How does the probability of genetic fixation change with decreasing population size? stays the same – genetic drift is selectively neutral increases decreases oscillates shows a random pattern
The correct answer and explanation is:
Correct Answer:
Increases
Explanation:
The probability of genetic fixation refers to the chance that a particular allele (variant of a gene) will become the only allele present in a population, effectively reaching a frequency of 100%. Genetic fixation is influenced by several factors, including natural selection, mutation, migration, and importantly, genetic drift.
Genetic drift is the random fluctuation of allele frequencies in a population due to chance events. It is especially powerful in small populations because random sampling effects can cause allele frequencies to change dramatically from one generation to the next.
How population size affects fixation probability:
- In large populations, genetic drift has less impact because the large number of individuals buffers against random changes in allele frequency. The allele frequency changes more slowly, and alleles are less likely to become fixed or lost by chance alone.
- In small populations, genetic drift is stronger because random sampling effects dominate. This means alleles can become fixed or lost purely by chance much more rapidly. So, the probability that any given allele will become fixed increases as population size decreases.
Why does fixation probability increase with decreasing population size?
- Random sampling effects are magnified: In smaller populations, fewer individuals mean that chance events (like which individuals reproduce) can drastically alter allele frequencies.
- Selective neutrality assumption: When genetic drift is the primary force (i.e., alleles are neutral or nearly neutral regarding fitness), fixation occurs purely by chance. The smaller the population, the more likely these chance events lead to fixation or loss.
- Time to fixation is shorter: Not only is the probability of fixation higher, but fixation occurs faster in smaller populations because allele frequencies fluctuate more widely generation to generation.
Summary:
- The probability of fixation increases as population size decreases.
- This is because genetic drift plays a bigger role in small populations.
- Genetic drift is selectively neutral, meaning it affects alleles regardless of their fitness.
- In large populations, alleles remain polymorphic (multiple alleles present) longer due to weaker drift effects.
- In small populations, alleles tend to fix or disappear quickly due to strong drift.
This principle is fundamental in population genetics and conservation biology, where small population sizes can lead to loss of genetic diversity through drift and fixation of alleles by chance rather than selection.