What are the three conditions that must be met for natural selection to occur? What should happen if those three conditions are met? Define each type of natural selection, and describe what you expect to happen over time if these types of natural selection occur. Do these types increase or decrease genetic diversity?
I know the conditions are variation for a given trait, trait variation is genetically inherited, and relationship between trait variants and fitness. I don't know how to answer the rest.
Increase
To answer the rest of the question, let's break it down step by step:
1. Three conditions for natural selection to occur:
a) Variation for a given trait: In any population, individuals exhibit variation in traits, such as height or color. This variation can arise due to genetic differences or environmental factors.
b) Trait variation is genetically inherited: The traits that show variation must be heritable, meaning they can be passed down from generation to generation through genes.
c) Relationship between trait variants and fitness: Different variants of a trait must have an impact on an individual's chances of survival and reproductive success (fitness). This means that certain traits are more advantageous in a given environment, allowing individuals with those traits to have higher fitness.
2. Types of natural selection:
a) Directional selection: This occurs when individuals with an extreme variation of a trait have higher fitness compared to the average trait values. Over time, the mean value of the trait tends to shift towards the extreme. For example, if a population of birds has longer beaks and shorter beaks, but food availability favors picking insects from deep crevices, birds with longer beaks will have higher fitness, leading to an increase in the average beak length.
b) Stabilizing selection: This occurs when individuals with intermediate trait values have higher fitness compared to those with extreme variations. As a result, the population becomes more clustered around the average trait value. An example would be human birth weight. Extremely low or high birth weights are associated with increased health risks, so intermediate birth weights have higher fitness and are more common.
c) Disruptive selection: This occurs when individuals with extreme variations have higher fitness compared to those with intermediate trait values. It leads to the population splitting into two or more distinct groups, each favoring a different extreme of the trait. For instance, if a population of butterflies has two different color patterns, and each pattern provides better camouflage against different predators, disruptive selection can lead to the population splitting and specializing in each color pattern.
3. Effects of natural selection on genetic diversity:
a) Directional selection tends to decrease genetic diversity over time since it favors one extreme trait value, reducing the variation in the population.
b) Stabilizing selection can also decrease genetic diversity, as it favors the intermediate trait values and reduces the frequencies of extreme variants.
c) Disruptive selection, on the other hand, tends to increase genetic diversity, as it promotes the development and maintenance of distinct trait variants in separate groups.
It's important to note that the impact of natural selection on genetic diversity can vary depending on other factors such as gene flow, mutation rates, and population size.