4.4 Genetic diversity and adaptation

Question 1

Genetic diversity is

Answer 1

1. the number of different alleles of genes in a population
2. a factor enabling natural selection to occur

Question 2

The principles of natural selection in the evolution of populations:

Answer 2

Random mutation can result in new alleles of a gene.
Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success.
The advantageous allele is inherited by members of the next generation.
As a result, over many generations, the new allele increases in frequency in the population.

Question 3

Directional selection, exemplified by antibiotic resistance in bacteria:

Answer 3

Random mutation introduces new alleles into a population of bacteria.
Frequent use of antibiotic creates selection pressure as antibiotic kills bacteria.
Any bacteria with a mutation giving resistance to the antibiotic are more likely to survive, so have a selective advantage over those that don’t.
Bacteria that survive reproduce, passing on the advantageous allele to the next generation.
Over many generations, as the frequency of the advantageous allele increases, more and more of the population are resistant to the antibiotic.
Natural selection is pushing the allele frequencies in a particular direction (up for antibiotic resistant alleles, down for antibiotic-susceptible alleles).

Question 4

Stabilising selection, exemplified by human birth weights:

Answer 4

Random mutation introduces new alleles into the population, resulting in a variety of birth weights.
Babies with high birth weight are less likely to survive the birth itself (as there is a limit to how wide the birth canal can be in humans, due to the constraints on the pelvis imposed by upright walking). So there is selective pressure against high birth weight.
Babies with low birth weight are less likely to survive the first weeks of life (even with medical intervention) than higher birth weight babies. So there is also selective pressure against low birth weight.
As both extremes of birth weight are being selected against, the mean birth weight stays the same over many generations.
Natural selection is keeping the allele frequency constant.

Question 5

Natural selection results in species that are better adapted to their environment. These adaptations may be:

Answer 5

anatomical, physiological or behavioural.