7.2 Natural Selection
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Selective Pressure | 选择压力 | Environmental factors (e.g., predators, climate, disease) that favor the survival of organisms with certain traits over others. |
| Adaptation | 适应/适应性特征 | A heritable trait that enhances an organism's ability to survive and reproduce in a specific environment. |
| Phenotypic Variation | 表型变异 | The observable differences in physical traits among individuals in a population, which natural selection acts upon directly. |
1. Acting on Phenotypes, Not Genotypes
It is a crucial distinction in AP Biology that natural selection acts directly on phenotypic variations in populations, not directly on the genotype (the DNA).
The environment (predators, weather, food availability) interacts with the physical traits (phenotypes) the organism presents. * An owl eating a mouse doesn't look at the mouse's DNA; it looks at the color of the mouse's fur. * If the fur color (phenotype) blends in with the soil, the mouse survives and reproduces, thereby passing on the underlying genes (genotype) that caused the dark fur.
2. The Power of Changing Environments
Environments are not static; they change constantly. As environments change, they apply different selective pressures to populations.
Some phenotypic variations significantly increase or decrease the fitness of an organism, but only in a particular environment. A trait that is highly advantageous today might become a lethal disadvantage tomorrow if the environment shifts.
Classic Example: The Peppered Moth
During the Industrial Revolution in England, coal soot blackened the trunks of trees. * Before the pollution: Light-colored moths were camouflaged against light tree bark. Dark moths were easily seen and eaten by birds (low fitness). * After the pollution: The environment changed. The tree bark turned black. Suddenly, the light-colored moths were highly visible and eaten, while the dark moths were camouflaged (high fitness).
The selective pressure (predation by birds) remained the same, but the changing environment reversed which phenotype was favored!