative fitness (w)

Answer 7

absolute fitness of each genotype divided by absolute fitness of fittest genotype

Question 8

absolute fitness

Answer 8

• viability * fertility
• viability is the probability of surviving to reproductive age
• fertility is the number or offspring that survive to reproductive age

Question 9

natural selection, directional selection

Answer 9

• one allele is always at an advantage e.g. peppered moths
• dominant allele advantage means allele will spread very quickly in a population (AA=Aa>aa)
• codominant dominant allele advantage means adv. allele spreads, disadv. allele gets selected out (AA>Aa>aa)
• recessive advantageous allele means allele will spread very slowly to start with, then quickly becomes only allele in population (aa>Aa=AA)

Question 10

natural selection, overdominance/balancing selection

Answer 10

• heterozygous advantage where there is codominance (AA<Aa>aa)</Aa>
• should maintain both alleles (stable polymorphism), but equilibrium frequency depends on relative fitness of each homozygote
• e.g. sickle-cell anaemia, homozygote is malaria resistant

Question 11

natural selection, underdominance

Answer 11

• heterozygous disadvantage where there is codominance (AA>Aa<aa)
• produces unstable equilibrium
• allele frequencies tend to move away from equilibrium point
• least common allele declines and eventually becomes extinct (less common means more likely to be heterozygous)
  e.g. chromosomal rearrangements in drosophilia

Question 12

genetic drift

Answer 12

• random changes in allele frequencies
• often makes populations less fit
• occurs more often in small populations as offspring produced are at a much lower frequency than alleles
• extreme cases caused by a temporary dramatic reduction in population size

Question 13

founder effect

Answer 13

• small group of individuals leave original population to found new one
• e.g. Eastern Pennsylvanian Amish population, founded by 200 German immigrants and married strictly within community. One individual carried recessive allele for Ellis van Creveld syndrome rare in German population but now common in Amish population

Question 14

genetic bottleneck

Answer 14

• random event wipes out most of population, leaving only a small group
  e.g. typhoon on Pingalap island in 1800s left 30 survivors, one carrying rare recessive allele for achromatopsia (complete colour blindness), 10% population are now colourblind

Question 15

measuring genetic diversity

Answer 15

• heterozygosity = likelihood individual will be heterozygous at a given locus, usually averaged over many genetic loci
• allelic diversity
  Number of alleles averaged across loci = total number of alleles over all loci / number of loc

Question 16

speciation

Answer 16

• genetic drift and/or local adaption produces reproductive isolation between populations
• cessation of gene flow allows species to evolve independently and accumulate different adaptations

Question 17

problems with the biological concept of species

Answer 17

• species = group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other organisms
• many recognised species hybridise or do not reproduce sexually

Question 18

calculating proportion of individuals expected to he homo/heterozygous for an allele in next generation

Answer 18

current fitness* current frequency / average fitness of population