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Natural Selection and Evolution
Natural Selection and Evolution
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Natural selection is the process by which certain inherited traits become more common in a population because they improve survival or reproduction in a specific environment. It is one of the main mechanisms that drives evolution, which is the change in populations over generations. Individuals do not evolve during their lifetimes, but populations can shift as some traits are passed on more often than others. Understanding natural selection helps explain adaptation, biodiversity, and the history of life on Earth.
Natural selection depends on variation, inheritance, and differences in reproductive success. If individuals in a population vary, if some of that variation is genetic, and if certain traits help organisms leave more offspring, then those traits tend to increase over time. Environmental pressures such as predators, climate, food supply, or disease can influence which traits are favored. Over many generations, these small changes can lead to major evolutionary patterns, including the formation of new species.
Key Facts
- Evolution = change in allele frequencies in a population over time.
- Natural selection requires variation + inheritance + differential reproductive success.
- Fitness refers to reproductive success, not strength alone.
- Adaptation = an inherited trait that increases survival or reproduction in a particular environment.
- If a trait increases survival and is heritable, its frequency is more likely to rise in the next generation.
- Natural selection acts on phenotypes, but evolution changes genotypes in populations.
Vocabulary
- Natural selection
- The process in which individuals with advantageous inherited traits leave more offspring than others.
- Evolution
- A change in the genetic makeup of a population across generations.
- Variation
- Differences in traits among individuals in the same population.
- Adaptation
- A heritable trait that improves an organism's ability to survive or reproduce in its environment.
- Fitness
- An organism's relative success in surviving and producing fertile offspring.
Common Mistakes to Avoid
- Thinking individual organisms evolve during their lifetimes, which is wrong because evolution refers to genetic change in populations across generations.
- Saying natural selection gives organisms what they need, which is wrong because selection acts only on existing heritable variation.
- Assuming the strongest organism is always the most fit, which is wrong because fitness means leaving more offspring, not simply being larger or more powerful.
- Believing all traits are adaptations, which is wrong because some traits may be neutral, harmful, or inherited along with other selected traits.
Practice Questions
- 1 In a population of 200 beetles, 60 are green and 140 are brown. Birds more easily spot green beetles, so brown beetles survive and reproduce more. After several generations, the population is 30 green and 170 brown. What happened to the frequency of the brown trait, and what process explains this change?
- 2 A mouse population lives on dark volcanic rock. At first, 20 out of 100 mice have dark fur. After many generations, 75 out of 100 mice have dark fur because predators more often catch light mice. By how many percentage points did the dark fur frequency increase?
- 3 Explain why natural selection can only act effectively if trait differences are heritable and not just caused by temporary environmental effects.