topic 8.3 - gene pools

Question 1

what is a gene pool

Answer 1

the sum of the alleles of all the genes of all the individuals in a population

Question 2

what is meant by allele frequency

Answer 2

how often an allele appears in the population

Question 3

how does selection affect allele frequencies

Answer 3

• some phenotypes make it more likely that an organism will survive & reproduce successfully
• the alleles that gave the adv are more likely to be passed on
• frequency of the allele in the population will increase (more individuals will show the characteristic)
• some alleles won’t be passed on & lost
• rate of change is usually very slow

Question 4

how can a new species be formed

Answer 4

• by directional and disruptive selection
• it can result in enough differences for a new species to be formed - speciation

Question 5

stabilising selection

Answer 5

• acts against change by making favourable characteristics more common in an unchanging environment
• reduces variety
• maintains continuity in a population

Question 6

disruptive selection

Answer 6

• leads to change by making extreme values of a characteristic more common
• this lead to two clear forms in the population (speciation)

Question 7

genetic drift

Answer 7

• change can sometimes be due to chance, not selection

Question 8

what is genetic drift

Answer 8

• changes in frequencies of alleles can be due to chance rather than selection
• chance can be due to random sexual reproduction or to accidents preventing reproduction
• alleles are not passed from a population in a way that reflects their frequency in the population
• the effects will be greater in small populations and may lead to complete loss of an allele if it isn’t passed on
• genetic drift is not directional

Question 9

population bottlenecks

Answer 9

• when the size of a population is dramatically reduced by an environmental disaster, a new disease, hunting by humans/predators or habitat destruction

Question 10

how can allele frequencies be influenced by population bottleneck

Answer 10

• it causes a severe decrease in the gene pool of the population
• many gene variants present in the original population are lost, so the gene pool shrinks
• only a small number of individuals survive a major event
• frequency of alleles is different within the surviving population

Question 11

founder effect

Answer 11

• the loss of genetic variation when a small number of individuals leave the main population and set up a separate new population

Question 12

how can allele frequencies be influenced by the founder effect

Answer 12

• the frequency of alleles is different within the new population
• any unusual genes in the founder members of the new population may become amplified as the population grows

Question 13

Hardy-Weinburg

Answer 13

• predicts the frequencies of alleles in a population will remain constant unless factors act to affect them

Question 14

what are the assumptions made about the Hardy-Weinburg equilibrium

Answer 14

• there are no random mutations
• there is random mating
• the population is very large
• there is no migration
• there is no selection pressure

Question 15

how do mutations affect allele frequency

Answer 15

• mutations are a source of new alleles in a population
• do not happen often as we have constant checking mechanisms in place but there are so many cells that they are bound to occur at some stage
• in animals, only mutations in germ line cells will affect alleles of next generation
• mutations in somatic body cells will die with their owner

Question 16

how does non-random mating affect allele frequency changes

Answer 16

• sexual selection occurs where mates choose individuals based on certain characteristics
• humans can interfere with some populations via selective breeding
• inbreeding in some population occurs - this increases the frequency of homozygotes

Question 17

how does gene flow affect allele frequency

Answer 17

• allele frequency can be affected by alleles entering or leaving a population via migration
• immigration could introduce new alleles to a population while emigration could cause some alleles to be lost from the population
• gene flow can also arise due to dispersal of seeds, pollen or spores

Question 18

how does genetic drift affect allele frequency

Answer 18

• in small populations you can get a much smaller gene pool and allele frequencies can change dramatically