Natural Selection and Evolution (Chapter 17)

Question 1

What is genetic drift?

Answer 1

A change in allele frequency that occurs by chance, because only some of the organisms of each generation reproduce

Question 2

What is the founder effect?

Answer 2

1) a small number of individuals are separated from the rest of a large population - they form only a small sample of the original populations and ∴ this founder population is unlikely to have the same allele frequencies as the large population
2) further genetic drift in the small population will alter the allele frequencies still more and evolution of this population may take a different direction from that of the larger parent population
3) the isolated populations will likely diverge genetically also due to different selection pressures in the new area and random mutations being different in separate populations

Question 3

What do selection pressures do?

Answer 3

Increase the chances of some allele being passed onto the next generation and decrease the chances of others e.g. predation of foxes

Question 4

What is natural selection?

Answer 4

The effects of selection pressures on the frequency of alleles in a population

Question 5

What does natural selection do?

Answer 5

Raises the frequency of alleles conferring an advantage and reduces the frequency of alleles conferring a disadvantage

Question 6

What is the general theory of evolution?

Answer 6

Organisms have changed over time

Question 7

What is stabilising selection?

Answer 7

• When selection pressures act against extremes, keeping the variation in a characteristic centred around the mean
• Keeping it the way it is

Question 8

What is directional selection?

Answer 8

• If a new environmental factor or new allele appears, then allele frequencies may also change
• e.g. if selection acts against smaller individuals, but not larger ones, the range of variation shifts towards larger size, resulting in a change of characteristic in a particular direction

Question 9

What is disruptive selection?

Answer 9

• This maintains different phenotypes (polymorphism) in a population
• Occurs when conditions favour both extremes of a population
• e.g. acts against those whose size is in the middle of the range

Question 10

What happens in the population when a particular phenotypic trait is controlled by two alleles of a single genes, A/a?

Answer 10

The population will be made up of three genotypes: AA, Aa and aa

Question 11

What do calculations based on the Hardy-Weinberg principle allow?

Answer 11

The proportions of each of these genotypes in a large, randomly mating population to be calculated

Question 12

State the Hardy-Weinberg principle

Answer 12

The frequency of a genotype is its proportion of the total population

Question 13

What are the two equations for the Hardy-Weinberg principle?

Answer 13

• p + q = 1

- p2 + 2pq + q2 = 1

Question 14

What does p stand for?

Answer 14

The frequency of A (in decimals)

Question 15

What does q stand for?

Answer 15

The frequency of a (in decimals)

Question 16

Describe how the second equation for the Hardy-Weinberg principle is obtained

Answer 16

1) the chance of offspring inheriting a dominant/recessive allele from both parents = p x p = p2 / q x q = q2
2) the chance of offspring inheriting a dominant allele from the mother/father and a recessive allele for the father/mother = p x q = pq ∴ chance = 2pq

Question 17

What are the 4 situations in which the Hardy-Weinberg population does not apply?

Answer 17

1) when the population is too small
2) when there is significant pressure against one of the genotypes
3) when there is migration of individuals carrying one of the two alleles into/out of the population
4) when there is non-random mating

Question 18

What can happen when the ratios of the different genotypes in a population have been determined using the Hardy-Weinberg principle?

Answer 18

1) their predicted ratios in the next generation can be compared with the observed values
2) any differences can be tested for significance using the chi squared test
3) if the differences are significant and migration/non-random mating can be discounted, then there is evidence that directional selection is occurring in the population

Question 19

What do all populations have?

Answer 19

The reproductive potential to increase their populations (exponentially), however under normal circumstances, this rarely happens

Question 20

What happens as a population increases?

Answer 20

Various factors (biotic and abiotic) keep numbers down e.g. if there are increasing numbers: food will be in short supply, there may be an increase in the number of predators, or overcrowding and ∴ it is easier for disease to spread

Question 21

What are biotic factors?

Answer 21

Factors caused by other living organisms e.g. through predation, competition for food or infection by pathogens

Question 22

What are abiotic factors?

Answer 22

Factors caused by non-living components of the environment e.g. water supply or nutrient levels in soil

Question 23

What do biotic and abiotic factors do?

Answer 23

They act to reduce the rate of growth of the population - only a small proportion of young will grow to adulthood and reproduce ∴ the population growth slows

Question 24

What happens if the pressure of the environmental factors is sufficiently great?

Answer 24

• The population will decrease
• Only when numbers have considerably fallen will the population be able to grow again
• ∴ over a period of time, the population will oscillate about a mean level

Question 25

What does variation within a population lead to?

Answer 25

• Some organisms within a species are born with a better chance of survival than others e.g. agouti colour is better than white colour at camouflaging
• ∴ some organisms will have features which give them an advantage in the ‘struggle for existence’

Question 26

What is fitness?

Answer 26

The capacity of an organism to survive and transmit its genotype to its offspring

Question 27

Give an example of how a new environmental factor would affect natural selection

Answer 27

If it suddenly got colder and there was more snow, white rabbits are more camouflaged ∴ have a selective advantage ∴ they are more likely to survive and reproduce and pass on the allele ∴ the frequency of white coat allele increases in population

Question 28

What are mutations?

Answer 28

Random events ∴ they can produce features that are harmful, neutral or useful

Question 29

What is the basis of evolution?

Answer 29

Changes in allele frequency in a population

Question 30

Why does evolution occur?

Answer 30

• Because natural selection gives some alleles a better chance of survival than others
• Over many generations, populations may gradually change, becoming better adapted to their environments

Question 31

What are the three examples of evolution?

Answer 31

1) antibiotic resistance
2) industrial melanism
3) sickle cell anaemia

Question 32

How does antibiotic resistance arise in a population?

Answer 32

1) there is a mutation in a bacterium causing an allele giving resistance
2) bacteria without this allele will be killed, but bacteria with this allele survive and reproduce as they have a selective advantage, passing on this allele to their offspring and rapidly through the population

Question 33

Why does the mutant allele have an immediate effect on the phenotype of the bacterium?

Answer 33

Because bacteria only have a single loop of DNA and ∴ only one copy of each gene

Question 34

What do antibiotics do?

Answer 34

Change the environmental factors which exert selection pressures on bacteria

Question 35

Where do alleles for antibiotic resistance often occur and what does this mean?

Answer 35

• They often occur on plasmids, which are often transferred from one bacterium to another, even between different species
• ∴ it is even possible for resistance to a particular antibiotic to arise in one species and be pass on to another

Question 36

What happened during the industrial revolution in industrial areas?

Answer 36

The number of black peppered moths, and ∴ the frequency of the allele for black colour, increased

Question 37

What happened before the industrial revolution and in non-industrial areas?

Answer 37

The frequency of the allele for speckled colouring was, and remained, the more common allele

Question 38

What was the selection pressure causing the change of allele frequency in industrial areas?

Answer 38

Predation by birds

Question 39

Describe the changes in allele frequency of peppered moths

Answer 39

1) in areas with unpolluted air, tree branches are often covered with lichen and ∴ the speckled moths are superbly camouflaged
2) however, because lichens are very sensitive to pollutants such as sulfur dioxide, they do not grow on trees near to or downwind of industries releasing pollutants into the air
3) ∴ trees in here areas have much darker bark, against which dark moths are better camouflaged and ∴ have a selective advantage
4) as air pollution from industry is reduced (from 1970s), the selective advantage swings back in favour of the speckled variety

Question 40

What is it important to remember about mutation and environmental factors?

Answer 40

• Changes in environmental factors only affect the likelihood of an allele surviving in a population - they do not affect the likelihood of such an allele arising by mutation
• The mutation was not caused by pollution
• Mutations have occurred all the time, just have not survived in the population

Question 41

What does possession of the HbSHbS genotype lead to?

Answer 41

• A great selective disadvantage

- People are less likely to survive, as it is lethal, and reproduce

Question 42

Describe what has happened to the sickle cell allele in parts of the world where malaria is found

Answer 42

• The frequency of the sickle cell allele is very high
• This is because people who are heterozygous are much less likely to suffer from a serious attack of malaria than people who are homozygous for the HbA allele
• HbSHbA people with malaria have roughly a third of the number of plasmodium in their blood than the HbAHbA homozygotes

Question 43

What are the two strong selection pressures acting on the two sickle cell alleles?

Answer 43

1) selection against people who are HbSHbS is very strong because they become seriously anaemic
2) selection against people who are HbAHbA is also very strong because they are more likely to die from malaria

Question 44

What happens where malaria is an important environmental factor?

Answer 44

• HbAHbS is advantageous as they do not suffer from sickle cell anaemia and are less likely to suffer badly from malaria
• ∴ both alleles remain in the population

Question 45

What happens where malaria was never present?

Answer 45

Selection against HbSHbS has almost completely removed the HbS allele from the population

Question 46

What is the change in allele frequency of the sickle cell allele, industrial melanism and antibiotic resistance an example of?

Answer 46

Natural selection showing the effect of a non-random process on the allele frequencies of a population of organisms

Question 47

What is an example of a random process that changes allele frequencies?

Answer 47

Genetic drift

Question 48

Describe what happened when the theory of evolution by natural selection was put forward

Answer 48

1) Charles Darwin and Alfred Russel Wallace both independently put forward the theory that natural selection might be a mechanism by which evolution could occur in 1856
2) they knew nothing of genes or mutation ∴ did not understand how natural variation could arise or be inherited - but they realised the significance of variation

Question 49

Describe the first deduction and the two observations leading to it by CD and ARW

Answer 49

O1: organisms produce more offspring than needed to replace the parents
O2: natural populations tend to remain stable in size over long periods
D1: there is competition for survival (a ‘struggle for existence’)

Question 50

Describe the second deduction and observation leading to it by CD and ARW

Answer 50

O3: there is variation among individuals of a given species
D2: the best adapted variants will be selected for by the natural condition operating at the time (i.e. natural selection will occur) - the best variants have a selective advantage ∴ survival of the fittest occurs

Question 51

What is the major difference between now and then about evolution, although it is the same idea?

Answer 51

We can now thick of natural selection as selecting particular alleles or groups of alleles

Question 52

What is evolution?

Answer 52

The change in allelic frequencies in a population over time

Question 53

When is genetic drift most noticeable?

Answer 53

When a small number of individuals are separated from the rest of a large population (founder effect)