Evolution: UNIT 2

Topic 2: Evolution

Introduction

Evolution is a central concept in biology, describing the genetic changes in populations over generations. This topic covers Darwin's foundational observations, mechanisms of evolution, evidence supporting evolutionary theory, and the mathematical principles underlying population genetics.

Science in the Age of Charles Darwin

Historical Perspectives

• Aristotle: Proposed that species are fixed and do not evolve.

• Judeo-Christian culture: Believed each form of life was individually created in its present-day form; Earth was considered 6000 years old.

• Western world: Assumed all living species appeared recently and do not change.

Charles Darwin and the Voyage of the Beagle

Darwin's Observations

• Five-year voyage on the HMS Beagle collecting fossils, plants, and animals.

• Questioned what makes organisms well suited to their environment.

Observations of Geographic Proximity

Galapagos Islands

• Geographic proximity is a better predictor of relationships among organisms than environmental similarity.

• Galapagos animals resemble South American species but are unique to the islands.

• Examples: Marine iguanas resemble South American land iguanas; each island has its own variety of giant tortoise.

Geological Changes

Influence of Geology

• Charles Lyell: Principles of Geology proposed Earth was shaped by gradual processes over millions of years.

• Darwin observed an earthquake in Chile, suggesting both Earth and organisms change over time.

Descent with Modification

Key Concepts

• Present-day species descend from ancient ancestors, accumulating differences over time.

• Natural selection: Individuals with advantageous traits are more likely to survive and reproduce.

• Adaptations: Traits that improve an organism's fit to its environment.

Darwin’s Writings

On the Origin of Species

• Outlined the theory of evolution by natural selection based on observations in biology, geology, and paleontology.

• Evolution: Genetic changes in a population from generation to generation.

Fossils and Evidence for Evolution

Types of Fossils

• Imprints or remains: Bones, teeth, shells.

• Casts: Empty molds filled with minerals.

• Imprints: Footprints, burrows, coprolites (fossilized feces).

• Entire organisms: Encased in a medium preventing decomposition.

Strata and Fossil Record

• Strata: Layers of rock; younger strata on top of older strata.

• Paleontologist: Scientist who studies fossils.

• Fossil record: Chronicle of evolution over millions of years, though incomplete.

Transitional Forms

Examples

• Fossils linking different groups: Whales from land mammals, amphibians from fish, birds from dinosaurs, mammals from reptiles.

Homology

Structural and Molecular Homology

• Homology: Similarity due to common ancestry.

• Homologous structures: Anatomical variations adapted to different functions (e.g., forelimbs of humans, cats, whales, bats).

• Molecular biology: Study of genes and gene expression; homologous genes have closely matched sequences inherited from a recent common ancestor.

Developmental Homology

• Similarities in early development (e.g., pharyngeal pouches in embryos) support common ancestry.

Vestigial Structures and Pseudogenes

• Vestigial structures: Remnants of features that served important functions in ancestors (e.g., pelvis in whales, eye remnants in cave fish).

• Pseudogenes: Genes that have lost their function (e.g., GLO enzyme for vitamin C synthesis).

Evolutionary Trees and Homology

Phylogenetic Trees

• Branch points represent common ancestors; hatch marks indicate shared homologous characters.

• Used to trace evolutionary relationships among major groups (e.g., tetrapods, reptiles, birds).

Mechanisms of Evolution

Natural Selection

• Variation among individuals and heritability of traits.

• Struggle for existence due to limited resources.

• Accumulation of adaptations over time; natural selection is adaptive evolution.

Examples

• Beak size changes in Galapagos finches.

• Pesticide resistance in insects.

• Antibiotic resistance in bacteria.

Key Notes

• Populations evolve, not individuals.

• Only heritable traits are subject to evolution.

• Evolution does not produce perfectly adapted organisms.

• Natural selection edits existing variation; it is contingent on time and place.

Genetic Variation

Sources of Variation

• Alleles: Variations of a trait; dominance and recessiveness.

• Mutations: Produce new alleles; can be beneficial or detrimental.

• Sexual reproduction: Shuffles alleles, increasing variation.

• Evolution: Changes in allele frequency over generations.

Example

• DDT resistance in houseflies: Mutation confers resistance but reduces growth rate; selection increases frequency when DDT is present.

Populations and Gene Pool

Definitions

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

• Gene pool: All copies of every allele at every locus in all members of the population.

• Microevolution: Changes in allele frequency within a population.

Frequency Calculations

• Genotype frequency: $\text{Genotype frequency} = \frac{\text{Number of animals with the genotype}}{\text{Total number of animals}}$

• Allele frequency: $\text{Allele frequency} = \frac{\text{Number of alleles in gene pool}}{\text{Total number of alleles}}$

Hardy-Weinberg Equation

Principle and Application

• Determines if a population is evolving.

• Hardy-Weinberg equilibrium: Allele frequencies remain constant if certain conditions are met.

Equation

• $p^2 + 2pq + q^2 = 1$

• $p^2$ = frequency of homozygous dominants

• $2pq$ = frequency of heterozygotes

• $q^2$ = frequency of homozygous recessives

Conditions for Equilibrium

• Very large population

• No gene flow

• No mutations

• Random mating

• No natural selection

Example Calculation

• If 1 in 10,000 people have a recessive disease (PKU):

• $q^2 = 1/10,000 = 0.0001$

• $q = \sqrt{0.0001} = 0.01$

• $p = 1 - q = 0.99$

• Carriers (heterozygotes): $2pq = 2 \times 0.99 \times 0.01 = 0.0198$

• 1.98% of the population are carriers.

Genetic Drift

Definition and Examples

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

• Bottleneck effect: Catastrophes reduce population size, altering genetic makeup.

• Founder effect: Small group colonizes new habitat, gene pool differs from original population.

Example Table: Genetic Drift Effects

Effect	Description	Example
Bottleneck	Population size reduced by catastrophe	Surviving population has different allele frequencies
Founder	Small group colonizes new area	High frequency of inherited disorders in isolated populations

Gene Flow

Definition and Examples

• Movement of alleles between populations via migration or gamete transfer.

• Reduces genetic differences between populations.

• Examples: Caribou herds mixing, human migration.

Relative Fitness

Definition

• Relative fitness: Individual's contribution to the next generation's gene pool compared to others.

• Measured by number of viable, fertile offspring produced.

• "Survival of the fittest" involves both direct and indirect competition.

Types of Natural Selection

Selection Processes

• Directional selection: Favors individuals at one phenotypic extreme.

• Stabilizing selection: Favors intermediate phenotypes.

• Disruptive selection: Favors individuals at both extremes of the phenotypic range.

Sexual Selection

Mechanisms

• Sexual selection: Traits increase mating success.

• Sexual dimorphism: Differences in appearance between males and females.

• Intrasexual selection: Competition among same sex for mates (e.g., combat, displays).

• Intersexual selection: One sex chooses mates based on traits (e.g., size, ornamentation).