Factors Increasing Extinction Risk

Species extinction is a natural process, but human activities and environmental changes are accelerating it dramatically. Several factors can increase the probability of a species becoming extinct. Understanding these factors is crucial for conservation efforts.

Let's examine the given reasons and their impact on the probability of extinction:

• (i) Increased homozygosity of alleles: When a population becomes small or isolated, related individuals may breed with each other (inbreeding). This leads to an increase in homozygosity, meaning individuals have two identical copies of an allele for many genes. Increased homozygosity can expose deleterious recessive alleles, leading to reduced fitness, lower reproductive success, and increased susceptibility to diseases. This phenomenon is known as inbreeding depression. Therefore, increased homozygosity significantly increases the risk of extinction.
• (ii) Increased heterozygosity of alleles: Heterozygosity refers to having different alleles for a gene. High heterozygosity indicates greater genetic diversity within a population. Genetic diversity is generally beneficial as it provides the raw material for adaptation to changing environments. A population with high heterozygosity is more likely to have individuals with traits that can survive new diseases or environmental conditions. Thus, increased heterozygosity typically *decreases* the probability of extinction, making the species more resilient.
• (iii) Decreasing population sizes: Small populations are inherently more vulnerable than large ones. They are more susceptible to random events (like a sudden environmental change or disease outbreak) that can wipe out a significant portion of the population or even the entire group. Small populations also suffer more from the effects of inbreeding and genetic drift, further reducing their ability to adapt and survive. Decreasing population sizes is a major driver of increased extinction probability.
• (iv) Increasing demographic stochasticity: Demographic stochasticity refers to random variations in birth rates, death rates, and sex ratios within a population. In large populations, these random fluctuations tend to average out. However, in small populations, random events like slightly more deaths than births in a given year, or a skewed sex ratio, can have a disproportionately large impact, potentially leading to a rapid decline towards extinction. Increasing demographic stochasticity, therefore, increases the extinction probability, especially in small populations.
• (v) Decreasing environmental stochasticity: Environmental stochasticity refers to unpredictable fluctuations in environmental conditions, such as variations in weather, availability of food or water, or incidence of natural disasters. Decreasing environmental stochasticity would imply the environment is becoming *more stable* or predictable. More stable conditions generally reduce the risk of extinction, as populations are less likely to face sudden, detrimental changes that they cannot adapt to. Increasing environmental stochasticity would increase extinction risk, but decreasing it would lower the risk.

Based on this analysis, the factors that contribute to a high probability of extinction are increased homozygosity (i), decreasing population sizes (iii), and increasing demographic stochasticity (iv). These three factors often interact, creating a downward spiral for small populations.

Let's consider the provided options:

• Option 1: (ii), (iii) and (v) - Includes increased heterozygosity (decreases risk) and decreasing environmental stochasticity (decreases risk). This combination is incorrect.
• Option 2: (i), (iii) and (iv) - Includes increased homozygosity (increases risk), decreasing population sizes (increases risk), and increasing demographic stochasticity (increases risk). This combination correctly identifies factors leading to high extinction probability.
• Option 3: (i), (ii) and (iii) - Includes increased heterozygosity (decreases risk). This combination is incorrect.
• Option 4: (ii), (iii) and (vi) - Includes increased heterozygosity (decreases risk) and an undefined factor (vi). This combination is incorrect.

Therefore, the correct combination of reasons leading to the highest probability of extinction of species is (i), (iii), and (iv).