Evolution

Question 1

What is evolution?

Answer 1

• The gradual process of change in the characteristics of a population of organisms from one generation to the next.
• A permanent change in a population’s gene pool from one gen to the next.
• Occurs on a species-wide level.

Question 2

What is a gene pool?

Answer 2

The combined genetic info (of alleles) and variabiltily present in a population.

Question 3

How can an allele frequency stay constant?

Answer 3

By having:

• a large population;
• a randomly-breeding population;
• no mutations;
• no migration; and
• no isolation.

This keeps the gene pool constant.

Question 4

What factors cause evolution?

Answer 4

Factors that can lead to evolution include:

• mutations;
• migration;
• random genetic drift;
• natural selection;
• isolation; and
• genetic diseases.

The gene pool is largely affected by these.

Question 5

How do mutations influence evolution?

Answer 5

• Biggest source of variation.
• Introduce completely new and different alleles into the gene pool.
• If the new allele is beneficial to the organism, they will reproduce and the allele will remain in the gene pool.

Question 6

What are phylogenetic trees/cladograms?

Answer 6

• A branching diagram used to represent relationships between groups of organisms derived from a common ancestor.
• The common ancestor lies at the bottom and then the hypothetical species are positioned on branches.

Question 7

Describe gene flow.

Answer 7

• Also called migration.
• The movement of individuals (and subsequently their genetic info) between certain populations and certain locations.
• Introduces new genetic info into other places.
• Certain barriers exist to prevent mixing.

Question 8

Give an example of gene flow.

Answer 8

The Rh- allele was introduced into China by European immigrants and sailors in the 16th century.

Question 9

What are the barriers to gene flow?

Answer 9

Sociocultural

• Influence of society’s constructs and norms on gene flow.
• Such as: Economic status, educational background, religious reasons, and distaste for people.
• E.g. Statistics show that people tend to marry within their own sociocultural class which restricts gene flow between the classes.

Geographical

• Physical and natural blockades preventing gene flow.
• Such as: Oceans, mountain ranges, canyons, deserts.
• E.g. The Australian Aboriginal peoples have been isolated from the now Papua New Guineans by the rising seawater levels.

Question 10

Random genetic drift

Answer 10

• The random fluctuation of allele frequencies in a population from one generation to the next.
• The random reproduction of two organisims produces offspring which could be randomly affected.
• Plays a large role in smaller populations.
• Non-directional change.

Question 11

What affects genetic drift?

Answer 11

• Mating choices;
• Survival of offspring; and
• the Effects of the environment.

Question 12

Give an example of random genetic drift.

Answer 12

E.g. the Dunkers do not breed with those outside of their religion. Therefore the genetic makeup of American Dunkers differs greatly to those of European and German descent.

Question 13

What is the founder effect?

Answer 13

• Extreme example of genetic drift.
• A small group of the population moves away from its homeland and populates an entirely new area.
• Only the alleles brought into the new location are kept in the new gene pool.
• This group naturally expands, causing the group to have a different allele frequency to the original population.

Question 14

Give an example of the founder effect.

Answer 14

The Ashkenazi Jews were a population isolated by geographical barriers and endogamy. People in that population have a 1 in 27 chance of carrying the Tay Sachs disease allele whilst only 1 in 300 non-Ashkenazi Jews have the chance. Since moving to Europe, the Ashkenazi population has become more cut off from the Jews in Israel.

Question 15

What is the bottleneck effect?

Answer 15

• An extreme case.
• Occurs when the size of a population is severely reduced (either by a natural disaster or another barrier).
• This reduces the allele frequency and variation within a species, enforcing endogamy, resulting in a limited no. of genes being passed on within the gene pool.

Question 16

Give an example of the bottleneck effect.

Answer 16

In 1975, a typhoon reduced the population of Pingelap to only 20 people. One person was heterozygous for achromatopsia which has resulted in there being 5% of the population being colourblind whilst only 0.0033% of the world’s population is.

Question 17

What are the key elements of the natural selection theory?

Answer 17

1. Some members of a species vary from other members (variation).
2. Organisms reproduce at a rate greater than the rate at which their resources increase (birth rate).
3. The no. of members of a species seems to always stay constant despite fluctuations (nature’s balance).
4. A high birth rate matched with limited resources creates a (struggle for existence).
5. Those ideally suited to the environment will have a greater chance of survival (survival of the fittest).
6. Surviving offspring get the chance to pass their alleles on (passing of alleles).
7. This eventually causes all members of the species to exhibit these ideal characteristics (species adaptaion).

Question 18

Why do some populations still carry harmful alleles?

Answer 18

• They may be maintained by heterozygote advantage.
• Mutations may affect them.
• Gene flow may not affect a certain population.
• There may have not been enough time to remove the harming alleles.
• Some genetic disorders only appear later on in life (post-reproduction).

Question 19

Mechanisms underpinning evolution by natural selection

Answer 19

• Variation.
• A struggle for existence/competition/survival of the fittest.
• Isolation (a lack of gene flow).
• Differential selection.
• Speciation.

Question 20

How does speciation occur?

Answer 20

• A new species can arise out of a group being isolated and developing differently.

1. Differences in members of a species exist (variation).
2. The population is split by a barrier/s which prevents interbreeding and genetic flow (isolation). Each population now has its own gene pool and diverges from each other.
3. The different environment brings about different selection pressures on the two populations (selection), creating a change in gene frequencies and a separate subspecies.
4. Changes may happen to the species which will disallow interbreeding between the two groups, thus classifying them as a new species (speciation).

Question 21

Isolating mechanisms

Answer 21

• Behavioural isolation - differences in mating rituals.
• Mechanical isolation - structural differences in the anatomy of organisms.
• Prevention of gamete fusion - gametes function poorly at the time of fertilisation.
• Geographical isolation - nature’s physical barriers.
• Temporal isolation - reproduction and mating occur at different times.
• Ecological isolation - even though organisms habitat the same area, they live in different parts, thus never cross paths.

Question 22

Theories for evolution

Answer 22

• Darwin and Wallace - created today’s accepted model of evolution by observing finches and tortoises on the Galapagos Islands. They observed that there was a struggle for existence within populations which was caused by the excessive birth rate and limited resources of populations. He also realised that only the fittest organisms survived whilst the others died off long before they could pass on these bad characteristics. He also noticed the role mutations play in evolution.
• Lamarck - created the Theory of Acquired Characteristics. After observing giraffes, Lamarck concluded that their necks grew longer because the first one used it more, and then each subsequent giraffe used their neck more and more which was then passed down to offspring. This is not accepted anymore because we now know that that action would not change the genetic info of the organism.