Unit 6: Evolutionary Theory
Evolutionary Theory
Understanding Evolution
Evolution: Change in a population over time.
Mechanism of Evolution: Natural selection is the primary process by which evolution occurs.
The World Before Darwin
Publication Context: Charles Darwin published "On the Origin of Species" in 1859, challenging the fundamental beliefs of Western culture regarding the age of the Earth and the origins of life.
Historical Belief: It was commonly believed that:
The Earth was only a few thousand years old.
Living organisms were created during a divine 7-day creation period.
Jean Baptiste Lamarck's Contributions
Lamarck's Theory: Proposed a model of evolution comprised of three key parts:
All organisms strive to survive.
The striving leads to improvements in certain body parts more than others.
Modifications acquired during an organism's life are inherited by offspring.
Examples:
The strength of a blacksmith's arms resulting from his work.
Giraffes developing longer necks over generations.
Charles Darwin: The Individual
Biography:
Born in England, 1809.
Dropped out of medical school and enrolled in divinity school.
Sailed on the H.M.S. Beagle to South America in 1831 to chart the coastline.
Conducted extensive observations of the diverse flora and fauna of South America, which contrasted greatly with European species.
Observations in the Galapagos Islands
Noticed that plants and animals on the Galapagos Islands were similar to those in South America but exhibited distinct variations.
Example: Finches on different islands varied in beak shape according to food availability.
Darwin’s Theory of Evolution by Natural Selection
Core Tenets:
Variations exist within populations.
Some variations provide advantages (i.e., favorable traits).
More offspring are produced than can survive each year.
Individuals with favorable traits have greater survival rates, leading to "survival of the fittest."
Over long periods, advantageous traits become more prevalent and disadvantageous traits diminish.
Evidence for Evolution
Various scientific evidences supporting evolution include:
Fossil Record
Biogeography
Comparative Anatomy
Comparative Embryology
Molecular Biology
Fossil Record
Definition: Fossils are preserved remnants or impressions of organisms from the past.
Formation Process: Organisms are compressed by sediments, preserving hard parts such as teeth and bones.
Historical Context:
The fossil record contradicts the idea that all species were created simultaneously; it shows increasing complexity over time.
Georges Cuvier: Founder of paleontology, noted that rock strata contain distinct fossil species.
Dating Methods:
Relative Dating: Dating fossils based on their position in layers of rock (strata).
Absolute Dating: Utilizes radioactive isotopes to determine ages, e.g., Carbon-14 for young fossils (with a half-life of approximately 5,600 years).
Biogeography
Overview: Biogeography is the study of the distribution of species across geographical areas.
Example: Species endemic to islands are closely related to those on nearby mainland or other islands.
Comparative Anatomy
Key Concept: Similar anatomical structures can indicate common ancestry.
Terminology:
Homology: Structural similarities due to shared ancestry.
Homologous Structures: Traits that demonstrate anatomical evolution from a common ancestor.
Vestigial Structures: Non-functional remnants of organs (e.g., human appendix).
Comparative Embryology
Definition: Closely related organisms exhibit similar stages during embryonic development.
Principle: "Ontogeny recapitulates phylogeny"—the development of an organism reflects its evolutionary history.
Molecular Biology
Findings: DNA sequences show significant similarity across diverse life forms.
A common genetic code exists across species (e.g., UUU codes for phenylalanine in both humans and bacteria).
Conservation of certain proteins exists even among vastly different organisms like humans and bacteria.
Phylogeny and Systematics
Phylogeny: Represents the evolutionary history of a species or group of species.
Systematics: An analytical approach for understanding relationships among organisms based on evolutionary criteria.
Molecular Systematics: Compares molecular data (DNA/RNA sequences) to infer evolutionary relationships.
The Tree of Life
Diagrammatic Representation: Phylogenetic trees visually represent the hypotheses regarding the evolutionary relationships among organisms.
Nature of Phylogenetic Trees: These trees are hypotheses and may need revision as new data becomes available.
Morphological and Molecular Homologies
Organisms that exhibit similar morphological traits or DNA sequences are likely to be closely related through their evolutionary history.
Convergent Evolution
Definition: Occurs when organisms from different evolutionary lineages develop similar adaptations in response to similar environmental pressures.
Example: Bats and birds both evolved wings; these structures are homologous in mammals and analogous in birds, indicating separate evolutionary paths despite similar functions.
Homologous vs. Analogous Structures
Homologous Structures: Similar anatomical features derived from a common ancestor (e.g., forelimbs of mammals).
Analogous Structures: Features that serve similar functions but evolved independently (e.g., wings of bats and birds).
Taxonomy
Definition: The ordered classification and division of organisms into categories based on shared characteristics.
Historical Context: Carolus Linnaeus is credited as the pioneer of this classification system in the 18th century.
Critique: Modern insights propose that his system primarily reflects morphological similarities rather than true evolutionary kinship.
Linnean Classification
Binomial Nomenclature: Naming system consisting of two parts, genus and species.
Hierarchical Classification: Taxonomic groups are arranged from broad to specific categories (e.g., Kingdom - Phylum - Class - Order - Family - Genus - Species).
Phylogenetic Trees
Definition: Diagrams that hypothesize evolutionary relationships among species based on shared traits.
Note: Serve as working hypotheses, subject to change with new findings.
Cladograms
Function: Illustrate patterns of shared traits to infer evolutionary relationships, distinguishing between homologous and analogous characteristics only.
Population Concepts
Population: A localized group of organisms of the same species.
Gene Pool: Consists of all genetic information within a population.
Bottleneck Effect
Description: Occurs when a significant reduction in population size results in a population less representative of the original group.
Implication: Some alleles may be lost, affecting genetic diversity.
Directional Selection
Definition: A type of natural selection that shifts the phenotypic traits in a specific direction.
Example: The average size of black bears in Europe increases during ice ages and decreases during warmer periods.
Stabilizing Selection
Concept: Favors the intermediate variants over extreme phenotypes, maintaining the status quo.
Example: Average human birth weight ranges between 3-4 kg.
Prezygotic Barriers
Forms of reproductive isolation preventing mating:
Habitat Isolation: Species live in different habitats.
Behavioral Isolation: Species-specific courtship rituals.
Allopatric Speciation
Definition: Speciation due to geographical separation.
Example: Different squirrel species on opposite rims of the Grand Canyon.
Adaptive Radiation
Explanation: This is the rapid diversification of species from a common ancestor, often observed in isolated environments like island chains (e.g., Darwin’s finches).
Sympatric Speciation
Occurs when new species arise within the geographical range of the parent population.
Mechanism: Reproductive isolation evolves without geographical separation.
Tempo of Speciation
Two main views of how speciation occurs:
Gradualism: Slow and steady divergence from a common ancestor.
Punctuated Equilibrium: Rapid changes occur during speciation followed by long periods of stability.
Microevolution
Definition: The generation-to-generation change in allele frequencies within a population.
Even minor changes in allele frequencies signify microevolution; for example, if one allele in a flower population changes.
Causes of Microevolution
Four main drivers:
Genetic Drift
Natural Selection
Gene Flow
Mutation
Genetic Drift
Explanation: Random changes in allele frequencies, particularly in small populations. Two types include:
Bottleneck effect: Major reduction in population size.
Founder effect: Small number of individuals establish a new population, possibly unrepresentative of the original.
Natural Selection
Definition: Individuals with variations better suited to their environment tend to produce more offspring, enhancing the gene pool.
Darwinian Fitness: Refers to an individual’s relative contribution to the next generation’s gene pool, encapsulated in the phrase "survival of the fittest."
Types of Natural Selection
Three predominant types:
Directional Selection
Diversifying/Disruptive Selection: Prioritizes extreme variants over intermediate individuals.
Stabilizing Selection
Gene Flow
Definition: The movement of alleles among populations through the migration of individuals or gametes.
Mutation
Explanation: Alterations in DNA sequences; though rare at specific loci, mutations across multiple loci can significantly influence genetic variation.
Genetic Variation
Integral to natural selection; reflected in:
Quantitative characters: Variations existing along a continuum (e.g., height).
Discrete characters: Traits governed by single genes (e.g., blood group types).
Preserving Genetic Variation
Mechanisms preventing reduction of genetic diversity against natural selection include:
Diploidy: Hides recessive alleles in heterozygotes.
Balanced polymorphism: Maintains both dominant and recessive alleles due to environmental advantages.
Frequency-dependent selection: The fitness of any variant decreases as it becomes more common.
Speciation
Description: The formation of new species from a parent population.
Species vs. Group of Species: Defined as populations that can interbreed to form viable offspring under natural conditions.
Reproductive Barriers
Types:
Prezygotic Barriers: Prevent mating.
Temporal Isolation: Breeding at different times.
Mechanical Isolation: Physical incompatibility.
Gametic Isolation: Incompatibility of sperm and egg.
Postzygotic Barriers: Prevent hybrids from developing into viable offspring.
Reduced Hybrid Viability: Hybrids die early in development.
Reduced Hybrid Fertility: Hybrids are sterile.
Hybrid Breakdown: Successive generations become sterile.
Modes of Speciation
Allopatric Speciation: Occurs when a population forms into separate species due to geographical separation.
Sympatric Speciation: Evolution of species from a subpopulation that becomes reproductively isolated within the same area as the parent population due to genetic changes.