Evolution

Question 1

How is variation in the phenotype caused by genetic factors?

Answer 1

• primary source of genetic variation is mutation = new alleles produced
• however, crossing over + independent segregation of chromosomes during meiosis + random fusion of gametes during fertilisation adds more variation bc creates new combinations of alleles

Question 2

How is variation in the phenotype caused by environmental factors?

Answer 2

• envi. variation is due to natural selection
• individuals w phenotypes giving them selective advantages to envi factors (e.g. predation, disease + competition which act as selection pressures) are more likely to survive + reproduce
• so can pass their alleles onto offspring causing a change in allele frequency

Question 3

What is the role of overproduction of offspring in natural selection?

Answer 3

• overproduction of offspring creates competition for resources, leading to the best adapted individuals surviving + passing on their alleles to offspring

Question 4

What is the role of variation in natural selection?

Answer 4

• variation results in individuals having diff phenotypes in a pop.
• so when selection pressures change, individuals w phenotypes giving them selective advantages, have a higher chance of survival so can reproduce + pass on advantageous alleles to offspring
• this differential reproductive success causes a change in allele frequencies within the gene pool

Question 5

What is genetic drift?

Answer 5

• change in allele frequency within a pop between generations
• smaller a pop. is, bigger the impact allele frequency changes have

Question 6

What are the 3 types of selection?

Answer 6

• stabilising
• directional
• disruptive

Question 7

What is stabilising selection?

Answer 7

• when modal trait has selective advantage so allele frequency remains constant unless there’s a change in envi.
• as a result, standard deviation dec. as individuals w extreme traits dec., dec genetic variation which prevents evolution

Question 8

What is directional selection?

Answer 8

• when there’s a change in envi., so 1 extreme has the selective advantage, causing the modal trait to change as allele frequency changes
• = an inc in genetic variation so contributes to evolution

Question 9

What is disruptive selection?

Answer 9

• when individuals, containing alleles coding for either extreme trait, are more likely to survive + pass on their alleles
• causing allele frequency to change so more individuals possess allele for extreme trait + middle trait allele becomes less frequent
• continued disruptive selection can lead to speciation + contributes to evolution

Question 10

What is the definition of evolution?

Answer 10

• when inherited characteristics of a pop. change over time through natural selection

Question 11

How are new species formed?

Answer 11

• when a pop. becomes reproductively isolated, both pops. don’t interchange genes when producing offspring
• so allele frequencies in gene pool of both pops. change independently + pops. eventually become genetically isolated
• this means they’re unable to interbreed to make fertile offspring + so are classed as 2 diff species

Question 12

What are the 2 ways a population can become reproductively isolated?

Answer 12

• geographically (allopatric speciation)
• bc of changes in reproductive mechanisms (sympatric speciation)

Question 13

Describe allopatric speciation.

Answer 13

• when a pop. becomes geographically isolated (e.g. new Mt range forms), it becomes reproductively isolated meaning gene pools are separate
• there is variation in both pops. due to mutations
• different selection pressures leads to differential reproductive success
• meaning organisms, w a selective advantage, survive + pass on their advantageous alleles to offspring when they reproduce
• this causes allele frequencies to change over time until they can no longer interbreed to create fertile offspring + so are classed as diff. species

Question 14

Describe sympatric speciation.

Answer 14

• when an isolating mechanism (e.g. diff. breeding seasons) causes a pop. to become reproductively isolated, meaning gene pools are separate
• there is variation in both pops. due to mutations
• different selection pressures leads to differential reproductive success
• meaning organisms, w a selective advantage, survive + pass on their advantageous alleles to offspring when they reproduce
• this causes allele frequencies to change over time until they can no longer interbreed to create fertile offspring + so are classed as diff. species

Question 15

What are e.g.s of isolating mechanisms for sympatric speciation?

Answer 15

• temporal: diff breeding seasons or feeding times
• ecological: diff niches/habitats/feeding areas
• behavioural: diff courtship displays
• mechanical: mismatch of reproductive parts
• gamete incompatibility: sperm killed in female’s reproductive tract
• hybrid infertility: offspring can’t reproduce