Which of these factors least affects population size?

Analyze the question: We are asked which factor has the least effect on population size. So we must find the option that does not directly change the number of individuals in a population.

Concept: Population size changes through four main processes:

• Natality (birth rate / rate of reproduction): increases the population
• Mortality (deaths): decreases it
• Immigration: increases it
• Emigration: decreases it

Limiting factors such as food supply, disease, predation, and competition also control population size, mainly by affecting births and deaths.

Step by step:

• A. Availability of food: Food shortage causes starvation, death, and lower reproduction. It is a major limiting factor, so it strongly affects population size.
• B. Disease: Epidemics can wipe out large numbers of individuals. It strongly affects population size.
• C. Immigration: Individuals moving into the area directly add to the population. It affects population size.
• D. Rate of reproduction: Births directly determine how fast a population grows. It strongly affects population size.
• E. Variation: Variation refers to differences among individuals of a species (in size, color, and so on). It matters for evolution and adaptation over long periods, but it does not directly add or remove individuals from a population. So it has the least direct effect on population size.

Answer: E. Variation

Tip: For "population size" questions, think of the four direct factors: births, deaths, immigration, emigration. Anything that is not one of these, or not a limiting factor like food, disease, or predators, is likely the "least affecting" option. Remember: variation relates to genetics and evolution, not head-count.