AP Biology Unit 7: Evolution

Overview

Unit 7 explores the central theme of evolution in AP Biology. It covers a broad range of topics, moving from the mechanisms of evolutionary change (selection) to the patterns of evolutionary change (phylogeny) and the processes that influence species diversity (speciation and extinction). The lecture series presented by Glenn Wolkenfeld (Mr. W), a retired AP Biology teacher, provides a comprehensive overview of these concepts. His emphasis on interactive learning and feedback is highlighted throughout the lecture.

Key Topics

Selection Types

Natural Selection

Developed by Charles Darwin in the 1800s, natural selection builds upon the concept of artificial selection. It's a process where organisms with heritable traits better suited to their environment survive and reproduce more successfully than those with less advantageous traits. This leads to an increase in the frequency of those advantageous traits within the population over time. Key components include:

• Inherited Variation: Differences among individuals, often stemming from genetic mutations and recombination.
• Overproduction: More offspring are produced than can possibly survive.
• Survival of the Fittest: Fitness is measured by reproductive success, not necessarily strength or speed. It encompasses the entire life cycle (feeding, breeding, juvenile survival, etc., as illustrated with the penguin example).
• Adaptation: The accumulation of advantageous traits within a population over generations. Examples mentioned included bat wings (structure and sonar), the satanic leaf gecko (camouflage), and enzyme-substrate interactions.

Artificial Selection

Also known as selective breeding, this is the process by which humans select for desirable traits in plants and animals over many generations. This creates a gene pool heavily biased towards those selected traits. Examples from the lecture include:

• Brassica oleracea family: Cauliflower, broccoli, Brussels sprouts, kale are all varieties of the same species, selectively bred for different traits (flower clusters, buds, lateral buds, leaves).
• Canis lupus: Dogs and wolves belong to the same species, with various breeds selectively bred for human purposes (protection, hunting, companionship).

Sexual Selection

This type of selection favors traits that enhance mating success. It often leads to sexual dimorphism (differences between males and females). Two types were highlighted:

• Intersexual Selection: Mate choice, where one sex (usually females) chooses mates based on certain traits. Examples include the elaborate tail feathers of male turkeys and peacocks.
• Intrasexual Selection: Competition among individuals of the same sex (usually males) for access to mates or resources. Examples include aggressive elephant seals, where males are significantly larger than females.

Population Genetics

This field studies how gene frequencies change within populations over time. Key concepts include:

• Allele Frequency: The proportion of a specific allele within a population's gene pool (the total collection of alleles in a population).
• Gene Pool: The sum of all alleles for all genes in a population.
• Evolution (in this context): A change in allele frequencies within a gene pool over time.
• Hardy-Weinberg Equilibrium: A model for a non-evolving population, described by two equations:
  • p + q = 1 (where 'p' is the frequency of the dominant allele and 'q' is the frequency of the recessive allele).
  • p² + 2pq + q² = 1 (representing the frequencies of homozygous dominant, heterozygous, and homozygous recessive genotypes, respectively).
• Factors that Disrupt Hardy-Weinberg Equilibrium:
  • Genetic Drift: Random changes in allele frequencies, especially pronounced in small populations (including bottlenecks and founder effects). Cheetahs were given as an example of a species with very low genetic diversity due to a past bottleneck event.
  • Natural Selection: Differential survival and reproduction based on advantageous or harmful alleles.
  • Sexual Selection: Non-random mating, departing from the Hardy-Weinberg assumption of random mating.
  • Gene Flow: Movement of alleles between populations.
  • Mutation: The ultimate source of new alleles. The example of sickle cell anemia and heterozygote advantage (protection against malaria) was discussed in detail.

Evidence for Evolution

This section explores various lines of evidence supporting evolutionary theory:

Homologous Traits

Structures in different species that share a common underlying structure and embryological origin, suggesting descent from a common ancestor. Examples included the forelimbs of humans, dogs, birds, and whales.

Adaptive Radiation

The rapid diversification of a single ancestral species into multiple descendant species, each adapted to a different ecological niche. The Galapagos finches were used as a classic example.

Vestigial Structures

Structures that have lost their original function in a species but are homologous to functional structures in related species (e.g., the pelvic bones in whales, the human coccyx).

Molecular Homologies

Similarities in molecules, such as DNA sequences and protein structures, across different species, indicating common ancestry. The example of hemoglobin similarities across various vertebrates was presented.

Biogeography

The geographic distribution of species, reflecting evolutionary history and patterns of dispersal and isolation. The example of marsupial mammals predominantly found in Australia was discussed. Convergent evolution, where unrelated species evolve similar traits due to similar environmental pressures (e.g., marsupial and placental moles), was also highlighted.

Fossil Evidence

Fossil records document the existence of extinct organisms and transitional forms, showing evolutionary change over time. The evolution of whales from land-dwelling mammals was discussed as an example. Relative dating (based on superposition in sedimentary rock layers) and absolute dating (using radioactive isotopes) were both explained. The continuing evolution of DDT resistance in mosquitoes, and antibiotic resistance in bacteria, serve as modern examples of evolution in action.

Speciation and Extinction

Species Concepts

The biological species concept defines a species as a group capable of interbreeding to produce viable and fertile offspring. However, limitations exist for asexual, extinct, and prokaryotic organisms, and for cases of hybridization between closely related species.

Reproductive Isolating Mechanisms

These prevent interbreeding between closely related species:

• Prezygotic Barriers: Prevent fertilization (habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation).
• Postzygotic Barriers: Result in hybrid inviability, sterility, or breakdown.

Modes of Speciation

• Allopatric Speciation: Geographic isolation leads to genetic divergence and reproductive isolation.
• Sympatric Speciation: Speciation occurs without geographic isolation (e.g., polyploidy in plants, sexual selection in cichlid fishes, microhabitat adaptation in lice).

Extinction

Extinction is a natural process, with over 99% of species that have ever lived now extinct. Extinction vortex, where small, isolated populations experience a positive feedback loop of decreasing genetic diversity and declining fitness, was discussed. Mass extinctions, caused by geological or astronomical events, create opportunities for adaptive radiation by opening ecological niches for surviving species. Human activities are currently causing a sixth mass extinction.

Phylogeny

Phylogenetic Trees

Branching diagrams representing evolutionary relationships among organisms. Key terms include:

• Clade: A group containing a common ancestor and all of its descendants.
• Shared Derived Character: A trait unique to a clade.
• Node: Point of divergence on a phylogenetic tree, representing a common ancestor.
• Sister Groups: Closely related clades that diverge from the same node.
• Outgroup: A more distantly related group used as a comparison.
• Ancestral Feature: A trait shared by a clade but also present in more inclusive clades.

The importance of focusing on the recency of common ancestry, rather than the proximity on the tree diagram, was emphasized. The use of molecular clocks (using mutation rates to estimate divergence times) was also explained.

Origin of Life

This section discusses the emergence of life on Earth:

• Key Steps: Formation of a habitable planet, abiotic synthesis of monomers, abiotic synthesis of polymers, formation of protocells (precursors to cells), and the emergence of self-replicating cells.
• Miller-Urey Experiment: Demonstrated the abiotic synthesis of amino acids under simulated early Earth conditions.
• RNA World Hypothesis: RNA, possessing both informational and catalytic properties, is hypothesized to have been the first genetic molecule. The progression from inorganic precursors to self-replicating RNA systems enclosed in lipid membranes (protocells) leading to the last universal common ancestor (LUCA) was described. The characteristics of LUCA (lipid bilayer, DNA, RNA, ribosomes, membrane channels, enzymes, ATP synthase) were summarized.

Study Resources

• Interactive tutorials and AP Biology exam reviews are available at learn-biology.com (mentioned repeatedly in the lecture).
• A checklist for topics 7.1 to 7.3 is downloadable from apbiosuccumbs.com (mentioned at the end of the lecture).

This expanded version provides more detail on the key concepts and examples from the lecture. Remember to review the lecture and your textbook for a more complete understanding.