Natural Selection and Microevolution: Unit 4, Topic 2

Question 1

What is natural selection?

Answer 1

• the process where organisms better adapted to their environment tend to survive and produce more offspring.
• transmitting favourable traits through generations, unfavourable traits die.
• phenotypes can confer a survival advantage in the changed environment, or they can give a disadvantage.

viability: the ability to survive or live successfully
fecundity: the ability to produce an abundance of offspring or new growth

positive selection: selection for a heritable trait
negative selection: selection against a heritable trait.

Question 2

What are environmental selection pressures?

Answer 2

climatic conditions: flood, drought, extreme temperature changes
competition for resources: availability of food and water, shelter, mate availability, predator abundance

Question 3

What is a gene pool?

Answer 3

• the sum total of all alleles found in a population

population: same species/ live in the same geographical area/ readily interbreed to produce fertile offspring.

The range of variation possible in a population is restricted by the alleles available in its gene pool

Fixed gene: genes that only have one possible allele in a gene pool and thus do not contribute to any variation.

Question 4

What is allele frequency?

Answer 4

• the relative proportion of a particular allele in a population (how common the allele is)
• can change overtime
• microevolution is a change in the frequency of alleles in a population
• selection of alleles can be positive or negative
  negative: limits the ability to survive and adapt to changes
  positive: helps ensure the survival and ability to adapt to changes of a species.

Question 5

what are the factors influencing allele frequency?

Answer 5

variation: genetic differences
viability: survival determined by environmental factors
reproduction
fecundity: rate of production and number of offspring
survival
environmental selection pressures.

Question 6

What are the three main types of phenotypic selection?

Answer 6

stabilising, directional, and disruptive.

stabilising selection:
- The environment of an organism is stable
- selective pressures will act against deleterious alleles that cause a departure from the optimal phenotype.
- favours organisms similar to their parents
- favours average

Directional selection:
- changes in the environment lead to selective pressures favouring organisms with one of two extreme traits.
- leads to change in traits over time
- one of two extreme phenotypes is favoured
- allele frequency shifts over time in the direction of that extreme phenotype

Disruptive selection:
- operates in favour of extreme and against intermediate forms
- selects against the average individuals in the population
- favours both extremes

Question 7

What is a mutation?

Answer 7

• the ultimate source of genetic variation as they introduce new alleles into the population.
• mutations can lead to phenotypic changes that limit long-term survival.
• may or may not lead to phenotypic change
• mutations that do not change the amino acid sequence would not affect phenotype.

Question 8

What is a gene flow?

Answer 8

• the transfer of alleles into or out of a population
• results from emigration and immigration
• a result of migration of individuals that reproduce in their new opulations, or movements of gametes such as pollen transfer.

Question 9

What is genetic drift?

Answer 9

• allele frequencies of a population change over generations due to chance.
• effects are strongest in small populations
• occur in small populations or when a large population is suddenly reduced.

Question 10

What is bottleneck effect?

Answer 10

• types of genetic drift
• caused by sudden disaster
• A catastrophic decrease in population size can result in the loss of some alleles from the gene pool.
  other genes can be preserved by chance

Question 11

What is the founder effect?

Answer 11

• occurs when a few individuals carry alleles to a new, isolated area = new population is formed with different allele frequencies from the original population

less genetic diversity than the original population - recessive alleles may have a higher chance of coming together.