Genetic diversity and adaptation (3.4.4)
On this page
Directional selection favours individuals at one extreme of the phenotype range, shifting the mean population in that direction over time.
It occurs when environmental conditions change, giving an advantage to phenotypes that were previously less common. Over time, allele frequencies shift as the favoured phenotype becomes more prevalent.
Directional selection can be exemplified by antibiotic resistance in bacteria:
- Random mutations produce some bacteria with alleles that confer antibiotic resistance.
- Antibiotics act as selection pressure.
- Non-resistant bacteria are killed.
- Resistant bacteria survive and reproduce.
- The frequency of the resistant allele increases in the population.
- Over time, most of the population carries the resistant allele, making the population predominantly antibiotic resistant.
Stabilising selection favours the average phenotype in a population and selects against individuals at both extremes.
It occurs when the environment is stable and the existing mean phenotype is well suited to conditions. Over time, variation in the population is reduced as extreme phenotypes are less likely to survive and reproduce.
For example, human birth weight is subject to stabilising selection – babies of intermediate weight have the highest survival rates, while very small or very large babies face greater risks.
Evolution by natural selection is a theory which suggests how species change over time.
Populations contain a gene pool with many different alleles. Random mutations can introduce new alleles into the gene pool (although most are neutral or harmful).
Occasionally, a mutation produces an advantageous allele that increases an individual’s chances of surviving and reproducing in a particular environment (they are better adapted). These individuals are more likely to pass the advantageous allele to their offspring. Over many generations, the frequency of the advantageous allele increases in the population, while less advantageous alleles become less common.
Anatomical (structural) adaptations are physical features of an organism that increase their likelihood of survival. They can be internal or external.
Anatomical adaptation examples include:
- Arctic hares have white fur in winter, which provides camouflage against snow and helps them avoid predators.
- Large ears in desert foxes facilitate heat loss.
- Spines on cacti reduce water loss and provide protection from herbivores.
Physiological (biochemical) adaptations are internal processes that increase chances of survival.
Psychological adaptation examples include:
- Desert lizards make concentrated urine to conserve water.
- Bacteria in hot springs have heat–stable enzymes.
- Some plants produce antifreeze proteins to survive freezing temperatures.
Behavioural adaptations are ways an organism acts to aid survival. These can be learned or inherited.
Behavioural adaptation examples include:
- Swallows migrate south in winter to avoid food shortages in northern ranges.
- Nocturnal desert animals avoid daytime heat.
- Penguins huddle together to conserve heat.