Evolution and Natural Selection

Lamarck’s Hypothesis of Evolution

Lamarck proposed one of the earliest theories of evolution, suggesting that organisms change over time by acquiring traits during their lifetime and passing them to offspring.

• Inheritance of Acquired Characteristics: Lamarck believed that traits developed through use or disuse (e.g., giraffes stretching their necks) could be inherited by the next generation.

• Use and Disuse: Organs used frequently become stronger, while those not used deteriorate.

• Example: A blacksmith’s muscular arms would be passed to his children (now known to be incorrect).

Additional info: Modern genetics has disproven Lamarck’s mechanism, but he was important for proposing that species are not fixed.

Darwin’s Hypothesis of Evolution

Charles Darwin proposed that evolution occurs through natural selection, where heritable traits that enhance survival and reproduction become more common in populations over generations.

• Variation: Individuals in a population vary in their traits.

• Heritability: Some of these variations are heritable.

• Overproduction: More offspring are produced than can survive.

• Differential Survival and Reproduction: Individuals with advantageous traits survive and reproduce more successfully.

• Example: Darwin’s finches on the Galápagos Islands showed beak variations suited to different food sources.

Adaptations and Natural Selection

Adaptations are inherited characteristics that enhance an organism’s ability to survive and reproduce in a specific environment.

• Natural Selection: The process by which individuals with favorable adaptations are more likely to survive and reproduce.

• Key Features:

  • Variation exists within populations.

  • Traits are heritable.

  • There is differential reproductive success.

• Evidence: Examples include antibiotic resistance in bacteria and the peppered moth’s coloration during the Industrial Revolution.

Convergent Evolution, Microevolution, and Macroevolution

• Convergent Evolution: Unrelated species evolve similar traits due to similar environmental pressures (e.g., wings in bats and birds).

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

• Macroevolution: Large-scale evolutionary changes that result in the formation of new species or groups.

Speciation and Reproductive Isolation

Biological Species Concept

The biological species concept defines a species as a group of populations whose members can interbreed and produce viable, fertile offspring.

• Prezygotic Barriers: Prevent mating or fertilization between species.

  • Examples: Habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation.

• Postzygotic Barriers: Prevent hybrid offspring from developing into viable, fertile adults.

  • Examples: Reduced hybrid viability, reduced hybrid fertility (e.g., mule), hybrid breakdown.

• Limitations: Cannot be applied to asexual organisms or fossils; some species hybridize in nature.

Allopatric and Sympatric Speciation

• Allopatric Speciation: Occurs when populations are geographically separated, leading to divergence (e.g., squirrels on opposite sides of the Grand Canyon).

• Sympatric Speciation: Occurs without geographic separation, often through polyploidy, habitat differentiation, or sexual selection (e.g., cichlid fish in African lakes).

Taxonomy and Phylogenetics

Linnaean Classification System

Carl Linnaeus developed a hierarchical system for classifying organisms, using binomial nomenclature (Genus species).

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

• Drawbacks: Does not always reflect evolutionary relationships; based on physical similarities.

Systematics and Taxonomy

• Systematics: The scientific study of the diversity and relationships among organisms.

• Taxonomy: The science of naming, describing, and classifying organisms.

Phylogenetics and Cladistics

Phylogenetics studies evolutionary relationships using evolutionary trees (phylogenies). Cladistics groups organisms by common ancestry.

• Phylogenetic Tree: Diagram showing evolutionary relationships.

• Clade: A group consisting of an ancestor and all its descendants (monophyletic group).

• Monophyletic: Includes ancestor and all descendants.

• Paraphyletic: Includes ancestor and some, but not all, descendants.

• Polyphyletic: Includes unrelated organisms from different ancestors.

• Homology: Similarity due to shared ancestry (e.g., vertebrate forelimbs).

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

• Parsimony: The simplest explanation (fewest evolutionary changes) is preferred.

Prokaryotes

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; DNA in nucleoid region; generally smaller (e.g., bacteria, archaea).

• Eukaryotic Cells: Have a nucleus and membrane-bound organelles; generally larger (e.g., plants, animals, fungi, protists).

Structure of a Prokaryotic Cell

• Cell Wall: Provides structure and protection; made of peptidoglycan in bacteria.

• Plasma Membrane: Controls entry and exit of substances.

• Nucleoid: Region containing circular DNA.

• Ribosomes: Sites of protein synthesis.

• Flagella: For movement (in some prokaryotes).

Genetic Diversity in Prokaryotes

• Conjugation: Transfer of DNA between bacteria via a pilus.

• Transformation: Uptake of foreign DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses).

• F Plasmids: Carry genes for conjugation (fertility factor).

• R Plasmids: Carry antibiotic resistance genes.

• Binary Fission: Asexual reproduction by cell division.

Energy Sources and Metabolic Cooperation

• Energy Sources: Prokaryotes can be phototrophs (light), chemotrophs (chemicals), autotrophs (CO2), or heterotrophs (organic compounds).

• Metabolic Cooperation: Prokaryotes may cooperate for mutual benefit (e.g., cyanobacteria form filaments with specialized cells for nitrogen fixation).

Archaean Diversity and Importance of Prokaryotes

• Archaea: Diverse group of prokaryotes; many live in extreme environments (extremophiles).

• Importance: Prokaryotes play key roles in nutrient cycles, such as the nitrogen cycle (e.g., nitrogen fixation by bacteria).

Protists

Primary and Secondary Endosymbiosis

• Primary Endosymbiosis: Eukaryotic cell engulfs a prokaryote (e.g., origin of mitochondria and chloroplasts).

• Secondary Endosymbiosis: Eukaryotic cell engulfs another eukaryotic cell that already contains endosymbionts.

• Evidence: Double membranes, DNA similarities, and ribosomes in organelles.

Four Supergroups of Eukaryotes

Eukaryotes are classified into four supergroups, each with unique features and representative protists.

Additional info: These supergroups reflect evolutionary relationships based on molecular and morphological data.