Chapter 14: Evolutionary Mechanisms

Question 1

• Define natural selection
• Define random genetic drift.
• Select a modern population and explain how random genetic drift is thought to have had an effect.
• Briefly describe the significance of the founder effect
• Describe an example of the founder effect
• Explain why geneticists think of migration as gene flow from one population to another.
• Describe an example of migration and how it affected a gene pool

Answer 1

• Process which a species become better adapted to its environment; those individuals with favourable characteristics have survival advantage and passes those characteristics on to subsequent generations.
• Occurrence of characteristics in a small population as a result of chance rather than natural selection.
• The ‘Dunkers’ are a small religious group that don’t reproduce with others and are an isolated breeding population. They have different allele frequencies from surrounding Americans due to their small size allowing certain characteristics to become common by chance.
• A new population is begun by a small number of people with allele frequencies different from the original population. The characteristics of the new population will then be different from the original.
• A typhoon reduced population of Pingelap to 20. 1 of the survivors had a form of colour blindness. Today the incidence of it on Pingelap is 5% whereas 0.0033% in other parts of the world.
• When a person leaves a population, their genes go with them.
• Chinese population all had Rh+ blood group. When European immigrants came, they introduced Rh- blood group to the population.

Question 2

• Describe the process of speciation in 4 steps
• Why is natural selection less likely to affect humans in present day?
• Outline the importance of gene flow for human evolution.
• Describe the past and present conditions required in a population to enable the founder effect to occur.
• Identify a population that has a high incidence of Tay-Sachs disease

Answer 2

1. Variation: A range of variation exist in a single population which shares a common gene pool with free gene flow.
2. Isolation: Geographical barrier forms dividing the population into two, inhibiting interbreeding. Each has a separate gene pool with no gene flow between them.
3. Selection: different selection pressures act on the 2 populations over time, bringing a change in the gene frequencies of each gene pool as different features are favourable.
4. Speciation: Over time, changes in the gene frequencies is great enough to prevent production of fertile offsprings between the 2 populations due to formation of 2 new species.
   - Modern culture allows humans to modify their environments to survive in harsh environments. Medical technology has enabled people with less favourable characteristics to survive.
   - Gene flow increases variation in a population. This is important as variation allows natural selection to occur so the population becomes better adapted to the environment.
   - Requires a small original population that is isolated from other breeding populations and with at least one individual carrying the allele of interest.
   - Ashkenazi Jewish population

Question 3

• What are the 2 types barriers that leads to the isolation of 1 gene pool from another, and give examples of each.
• Another explanation of why the Jewish population has a higher incidence of Tay-Sachs disease is that it offers a survival advantage. Explain.
• In terms of genetic diseases, explain why it is dangerous to interbreed with close relatives?
• Describe how the link between sickle-cell allele and malaria can lead to changes in the allele frequencies in a population.
• What term is used to describe the process of changing allele frequency in a population?
• List three features required for natural selection to occur in a population.
• Two artefacts were found in the same location: a stone tool, which has not been previously disturbed, and a wooden handle. The wooden handle could be dated using carbon-14 technique. Name a method that could be used to determine the age of the other artefact.

Answer 3

1. Geographical barriers: Mountain ranges, oceans and lakes
2. Sociocultural barriers: Religion, language and social position.
   - Those who are heterozygous have resistance to tuberculosis thus a survival advantage and likely to pass on their alleles. However, Individuals with 2 normal alleles would die of TB and individuals with 2 Tay-Sachs alleles would die of Tay-sachs.
   - The related parents have received genes from a common ancestor and therefore have a greater chance of being carriers of an allele for the same condition. Probability of having a child with a recessive genetic disease therefore increases.
   - Malaria reduces reproduction of normal cell individuals due to death. Sickle-cell allele provides survival advantage to malaria therefore is favoured and carriers for sickle cell reproduce at greater rates, increasing the sickle cell allele frequency.
   - Natural selection
   - Variation, high birth rate and competition for survival.
   - Stratigraphy

Question 4

• What is sickle-cell anaemia?
• List the advantage and disadvantage of having sickle-cell trait in an area where malaria is prevalent.
• Define evolution
• Survival of the fittest is possible because of what?
• Explain Darwin’s 6 principles of evolution through natural selection
• Explain what is meant by a selectively advantageous mutation

Answer 4

• A recessive mutated form of haemoglobin that results in distortion of RBC into a sickle shape. This reduces the surface area and O2 carrying capacity of RBC. Is lethal and can be fatal.
• Ad: malarial resistance Dis:RBC sickle when O2 concentration is low
• Gradual change in the characteristics of a species over time.
• Variation in species.
  1. There is variation of characteristics within a species.
  2. More offspring of a specie are produced than can survive to maturity
  3. Due to excessive birth rate and limited resources, there is a struggle for existence - competition for survival.
  4. Individuals with characteristics best suited to the environment have more chance of surviving and reproducing - survival of the fittest.
  5. Favourable characteristics are passed on to next generations.
  6. Proportion of alleles with favourable characteristics increases in gene pool.
• They are a change in DNA that provides a survival advantage, to a particular genotype, under a particular selective pressure.

Question 5

• People of short stature tend to live in cold climates, and people with long limbs and short trunks tend to live in hot climates. Explain how these adaptions to cold and hot environments could have come about.
• How could isolation of a population from another lead to selection and speciation?
• People who carry only one sickle-cell allele are said to have the sickle-cell trait. State 2 disadvantage sickle-cell trait has for people with this condition.
• Describe 2 ways in which random genetic drift differs from natural selection in its effect on changes in allele frequencies.

Answer 5

• Range of variation for stature in human populations. Through natural selection, those with favourable characteristics would have survived and pass on their favourable alleles. Having long bodies and short limbs provides a smaller surface area in relation to body volume therefore lose less heat in cold environments and so have a survival advantage and vice verser.
• Isolation over a long period of time may result in a population undergoing many changes in allele frequency through natural selection, as well as the possibility of genetic drift and mutation. This may result in that population becoming so different for the original that it is no longer able to reproduce with the original population. It has become a different species.
• May cause breathing issues when O2 is low in supply and if reproduce with another sickle-cell trait individual, could have offspring with sickle-cell anaemia.
• Chance occurrence and non-directional change