Theory of Evolution

Overview of Evolution

The theory of evolution is a fundamental concept in biology, explaining the diversity of life on Earth. It posits that all living organisms are related and descended from a common ancestor that existed approximately 3 billion years ago. Evolution is responsible for the nearly 2 million different forms of life observed today.

• Descent over time: Populations change slowly over generations, becoming different from their ancestors.

• Modification: Changes in phenotype occur due to genetic mutations or gene reshuffling during meiosis.

• Unpredictable and natural: Evolution is influenced by chance, natural selection, historical events, and environmental changes.

Evidence for Evolution

Types of Scientific Evidence

Multiple lines of evidence support the theory of evolution, including:

• Fossils & fossil record

• Comparative anatomy, embryology & biochemistry

• Biogeography

Fossil Evidence

Formation and Importance of Fossils

Fossils are preserved remains of organisms, typically formed when remnants are quickly covered by sediment or volcanic ash. Over time, these remains become mineralized, leaving rock-like impressions of hard tissues such as bones and teeth. Fossils provide a rich, though incomplete, record of past life and allow scientists to observe changes in organisms over time.

• Fossilization process: Hard elements of organisms become mineralized by the same minerals that comprise rocks.

• Age determination: Fossils are dated using radiometric methods (e.g., potassium or carbon-14 dating).

• Limitations: Fossil record is incomplete, especially for organisms without hard tissues.

Occasionally, erosion, uplifting of the earth's crust, or human excavation exposes fossils to the surface, allowing scientists to study them.

Comparative Anatomy

Homologous, Analogous, and Vestigial Structures

Comparing anatomical structures among organisms reveals evolutionary relationships:

• Homologous structures: Body parts that share a common ancestor, such as the human arm, dog leg, whale flipper, and bird wing. Their resemblance indicates evolutionary relatedness.

• Analogous structures: Structures that serve similar functions but do not arise from a common ancestor (e.g., bird and insect wings).

• Vestigial structures: Structures with little or no function, often homologous to functional body parts in other organisms (e.g., human tailbone, appendix, ear muscles, wisdom teeth).

Comparative Embryology

Embryonic Development Among Vertebrates

Early embryonic development is remarkably similar among vertebrates, indicating common ancestry. All vertebrates develop:

• Notochord: Becomes the core of intervertebral discs.

• Somites: Series of folds that become bone, muscle, and skin.

• Gill or pharyngeal arches: Develop into gills or parts of the face, middle ear, and mouth.

Comparative Biochemistry

Protein and Gene Similarities

Biochemical comparisons examine similarities between proteins and genes of different species. Identical or nearly identical molecules ("homologous molecules") suggest common ancestry. For example, the protein cytochrome c is identical in humans and chimps, with increasing differences in more distantly related species.

• Example: Cytochrome c amino acid differences: humans and chimps (0), rhesus monkey (1), chicken (16), yeast (50).

• The greater the difference, the further back the divergence occurred.

Biogeography

Distribution of Species

Biogeography studies the distribution of plants and animals worldwide. Physical barriers (oceans, mountains) and environmental conditions (temperature extremes) influence species migration and survival.

• Continental drift: The breakup of Pangea isolated groups, leading to separate evolutionary paths.

Mechanisms of Evolution

Mutations, Natural Selection, Genetic Drift, Gene Flow

Evolution occurs through several processes:

• Mutations: Random, rare changes in DNA, often due to replication errors or epigenetics. Accumulation of mutations can lead to speciation.

• Natural selection: Individuals with traits better suited to their environment are more likely to survive and reproduce, increasing the frequency of beneficial alleles.

• Genetic drift: Random changes in allele frequency, especially in small populations. Includes bottleneck and founder effects.

• Gene flow: Redistribution of alleles due to immigration or emigration, mixing gene pools.

Genetic Drift: Bottleneck and Founder Effects

• Bottleneck effect: Catastrophic events reduce population size, decreasing genetic variability.

• Founder effect: A few individuals establish a new population, which may not represent the original gene pool.

Other Factors

• Antigenic shift: Rapid genetic changes in viruses due to recombination, leading to new, potentially more infective strains (e.g., bird flu, swine flu).

• Extinction: Complete disappearance of species, often due to environmental changes. At least five mass extinctions have occurred, with the most recent at the end of the Cretaceous period.

Evolutionary Trees and Adaptive Radiation

Depicting Divergence and Speciation

Evolutionary trees illustrate points of divergence, extinction, and adaptive radiation. Adaptive radiation refers to the rapid development of new species from a single ancestor, shown as branching patterns on evolutionary trees.

Key Terms

• Mutation: Change in DNA sequence

• Natural selection: Differential survival and reproduction

• Genetic drift: Random changes in allele frequency

• Gene flow: Movement of alleles between populations

• Extinction: Complete disappearance of a species

• Adaptive radiation: Rapid speciation from a common ancestor

Summary Table: Types of Evidence for Evolution

Additional info: This guide expands on lecture notes by providing definitions, examples, and a summary table for exam preparation.