7.2 Natural Selection

Unit 7.2: Natural Selection – AP Biology

Introduction to Natural Selection

Natural selection is a fundamental process in nature, shaping the traits and characteristics of populations over time. It is the mechanism by which organisms that are better adapted to their environment tend to survive and reproduce more successfully. As environments change, selective pressures act on organisms, favoring those with advantageous adaptations. These adaptations increase their evolutionary fitness, meaning these organisms are more likely to pass on their traits to future generations. Over time, natural selection can even lead to the emergence of new species, driving evolutionary change.

Phenotypic Variation in Populations

To understand natural selection, it is crucial to recognize that populations contain phenotypic variation. A phenotype is an observable characteristic of an organism—such as eye color, hair color, or height in humans. In every population, different individuals will exhibit different phenotypes. For example, among humans, while brown and blonde hair colors are common, red hair is another less common phenotype.

In the natural world, certain phenotypes can significantly influence an individual’s ability to survive and reproduce, particularly in the face of environmental challenges. These phenotypic differences become the basis upon which natural selection acts, allowing organisms with traits that provide survival advantages to thrive and pass those traits on to the next generation. To illustrate this concept, let’s explore the classic example of peppered moths.

The Peppered Moth – A Case Study in Natural Selection

The peppered moth (Biston betularia) is a well-known example that clearly demonstrates natural selection in action. Originally, peppered moths had a light-colored phenotype with black speckles on their wings, helping them blend into lichen-covered trees in rural England during the early 19th century. This camouflage provided effective protection from predators, such as birds, by allowing them to remain inconspicuous.

However, during the Industrial Revolution, the environment in England began to change dramatically. Soot from factories covered the once-light trees, darkening their surfaces. Suddenly, the light-colored peppered moths that had previously been well camouflaged stood out against the now-sooty bark. This made them easier targets for predators, drastically reducing their survival rates in these areas.

Meanwhile, a random mutation occurred in an individual peppered moth, resulting in a dark-colored phenotype. In the newly soot-darkened environment, these dark-colored moths now had an advantage—they could blend in with the darkened trees, making them less visible to predators. As a result, these moths had increased evolutionary fitness. They were more likely to survive, reproduce, and pass on their dark-coloring trait to their offspring.

Natural Selection in Action

As the Industrial Revolution continued, the dark form of the peppered moth became increasingly common in areas affected by soot and pollution. Over time, the allele frequency for dark coloration increased in these moth populations, while the allele frequency for light coloration decreased. This process exemplifies natural selection: the environment created selective pressure that favored the dark phenotype, leading to an increase in the population of dark-colored moths.

It is important to note that if the environment had not changed, the dark-colored phenotype might not have been advantageous. In fact, it may have been detrimental, as it would have made the moths stand out against the lighter trees, leading to higher predation rates. The story of the peppered moth is a powerful reminder of how changing environments can shape the characteristics of populations and drive evolutionary change.

Key Takeaways on Natural Selection

• Natural selection is a process where organisms better adapted to their environment have a higher likelihood of surviving and reproducing.

• Phenotypic variation in a population provides the raw material for natural selection to act upon.

• Selective pressure can favor certain phenotypes over others, increasing their prevalence in a population.

• The example of the peppered moth illustrates how environmental changes can influence which traits are advantageous, leading to shifts in allele frequencies over time.

Conclusion

Natural selection is one of the driving forces behind evolution, allowing populations to adapt to their changing environments. As seen in the peppered moth example, the interplay between phenotype and environment plays a crucial role in determining which traits are passed on to future generations. Understanding natural selection is fundamental to understanding the broader concepts of evolution and species diversity.

By grasping how natural selection works, we can better understand the resilience of life and the adaptive strategies that organisms develop to survive in a constantly changing world.