ap biology unit 7 notes

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01-02-2025

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Unit 7 of AP Biology delves into the fascinating world of natural selection and the mechanisms driving evolutionary change. This comprehensive guide provides detailed notes covering key concepts, ensuring you're well-prepared for the exam.

I. Natural Selection: The Driving Force of Evolution

Natural selection, the cornerstone of evolutionary theory, explains how populations change over time. It's not simply "survival of the fittest," but rather a process based on several key principles:

• Variation: Individuals within a population exhibit variation in their traits. This variation arises from genetic mutations, sexual reproduction (recombination), and other factors.
• Inheritance: These traits are heritable, meaning they can be passed down from parents to offspring. Understanding Mendelian genetics is crucial here.
• Overproduction: Populations produce more offspring than can possibly survive. This creates competition for limited resources.
• Differential Survival and Reproduction: Individuals with traits better suited to their environment are more likely to survive and reproduce, passing those advantageous traits to their offspring. This is where "fitness" comes into play – reproductive success, not necessarily physical strength.

Key Concepts Related to Natural Selection:

• Adaptation: A heritable trait that enhances survival and reproduction in a specific environment. Adaptations are not "chosen" but arise through random variation and are selected for by environmental pressures.
• Fitness: A measure of an individual's reproductive success. Higher fitness means more offspring.
• Directional Selection: Selection favors one extreme phenotype over others.
• Stabilizing Selection: Selection favors the intermediate phenotype, reducing variation.
• Disruptive Selection: Selection favors both extreme phenotypes, leading to increased variation.
• Sexual Selection: Selection based on mate choice, often leading to traits that enhance mating success but may not improve survival.

Examples of Natural Selection in Action:

• Peppered Moths: A classic example illustrating how environmental changes (industrial melanism) can drive changes in allele frequencies.
• Antibiotic Resistance in Bacteria: The rapid evolution of antibiotic resistance highlights the power of natural selection in the face of human intervention.
• Darwin's Finches: The diverse beak shapes of Galapagos finches showcase adaptive radiation and the role of natural selection in shaping species diversity.

II. Mechanisms of Evolution

Beyond natural selection, several other mechanisms contribute to evolutionary change:

• Genetic Drift: Random fluctuations in allele frequencies, particularly impactful in small populations. Includes the bottleneck effect and founder effect.
• Gene Flow: The movement of alleles between populations through migration. This can introduce new alleles or alter existing allele frequencies.
• Mutation: The ultimate source of new genetic variation. While individual mutations are often small, their accumulation over time can lead to significant evolutionary change.

Understanding the interplay between these mechanisms is crucial for comprehending the complexities of evolutionary processes.

III. Speciation and Reproductive Isolation

Speciation, the formation of new and distinct species, occurs when populations become reproductively isolated. Reproductive isolation can arise through various mechanisms:

• Prezygotic Barriers: Prevent mating or fertilization from occurring. Examples include habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation.
• Postzygotic Barriers: Prevent a hybrid zygote from developing into a fertile adult. Examples include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

IV. Phylogenetic Trees and Evolutionary Relationships

Phylogenetic trees depict the evolutionary relationships among different species or groups of organisms. Understanding how to interpret and construct these trees is essential for understanding evolutionary history. Key concepts include:

• Clades: Groups of organisms that share a common ancestor.
• Homologous Structures: Structures derived from a common ancestor, reflecting shared ancestry even if their functions differ.
• Analogous Structures: Structures that have similar functions but evolved independently, not reflecting shared ancestry.
• Convergent Evolution: The independent evolution of similar traits in different lineages due to similar environmental pressures.

This comprehensive overview of AP Biology Unit 7 provides a solid foundation for understanding natural selection and the mechanisms driving evolutionary change. Remember to consult your textbook and class materials for further detail and practice problems. Good luck with your studies!