7.3 Evolution may lead to speciation

Question 1

What is genetic variation?

Answer 1

differences within a species because of different genes/alleles

Question 2

What is environmental variation?

Answer 2

differences within a species because of different conditions during upbringing

Question 3

What genetic factors cause phenotypic variation within a species?

Answer 3

• mutations of alleles
• random fertilisation
• meiosis

Question 4

Other than genetic factors, why else may phenotypes vary within a species?

Answer 4

environmental influences

Question 5

What are mutations?

Answer 5

sudden changes to genes and chromosomes that may or may not be passed on to the next generation

Question 6

What is meiosis?

Answer 6

special form of nuclear division produces new combinations of alleles before they are passed into the gametes - all of which are therefore different, independent assortment

Question 7

What is random fertilisation of gametes?

Answer 7

in sexual reproduction this produces new combinations of alleles - therefore offspring are different to parents

Question 8

What are environmental variations?

Answer 8

• in plants it could be poor light, low soil nitrates, temperature, rainfall, pH etc
• For humans - diet, parental influence, life circumstances etc

Question 9

What is discontinuous variation? - definition, example and graph

Answer 9

• a characteristic of any species with only a limited number of possible values
• eye colour, sex, blood group
• Bar chart

Question 10

What is continuous variation? - definition, example and graph

Answer 10

• a characteristic that changes gradually over a range of values
• height, weight, hand span
• line graph

Question 11

What is selection pressure?

Answer 11

environmental factors that limit the population of a species. Selection pressures may vary from time to time and place to place

Question 12

Why does natural selection occur?

Answer 12

1. predation
2. disease
3. competition
   - all resulting in differential survival and reproduction
   - they determine the frequency of all alleles within the gene pool

Question 13

What is evolution?

Answer 13

a change over time which happens because of natural selction

Question 14

What factors does natural selection depend on?

Answer 14

• organisms produce more offspring than can be supported by the available supply of food, light space etc
• there is a genetic variety within the population of all species
• a variety of phenotypes that selection operates against

Question 15

How does natural selection cause a change in a populations gene pool over generations?

Answer 15

in every species there are some organisms which are better suited for their environment than others. Those organisms which are less suited to their environment will die before they can reproduce. Those organisms which are suited will survive and pass on their genes to the next generation

Question 16

What prevents an organism from reproducing?

Answer 16

• geographical isolation = natural disasters
• reproductive isolation = gametes aren’t going to fuse, when individuals cannot mate to produce fertile offspring

Question 17

What is stabilising selection?

Answer 17

• occurs when environmental conditions stay the same/are constant over long periods of time
• individuals closest to the mean are favoured, and any new characteristics are selected against
• eliminates the extremes of the phenotype and with it the capacity for evolutionary change
• results in low diversity

Question 18

What is directional selection?

Answer 18

• when environmental conditions change so will optimum value for survival
• individuals with phenotypes suited to the new conditions will survive and pass on their genes
• overtime the mean of the population will move towards these characteristics

Question 19

What is disruptive selection?

Answer 19

• the opposite of stabilising selection, in that both extremes of the normal distribution are favoured over the mean
• overtime, the population becomes phenotypically divided
• brings about evolutionary change
• occurs when an environmental factor takes two or more distinct forms

Question 20

What is speciation?

Answer 20

the formation of a new species from an existing one

Question 21

What needs to happen for speciation to occur?

Answer 21

• there must be something that prevents populations from interbreeding - known as the reproductive barrier
• if 2 groups are reproductively isolated from one another, they can accumulate more differences in their alleles and genes through repeated cycles of mutation and natural selection
• Populations tend to become more different over time, especially as they specialise to their particular habitats

Question 22

What are some reproductive barriers?

Answer 22

• geography
• behaviour
• timing
• anatomy
• genetic incompatiability

Question 23

What is allopatric speciation?

Answer 23

• relies on physical barriers e.g. river, mountains to separate populations
• the environments occupied by the two groups are different and therefore different alleles are favoured

Question 24

What is sympatric speciation?

Answer 24

• Speciation of organisms within the same habitat
• resulting from a non-physical barrier e.g. a mutation no longer allows 2 organisms to produce fertile offspring
• any changes in anatomy or behaviour may also prevent breeding
• occurs without geographical isolation

Question 25

What is genetic drift?

Answer 25

• a change in a population’s allele frequencies that occurs due to chance rather than selective pressures/environmental factors
• it is caused by ‘sampling error’ during reproduction

Question 26

Why does genetic drift affect small populations more than larger ones?

Answer 26

the gene pool is smaller, so there are less alleles available and any change in frequency becomes pronounced very quickly

Question 27

Example question: the 2 species both inhabit the forest of Madagascar located in the east of the island. Explain how the Aye-Aye could have evolved (6)

Answer 27

Sympatric speciation has occurred, as both species inhabit the same environment. The mechanism behind this change was behavioural, as nothing physical separated the 2 populations. A mutation could have occurred in the ancestor of the lemurs, which gave it a longer middle finger. Individuals with this mutation were able to catch food, therefore food is the selection pressure. Able to survive so can pass on genes therefore frequency increases