Session 2: Mechanisms for Evolution

Question 1

What is a gene pool?

Answer 1

The concept of a gene pool usually refers to the sum of all alleles within the genes of a population of a single species. The composition of a population’s gene pool can change over time through evolution. This can occur by a variety of mechanisms, such as selection, mutations, and genetic drift.

Question 2

Describe natural selection in relation to gene pools.

Answer 2

Populations with large and diverse gene pools tend to have increased biological fitness (organism’s ability to survive) and are usually able to adapt to and survive pressures caused by rapid changes in their environment. Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.

Question 3

What is sexual selection?

Answer 3

Selection where the presence of a particular trait provides greater success in obtaining mates and reproducing. These traits may relate to fitness (for example bigger, stronger males), but in some cases they could actually be counterproductive to survivability (for example bright plumage in birds that makes them more visible to predators). It can lead to a situation referred to as non-random mating, where
not all individuals have the same opportunity to pass on their alleles/genes.

Question 4

What is artificial selection?

Answer 4

The identification by humans of desirable traits in plants and animals, and the steps taken to enhance and perpetuate those traits in future generations.

Question 5

Describe the 3 “effects of selection”.

Answer 5

Selection can be categorized by its effect on a species’ phenotype distribution.

Stabilising: narrows width of distribution, cuts extreme variations.

Directional selection favours one extreme (shifts distribution left or right)

Disruptive selection: favours both extremes but not the middle, creates bimodal distribution.

Question 6

Describe mutations in relation to selection.

Answer 6

Mutations are the source of all new alleles. Mutations can therefore change the frequency of existing alleles by competing with them. Recurrent spontaneous mutations may become common in a population if they are not harmful and are not eliminated.

Question 7

What is genetic drift?

Answer 7

Due to random chance, not all individuals in a gene pool pass on their genes to the next generation. This causes random changes in the gene frequencies. Alleles may become lost or fixed (present in all individuals). Genetic drift is greater in small populations. Although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.

Question 8

What is the founder effect?

Answer 8

When a small number of individuals migrate away or become isolated from their original population they will have a small and probably non-representative sample of alleles from the parent population’s gene pool. The colonizing population may evolve in a different direction than the parent population.

Question 9

What is a population bottleneck?

Answer 9

This occurs when the breeding population is reduced by 50% or more. Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to the population size. They also decrease genetic diversity.

Question 10

What is coevolution?

Answer 10

Where two (or more) species affect each other’s evolution. Common examples: flowering plants and pollinators and predator/prey relationships.

Question 11

What are the two types of genetic isolation?

Answer 11

Prezygotic isolation – occurs before fertilisation can occur (no offspring are produced). And Postzygotic isolation – occurs after fertilisation (offspring are either not viable or infertile)

Question 12

What are the 4 types of pre-zygotic isolating mechanisms?

Answer 12

Temporal (when two species mate at different times of the year), ecological (when two species occupy different habitats), behavioural (when two species have different courtship behaviours), and mechanical (when physical differences prevent copulation/pollination)

Question 13

What the three types of post-zygotic mechanisms?

Answer 13

hybrid inviability (when hybrids are produced but fail to develop to reproductive maturity), hybrid infertility (when hybrid fail to produce functional gametes), and hybrid breakdown (F1 hybrids are fertile, but F2 generation fails to develop properly)

Question 14

What is allopatric speciation?

Answer 14

Speciation that occurs when a physical barrier divides a population, e.g. ocean, mountains, etc.

Question 15

What is sympatric speciation?

Answer 15

Speciation that occurs when species within the same area become reproductively isolated, usually due to some sort of behavioural change. For example, herbivorous insects try out a new host plant.

Question 16

Define splitting and budding.

Answer 16

Splitting: A species could split into two populations that evolve differently until they eventually become separate species. Budding: A small part of the species population could “bud off” from the main part and evolve rapidly to form a new species.

Question 17

What is divergent evolution?

Answer 17

When a common ancestral species evolves into a number of new species. These species have adaptations allowing them to occupy different niches.

Question 18

What is adaptive radiation?

Answer 18

Common example of divergent evolution, where rapid evolutionary diversification of a single ancestral line occurs. e.g. dinosaurs who occupied many niches became extinct, so mammals, etc. can now occupy those niches and diversify (radiate/spread out and adapt = adaptive radiation)

Question 19

What is convergent evolution?

Answer 19

Where similar niches and selection pressures exist, species without a recent common ancestry may evolve to have similar physical characteristics, to suit the similar environment.

Question 20

What is phyletic gradualism?

Answer 20

If the environment remains stable, organisms may undergo very little change over time and evolution is slow and gradual. The process is considered a smooth and continuous process and any big changes are the result of the accumulation of many small changes over a long period of time.

Question 21

What is punctuated equilibrium?

Answer 21

After long periods with little or no change, an isolation event separates a small population of individuals. With a different gene pool or a burst of mutations, this
isolated group rapidly evolves into a separate species.

Question 22

What is microevolution?

Answer 22

Microevolution describes evolutionary changes that occur over a relatively short period of geologic time (such as between generations) and results in diversification within a species. Microevolution is evolution in a single population. It works within a group of organisms that interbreed with each other and share a gene pool. It can be detected through changes in gene frequency over time. These mechanisms for microevolution can directly affect gene frequencies in a population: mutations, gene flow, genetic drift, and selection.

Question 23

What is macroevolution?

Answer 23

Macroevolution is evolution on a large scale over longer periods of time, generally above the species level. Macroevolution encompasses larger trends and transformations in evolution, such as the origin of mammals and the radiation of flowering plants. The basic mechanisms of microevolution - mutation, gene flow, genetic drift, and natural selection - can produce macroevolutionary change if given enough time.

Question 24

What are the macroevolution patterns?

Answer 24

Stasis - some lineages remain unchanged for long periods of time. Characteristic changes - for example gaining or losing body parts. Speciation - the rate and frequency at which branching occurs. Extinctions - this can be rare, frequent, or across many lineages (mass extinction events)

Question 25

What is population genetics?

Answer 25

The study of genetic variation within populations, and involves the examination and modelling of changes in the frequencies of genes and alleles in populations over space and time.

Question 26

What is the Hardy-Weinberg Equilibrium?

Answer 26

A principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors. When mating is random in a large population with no disruptive circumstances, the law predicts that both genotype and allele frequencies will remain constant because they are in equilibrium.

Question 27

What can disturb the Hardy-Weinberg Equilibrium?

Answer 27

Mutations - adds new alleles and is a source of variation. Natural selection - can increase or decrease certain phenotypes. Non-random mating (sexual selection) - not all individuals will get to pass their alleles on to the next generation. Genetic drift - can randomly alter allele frequencies. Gene flow - can add or remove alleles in a population

Question 28

What are the conditions for the Hardy-Weinberg Equilibrium to occur.

Answer 28

Random mating (no sexual selection). No mutations (creation of new alleles). Large population size (decreases effect of genetic drift). No migration (introduces or removes alleles). No natural selection (no alleles are “fitter” than others). If these conditions aren’t met, the principle doesn’t apply and over time evolution can occur.

Question 29

What is the equation for the Hardy-Weinberg equilibrium?

Answer 29

(p+q)^2 = p^2 + 2pq + q^2 = 1

p = frequency of one allele
q = frequency of the second allele