自然淘汰とは？

Natural selection requires three conditions: variation in traits among individuals within a population, heritable traits that can pass from parent to offspring, and some trait forms providing a survival or reproductive advantage. For example, green beetles camouflaged on grass survive better than yellow beetles, so green color becomes more common across generations.

Random genetic mutations create trait variations in populations, regardless of whether mutations benefit organisms. Natural selection then acts on these variations, preserving advantageous traits. Mutations provide the raw material for evolution, but selection determines which traits persist and become more common over generations.

An adaptation is a trait that provides a survival or reproductive advantage in a specific environment. Green color in beetles living on green grass is an adaptation because it improves camouflage, allowing more green beetles to survive and reproduce than yellow beetles, making the trait increasingly common in the population.

When individuals with advantageous traits survive and reproduce at higher rates, those traits become more common in subsequent generations. Snowshoe hares with white winter coats survive better in snow than brown-coated hares, so white coat frequency increases. This process of adaptive evolution typically occurs over many generations within a population.

Heritability ensures that advantageous traits pass from parents to offspring, allowing beneficial variations to accumulate in populations. Without heritable traits, even highly advantageous characteristics would disappear when individuals die. For snowshoe hares, variation in the Agouti gene controls coat color and is heritable, enabling selection to act on this trait across generations.

Changing environmental conditions shift selection pressures, favoring different traits. Climate change is altering snow seasons, causing some snowshoe hare populations to revert to brown coats year-round because white coats no longer provide camouflage advantages. This demonstrates how types of selection can change when environmental conditions shift.

Natural selection generates matches between organisms and their environments by favoring traits suited to specific habitats. Coyotes have teeth for eating birds, while birds have wings for escaping predators. These complementary adaptations arise because individuals with traits improving survival in their environment reproduce more successfully, spreading those advantageous characteristics throughout populations.