AP Biology Unit 7: Natural Selection and Evolutionary Processes

Natural Selection and Evolution

Selection

Evolution is defined as the change in the genetic makeup of a population over time, driven by various mechanisms and supported by multiple lines of evidence. Natural selection is a primary mechanism of evolution, shaping populations through differential survival and reproduction.

• Causes of Natural Selection: Competition for limited resources leads to differential survival. Individuals with advantageous phenotypes are more likely to survive and reproduce, passing those traits to the next generation.

• Effects on Populations: Over time, favorable traits become more common, while less advantageous traits diminish.

• Phenotypic Variation: Variation in traits within a population is essential for natural selection to act. Without variation, evolution cannot occur.

• Human Impact: Through artificial selection, humans can influence the diversity and traits within populations (e.g., selective breeding in agriculture).

• Environmental Changes: Shifts in the environment alter selective pressures, leading to evolutionary changes in populations.

• Evolutionary Fitness: Measured by reproductive success—the ability to survive and produce fertile offspring.

• Convergent Evolution: Similar selective pressures can lead to similar adaptations in unrelated species (e.g., wings in bats and birds).

Example: The development of antibiotic resistance in bacteria is a result of natural selection acting on genetic variation.

Evolution of Populations

Genetic Variation and Evolutionary Mechanisms

Population genetics studies the genetic composition of populations and how it changes over time. Evolution is influenced by both selective and random processes.

• Random Processes:

  • Mutation: Random changes in DNA that introduce new genetic variation.

  • Genetic Drift: Random fluctuations in allele frequencies, especially significant in small populations. Includes bottleneck and founder effects.

  • Gene Flow (Migration): Movement of alleles between populations, which can introduce new genetic material.

• Hardy-Weinberg Equilibrium: A model describing allele and genotype frequencies in a non-evolving population. The five conditions are:

  1. Large population size

  2. No migration

  3. No mutations

  4. Random mating

  5. No natural selection

  Equations:

  • Allele frequencies: $p + q = 1$

  • Genotype frequencies: $p^2 + 2pq + q^2 = 1$

• Genetic Diversity: Populations with higher genetic diversity are more resilient to environmental changes. Low diversity increases risk of decline or extinction.

• Adaptive vs. Deleterious Alleles: An allele beneficial in one environment may be harmful in another due to changing selective pressures.

Example: The cheetah population experienced a genetic bottleneck, reducing genetic diversity and increasing vulnerability to disease.

Evidence of Evolution, Common Ancestry, and Phylogeny

Lines of Evidence and Evolutionary Relationships

Multiple scientific disciplines provide evidence for evolution and common ancestry among organisms. Phylogenetic trees and cladograms are tools for visualizing evolutionary relationships.

• Types of Evidence:

  • Fossil Record: Shows changes in organisms over time; fossils can be dated using rock layers, isotope decay (e.g., carbon-14), and geographical data.

  • Morphological Homologies: Shared anatomical features (including vestigial structures) indicate common ancestry.

  • Molecular Evidence: DNA and protein sequence comparisons reveal evolutionary relationships.

  • Conserved Cellular Features: All life shares fundamental molecular and cellular processes, such as genetic code and metabolic pathways.

  • Structural Evidence in Eukaryotes: Presence of membrane-bound organelles, linear chromosomes, and genes with introns support common ancestry.

• Ongoing Evolution: Examples include genomic changes, continuous fossil record transitions, and the evolution of resistance in pathogens.

• Phylogenetic Trees and Cladograms:

  • Show evolutionary relationships; phylogenetic trees can indicate the amount of change over time.

  • Constructed using shared and derived characters; out-groups help root the tree.

  • Molecular data is generally more reliable than morphological traits for constructing trees.

  • Nodes represent the most recent common ancestor of lineages.

  • Trees are hypotheses and may be revised with new evidence.

Example: The similarity in cytochrome c protein sequences among mammals supports their common ancestry.

Speciation and Extinction

Mechanisms and Patterns of Speciation and Extinction

Speciation is the process by which new species arise, while extinction is the loss of species. Both processes shape biodiversity.

• Speciation:

  • Occurs when populations become reproductively isolated.

  • Biological Species Concept: Species are groups of interbreeding populations that produce viable, fertile offspring.

  • Modes of Speciation:

    • Allopatric Speciation: Occurs due to geographic separation.

    • Sympatric Speciation: Occurs without geographic separation, often via polyploidy or behavioral isolation.

  • Reproductive Isolation Mechanisms: Prezygotic (before fertilization) and postzygotic (after fertilization) barriers prevent gene flow.

  • Patterns of Evolution:

    • Punctuated Equilibrium: Rapid evolution after long periods of stasis.

    • Gradualism: Slow, steady evolutionary change.

    • Divergent Evolution: Adaptation to new habitats leads to phenotypic diversification.

    • Adaptive Radiation: Rapid speciation as new niches become available, often after extinction events.

• Extinction:

  • Can be caused by ecological stress, environmental changes, or human activity.

  • Extinction rates influence ecosystem diversity; high rates can open niches for adaptive radiation.

Example: The diversification of mammals after the extinction of dinosaurs is an example of adaptive radiation.

Origins of Life on Earth

Scientific Models and Evidence for the Origin of Life

The origin of life on Earth is explained by several scientific hypotheses, supported by geological and experimental evidence.

• Geological Evidence: Earth formed about 4.6 billion years ago; earliest life appeared between 3.9 and 3.5 billion years ago.

• Models for the Origin of Life:

  • Primitive Earth had inorganic precursors and free energy, allowing synthesis of organic molecules in the absence of oxygen.

  • Organic molecules may have arrived via meteorites (panspermia hypothesis).

• Chemical Experiments: Laboratory simulations (e.g., Miller-Urey experiment) show that complex organic molecules can form from inorganic precursors.

• Formation of Polymers: Organic monomers (amino acids, nucleotides) joined to form polymers capable of replication and information storage.

• RNA World Hypothesis: Proposes that RNA was the first genetic material, capable of self-replication and catalysis.

Example: The Miller-Urey experiment demonstrated the abiotic synthesis of amino acids under early Earth conditions.

Additional info: These notes expand on the AP Biology Unit 7 outline, providing academic context, definitions, and examples for each topic. For further detail, refer to Campbell Biology Chapters 22-26 as indicated in the resources.