AP Biology Unit 7: Natural Selection and Evolutionary Processes

Natural Selection and Evolution

Selection

Evolution is defined as a change in the genetic makeup of a population over time, supported by multiple lines of evidence. Natural selection is a primary mechanism driving evolutionary change.

• Causes of Natural Selection: Competition for limited resources leads to differential survival and reproduction.

• Effect on Populations: Individuals with favorable phenotypes are more likely to survive and reproduce, passing advantageous traits to the next generation.

• Phenotypic Variation: Variation in traits within a population is essential for natural selection to act upon.

• Human Impact: Humans can influence diversity through artificial selection, altering the genetic makeup of populations.

• Environmental Change: Changes in the environment apply selective pressures, influencing which traits are advantageous.

Key Concepts:

• Natural selection acts on phenotypic variation.

• Evolutionary fitness is measured by reproductive success.

• Both biotic (living) and abiotic (non-living) environmental factors influence the rate and direction of evolution.

• Convergent evolution occurs when similar selective pressures result in similar adaptations in unrelated species.

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

Evolution of Populations

Genetic Variation and Population Genetics

Evolution within populations is influenced by both selective and random processes, which affect allele and genotype frequencies over time.

• Random Processes:

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

  • Genetic Drift: Random fluctuations in allele frequencies, especially significant in small populations (e.g., 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 frequencies in a non-evolving population. The five conditions are:

  1. Large population size

  2. No migration

  3. No net mutations

  4. Random mating

  5. No natural selection

  Equations:

  • Allele frequency: $p + q = 1$

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

• Genetic Diversity: Populations with higher genetic diversity are more resilient to environmental changes and less likely to go extinct.

Example: The cheetah population has low genetic diversity due to a historical bottleneck, making it vulnerable to disease and environmental changes.

Evidence of Evolution, Common Ancestry, and Phylogeny

Lines of Evidence

Multiple types of data support the theory of evolution and common ancestry among organisms.

• Fossil Record: Fossils provide chronological evidence of evolutionary change and can be dated using rock layers and isotope decay (e.g., carbon-14 dating).

• Morphological Homologies: Similar structures (including vestigial organs) indicate shared ancestry.

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

• Conserved Features: All eukaryotes share membrane-bound organelles, linear chromosomes, and genes with introns.

• Ongoing Evolution: Examples include the evolution of resistance to antibiotics, pesticides, and the emergence of new diseases.

Phylogenetic Trees and Cladograms:

• Show evolutionary relationships among lineages.

• Phylogenetic trees can indicate the amount of change over time (calibrated by fossils or molecular clocks).

• Shared, derived characters are used to construct trees; the out-group is the least closely related lineage.

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

• Nodes represent the most recent common ancestor of two groups.

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

Example: The presence of similar DNA sequences in humans and chimpanzees supports their close evolutionary relationship.

Speciation and Extinction

Mechanisms and Patterns

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 that can interbreed and produce fertile offspring.

  • Allopatric Speciation: Occurs due to geographic separation.

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

  • Prezygotic and Postzygotic Barriers: Mechanisms that prevent gene flow between populations.

  • Punctuated Equilibrium: Rapid evolutionary change 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 environmental changes, ecological stress, or human activity.

  • Extinction rates influence ecosystem diversity and open niches for new species.

Example: The mass extinction of dinosaurs allowed mammals to diversify and occupy new ecological niches.

Origins of Life on Earth

Scientific Models and Evidence

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

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

• Abiotic Synthesis: Early Earth conditions allowed inorganic molecules to form organic molecules (e.g., amino acids, nucleotides) due to available energy and low oxygen.

• Extraterrestrial Hypothesis: Organic molecules may have been delivered to Earth by meteorites.

• Polymerization: Monomers joined to form polymers capable of replication and information storage.

• RNA World Hypothesis: RNA was likely the first genetic material, capable of both storing information and catalyzing reactions.

Example: The Miller-Urey experiment demonstrated that amino acids could form under simulated early Earth conditions.

Additional info: These notes are based on AP Biology Unit 7 and reference Campbell Biology chapters 22-26. For further detail, consult the recommended textbook sections and AP Classroom resources.