Theory of Evolution

Overview of Evolution

The theory of evolution is a central concept in biology, proposing that all living organisms on Earth are related and have descended from a common ancestor. Over approximately 3 billion years, life has diversified into nearly 2 million different forms. The diversity of life is explained by evolutionary processes that result in gradual changes in populations over generations.

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

• Modification: Changes in phenotype arise from 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 scientific evidence support the theory of evolution. These include:

• Fossils and the fossil record

• Comparative anatomy, embryology, and biochemistry

• Biogeography

Fossil Evidence

Fossils are preserved remains or traces of organisms, typically found in sedimentary rock. Fossilization occurs when an organism is rapidly buried by sediment, ash, or soil, and over time, the hard elements become mineralized. Fossils provide a record of past life and allow scientists to observe changes in organisms over time.

• Fossils are most commonly found for organisms with hard tissues (bones, teeth).

• The fossil record is incomplete but remains the richest source of evolutionary information.

• Fossil age is determined by radiometric dating (e.g., potassium or carbon-14 dating).

Comparative Anatomy

Comparing anatomical structures among organisms reveals evolutionary relationships. Structures are classified as:

• Homologous structures: Body parts that share a common ancestry but may serve different functions (e.g., human arm, whale flipper).

• Analogous structures: Body parts that serve similar functions but do not share a common ancestry (e.g., bird wing and insect wing).

• Vestigial structures: Structures with little or no function, remnants of ancestral features (e.g., human tailbone, appendix).

Comparative Embryology

Embryological development is remarkably similar among vertebrates, indicating common ancestry. Early embryos of different species share features such as:

• Notochord: Becomes the core of intervertebral discs.

• Somites: Folds that develop into bone, muscle, and skin.

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

Comparative Biochemistry

Biochemical comparisons examine similarities in proteins and genes. Closely related species have more similar molecules ("homologous molecules"). For example, the protein cytochrome c is nearly identical in humans and chimpanzees, with increasing differences in more distantly related species. The greater the molecular difference, the more distant the common ancestor.

• Example: Cytochrome c amino acid differences—0 between humans and chimps, 1 in rhesus monkeys, 16 in chickens, 50 in yeast.

Biogeography

Biogeography is the study of the geographic distribution of species. Physical barriers (oceans, mountains) and environmental conditions (temperature, humidity) influence where species can live and migrate. Continental drift (movement of tectonic plates) has separated populations, leading to independent evolutionary paths.

• Pangea: Supercontinent that existed 200 million years ago; its breakup isolated groups, leading to parallel evolution.

Mechanisms of Evolution

Mutations

Mutations are random, rare changes in DNA that can be inherited. They may result from replication errors or epigenetic changes. Over long periods, accumulated mutations can lead to the divergence of species.

Natural Selection

Proposed by Charles Darwin, natural selection is the process by which individuals with traits better suited to their environment are more likely to survive and reproduce. Beneficial alleles increase in frequency, while harmful alleles decrease. Natural selection, combined with mutation, drives evolutionary change.

Genetic Drift

Genetic drift refers to random changes in allele frequencies, especially in small populations. Two main types are:

• Bottleneck effect: A catastrophe drastically reduces population size, leaving a non-representative gene pool.

• Founder effect: A few individuals establish a new population, which may not reflect the genetic diversity of the original group.

Gene Flow

Gene flow is the movement of alleles between populations due to immigration or emigration, resulting in genetic mixing.

Antigenic Shift

In viruses, antigenic shift is a rapid, dramatic change caused by the combination or exchange of genetic material between viruses, producing new, potentially more infectious strains (e.g., bird flu, swine flu).

Extinction

Extinction occurs when a species dies out completely. There have been at least five mass extinctions in the past 530 million years, often due to environmental changes. The most recent mass extinction occurred at the end of the Cretaceous period, likely caused by an asteroid impact. Human activity is now contributing to a possible sixth mass extinction.

Evolutionary Trees and Adaptive Radiation

Evolutionary trees (phylogenies) depict points of divergence between species, showing evolutionary relationships, extinction events, and adaptive radiation—where new species rapidly evolve from a common ancestor.

Key Terms and Concepts

• Descent with modification: The process by which descendants of ancestral organisms spread into various habitats and accumulate adaptations.

• Homologous structures: Anatomical features inherited from a common ancestor.

• Analogous structures: Features with similar functions but different evolutionary origins.

• Vestigial structures: Reduced or nonfunctional features inherited from ancestors.

• Gene pool: The total collection of genes in a population at any one time.

• Allele frequency: The proportion of a specific allele among all alleles in a population.

Summary Table: Mechanisms of Evolution