Gene frequencies, selection and speciation

Question 1

What is a species? (3)

Answer 1

Organisms that can breed together to produce fertile offspring,
Organisms in the same species are similar in morphology, behaviour and biochemistry, and have the same ecological niche.
Organisms in the same species share a common ancestor.

Question 2

What is speciation?

Answer 2

New species arise from an existing species where reproductively isolated populations have resulted (thus, there is no gene flow between the populations).

Question 3

What is allopatric speciation?

Answer 3

This most commonly happens when the two populations become physically separated

Question 4

What is sympatric speciation?

Answer 4

If the speciation occurs when the organisms are occupying the same geographical area but are reproductively isolated it is sympatric speciation. Common in plants (flower at different times, polyploidy) less so in animals (but could arise due to differences in reproductive organs, differences in mating ritual)

Question 5

How does speciation occur?

Answer 5

Each population experiences different environmental conditions, accumulates different mutations and over a long period of time natural selection changes the allelic frequencies of each population in different ways
This makes the organisms become so different they can no longer interbreed

Question 6

What is meant by reproductive isolation?

Answer 6

Organisms cannot interbreed/ breed or mate or reproduce with another group
Due to incompatible gametes/ wrong courtship behaviour/ other valid reason

Question 7

Copper-tolerant plants flower at a different time from those which are not copper-tolerant. Explain how this might eventually lead to the production of a new species of plant. (5)

Answer 7

1. Reproductively isolated due to different flowering times
2. Different selection pressures for two populations
3. Different features or plants are selected or survive /different adaptations;
4. Populations become (genetically) different;
5. Unable to produce fertile offspring;

Question 8

Explain how geographical isolation can lead to the formation of new species. (Allopatric speciation) (6)

Answer 8

1. Populations are isolated by geographical barrier (river, mountain, desert or ocean);
2. No gene flow between populations
3. Variation exists within the populations
4. Each population faces different selection pressures due to different environments (climatic, food, predators)
5. Mutation in one group (different from other group)
6. Natural selection for specific alleles means populations become adapted to local environment;
7. The best suited organisms survive and reproduce pass on their alleles;
8. Change in allele frequencies over a long period of time
9. Isolated populations become so different they can no longer interbreed;

Question 9

Darwin’s theory of evolution based on natural selection was based on four observations. What were they?

Answer 9

• Individuals within a species differ– there is variation.
• Offspring resemble their parents – characteristics are inherited.
• More offspring are generally produced than survive to maturity – most organisms die young from predation, disease and competition. Those that survive have better characteristics and thus reproduce and pass on these genes
• Populations are usually fairly constant in size.

Question 10

Differences between reproductive successes of individuals affect the allele frequency in a population. How does this work?

Answer 10

More organisms are produced than the environment can support
Populations remain a constant size (relatively)
Thus competition exists between members of a species to survive

In any population there is a gene pool (all the alleles of all the genes in that)
Some organisms will have allele combinations that make them better for competing thus they are more likely to survive and thus reproduce and pass on their alleles.

Thus the advantageous alleles the parents had are more likely to be passed on and the offspring are in turn more likely to survive as they have advantageous alleles

Over generations the number of individuals with advantageous alleles increases compared to the dwindling number of disadvantageous alleles

So the allele frequency changes, advantageous alleles are more common.

Question 11

Tell me about natural selection in the peppered moth? (5)

Answer 11

These light-coloured moths are well camouflaged from bird predators against pale lichen-covered bark of trees, while rare mutant dark moths are easily picked off. During the industrial revolution in the 19th century, birch woods near industrial centres became black with pollution. In this changed environment the black moths had a selective advantage and became the most common colour, while the pale moths were easily predated and became rare. Kettlewell tested this by releasing dark and light moths in polluted and unpolluted environments and observing selective predation. Since pollution has cleared up in the 20th century the selection has revered again and pale moths are now favoured again over dark ones.

Question 12

How is antibiotic resistance an example of natural selection? (5)

Answer 12

Antibiotics kill bacteria, but occasionally a chance mutant bacterium appears that is resistant to an antibiotic. In an environment where the antibiotic is often present, this mutant has an enormous selective advantage since all the normal (wild type) bacteria are killed leaving the mutant cell free to reproduce and colonise the whole environment without any competition. Some farmers routinely feed antibiotics to their animals to prevent infection, but this is a perfect environment for resistant bacteria to thrive. The best solution is to stop using the antibiotic so that the resistant strain has no selective advantage, and may die out.

Question 13

Summarise the key points in Natural selection (4)

Answer 13

1. There is genetic variation in the characteristics within a population
2. Individuals with characteristics that make them less well adapted to their environment will die young from predation, disease or competition.
3. Individuals with characteristics that make them well adapted to their environment will survive and reproduce.
4. The allele frequency will change in each generation.

Question 14

Imagine you are Darwin! How might you explain how a giraffe got a long neck?

Answer 14

1. In a population of animals there would be random genetic variation in neck length.
2. In an environment where there were trees and bushes, the longer-necked animals were slightly better adapted as they could reach more leaves, and so competed well compared to their shorter-necked relatives. These longer-necked animals lived longer, through more breeding seasons, and so had more offspring.
3. The shorter-necked animals would be more likely to lose the competition for food, so would be poorly nourished and would probably die young from predation or disease. They would have few, if any, offspring.
4. So in the next generation there were more long-neck alleles than short-neck alleles in the population. If this continued over very many generations, then in time the frequency of long-neck alleles would increase and so the average neck length would increase.

Question 15

What are the 3 types of natural selection?

Answer 15

Directional, stabilising and disruptive selection

Question 16

What is directional selection and explain how it happens?

Answer 16

Directional Selection occurs when one extreme phonotype (e.g. tallest) is favoured over the other extreme (e.g. shortest). This happens when the environment changes in a particular way. “Environment” includes biotic as well as abiotic factors, so organisms evolve in response to each other. e.g. if predators run faster there is selective pressure for prey to run faster, or if one tree species grows taller, there is selective pressure for other to grow tall. Most environments do change (e.g. due to migration of new species, or natural catastrophes, or climate change, or to sea level change, or continental drift, etc.), so directional selection is common.

Question 17

What is disruptive selection and how does it happen?

Answer 17

Disruptive (or Diverging) Selection. This occurs when both extremes of phenotype are selected over intermediate types. For example in a population of finches, birds with large and small beaks feed on large and small seeds respectively and both do well, but birds with intermediate beaks have no advantage, and are selected against.

Question 18

What is stabilising selection and how does it happen?

Answer 18

Stabilising (or Normalising) Selection. This occurs when the intermediate phenotype is selected over extreme phenotypes, and tends to occur when the environment doesn’t change much. For example birds’ eggs and human babies of intermediate birth weight are most likely to survive. The mean will not change, the standard deviation will decrease as it selects against the extremes.

Question 19

Explain how selection results in an insecticide resistance. (5)

Answer 19

Insecticide resistance already in population;
(resulting) from mutation;
resistant insects are not killed (by insecticide)/survive;
reproduce/breed;
passing on the relevant allele to the next generation/offspring;
resulting in increasing frequency of resistance allele in population

Question 20

Explain what is meant by stabilising selection and describe where it takes place. (5)

Answer 20

1. Occurs in an unchanging environment;
2. (Initial range of values in which) mean is best adapted;
3. Selection against extremes / selection for the mean;
4. Mean/median/mode unaltered
5. Range/S.D is reduced;
6. Repeated over many generations;
7. Increasing proportion of populations becomes well adapted to environment;

Question 21

Explain how resistance to an antibiotic could become widespread in a bacterial population following a gene mutation conferring resistance in just one bacterium. (5)

Answer 21

1. Frequent use of antibiotic creates selection pressure
2. Bacteria with mutation/ resistance have advantage over others
3. (Survive to) reproduce more than other types
4. Pass on advantageous allele/ mutated allele in greater numbers
5. Frequency of (advantageous) allele increases over generations

Question 22

Explain how natural selection favours the evolution of bottom dwelling bacteria containing photosynthetic pigment that absorbs red and blue light most effectively rather than green absorbed by those living near the surface of the water

Answer 22

Little green light reaches bottom as absorbed by surface dwellers
Red and blue not absorbed and so penetrate;
Variation in pigments of sediment dwellers;
Bacteria with chlorophyll at an advantage;
As chlorophyll absorbs red and blue;
(Survive to) reproduce in greater numbers;
Pass on advantageous alleles/genes in greater numbers / increase in
frequency of advantageous alleles in subsequent generations;
Increase in frequency/numbers of bacteria with chlorophyll;

Question 23

What is a gene pool in terms of alleles?

Answer 23

The sum of all the alleles of all the genes of all the individuals in a population is called the gene pool

Question 24

The Hardy-Weinberg Principle
The frequencies of dominant and recessive alleles in a population remain constant over time, so long as five key conditions about the population were met. What are they?(5)

Answer 24

1. There are no mutations, so no new alleles are created.
2. There is no immigration /emigration, so no new alleles are introduced/ lost.
3. There is no selection, so no alleles are favoured or eliminated.
4. Mating is random, so alleles are mixed randomly.
5. The population is large, so there are no genetic bottlenecks.

Question 25

Explain the relevance of Hardy Weinberg and what people thought about allele frequencies before?

Answer 25

These conditions mean that there is nothing to disturb the gene pool; the allele frequencies in the population will remain constant from generation to generation.
Before this it was thought that dominant alleles would increase in frequency over time, and recessive alleles would decrease in frequency, but this intuitive idea is wrong. Dominant alleles need not be common. For example the dominant allele for Huntington’s disease is very rare in the population and almost everyone is homozygous recessive.

Question 26

What does Hardy - Weinberg allow us to do?

Answer 26

Hardy-Weinberg principle provides a means of detecting evolution, and quantifying the rate of evolutionary change.

Question 27

What are the 2 equations to use when doing Hardy Weinberg calculations?

Answer 27

1) p + q = 1 (use this when given allele frequencies)

2) p2 + 2pq + q2 = 1 (use when given phenotype/genotype frequencies)

Question 28

In a study of people, the frequency of the IO allele was found to be 0.55 and that of the IA allele, 0.18. What was the frequency of the IB allele in this population?

Answer 28

Sum of allele frequencies must be 1 (p + q = 1) so
0.55 + 0.18 + IB = 1
IB = 0.27

Question 29

A population of 1000 cats has 840 black cats and 160 white cats. Black is dominant and white is recessive. Calculate how many of the black cats were homozygous and heterozygous. You might need a pen and paper for this one and a calculator!!! (5)

Answer 29

We know that q2 = 0.16 (160/1000)
We know that p2 + 2pq = 0.84 (840/1000)

We can work out the frequency of the dominant and recessive alleles
q2 = 0.16
q = 0.4
p + q = 1
p = 1 – 0.4
p = 0.6

Now we can calculate the genotype frequencies

p2 so homozygous dominant black cats = 0.6 ×0.6 = 0.36
2pq, heterozygous cats = 2 × 0.6 × 0.4 = 0.48
q2, homozygous recessive white cats = 0.16

So: there were 360 homozygous black cats and 480 heterozygous making the 840 black cats.