Natural Selection and Adaptation

A population contains genetic variation, including different alleles that may affect phenotype. New alleles can arise through random mutation. Under particular environmental conditions, some phenotypes lead to greater survival and reproductive success than others.

Individuals carrying advantageous heritable variants can contribute more offspring and pass those alleles on. Across generations, allele frequencies can change. The environment selects among variants; it does not instruct a needed DNA change. A trait advantageous in one setting can be neutral or costly in another, so adaptation is always considered relative to conditions.

Directional selection favours one end of a phenotypic range, tending to move the population mean over generations. Exposure to an antibiotic can favour resistant bacteria if susceptible cells are inhibited and resistant cells leave more descendants.

Stabilising selection favours intermediate phenotypes and acts against extremes, tending to reduce variation around a favourable range. Human birth mass is a traditional example involving survival trade-offs, but real human outcomes depend on many biological and environmental factors. The selection pattern describes relative success across a distribution, not a guarantee about every individual.

A resistance-conferring mutation may already be present or arise spontaneously; antibiotic exposure does not purposefully create a useful mutation in every cell. Resistant bacteria can reproduce while susceptible competitors are inhibited, increasing the resistant allele’s frequency. Resistance genes can also spread between bacteria, but selection remains a difference in reproductive contribution.

Anatomical, physiological and behavioural adaptations all require an explanation linking phenotype to function and reproductive success. A change observed within one individual during its lifetime may be acclimation or learning rather than evolutionary allele-frequency change. Use appropriate time scales and distinguish evidence from an adaptive story that has not been tested.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.