7.3 Evolution and speciation

Question 1

define variation

Answer 1

the differences that exist between individuals in a population, this is mostly due to different allele combinations producing different phenotypes

Question 2

define selection pressures giving 3 examples

Answer 2

where organisms die or fail to reproduce
- predation
- disease
- competition for resources

Question 3

model 6 marker on how new species is formed

Answer 3

1. geographical location
2. separate gene pools
3. variation due to mutation
4. different biotic and abiotic conditions
5. different reproductive success (advantage) so they survive
6. Leads to changes in allele frequency as they reproduce and new generations inherit these genes

Question 4

describe stabilising selection

Answer 4

individuals with characteristics towards the middle are more likely to survive and reproduce

Question 5

when does stabilising selection occur

Answer 5

when the environment isn’t changing

Question 6

what happens to the mean and range in stabilising selection

Answer 6

the mean stays in the middle but the range of phenotypes is reduced

Question 7

describe directional selection

Answer 7

individuals with alleles for a single extreme phenotype are more likely to survive and reproduce

Question 8

when does directional selection occur

Answer 8

change in environment

Question 9

what happens to the mean and range in directional selection

Answer 9

means move in a direction away from the original

Question 10

describe disruptive selection

Answer 10

individuals with alleles for extreme phenotypes are more likely to survive and reproduce

Question 11

when does disruptive selection occur

Answer 11

in fluctuating environments which favour more than one phenotype

Question 12

what happens to the mean and range in disruptive selection

Answer 12

mean in the middle decreases and the range increases

Question 13

define speciation

Answer 13

when a new species arises from an existing species due to certain conditions

Question 14

when does speciation occur

Answer 14

when populations of the same species become reproductively isolated from each other so can no longer interbreed to produce fertile offspring

Question 15

5 steps of allopatric speciation

Answer 15

• geographical isolation so no interbreeding hence separate gene pool
• different environmental conditions can occur causing the populations to be affected by different selection pressures
• different mutations will occur so different advantageous alleles arise
• this leads to a change in the allele frequency
• eventually phenotype frequencies change and the populations become reproductively isolated

Question 16

5 steps of sympatric speciation

Answer 16

• formation of a new species from a population without geographical isolation
• random mutation or change in behaviour prevents individuals from breeding with each other
• so no interbreeding
• over time the reproductive isolation leads to a change in allele frequency in the different populations
• eventually different phenotype frequencies change

Question 17

what are the three changes which can cause reproductive isolation

Answer 17

temporal changes - different breeding seasons / different feeding times
mechanical changes - physical mismatch of reproductive parts / gamete incompatibility such as sperm killed in females tract
behavioural changes - different courtship displays such as change in song so no attraction

Question 18

3 steps of genetic drift

Answer 18

1. individuals in a population show variation in genotypes
2. by chance an allele for one genotype is passed on to more offspring than the others, so the number of individuals with this allele increases in frequency in the population
3. if by chance this same alley is passed on more often over repeated generations then this can leads to evolution as the allele changes in frequency in the population