Evolution

Key Terminology

• Evolution: Heritable change in characteristics of a population over generations.

• Microevolution: Small-scale changes in allele frequencies within a population.

• Macroevolution: Large-scale formation of new species.

• Species: Group of organisms that share a distinctive form.

• Population: Members of the same species in the same area that can interbreed.

Important Scientists

• John Ray (1600s): Developed an early classification system, introduced the species concept.

• Carl Linnaeus: Expanded Ray’s work, created binomial nomenclature (e.g., Homo sapiens).

• George Buffon (1700s): Suggested species change over time.

• Jean-Baptiste Lamarck: Proposed species evolve by adapting to their environment over generations.

Lamarck’s Theory (Incorrect)

• Organisms change behavior in response to environment → Traits modify → Passed to offspring.

• Example: Giraffes stretch necks → Longer necks inherited.

Influence on Evolutionary Thought

• Uniformitarianism (Geology): Earth changes gradually over time (older than 6,000 years).

• Thomas Malthus (Economics): Not all individuals survive and reproduce due to competition.

Charles Darwin

• Developed natural selection theory.

• Influenced by:

  • Voyage on HMS Beagle (1831-1836).

  • Galápagos finches (variation in beak shape).

• "On the Origin of Species" (1859): Evolution by natural selection.

Darwin’s Theory of Evolution

1. Variation in Traits – Inherited differences exist in a population.

2. Natural Selection – Individuals with favorable traits survive and reproduce, passing traits to offspring.

Genetic Variation & Mutations

• Source: Random mutations, recombination, and random mating.

• Importance: Enables adaptation and survival.

• Darwin vs. Lamarck on Elephant Trunks:

  • Lamarck: Elephants stretched trunks → Offspring inherited longer trunks.

  • Darwin: Elephants had different trunk lengths → Those with longer trunks survived and reproduced.

Beak Depth on Daphne Major

• Medium ground finches studied for 40+ years as a model of natural selection.

• Beak depth is heritable (measured in parents & offspring).

• 1977 drought: Fewer small seeds → finches ate larger, harder seeds.

• 1978 offspring had larger beaks than 1976 → evidence of natural selection.

Selective Breeding (Artificial Selection)

• Humans select desirable traits in domesticated species.

• Differs from natural selection: Parents chosen based on phenotype.

• Examples: Darwin’s pigeons, wild mustard breeding.

Biogeography

• Study of species distribution.

• Isolated regions evolve unique species (e.g., endemic species).

• Example: Island fox on Channel Islands evolved from mainland gray fox.

• Australia: Home to marsupials due to geographic isolation.

Convergent Evolution

• Unrelated species evolve similar traits due to similar environments.

• Traits = analogous structures (not due to shared ancestry).

• Example: Fishapod, whale evolution.

Homology (Common Ancestry)

1. Anatomical Homology

   • Vertebrate forelimbs share a common structure but serve different functions.

   • Vestigial structures: No function but resemble ancestral traits.

2. Developmental Homology

   • Similar embryonic features in different species (e.g., human embryos have gill ridges and tails).

3. Molecular Homology

   • DNA similarities indicate common ancestry.

   • Shared biochemical pathways across species.

   • Genes more similar between closely related species.

Homologous Genes

• Genes from the same ancestral gene.

• Orthologs: Same gene in different species.

• Paralogs: Gene duplications within a species (e.g., globin gene family).

Horizontal Gene Transfer

• Genes transferred between organisms (not inherited from parents).

• Occurs in bacteria and across species.

Evolution at the Genome Level

• Chromosome structure & number changes can drive speciation.

• Affects ability of species to interbreed.