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.
Natural Selection: The Engine of Evolution
Natural selection, the cornerstone of evolutionary biology, is the process by which organisms better adapted to their environment tend to survive and produce more offspring. This differential reproductive success is driven by several key factors:
1. Variation: The Raw Material of Selection
- Genetic variation: Differences in genes among individuals of a population, arising from mutations, sexual reproduction (meiosis and fertilization), and gene flow. This variation provides the raw material upon which natural selection acts.
- Phenotypic variation: Observable differences in traits resulting from genetic variation and environmental influences. These traits can be morphological, physiological, or behavioral.
2. Inheritance: Passing Traits to Offspring
Heritable traits, determined by genes, are passed from parents to offspring. The mechanism of inheritance, explored in earlier units, is crucial to understanding how advantageous traits persist over generations.
3. Differential Survival and Reproduction: The "Survival of the Fittest"
Individuals with traits better suited to their environment have a higher chance of survival and reproduction, leaving behind more offspring. "Fitness," in this context, refers to reproductive success, not necessarily physical strength or athletic ability.
4. Adaptation: The Outcome of Selection
Over time, the frequency of advantageous traits increases within a population, leading to adaptation—the process by which organisms become better suited to their environment. Adaptations can be structural, physiological, or behavioral.
Mechanisms of Evolution Beyond Natural Selection
While natural selection is a major driver of evolution, several other mechanisms contribute to evolutionary change:
1. Genetic Drift: Random Changes in Allele Frequencies
- Bottleneck effect: A drastic reduction in population size due to a random event (e.g., natural disaster), leading to a loss of genetic diversity and potentially significant changes in allele frequencies.
- Founder effect: A small group of individuals establishes a new population, carrying only a subset of the original population's genetic variation. This can lead to a different allele frequency distribution in the new population.
2. Gene Flow: Movement of Genes Between Populations
Migration of individuals between populations introduces new alleles into a population, altering allele frequencies and potentially increasing genetic diversity.
3. Mutation: The Ultimate Source of Variation
Mutations, changes in DNA sequence, are the ultimate source of new genetic variation. While most mutations are neutral or deleterious, some can be advantageous, providing the raw material for natural selection to act upon.
4. Non-random Mating: Sexual Selection
Sexual selection favors traits that increase an individual's mating success, even if these traits reduce survival chances. This often leads to sexual dimorphism—differences in appearance between males and females.
Evidence for Evolution
Numerous lines of evidence support the theory of evolution:
- Fossil record: Provides a chronological record of life on Earth, showing transitional forms and extinct species.
- Biogeography: The geographical distribution of species reflects evolutionary history and patterns of continental drift.
- Comparative anatomy: Homologous structures (similar structures with different functions) and vestigial structures (structures with reduced function) reveal evolutionary relationships.
- Molecular biology: Similarities in DNA sequences and protein structures across species reflect shared ancestry.
- Direct observation: Evolution can be observed directly in populations with short generation times, such as bacteria and insects.
Speciation: The Formation of New Species
Speciation is the process by which one species splits into two or more distinct species. This often occurs through reproductive isolation, preventing gene flow between populations. Various mechanisms can lead to reproductive isolation, including:
- Geographic isolation: Physical barriers separate populations.
- Temporal isolation: Breeding seasons or times of day differ.
- Behavioral isolation: Differences in mating rituals prevent interbreeding.
- Mechanical isolation: Incompatibility of reproductive structures.
- Gametic isolation: Incompatibility of eggs and sperm.
This detailed overview provides a solid foundation for understanding the core concepts within AP Biology Unit 7. Remember to consult your textbook and class materials for further clarification and practice problems. Good luck with your studies!
