Natural Selection Define natural selection: Explain how variations (differences) within a population lead to changes in the population over time.

**Natural Selection** **Definition:** Natural selection is a fundamental mechanism of evolution first articulated by Charles Darwin. It is the process by which heritable traits that confer a survival or reproductive advantage become more common in successive generations within a population. Essentially, individuals with traits better suited to their environment are more likely to survive, reproduce, and pass those advantageous traits to their offspring, leading to gradual changes in the population over time. **How Variations Within a Population Lead to Changes Over Time:** 1. **Genetic Variation:** - **Sources of Variation:** Genetic diversity within a population arises from mutations (random changes in DNA), gene flow (transfer of genes between populations), and sexual reproduction (which combines genes from two parents in new ways). - **Importance:** Without genetic variation, a population cannot adapt to changing environments, making it vulnerable to extinction. 2. **Variation in Traits:** - Individuals in a population exhibit differences in their physical and behavioral traits, such as size, coloration, speed, or resistance to disease. - These variations can be subtle or significant and are often influenced by genetic factors. 3. **Differential Survival and Reproduction:** - **Survival:** In a given environment, certain traits may enhance an individual's ability to survive. For example, a moth species where darker-colored moths are better camouflaged against predators in a polluted environment will have higher survival rates than lighter-colored moths. - **Reproduction:** Individuals with advantageous traits are more likely to find mates and produce offspring. Thus, these traits become more prevalent in the next generation. 4. **Inheritance of Traits:** - For natural selection to drive evolutionary change, the advantageous traits must be heritable, meaning they can be passed from parents to offspring through genes. - Over successive generations, the frequency of these beneficial traits increases within the population. 5. **Adaptation:** - As beneficial traits become more common, the population becomes better adapted to its environment. This adaptation enhances the overall fitness of the population, improving its chances of long-term survival. **Example: The Peppered Moth** A classic example illustrating natural selection is the case of the peppered moth (*Biston betularia*) in England: - **Pre-Industrial Revolution:** Most peppered moths had light-colored wings, which camouflaged them against lichen-covered trees. The dark-colored (melanic) moths were more visible to predators and thus had lower survival rates. - **Industrial Revolution:** Pollution killed the light-colored lichens and darkened the tree trunks with soot. In this new environment, the dark-colored moths became better camouflaged, while the light-colored moths became more visible to predators. - **Result:** Over time, the frequency of dark-colored moths increased in polluted areas because they had a selective advantage, demonstrating natural selection in action. **Conclusion:** Natural selection drives the evolution of species by favoring individuals with traits that enhance survival and reproduction in their specific environments. Genetic variations within a population provide the raw material for this process, enabling populations to adapt and evolve over time in response to changing environmental pressures.