Evolutionary Mechanisms

Genetic Variation in Populations

Genetic variation is the foundation of evolution and is essential for populations to adapt to changing environments.

• Source of genetic variation: Includes mutations, gene flow, and sexual reproduction.

• Mutation: Random changes in DNA that introduce new alleles.

• Gene flow: Movement of alleles between populations through migration.

• Sexual reproduction: Shuffles alleles to produce new genetic combinations.

• Example: Sickle cell allele in human populations provides resistance to malaria.

Evidence for Common Origin of Life

Multiple lines of evidence support the idea that all life shares a common ancestor.

• Universal genetic code: All organisms use DNA and RNA to store and transmit genetic information.

• Homologous structures: Similar anatomical features in different species due to shared ancestry.

• Example: Forelimbs of mammals (human arm, whale flipper, bat wing).

Lamarck vs. Darwin: Theories of Evolution

Lamarck and Darwin proposed different mechanisms for evolutionary change.

• Lamarck: Proposed inheritance of acquired characteristics; organisms change during their lifetime and pass changes to offspring.

• Darwin: Proposed natural selection; variation exists, and advantageous traits increase in frequency over generations.

• Example: Giraffe neck length: Lamarck - stretching necks; Darwin - selection for longer necks.

Descent with Modification

Darwin's concept that species change over time, giving rise to new species while retaining some ancestral traits.

• Definition: Evolutionary process where descendants differ from ancestors.

• Example: Finches on the Galápagos Islands evolving different beak shapes.

Darwin's Observations and Natural Selection

Darwin observed variation, overproduction, competition, and differential survival, leading to his theory of natural selection.

• Natural selection: Process where individuals with advantageous traits survive and reproduce more successfully.

• Key observations: Variation exists; more offspring are produced than can survive; competition for resources; differential survival and reproduction.

Types of Selection

Selection can be classified based on its effect on trait distribution.

• Directional selection: Favors one extreme phenotype.

• Disruptive selection: Favors both extremes over intermediate phenotypes.

• Stabilizing selection: Favors intermediate phenotypes.

• Balancing selection: Maintains genetic diversity in a population.

Individual Organisms and Evolution

Evolution acts on populations, not individuals. Individuals do not evolve; populations do over generations.

Heritability and Natural Selection

Natural selection only acts on traits that are heritable (genetically passed from parents to offspring).

• Non-heritable traits: Traits acquired during an organism's lifetime are not subject to natural selection.

Homologous vs. Analogous Structures

Comparing anatomical features helps distinguish evolutionary relationships.

• Homologous structures: Similar due to shared ancestry (e.g., vertebrate limbs).

• Analogous structures: Similar due to convergent evolution, not common ancestry (e.g., wings of birds and insects).

Convergent Evolution

Convergent evolution occurs when unrelated species evolve similar traits independently.

• Example: Dolphins (mammals) and sharks (fish) both have streamlined bodies for swimming.

Acquired Characteristics

Traits gained during an organism's lifetime (not inherited genetically).

• Significance: Not passed to offspring; not a mechanism for evolution.

Population Genetics and Hardy-Weinberg Equilibrium

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle describes a non-evolving population where allele and genotype frequencies remain constant.

• Equation:

• Allele frequencies:

• Conditions: No mutation, random mating, no gene flow, infinite population size, no selection.

Microevolution Mechanisms

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

• Natural selection: Differential survival and reproduction.

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

• Gene flow: Movement of alleles between populations.

Founder Effect and Bottleneck Effect

Both are forms of genetic drift that reduce genetic diversity.

• Founder effect: Small group establishes a new population; allele frequencies may differ from original population.

• Bottleneck effect: Sudden reduction in population size due to environmental events; surviving population has reduced genetic diversity.

Population, Species, and Gene Pool

• Population: Group of individuals of the same species in a given area.

• Species: Group of organisms capable of interbreeding and producing fertile offspring.

• Gene pool: All alleles present in a population.

Random Fertilization and Hardy-Weinberg

Random fertilization ensures allele frequencies remain constant if Hardy-Weinberg conditions are met.

Calculating Allele and Genotype Frequencies

• Allele frequency: Proportion of a specific allele in the gene pool.

• Genotype frequency: Proportion of a specific genotype in the population.

• Example: For alleles p and q, genotype frequencies are (homozygous dominant), (heterozygous), (homozygous recessive).

Selection Types and Effects

• Directional selection: Shifts allele frequencies toward one extreme.

• Disruptive selection: Favors both extremes.

• Stabilizing selection: Favors intermediate phenotypes.

Balancing Selection

Maintains multiple alleles in a population.

• Heterozygote advantage: Heterozygotes have higher fitness than either homozygote (e.g., sickle cell trait).

• Frequency-dependent selection: Fitness depends on allele frequency.

Speciation and Macroevolution

Macroevolution

Large-scale evolutionary changes that result in new species or groups above the species level.

Species Concepts

• Biological species concept: Species are groups of interbreeding natural populations.

• Morphological species concept: Species defined by physical traits.

• Ecological species concept: Species defined by ecological niche.

• Phylogenetic species concept: Species defined by evolutionary history.

Speciation Mechanisms

• Allopatric speciation: Occurs when populations are geographically separated.

• Sympatric speciation: Occurs without geographic separation, often via polyploidy or habitat differentiation.

Reproductive Isolation

Prevents gene flow between species.

• Prezygotic barriers: Prevent mating or fertilization (e.g., habitat, temporal, behavioral isolation).

• Postzygotic barriers: Prevent viable, fertile offspring (e.g., hybrid sterility).

Hybrid Zones and Speciation Outcomes

• Hybrid zone: Region where different species meet and mate.

• Outcomes: Reinforcement (strengthening reproductive barriers), fusion (species merge), stability (hybrids persist).

Punctuated vs. Gradual Speciation

• Punctuated equilibrium: Rapid bursts of change followed by long periods of stability.

• Gradualism: Slow, steady accumulation of changes.

Adaptive Radiation and Evolution of Novel Traits

Adaptive Radiation

Rapid evolution of diversely adapted species from a common ancestor, often following environmental changes.

• Factors: Mass extinctions, novel environments, colonization of new areas.

• Example: Darwin's finches on the Galápagos Islands.

Evo-Devo, Heterochrony, and Paedomorphosis

• Evo-devo: Evolutionary developmental biology; studies how changes in development affect evolution.

• Heterochrony: Changes in timing or rate of developmental events.

• Paedomorphosis: Retention of juvenile traits in adult organisms.

Hox Genes and Novel Traits

Hox genes control body plan development; changes can lead to major morphological differences.

• Exaptation: Trait that evolves for one function and is co-opted for another.

Systematics and Phylogeny

Phylogeny and Systematics

Systematics is the study of evolutionary relationships; phylogeny is the evolutionary history of a group.

• Clade: Group of organisms with a common ancestor.

• Monophyletic group: Includes ancestor and all descendants.

• Paraphyletic group: Includes ancestor and some descendants.

• Polyphyletic group: Includes unrelated organisms.

Linnaean System and Taxonomy

• Binomial system: Two-part scientific names (Genus species).

• Taxon: Group of organisms at any taxonomic rank.

Branching and Evolutionary Relationships

• Branching: Represents speciation events in phylogenetic trees.

• Outgroup: Species outside the group of interest, used for comparison.

Homology and Gene Duplication

• Homologs: Genes or traits inherited from a common ancestor.

• Paralogs: Genes related by duplication within a genome.

• Orthologs: Genes in different species that evolved from a common ancestral gene.

• Gene duplication: Major source of genetic innovation and increase in genome size.

Table: Species Concepts Comparison

Table: Types of Selection

Table: Reproductive Isolation Mechanisms

Additional info: Some definitions and examples have been expanded for clarity and completeness.