Understanding Natural Selection in AP Biology
Natural selection is a fundamental concept in AP Biology, explaining how traits that enhance survival and reproduction become more common in populations over time. Proposed by Charles Darwin, this mechanism is the driving force of evolution. In this guide, we will cover the principles of natural selection, its types, and real-world examples to help you ace the AP Bio exam.
1. What is Natural Selection?
Natural selection is the process by which organisms with favorable traits are more likely to survive and reproduce, passing these traits to the next generation. It is based on variation, heritability, differential survival, and reproduction.
Key Points:
- Variation: Individuals within a population have different traits.
- Heritability: Traits must be inheritable for natural selection to act on them.
- Differential Survival: Organisms with advantageous traits survive and reproduce more successfully.
2. Darwin’s Theory of Natural Selection
Charles Darwin introduced the concept of natural selection in his book “On the Origin of Species” (1859). His theory is summarized by the phrase “survival of the fittest”.
Key Points:
- Overproduction: More offspring are produced than can survive.
- Struggle for Existence: Limited resources create competition.
- Descent with Modification: Beneficial traits become more common over generations.
3. Mechanisms of Natural Selection
Natural selection can occur through various mechanisms:
- Stabilizing Selection: Favors intermediate traits, reducing variation.
- Directional Selection: Favors one extreme trait, shifting the population mean.
- Disruptive Selection: Favors both extremes, leading to two distinct phenotypes.
- Sexual Selection: Traits that improve mating success become more common.
4. Types of Natural Selection in Detail
1. Stabilizing Selection:
- Reduces variation by favoring the average phenotype.
- Example: Human birth weights, where extremely high or low weights reduce survival rates.
2. Directional Selection:
- Shifts traits in a specific direction due to environmental changes.
- Example: Antibiotic resistance in bacteria.
3. Disruptive Selection:
- Favors extreme traits over intermediate ones.
- Example: Darwin’s finches with different beak sizes suited for specific food sources.
5. Evidence Supporting Natural Selection
1. Fossil Record:
- Shows gradual changes in species over time.
2. Molecular Biology:
- DNA and protein similarities among species indicate common ancestry.
3. Comparative Anatomy:
- Homologous structures suggest evolutionary relationships.
6. Practice Questions for AP Biology
- Explain how directional selection affects allele frequencies in a population.
- Differentiate between stabilizing and disruptive selection with examples.
- Provide evidence supporting Darwin’s theory of natural selection.
Brief Answers to Practice Questions
- Directional selection increases the frequency of advantageous alleles by favoring one extreme trait.
- Stabilizing selection favors intermediate traits (e.g., human birth weight), while disruptive selection favors extremes (e.g., beak sizes in finches).
- Evidence includes: fossil records, molecular biology, and comparative anatomy.
Conclusion: Mastering Natural Selection for the AP Bio Exam
Grasping the concepts of natural selection is crucial for scoring high on the AP Biology exam. Focus on understanding the mechanisms, types, and supporting evidence. This guide is designed to streamline your study process and enhance your preparation.
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