Variation and Evolution

Question 1

three types of variation (+2 more)

Answer 1

• discontinuous (discrete)
• continuous
• phenotypic

and intra/inter specific variation

Question 2

link between genes and discontinuous variation

Answer 2

controlled by a single gene (monogenic) OR multiple genes in epistasis

Question 3

link between genes and continuous variation

Answer 3

• controlled by several genes (polygenic)
• genes must not be linked/carried on the same chromosome so can be shuffled during meiosis (crossing over could however happen if genes are on the same chromosome)

Question 4

what two things contribute to phenotypic variation?

Answer 4

• genotype
• environment

Question 5

which type of phenotypic variation is more affected by environmental factors?

Answer 5

continuous variation

Question 6

three types of selection

Answer 6

• stabilising
• directional
• disruptive

Question 7

stabilising vs directional vs disruptive selection

Answer 7

stabilising - selects for the norm (extremes most affected by selective pressure)

directional - selection in response to environmental change (some characteristics may have a selective advantage)

disruptive - selects for the extremes (norms most affected by selective pressure)

Question 8

example of stabilising selection

Answer 8

• clutch size in birds (number of eggs)
• too few will increase the probability that all will die before reproductive age
• too many will cause food availability to become a selective pressure

Question 9

example of directional selection

Answer 9

• peppered moth more abundant before industrial revolution
• melanated moths more abundant after industrial revolution as they have a selective advantage (camoflauge for the soot selective pressure)

OR antibacterial resistance

Question 10

genetic drift

Answer 10

change in allele frequency caused by chance (often seen in small populations)

Question 11

two causes of genetic drift

Answer 11

• founder effect
• bottlenecks

Question 12

example of disruptive selection

Answer 12

• young male birds with either dull or bright blue feathers are selected for
• dull are seen as non-threatening
• bright are seen as threatening
• both are left alone (not attacked/less competition)

Question 13

founder effect

Answer 13

few individuals of a species colonise a new area leading to a loss in genetic variation and rare alleles can become more common in the population

Question 14

two types of limiting factors in population size

Answer 14

• density dependent factors (affect different sized populations differently): competition, predation, parasitism, communicable disease
• density independent factors (affect populations of all sizes the same): climate change, natural disaster, seasonal change, human activity

Question 15

bottleneck

Answer 15

catastrophic events significantly reduce the population size (lasting for at least one generation) which reduced genetic variation

Question 16

speciation

Answer 16

formation of new species through evolution

Question 17

three types of isolation mechanisms

Answer 17

• ecological (geographical) isolation
• seasonal (temporal) isolation
• reproductive isolation

Question 18

allopatric speciation

Answer 18

• caused by ecological/geographical isolation
• members of a population are separated by a physical barrier (rivers, mountain ranges, sea)
• different areas have different selection pressures
• different genes will provide different selective advantages leading to different adaptations
• differences accumulate until it is impossible for successful interbreeding

Question 19

what other form of genetic drift can occur with ecological isolation?

Answer 19

founder effect

Question 20

sympatric speciation (seasonal and reproductive)

Answer 20

• caused by seasonal and reproductive isolation
• occurs within a population sharing the same habitat
• less frequent than allopatric speciation
• more frequent in plants than animals

Question 21

two types of reproductive isolation

Answer 21

• prezygotic
• postzygotic

Question 22

prezygotic reproductive barriers (3)

Answer 22

• courtship behaviour: unrecognisable courtship rituals can prevent mating
• mechanical barriers: different sized genitalia
• physiological barriers: pollen tubes may not grow at the same time, preventing fertilisarion

Question 23

postzygotic reproductive barriers (2)

Answer 23

• sterile hybrid: mules have odd numbers of chromosomes so cannot pair in meiosis to produce gametes/plants can double chromosomes (polyploidy) to enable gamete production
• hybrid not viable: fertilisation occurs but embryo fails to develop and onaborts

Question 24

polyploidy

Answer 24

mutation which doubles chromosomes so that they can pair up to produce gametes (usually to make infertile plant hybrids fertile)