Evolution and Natural Selection

Lamarck’s Hypothesis of Evolution

Lamarck proposed that organisms evolve through the inheritance of acquired characteristics. He suggested that traits developed during an organism’s lifetime could be passed to offspring.

• Key Point: Use and disuse of organs leads to changes; these changes are inherited.

• Example: Giraffes stretching their necks to reach leaves, resulting in longer necks in offspring.

• Additional info: Modern genetics disproves inheritance of acquired traits; genetic changes must occur in gametes.

Darwin’s Hypothesis of Evolution

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

• Key Point: Variation exists within populations; some traits confer advantages.

• Key Point: Advantageous traits increase in frequency due to differential survival and reproduction.

• Example: Finches on the Galápagos Islands with beak shapes suited to available food sources.

Adaptations and Natural Selection

Adaptations are inherited traits that improve an organism’s ability to survive and reproduce in a specific environment. Natural selection is the process that leads to adaptation.

• Key Point: Adaptations arise from genetic variation and selective pressures.

• Example: Camouflage in peppered moths during the Industrial Revolution.

Principles and Features of Natural Selection

Natural selection operates based on several principles:

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

• Inheritance: Traits are passed from parents to offspring.

• Differential Survival: Some individuals are more likely to survive and reproduce.

• Key Features: Natural selection acts on phenotypes, not genotypes; it is not goal-directed.

Evidence for Natural Selection

• Fossil Record: Shows gradual changes in species over time.

• Homologous Structures: Similar anatomy in different species due to common ancestry.

• Example: Antibiotic resistance in bacteria.

Convergent Evolution, Microevolution, and Macroevolution

Convergent evolution occurs when unrelated species evolve similar traits due to similar environmental pressures. Microevolution refers to changes within a population, while macroevolution involves changes that lead to new species or higher taxonomic groups.

• Convergent Evolution Example: Wings in bats and birds.

• Microevolution Example: Changes in allele frequency in a population.

• Macroevolution Example: Origin of mammals from reptilian ancestors.

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.

• Key Point: Reproductive isolation is essential for maintaining species boundaries.

Prezygotic and Postzygotic Barriers

Barriers to reproduction prevent gene flow between species.

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

• Postzygotic Barriers: Prevent development of viable, fertile offspring (e.g., hybrid inviability, hybrid sterility, hybrid breakdown).

• Example: Mules (hybrid sterility) from horse and donkey parents.

Limitations of the Biological Species Concept

• Key Point: Cannot be applied to asexual organisms or fossils.

• Additional info: Other species concepts include morphological and ecological species concepts.

Allopatric and Sympatric Speciation

Speciation can occur in different ways:

• Allopatric Speciation: Occurs when populations are geographically separated.

• Sympatric Speciation: Occurs without geographic separation, often due to polyploidy, habitat differentiation, or sexual selection.

• Example: Allopatric: Squirrels on opposite sides of the Grand Canyon; Sympatric: Polyploidy in plants.

Taxonomy and Phylogenetics

Linnaeus Classification System

Linnaeus developed a hierarchical system for classifying organisms based on shared characteristics.

• Key Point: Levels: Kingdom, Phylum, Class, Order, Family, Genus, Species.

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

Systematics and Taxonomy

Systematics is the study of biological diversity and relationships among organisms. Taxonomy is the science of naming and classifying organisms.

• Drawbacks: Traditional taxonomy may not reflect evolutionary relationships.

Phylogenetics and Cladistics

Phylogenetics uses evolutionary trees to depict relationships. Cladistics groups organisms by common ancestry.

• Parsimony: The simplest explanation is preferred.

• Reading a Phylogenetic Tree: Branch points represent common ancestors; tips represent current species.

• Terminologies: Homology (shared ancestry), Analogy (convergent traits), Clade (group with common ancestor).

Classification of Groups

Prokaryotes

Prokaryotic vs. Eukaryotic Cells

Prokaryotes lack a nucleus and membrane-bound organelles, while eukaryotes have both.

• Prokaryotes: Bacteria and Archaea

• Eukaryotes: Plants, animals, fungi, protists

Structure of a Prokaryotic Cell

• Cell Wall: Provides shape and protection

• Plasma Membrane: Controls entry and exit of substances

• Nucleoid: Region containing DNA

• Ribosomes: Protein synthesis

• Additional info: Some have flagella for movement, pili for attachment

Genetic Diversity in Prokaryotes

Prokaryotes achieve genetic diversity through several mechanisms:

• Conjugation: Transfer of DNA via direct contact

• Transformation: Uptake of DNA from environment

• Transduction: Transfer of DNA by bacteriophages

• F Plasmids: Fertility plasmids involved in conjugation

• R Plasmids: Resistance plasmids carrying antibiotic resistance genes

Binary Fission

Prokaryotes reproduce by binary fission, a simple cell division process.

• Equation: Population doubles each generation:

Energy Sources and Metabolic Cooperation

• Energy Sources: Phototrophs (light), Chemotrophs (chemicals)

• Metabolic Cooperation: Prokaryotes may cooperate, e.g., biofilms, nitrogen fixation in cyanobacteria

• Example: Anabaena forms filaments with specialized cells for nitrogen fixation

Archaean Diversity and Importance

• Archaea: Diverse group, includes extremophiles (e.g., thermophiles, halophiles)

• Importance: Prokaryotes play key roles in nutrient cycles, especially nitrogen cycle

Protists

Primary and Secondary Endosymbiosis

Endosymbiosis is the process by which one cell engulfs another, leading to organelles like mitochondria and chloroplasts.

• Primary Endosymbiosis: Eukaryote engulfs prokaryote (origin of mitochondria, chloroplasts)

• Secondary Endosymbiosis: Eukaryote engulfs another eukaryote with organelles

• Evidence: Multiple membranes around chloroplasts, DNA similarities

Four Supergroups of Eukaryotes

Eukaryotes are classified into four supergroups based on molecular and structural characteristics.

Additional info: These supergroups reflect evolutionary relationships and diversity among eukaryotes.