Natural Selection Changes Allele Frequencies in Populations
- Natural selection alters allele frequencies by favoring traits that improve survival and reproduction.
- Over generations, these advantageous alleles become more common in the gene pool.
Note
Allele frequencies are expressed as a decimal or percentage.
Natural Selection Acts on Phenotypes but Changes the Gene Pool
- Natural selection acts on phenotypes, but its effects are seen in the gene pool.
- Traits that improve survival and reproduction become more common, increasing the frequency of their associated alleles.
Example
Peppered Moths: During the Industrial Revolution, dark moths had a survival advantage in polluted areas, leading to an increase in the allele for dark coloration.
Key Mechanisms That Change Allele Frequencies
1. Variation in Traits Fuels Selection
- Variation is essential for natural selection.
- It arises from:
- Mutations: New alleles are introduced.
- Sexual Reproduction: Meiosis and fertilization create unique allele combinations.
Example
In green sea turtles, genetic diversity increases offspring survival in changing environments.
2. Selection Pressures Shape Traits
- Selection pressures determine which traits are advantageous.
- These can be:
- Biotic factors: Predation, competition, disease.
- Abiotic factors: Temperature, salinity, availability of resources.
Example
In some elephant populations, tusklessness has increased due to poaching, as tuskless elephants are less likely to be hunted.
3. Survival and Reproduction Drive Gene Pool Changes
- Individuals with advantageous traits are more likely to survive and reproduce.
- Their alleles contribute more to the next generation's gene pool.
Example
In bighorn sheep, hunting of males with large horns has reduced horn size over generations.
4. Traits Must Be Heritable for Selection to Work
- For natural selection to affect allele frequencies, traits must be heritable.
- Acquired traits (e.g., muscle strength from exercise) are not passed to offspring.
Hint
Recall that traits acquired during an organism's lifetime cannot be inherited.
