7B - Populations and Evolution

Question 1

What is the definition of ‘species’?

Answer 1

• group of similar organisms that can reproduce to give fertile offspring

Question 2

What is a ‘population’?

Answer 2

• group of organisms of same species living in a particular area at a particular time

Question 3

What is a ‘gene pool’?

Answer 3

• complete range of alleles present in a population

Question 4

What is meant by ‘allele frequency’?

Answer 4

• how often an allele occurs in a population

- given as a percentage

Question 5

What does the Hardy-Weinberg Principle predict?

Answer 5

• that frequencies of alleles in a population won’t change from one generation to the next
• but prediction is only true under certain conditions - large population w/no immigration, emigration, mutations or natural selection

Question 6

What Hardy-Weinberg equation is used to predict allele frequency?

Answer 6

• p + q = 1
• p = frequency of dominant allele
• q = frequency of recessive allele

Question 7

What Hardy-Weinberg equation is used to predict genotype and phenotype frequency?

Answer 7

• p^2 + 2pq + q^2 = 1
• p^2 = frequency of homozygous dominant genotype
• 2pq = frequency of heterozygous genotype
• q^2 = frequency of homozygous recessive genotype

Question 8

What is meant by ‘variation’?

Answer 8

• differences that exist between individuals

Question 9

What causes genetic variation?

Answer 9

• individuals of same species have same genes, but have different alleles

Question 10

What factors are there in genetic variation?

Answer 10

• mutations
• meiosis (crossing over of chromatids and independent segregation of chromosomes)
• random fertilisation of gametes during sexual reproduction

Question 11

What is the process of natural selection?

Answer 11

• predation, disease and competition create struggle for survival
• some are better adapted to selection pressures than others
• individuals w/phenotype that increases chance of survival are more likely to survive, reproduce and pass on beneficial alleles
• greater proportion of next gen inherit beneficial alleles

Question 12

What is meant by stabilising selection?

Answer 12

• individuals w/alleles for characteristics towards middle of the range are more likely to survive and reproduce
• occurs when the environment isn’t changing, it reduces the range of possible phenotypes

Question 13

What is meant by directional selection?

Answer 13

• individuals w/alleles for a single extreme phenotype are more likely to survive and reproduce
• could be in response to environmental change

Question 14

What is meant by disruptive selection?

Answer 14

• individuals w/alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce
• opposite of stabilising selection as characteristics in middle of range are lost
• occurs when environment favours more than one phenotype

Question 15

What is ‘speciation’?

Answer 15

• development of a new species from an existing species

Question 16

What is reproductive isolation?

Answer 16

• changes in allele frequency cause changes in phenotype, which mean they can no longer interbreed to produce fertile offspring

Question 17

What is geographical isolation?

Answer 17

• a physical barrier e.g. a flood or earthquake divides a population of species, causing some individuals to become separated from main population

Question 18

What does geographical isolation lead to?

Answer 18

• allopatric speciation

Question 19

What is sympatric speciation?

Answer 19

• when a population becomes reproductively isolated without any physical separation

Question 20

How does allopatric speciation occur?

Answer 20

Geographical isolation occurs between two populations

• climate will be different on either side with different selection pressures
• means different alleles will be favourable and due to variation different alleles will survive and be passed on
• after time the two populations will become so different that they will become reproductively isolated and not be able to produce fertile offspring

Question 21

What makes sympatric speciation different from allopatric speciation?

Answer 21

• doesn’t require geographical isolation
• instead, random mutations could occur within a population, preventing members of that population breeding with other members of the species

Question 22

What are the 3 examples of reproductive isolation?

Answer 22

• seasonal; individuals of same pop develop diff flowering or mating seasons or more sexually active at diff times of year
• mechanical; changes in genitalia prevent successful mating
• behavioural; group of individuals develop courtship rituals that aren’t attractive to main population

Question 23

What is genetic drift?

Answer 23

• when chance, rather than environmental factors, dictates which individuals survive, breed and pass on their alleles

Question 24

How does genetic drift work?

Answer 24

• individuals within pop show variation in genotypes
• by chance, allele for one genotype is passed on to offspring more often than others
• so no. of individuals with allele increases
• changes in allele frequency in two isolated pops could eventually lead to reproductive isolation and speciation

Question 25

Why does genetic drift have greater effect on smaller populations?

Answer 25

• chance has greater influence

- in larger populations, any chance variations in allele frequency tend to even out across the whole population