Natural Selection: Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. Allele frequencies change because individuals with advantageous alleles are more likely to survive, reproduce, and pass on those alleles to the next generation. This leads to an increase in the frequency of the advantageous allele and a decrease in the frequency of less advantageous alleles.

Example: Consider a population of moths living in a forest. Initially, there is variation in wing colour (light and dark). If the tree bark becomes darker due to pollution, the dark-winged moths will have better camouflage and be less likely to be preyed upon. As a result, the frequency of the dark-winged allele will increase in the population over time.

Founder Effect: The founder effect occurs when a small group of individuals from a larger population colonizes a new area. The allele frequencies in the founding population may not be representative of the original population. This can lead to a significant difference in allele frequencies in the new population compared to the source population. Rare alleles in the original population may be overrepresented in the new population, and common alleles may be underrepresented.

Example: The Amish population in North America is descended from a small number of founders who migrated from Europe. They have a higher frequency of certain genetic disorders, such as Ellis-van Creveld syndrome (a form of dwarfism), than the general European population. This is because the founders carried the allele for this disorder, and due to the small population size, the allele became more common in their descendants.

Genetic Drift (including Bottleneck Effect): Genetic drift is the random fluctuation of allele frequencies due to chance events. It is particularly pronounced in small populations. The bottleneck effect is a specific type of genetic drift that occurs when a population experiences a drastic reduction in size, often due to a natural disaster or human activity. This reduction in size leads to a loss of genetic diversity, and the allele frequencies in the surviving population may be very different from those in the original population.

Example: The Northern elephant seal population was hunted to near extinction in the 19th century. The population was reduced to as few as 20 individuals. As a result, the current population has very low genetic diversity compared to other seal species. They have a reduced ability to adapt to environmental changes.

Mutations are random changes in the bacterial DNA sequence. While most mutations are harmful or neutral, some can occur in genes that code for proteins involved in antibiotic susceptibility. If a mutation alters the structure of a protein that the antibiotic targets, it can reduce or eliminate the antibiotic's ability to bind and inhibit the protein's function. This results in resistance.

A common example is mutations in the gene encoding for enzymes that break down antibiotics. For instance, some bacteria develop mutations in genes that code for beta-lactamases. Beta-lactamases are enzymes that hydrolyze the beta-lactam ring in penicillin and related antibiotics, rendering the antibiotic ineffective. The mutation might result in a change in the enzyme's active site, making it less susceptible to inhibition by other molecules or increasing its catalytic efficiency. This allows the bacteria to survive in the presence of the antibiotic because the antibiotic cannot effectively degrade the bacterial cell wall synthesis inhibitors.

A prolonged drought would create a strong selective pressure favouring plants with adaptations that allow them to survive with limited water. This would act as a directional force, shifting the population's characteristics towards drought tolerance.

Several phenotypic changes might arise. Plants with deeper root systems would be better able to access water deeper in the soil, giving them a survival advantage. Those with thicker leaves or waxy coatings would reduce water loss through transpiration. Some plants might develop physiological adaptations like CAM photosynthesis, which allows them to open their stomata at night to take in carbon dioxide and reduce water loss during the day. Increased seed dormancy would also be advantageous, allowing plants to survive the drought period and germinate when rainfall returns.

The long-term evolutionary consequences could be significant. Over many generations, the population would become increasingly dominated by plants with these drought-tolerant traits. This could lead to a substantial shift in the overall morphology and physiology of the plant population. If the drought conditions persist for extended periods, the population might diverge significantly from its ancestors, potentially leading to the formation of a new, drought-adapted species. The selective pressure of drought would essentially drive the evolution of a new phenotype that is highly adapted to arid conditions. This demonstrates how environmental changes can drive substantial evolutionary change over time.