Natural selection and adaptationAQA A-Level Biology: Revision notes

Genetic diversity is the number of different alleles of genes in a population. It is a factor that enables natural selection to occur, because selection can only act where individuals differ in their characteristics. A population with little genetic diversity has few alleles for selection to act on, so it may not be able to adapt if the environment changes.

The steps of natural selection in the evolution of populations are:

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

The environment provides the selection pressure. Individuals with the advantageous allele survive and reproduce more; they are not chosen, and they do not change to suit the environment.

Directional selection favours one extreme of a characteristic, so the mean moves in that direction. For example, antibiotic resistance in bacteria: when an antibiotic is used, bacteria with a resistance allele survive and reproduce, and the allele becomes more frequent.

Stabilising selection favours the mean and acts against both extremes, so the range of the characteristic is reduced and the mean stays the same. For example, human birth weight: babies of about 3.5 kg have the lowest mortality, and those much lighter or heavier have higher mortality.

Natural selection results in a species being better adapted to its environment. An adaptation can be:

• Anatomical: a structural feature, such as beak depth
• Physiological: a process inside the body, such as producing concentrated urine to save water
• Behavioural: how an organism acts, such as feeding at a cooler time of day

Adaptation and selection are major factors in evolution. Populations in different environments are exposed to different selection pressures, so different alleles are favoured, and over many generations this contributes to the diversity of living organisms.

In exam questions, apply the same sequence to the new context: variation (different alleles) → selection pressure → survivors with the advantageous allele reproduce more → allele passed on → frequency rises over generations.

For data, quote figures and calculate where asked. For example, if mean beak depth rises from 9.2 mm to 10.1 mm, the percentage change is $(10.1 - 9.2) \div 9.2 \times 100 = 9.8\%$. If a graph or table shows the mean staying the same but the range narrowing, that suggests stabilising selection.

Use aseptic technique to investigate the effect of antimicrobial substances (such as antibiotics, antiseptics or disinfectants) on bacterial growth.

• Wash hands and disinfect the bench to avoid contamination.
• Sterilise the inoculating loop or spreader in a Bunsen flame; flame the neck of the culture bottle.
• Lift the Petri dish lid only briefly and spread the bacteria evenly over the agar.
• Place sterile paper discs soaked in different antimicrobial substances on the agar, using sterile forceps. Add a control disc soaked in sterile water.
• Tape the lid with two or four pieces of tape, not sealed completely, and incubate at about 25 °C (not 37 °C) so that human pathogens do not grow.
• Measure the diameter of each clear zone (the zone of inhibition), where bacteria did not grow. Calculate the area using $\pi r^2$. A larger area means a more effective substance.