Evolution: Patterns and Processes

Definition and Overview

Evolution is the process by which species accumulate differences from their ancestors as they adapt to different environments over time. It involves changes in the genetic composition of populations from generation to generation.

• Evolutionary change can be observed as both a pattern (the history of life) and a process (the mechanisms that drive change).

• Descent with modification is Darwin's phrase summarizing evolution, indicating that species change over time and give rise to new species.

Key Observations About Life

• Unity of life: Shared characteristics among organisms.

• Diversity of life: Wide variety of forms and functions.

• Fit between organisms and environment: Adaptations that suit organisms to their habitats.

Classification and Taxonomy

Organisms are classified into hierarchical categories based on shared characteristics.

• Taxonomic ranks: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Binomial nomenclature: Each species has a two-part scientific name (Genus species).

• Carolus Linnaeus: Developed the modern system of taxonomy.

Theories of Evolution

• Catastrophism: Many species were created originally; disasters caused extinctions, and fossils are remnants.

• Lamarck's hypothesis: Species evolve through use and disuse of body parts and inheritance of acquired characteristics. Additional info: This theory is not supported by modern genetics.

• Darwin's theory: Natural selection acts on individuals, favoring those with advantageous traits, leading to adaptation and speciation.

Mechanisms of Evolution

Mutation and Genetic Variation

Genetic variation is the raw material for evolution, arising from mutations, gene duplication, and other processes.

• Mutation: Changes in DNA sequence; can be point mutations (single nucleotide changes).

• Genetic variation: Differences in alleles and genes among individuals.

• Phenotypic variation: Observable differences in traits, often reflecting underlying genetic variation.

• Mutation rates: Generally low in animals and plants, higher in prokaryotes.

Natural Selection

Natural selection is the process by which individuals with favorable inherited traits are more likely to survive and reproduce.

• Adaptive evolution: Increase in frequency of advantageous traits over time.

• Artificial selection: Humans selectively breed organisms for desirable traits.

Genetic Drift and Gene Flow

• Genetic drift: Random fluctuations in allele frequencies, especially in small populations.

• Founder effect: Small group becomes isolated, leading to different allele frequencies than the original population.

• Bottleneck effect: Sudden reduction in population size due to environmental events, reducing genetic diversity.

• Gene flow: Movement of alleles between populations, reducing genetic differences among them.

Evidence for Evolution

Direct Observation

• Natural selection in response to introduced species.

• Evolution of drug-resistant bacteria.

Homology and Analogy

• Homologous structures: Features with the same evolutionary origin, even if their function differs.

• Vestigial structures: Inherited from ancestors but have lost their original function.

• Comparative embryology: Reveals anatomical homologies not visible in adult organisms.

• Convergent evolution: Evolution of analogous features in distantly related groups.

• Analogous structures: Similar function but different evolutionary origin.

Fossil Record

• Fossils: Document patterns of evolution, origins of new groups, and important transitions.

• Endemic species: Species found only in one specific location.

Systematics and Phylogeny

Phylogeny

Phylogeny is the evolutionary history of a species or group of related species.

• Systematics: Discipline that classifies organisms and determines evolutionary relationships.

• Phylogenetic tree: Diagram showing evolutionary relationships; each branch point represents divergence from a common ancestor.

• Sister taxa: Groups sharing an immediate common ancestor.

• Basal taxon: Diverged early from all other species.

Microevolution

Definition and Processes

Microevolution refers to changes in allele frequencies within a population over time.

• Mutation: Source of genetic variation.

• Natural selection: Adaptive changes in populations.

• Genetic drift: Random changes in allele frequencies.

• Gene flow: Exchange of genes between populations.

Animal Diversity and Evolution

Origins and Classification

Animals likely evolved from single-celled eukaryotes similar to present-day choanoflagellates. Over 1.3 million animal species have been named.

• Sponges and cnidarians: Diverged early; sponges lack true tissues.

• Amoebocytes: Cells in sponges that carry nutrients and waste.

• Bilaterians: Animals with bilateral symmetry and a complete digestive tract.

Germ Layers and Body Plans

• Diploblasts: Two germ layers (ectoderm and endoderm).

• Triploblasts: Three germ layers (ectoderm, mesoderm, endoderm).

• Body cavity: Presence or absence of a coelom (body cavity).

Major Animal Groups

• Invertebrates: Includes lophotrochozoa, ecdysozoa, and some deuterostomes.

• Vertebrates: Formed around 500 million years ago; first lineage was jawless.

• Gnathostomes: Jawed vertebrates.

• Dipnoi: Lungfish with both gills and lungs, bridging aquatic and terrestrial animals.

Adaptations for Life on Land

• Higher oxygen levels in the atmosphere compared to aquatic environments.

• New food sources and fewer competitors.

• Challenges: Scarcity of water, temperature fluctuations, lack of support against gravity.

• Arthropods: First animals to colonize land (~400 million years ago).

Evolution of Flight and Terrestrial Vertebrates

• Wings: Extension of the cuticle in insects; did not lose legs when wings evolved.

• Evolution of limbs and feet: Key event in tetrapod colonization of land.

• Tiktaalik: Transitional fossil bridging aquatic and terrestrial vertebrates.

• First tetrapods: Appeared about 365 million years ago.

Amphibians and Reptiles

• Amphibians: Three categories: salamanders (tailed), frogs (tailless), caecilians (legless).

• Double life: Many amphibians start in water and transition to land.

• Reptiles: Lived around 310 million years ago; most are ectothermic (regulate body temperature using external heat).

• Birds: Endothermic (regulate body temperature via metabolism); evolution of flight occurred about 160 million years ago.

Summary Table: Key Evolutionary Concepts

Key Equations

• Hardy-Weinberg Equation:

• Allele Frequency:

Additional info: These equations are used to calculate genetic variation in populations.