Disruptive selection, also known as diversifying selection, is a type of natural selection that favors extreme phenotypes in a population while eliminating the intermediate phenotypes. It occurs when individuals with both high and low values of a trait have a higher fitness than individuals with intermediate values. Disruptive selection can lead to the formation of two distinct peaks in the distribution of a trait, representing the two extreme phenotypes.

There are several reasons why disruptive selection can lead to two peaks in a population:

1. Selective pressure: Disruptive selection occurs when there is strong selective pressure against individuals with intermediate phenotypes. This can be due to environmental factors such as different available resources or predation pressures. Individuals with extreme phenotypes are better adapted to these specific conditions, resulting in a higher fitness.

2. Genetic variation: For disruptive selection to occur, there must be sufficient genetic variation in the population. This genetic variation can arise through mutations, genetic recombination, or gene flow from other populations. The presence of genetic variation allows for the expression of different phenotypes, some of which may be favored by disruptive selection.

3. Non-random mating: Disruptive selection can be reinforced if individuals with similar phenotypes preferentially mate with each other. This can lead to assortative mating, where individuals with similar phenotypes are more likely to reproduce together. Assortative mating can further enhance the divergence between the extreme phenotypes, resulting in two distinct peaks in the trait distribution.

4. Genetic drift: Random fluctuations in allele frequencies, known as genetic drift, can also contribute to the formation of two peaks in a population undergoing disruptive selection. If the population size is small, genetic drift can have a significant impact on the distribution of traits. Over time, genetic drift can lead to the fixation of extreme phenotypes, resulting in two distinct peaks.

In conclusion, disruptive selection can lead to the formation of two peaks in the distribution of a trait. This occurs when extreme phenotypes have a higher fitness than intermediate phenotypes, and there is sufficient genetic variation, non-random mating, and genetic drift in the population.