The Three Essential Ingredients for Natural Selection

2. Heritability: Passing the Blueprint

The second requirement is heritability. It is not enough for an individual to possess a beneficial trait; that trait must be capable of being passed from parents to offspring through genetic material.

We must distinguish between acquired characteristics and inherited traits. For example, if a weightlifter develops massive muscles through exercise, those muscles are not heritable; their children will not be born with increased muscle mass. In contrast, if a bird is born with a slightly longer beak due to its DNA, that beak shape can be passed to the next generation. Natural selection only works on traits that have a genetic basis because only genetic traits can change the composition of a population over time.

3. Differential Survival and Reproduction

The final ingredient is differential survival and reproduction, often referred to as "fitness." In a world of limited resources, not every individual born will survive to adulthood, and certainly not every individual will produce the same number of offspring.

In biology, "fitness" is not necessarily about being the strongest or fastest in a vacuum; it is about how well an organism's traits match its specific environment. An organism has high fitness if it survives long enough to reproduce and leaves behind more offspring than other members of the population.

When certain traits provide a survival or reproductive advantage, the individuals possessing those traits contribute a larger proportion of genes to the next generation.

Putting It All Together: A Worked Example

To see how these three ingredients work as a single mechanism, let's look at a hypothetical population of desert mice.

1. Variation: Within the population, some mice have light-colored fur, while others have dark-colored fur.
2. Heritability: The color of the fur is determined by genes, meaning light-colored parents are likely to have light-colored offspring.
3. Differential Reproduction: The mice live on light-colored sand. Dark-colored mice are easily seen by hawks and are eaten more frequently. Because the light-colored mice survive more often, they have more opportunities to mate and pass on their "light fur" genes.

The Result: Over many generations, the frequency of the "light fur" allele will increase in the population. The population has evolved.

Summary Table