Populations

Question 1

Species

Answer 1

a group of similar organisms which are able to interbreed to produce fertile offspring

Question 2

Population

Answer 2

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

Question 3

gene pool

Answer 3

all the alleles of all the genes of all the individuals in a population at a particular time

Question 4

allele frequency

Answer 4

how often an allele occurs within the gene pool

Question 5

What is the Hardy-Weinberg principle, and what does it predict?

Answer 5

a mathematical model which predicts that allele frequencies in a population won’t change from 1 generation to the next.

Question 6

Conditions needed for the HW principle to be true

Answer 6

• large population size
• no immigration/ emigration
• mating is random
• no mutations occur
• each allele is equally likely to be passed on to the next generation - there is no natural selection.

Question 7

Equation to work out the allele frequency of one allele from the other

Answer 7

p + q=1
p = dominant
The total frequency of all possible alleles for a characteristic in a population must add up to 1.0, so frequencies of all individual alleles also add to 1.0

Question 8

Equation to predict phenotype and genotype frequency

Answer 8

p^2 + 2pq + q^2 = 1

The frequencies of all individual genotypes must add to 1.

Question 9

p^2 + 2pq + q^2 = 1

What do each of the different terms represent?

Answer 9

p^2 = the frequency of the homozygous dominant genotype
2pq = the frequency of the heterozygous genotype
q^2 = the frequency of the homozygous recessive genotype

Question 10

How do you know if external factors have affected allele frequency?
give examples of factors

Answer 10

allele frequency changes between generations, the HW principle doesn’t apply - a factor such as immigration, emigration, mutation or natural selection must have influenced allele frequency.

Question 11

Which 2 factors cause most variation in phenotype?

Answer 11

genetic and environmental factors

Question 12

What are the main sources of genetic variation? (3)

Answer 12

• mutation
• meiosis (crossing over, independent segregation)
• the random fertilisation of gametes

Question 13

Describe how natural selection occurs

Answer 13

• there is genetic variation within a population due to mutation
• selection pressures
• some individuals are better adapted so are more likely to survive, reproduce and pass their advantageous alleles on to offspring.
• a greater proportion of the next generation inherits the advantageous alleles, so their frequency in the gene pool increases.

Question 14

which factors can lead to differential survival and reproduction (natural selection)?

Answer 14

predation, disease, competition

Question 15

what is evolution?

Answer 15

a change in allele frequency over time.

Question 16

what are the 2 mechanisms of evolution?

Answer 16

natural selection, genetic drift

Question 17

What happens in stabilising selection?

Answer 17

when individuals with phenotypes around the mean are selected for and extreme phenotypes are selected against. This preserves
a population’s characteristics. This generally occurs when the environment isn’t changing. The normal distribution curve becomes narrower and taller.

Question 18

What happens in directional selection?

Answer 18

Phenotypes at 1 extreme are selected for, while phenotypes at the other extreme are selected against. This changes the characteristics of the population. The normal distribution curve shifts in the direction ofthe phenotype that is selected for. This can occur in response to environmental change.

Question 19

What happens in disruptive selection?

Answer 19

Phenotypes at both extremes are selected for and individuals with phenotypes around the mean are selected against. This can lead to speciation.

Question 20

What is speciation?

Answer 20

formation of a new species from an existing one.

Question 21

What happens in allopatric speciation?

Answer 21

1) 2 populations become geographically isolated by a physical barrier.
2) mutations occur in each population’s gene pool.
3) environmental conditions either side of the barrier vary, so there are differentselection pressures.
4) different alleles are selected for and different changes in allele frequency occur.
* **

Question 22

What happens in sympatric speciation?

Answer 22

1) occurs in the same population (so no geographical isolation)
2) mutation causes variation
3) reproductive isolation
4) different alleles selected for and more likely to be passed on to the next generation. different changes in allele frequency occur
5) disruptive selection
6) eventually different species form - cannot interbreed to produce fertile offspring.

Question 23

What is sympatric speciation?

Answer 23

Formation of a new species from an existing one when there is reproductive isolation without geographical isolation

Question 24

What factors can cause reproductive isolation and lead to sympatric speciation?

Answer 24

• different breeding seasons
• different courtship behaviour
• mechanical - anatomical differences so a mismatch of reproductive parts.

Question 25

What happens in genetic drift?

Answer 25

chance determines which alleles are passed on to the next generation. Some alleles are passed on more frequently than others, leading to changes in the characteristics of a population.

Question 26

In what population size does genetic drift have a larger influence and why?

Answer 26

small population size because chance has a greater influence here. In larger populations, chance variations in allele frequency tend to even out across the whole population.

Question 27

At what stage has speciation occured?

Answer 27

new species arises when genetic differences that have accumulated in the gene pools of the 2 populations mean that members of the populations are unable to interbreed and produce fertile offspring.