Genetic diversity

Individuals in a population can carry different versions of the same gene. These versions are called alleles. A population is a group of organisms of the same species living in the same area.

Genetic diversity is the number of different alleles of genes present in a population. A population with more different alleles has greater genetic diversity than one with fewer.

Different alleles can contribute to differences between individuals, and alleles can be passed from parents to offspring. Genetic diversity therefore provides heritable variation within a population.

If an allele affects reproductive success in a particular environment, its frequency in the population can change through natural selection. For example, individuals carrying an allele that helps them produce more offspring may pass it on more often. Over generations, that allele can become more common in the population.

Natural selection

Individuals in a population can inherit different versions of a gene, called alleles. If an inherited allele helps individuals leave more offspring in a particular environment, that allele can become more common in the population over generations. This change in allele frequency is natural selection.

Random mutations can produce new alleles. Many mutations are harmful, but an allele may give an individual an advantage under certain environmental conditions. For example, it may make the individual more likely to survive long enough to reproduce. Individuals with the advantageous allele may then produce more offspring and pass the allele to them.

If this difference in reproductive success continues over many generations, the advantageous allele can make up a greater proportion of the alleles in the population. Natural selection therefore changes a population across generations; it does not change an individual’s inherited alleles during its lifetime.

Natural selection depends on the environment

A mutation does not arise because an organism needs it. Mutations occur randomly, while the environment affects whether a particular allele is advantageous. An allele that improves reproductive success in one environment may offer no advantage, or may be harmful, in another.

Changes in allele frequencies contribute to evolution: populations change over generations. Natural selection thereby contributes to the diversity of living organisms.

Interpreting evidence of natural selection

To explain selection in an unfamiliar situation, connect the steps: identify the heritable variation, relate it to the environmental conditions, explain how it could lead to differences in reproductive success, and predict how allele frequency could change over generations.

Population data can be examined by comparing allele frequencies or inherited characteristics over time. For example, if an allele becomes more common over several generations in an environment where it appears to help its carriers reproduce, the change is consistent with natural selection. The change alone does not prove that selection caused it; other factors could also affect the population.

Directional selection

Individuals in a population can vary in a heritable characteristic. If individuals towards one end of the range leave more offspring than others, that characteristic becomes more common at that end of the range over generations. This shift in the population is called directional selection.

Directional selection in antibiotic resistance

A bacterial population may contain some bacteria with an allele that makes them resistant to an antibiotic. When the population is exposed to the antibiotic, susceptible bacteria are killed or their reproduction is inhibited. A resistant bacterium may survive and reproduce, passing the resistance allele to its descendants. As a result, resistant bacteria can become a larger proportion of the population over generations.

The antibiotic selects resistant bacteria: it gives them a reproductive advantage in that environment. It does not cause a resistance mutation because the bacteria need it.

Stabilising selection

Individuals in a population can differ in a characteristic, such as body size. If the characteristic is heritable, some of this variation can be passed from parents to offspring.

Stabilising selection occurs when individuals with intermediate values of a heritable characteristic have greater reproductive success than individuals at either extreme. They leave more offspring, so the intermediate form tends to remain common in the population. Over generations, selection against both extremes tends to reduce variation in the characteristic.

Human birth weight illustrates this pattern. Historically, babies with very low or very high birth weights have been less likely to survive than babies with intermediate birth weights. Lower survival reduces the chance of reaching reproductive age, so both extremes are selected against.

Adaptation

An adaptation is an inherited feature that makes an organism better suited to its environment. It can help the organism survive or reproduce, increasing its reproductive success: the chance of producing offspring that themselves survive and reproduce.

An adaptation is useful in relation to a particular environment. For example, a feature that reduces water loss can be advantageous where water is scarce, but may offer little advantage where water is plentiful. If the environment changes, a feature that was once advantageous may become less useful.

Types of adaptation

Adaptations can be grouped according to the kind of feature involved:

• Anatomical adaptations are body structures. For example, a thick, waxy surface on a plant can reduce water loss.
• Physiological adaptations are internal processes. For example, producing concentrated urine allows some animals to conserve water.
• Behavioural adaptations are inherited patterns of action. For example, being active mainly at night can help some desert animals avoid the heat of the day.

Adaptation and natural selection

Individuals in a population can inherit different versions of genes, called alleles. Some of these differences affect features that influence reproductive success in the environment.

If an inherited feature gives its bearers an advantage, they are more likely to reproduce and pass on the alleles associated with it. Over generations, those alleles can become more common in the population. This process is natural selection. As a result, the population can become better adapted to its environment.

Adaptations do not appear because an individual needs them, and individuals do not change their inherited features simply by trying to adapt. Natural selection acts on inherited differences already present in a population.

Changes in inherited characteristics across generations are part of evolution. Because different environments favour different features, adaptation through natural selection contributes to the diversity of living organisms.

Investigating antimicrobial effects using aseptic technique

An antimicrobial substance can reduce or prevent the growth of microorganisms. Its effect can be investigated by applying it to a microorganism growing on a suitable culture medium and observing whether visible growth occurs nearby.

Aseptic technique minimises contamination of both the culture and its surroundings. Sterile equipment and media are used, and sterile materials are exposed to the air only briefly. Cultures are handled and disposed of safely according to local procedures.

Antimicrobial discs on agar

A suitable microorganism is spread over an agar plate to produce an even layer of growth. Sterile discs containing the antimicrobial substances are placed on the inoculated agar. An appropriate control disc, treated in the same way but without the antimicrobial substance, is also placed on the plate. For example, if a substance is dissolved in a solvent, the control disc contains the solvent alone. This helps identify whether the solvent itself affects growth.

The plate is incubated under suitable safe conditions. If an antimicrobial substance inhibits growth, a clear zone may appear around its disc where no microbial growth is visible. The diameter of this inhibition zone can be measured and compared with the control and with the zones around other treatment discs. The plate should be kept closed when measuring the zones and disposed of safely afterwards.

Interpreting inhibition zones

A larger zone shows that visible growth was inhibited over a greater distance from the disc. However, zone size depends not only on the substance’s antimicrobial effect but also on how readily it diffuses through the agar. A larger zone therefore does not, by itself, prove that one substance is more effective than another. A clear zone shows inhibition of visible growth; it does not necessarily mean that all the microorganisms there have been killed.

For a fair comparison, conditions should be kept consistent, including the microorganism used, the amount of inoculum and the incubation conditions. The amounts applied to the discs should also be controlled. Repeating the investigation helps establish whether the same pattern occurs consistently and improves confidence in the comparison.