Genetic diversity and adaptation

Assessed definition: Genetic diversity means the number of different alleles of genes present within a population.

Useful definition: Alleles are different forms of a gene that might produce a protein that functions in a different way, or not at all. 

Genetic diversity is essential for the evolution and survival of a species. Low genetic diversity makes a species more vulnerable to extinction. Generally, larger populations tend to have higher genetic diversity, while smaller populations often experience lower diversity. Certain populations may suffer from low genetic diversity due to natural events or human activities. The founder effect, where a small group of individuals emigrates and establishes a new population, also reduces genetic diversity.

Species with high genetic diversity are more likely to have individuals with traits that allow them to adapt to environmental changes, ensuring that at least some members will survive.

Charles Darwin and Alfred Russel Wallace first described the concept of natural selection. Since then, our understanding has evolved with advances in genetics:

• Random mutations can introduce new alleles of a gene.

• While many mutations are harmful, in certain environments, a new allele may provide an advantage to its carrier.

• Beneficial alleles can lead to greater reproductive success.

• These advantageous alleles are passed down to future generations.

• Over time, the frequency of the beneficial allele increases in the population.

Adaptation and selection

Natural selection can result in adaptations related to an organism's behavior (how it acts), physiology (its internal functions), or anatomy (its physical features). Depending on the environment, selection can lead to different outcomes. 

Directional selection occurs when environmental changes favor individuals with traits that deviate from the average. This type of selection is common when the environment undergoes significant changes.

An example of directional selection is antibiotic resistance in bacteria. Antibiotics, which are naturally produced by other microbes like fungi or bacteria, usually target unique enzymes in prokaryotic cells. For instance, penicillin interferes with an enzyme that helps produce murein for bacterial cell walls, weakening the walls and leading to bacterial death via osmotic lysis. Mutations that slightly alter these enzymes, making them resistant to antibiotics, allow only resistant bacteria to survive. As these resistant traits are passed on, the population evolves in a new direction.

Stabilising selection occurs when environmental conditions remain stable, favoring individuals with traits closer to the average.

Human birth weight illustrates stabilizing selection. Extremely small babies may struggle to regulate body temperature, while very large babies face complications during birth. As a result, the average birth weight stabilizes around a middle range, where survival is most likely.

Directional selection (left) and stabilising selection (right)

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