Unit 2 - Evolution

Question 1

Evolution

Answer 1

The proportions of individuals in a population with different inherited traits (frequency) changes over a number of generations.

Evolution is caused by genetic drift, natural selection and sexual selection.

Question 2

Mutations

Answer 2

Different alleles have different DNA base sequences as a result of mutation, which changes the amino acids present in a polypeptide.

Novel alleles are only produced by mutation.

Question 3

Mutations and evolutionary fitness

Answer 3

Many mutations have no effect on fitness - they are neutral.

Some are harmful and reduce an organism’s evolutionary fitness.

Rarely, mutations are beneficial to the fitness of an individual.

Question 4

Genetic drift

Answer 4

The random increase or decrease in the frequency of inherited traits over a number of generations.

Chance events cause unpredictable fluctuations in allele frequencies from one generation to the next, altering the genepool.

Genetic drift has a greater effect in small populations.

Question 5

Causes of genetic drift

Answer 5

Population bottlenecks, where population size is drastically reduced, results in genetic drift if rare alleles are not selected for at this time.

In the founder effect, a new population is created from a random sample of the original population, which may not be representative.

Question 6

Natural selection

Answer 6

A non-random process that acts on genetic variation in a population.

Beneficial alleles increase survival chances, and are more likely to be passed to the next generation, increasing their frequency.

Alleles that reduce evolutionary fitness, decrease survival chances and will decrease in frequency in subsequent generations.

Question 7

Sexual selection

Answer 7

A non-random increase in the frequency of alleles that make mating and reproduction more likely.

Selected alleles do not increase survival chances, but give the organism a better chance of reproducing and passing these alleles on. eg. male peacock tail.

Sexual selection may lead to sexual dimorphism and the evolution of elaborate courtship displays.

Question 8

Male-male rivalry

Answer 8

Found in many sexually dimorphic species.

Males fight each other for dominance and access to females.

Successful males are larger, stronger or have greater weaponry (eg. antlers).

These characteristics give offspring the same advantage, so are passed on, improving their fitness.

Question 9

Honest signals

Answer 9

Female choice drives the evolution of conspicuous markings, structures or behaviours in males.

Males display ‘honest signals’ - characteristics that allow a potential mate to assess parasite burden, genetic quality or fitness of the males.

Good phenotype indicates favourable alleles that increase the survival chances of the offspring.

Question 10

Sexual dimorphism

Answer 10

Differences in characteristics between the two sexes of the same species (other than differences in the sex organs).

Caused by sexual selection.

Males and females differ in size - the male is larger and heavily armoured

Males may have conspicuous marking, structures or behaviours.

Question 11

Reversed sexual dimorphism

Answer 11

The female is larger or more brightly coloured than the male.

Found in some species of birds.

Question 12

Selection pressures

Answer 12

The environmental factors that influence which individuals in a population pass on their alleles.

Beneficial traits are favoured, harmful ones are reduced.

Strong selection pressures lead to rapid evolution. eg antibiotic resistance.

Selection pressures can be biotic - competition, predation, disease or abiotic - temperature, light, salinity, pH.

Question 13

Fitness

Answer 13

An indication of an individuals ability to succeed at surviving and reproducing.

It is a measure of the tendency of some organisms to produce more surviving offspring than competing members of the same species.

Question 14

Absolute fitness

Answer 14

The ratio between the frequency of individuals of a particular genotype after selection and those before selection.

(ie. the frequency of a genotype after selection, divided by the frequency before)

Question 15

Relative fitness

Answer 15

The ratio of the number of surviving offspring per individual of a particular genotype, compared to the number of surviving offspring of the most successful genotype.

(ie. number of surviving offspring divided by the number of surviving offspring of the most successful)

Question 16

The Hardy-Weinberg principle

Answer 16

States that in the absence of evolution, allele and genotype frequencies in a population will stay constant over the generations.

This is known as Hardy-Weinberg equilibrium.

The HW principle can be used to calculate allele, genotype and phenotype frequencies in a population, using the formula :

p2 + 2pq + q2 = 1

(It is assumed that there are 2 alleles for a gene, p and q, so p + q = 1)

HW is used to detect evolution, by looking for changes from the expected frequencies.

Question 17

Co-evolution

Answer 17

Two species with frequent or close interactions, evolve in response to selection pressures imposed by each other.

A change in the traits of one species acts as a selection pressure on the other.

Often found in symbiotic relationships.

Question 18

Red Queen hypothesis

Answer 18

Explains the idea of co-evolution using the Red Queen analogy from ‘Through the Looking Glass’ by Lewis Carroll.

Alice chases the Red Queen, but when they stop they are in the same place. ie. they are running to stand still.

There is an ‘arms race’ between predators and their prey and between parasites and their hosts.

Question 19

Parasitism

Answer 19

The parasite gains in terms of energy or nutrients for growth and reproduction, at the expense of its host. (+/- symbiosis)

The host is harmed, loses energy or resources and incurs further costs when defending its tissues from parasites.

Co-evolution often results in high host-parasite specificity.

The relationship is usually stable - the fittest parasites do not kill their host, but maintain maximum reproduction rates, while the fittest hosts can tolerate or resist parasites.

Question 20

Commensalism

Answer 20

One species gains a nutritional benefit, the other is unaffected (+/0 symbiosis)

Either dead or waste parts of the host are used, or the commensal takes advantage of the host altering the environment in some way.

Question 21

Mutualism

Answer 21

Both species benefit, and are often interdependent, with structural compatibility or exchange of metabolic products.

eg. zooxanthellae and coral polyps, lichens.

It is a +/+ symbiotic relationship, involving co-evolution.

Question 22

Hardy-Weinberg conditions

Answer 22

The conditions for maintaining HW equilibrium are :

no natural selection, random mating, no mutation, a large population size, no gene flow by migration