Unit 7: Darwinian Evolution, Speciation, and Biodiversity

Darwinian Evolution

Darwin’s Influences and Theory of Evolution

Charles Darwin’s theory of evolution was shaped by scientific and cultural changes, as well as his travels on the HMS Beagle. His work, On the Origin of Species, challenged prevailing ideas about life on Earth.

• Main Idea: Darwin introduced evolution by natural selection ("descent with modification").

• Prevailing Beliefs:

  • Earth was only a few thousand years old.

  • Species were unchanging.

  • All organisms were created as they are now.

• Darwin’s Impact: His ideas transformed biology and changed how scientists understand life on Earth.

Natural Selection and Unequal Reproductive Success

Evolution by natural selection is a logical consequence of observable natural phenomena. Evidence of this mechanism is found in the world around us.

• Evolution = "Descent with Modification": All living species descend from earlier ancestral species.

• Natural Selection: Organisms gain small changes (modifications) that help them survive in their environment.

• Key Points:

  • Organisms vary.

  • More offspring are produced than can survive.

  • Resources are limited.

  • Individuals with helpful traits survive and reproduce more often.

  • Over generations, populations change—this is evolution by natural selection.

• Modern Examples: Bacteria becoming resistant to antibiotics, insects evolving resistance to pesticides, viruses changing over time, and animals/plants adapting to changing environments.

Important Points About Evolution

• Evolution happens to populations, not individuals.

• Natural selection does not have a goal. It does not work toward "perfect" organisms, but favors traits that help organisms survive in their current environment.

• Evolution is based on existing variation. Variation comes from mutations and genetic mixing.

Evolution in Everyday Life

Artificial Selection

Artificial selection is when humans choose which organisms get to reproduce based on desired traits.

• How it works: Humans pick parents with desirable traits (e.g., big fruit, calm behavior).

• Only those individuals get to breed, so their traits become more common over generations.

• Darwin’s Insight: Artificial selection is human-controlled evolution, similar to natural selection but guided by preferences.

• Result: Domesticated plants and animals often look very different from their wild ancestors.

Antibiotic and Pesticide Resistance

• Antibiotic Resistance: Bacteria evolve resistance to antibiotics, making some drugs less effective (e.g., MRSA).

• Pesticide Resistance: Mosquitoes evolve resistance to pesticides, making malaria control more difficult.

Diversity and Unity of Life

• Diversity: Life on Earth contains a huge variety of species.

• Unity: All living organisms share similar cell structures and DNA composition.

• Why? All species descended from a common ancestor and changed over time through evolution.

Evidence for Evolution

The Fossil Record

Fossils form through several mechanisms and provide evidence for evolution by showing transitional forms and changes over time.

• Darwin’s Insight: Fossils of extinct species resembled living species but were not identical, suggesting change over time.

• Types of Fossils:

  • Sedimentary fossils: minerals replace tissues (e.g., Confuciusornis, a 120-million-year-old bird).

  • Amber: fossilized tree sap traps and preserves organisms.

  • Ice: organisms can freeze (e.g., "Lyuba," a 42,000-year-old baby mammoth).

  • Trace fossils: footprints or tracks (e.g., Apatosaurus tracks).

• Radiometric Dating: Scientists determine the age of fossils using radioactive isotopes (e.g., carbon-14).

Evidence Beyond Fossils

• Biogeography: Study of where species live around the world. Example: Australia dominated by marsupials, other continents by placental mammals.

• Comparative Anatomy: Comparing body structures across species. Homologous structures (e.g., bat wing, human arm) show common ancestry.

• Embryology: Comparing embryos shows similarities (e.g., human and chicken embryos both have tails and pharyngeal pouches).

• Bioinformatics: Using DNA and protein sequence data to study relationships. Example: DNA sequences show chimpanzees are most closely related to humans.

Populations and Mechanisms of Evolution

Populations as Units of Evolution

Evolution acts on populations by altering the gene pool (all versions of all genes in the population). Microevolution is generation-to-generation change in the gene pool.

• Gene Pool: All versions of all genes in a population.

• Microevolution: Small, generation-to-generation changes in the gene pool.

• Sources of Variation:

  • Mutations: random changes in DNA.

  • Sexual reproduction: mixes genes into new combinations.

Mechanisms That Change Gene Pools

• Natural Selection: Traits that improve survival or reproduction become more common.

• Genetic Drift: Random changes in gene frequencies, especially in small populations.

  • Bottleneck Effect: Population size drops drastically, reducing genetic diversity.

  • Founder Effect: A few individuals start a new population in a new location.

• Gene Flow: Movement of individuals into or out of a population, making populations more genetically similar.

• Sexual Selection: Traits that attract mates (e.g., bright feathers) get passed on.

Geologic Record and Macroevolution

The Geologic Record

Earth’s history is divided into eras and periods, marked by major changes in life and biodiversity. Plate tectonics and continental drift have shaped the evolution of life.

• Earth’s Age: Over 4 billion years old.

• Plate Tectonics: Movement of plates causes earthquakes, volcanoes, mountain formation, and continental drift.

• Pangaea: About 250 million years ago, all continents were joined as one supercontinent. Its breakup led to species isolation and evolutionary divergence.

Macroevolution

Macroevolution refers to major changes in the history of life, including the formation of new species (speciation), mass extinctions, and large-scale diversification.

• Speciation: Formation of new species increases biodiversity.

• Types of Speciation:

  • Nonbranching: One species changes into a new species.

  • Branching: One species splits into two or more new species.

• Mass Extinctions: Earth has experienced several major mass extinctions, with 50–90% of all species dying out during each event.

Species and Speciation

Species and Reproductive Barriers

Species are defined as groups of individuals capable of successfully interbreeding. Reproductive barriers prevent members of different species from producing healthy offspring.

• Biological Species Definition: A species is a population whose members can interbreed naturally and produce healthy offspring.

• Reproductive Barriers:

  • Behavioral isolation: Unique mating rituals.

  • Mating time (temporal isolation): Different times of day or year.

  • Habitat isolation: Different habitats.

  • Mechanical incompatibility: Body parts don’t fit together.

  • Gametic incompatibility: Sperm and egg from different species cannot fuse.

  • Hybrid weakness: Offspring are sterile or weak.

Speciation Mechanisms

• Gradualism: Species change slowly and steadily over time.

• Punctuated Equilibrium: Species remain unchanged for long periods, followed by rapid bursts of change.

• Allopatric Speciation: Occurs when populations are separated by a physical barrier (mountains, rivers).

• Sympatric Speciation: New species arise without geographic separation, often through large genetic changes (e.g., polyploidy in plants).

Taxonomy and Classification

Taxonomy

Taxonomy is the science of identifying, naming, and classifying organisms. It provides a universal language for scientists.

• Taxonomic Hierarchy:

  1. Domain

  2. Kingdom

  3. Phylum

  4. Class

  5. Order

  6. Family

  7. Genus

  8. Species

• Three-Domain System:

  • Bacteria: Prokaryotes, no nucleus.

  • Archaea: Prokaryotes, often in extreme environments.

  • Eukarya: Eukaryotes (cells with a nucleus), includes plants, fungi, animals, and protists.

• Binomial Nomenclature: Uses two Latin names (Genus species), e.g., Homo sapiens.

Phylogenetic Trees and Systematics

Phylogenetic Trees

Phylogenetic trees present hypotheses about the shared evolutionary history of groups of organisms. They use evidence from fossil records, DNA sequences, and physical traits.

• Systematics: The branch of biology that classifies organisms based on evolutionary relationships.

• Phylogenetic Tree: Shows branches (evolutionary paths) and nodes (common ancestors).

• Cladistics: Groups organisms into clades based on shared characteristics inherited from a common ancestor.

• Ingroup vs. Outgroup: Ingroup shares a common ancestor; outgroup branched off earlier.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Example
Natural Selection	Traits that improve survival/reproduction become more common	Antibiotic resistance in bacteria
Genetic Drift	Random changes in gene frequencies	Bottleneck effect in cheetahs
Gene Flow	Movement of genes between populations	Pollen drifting between plant populations
Sexual Selection	Traits that attract mates get passed on	Bright feathers in birds of paradise

Additional info: These notes expand on the original content with definitions, examples, and academic context to ensure completeness and clarity for exam preparation.