How Populations Evolve

Historical Theories Anaximander (~2500 yrs ago) Aristotle Georges Buffon (1700’s) Jean Baptist Lemark (late 1700’s - early1800’s) Erasmus Darwin

Charles Darwin

Voyage of the HMS Beagle

On the Origin of Species… Descent With Modification By means of Natural Selection

Support for Descent with Modification Biogeography Fossil Record Molecular Biology, Biochemistry, Cell Biology Comparative Anatomy

Homology of Structures

More Support Embryology Molecular Biology

How Did Darwin Come Up With His Ideas? Scientific Method Key observations –All species tend to produce excessive amounts of offspring (Thomas Malthus) –Individuals in a population vary –Many of the varying traits are passed from 1 generation to the next

Recap Limited resources Overproduction of offspring Heritable individual variation –Therefore, survival depends partly on inherited features

Darwin’s Theory of Evolution In a varied population, individuals whose inherited characters best adapt them to the environment are more likely to survive and reproduce. Therefore, they tend to leave more offspring than less fit individuals. Natural Selection is mechanism –Reproduction (differential) is Key

Natural Selection

Examples of Natural Selection English Peppered Moths

Artificial Selection

Population Genetics Modern Synthesis Species concept –Species = a group of populations whose individuals have the potential to: Interbreed Produce fertile offspring

Micro-Evolution Changes in the numbers (frequency) of alleles in a population Gene pool –All alleles of each gene in all individuals Ex. Peppered Moth –Before pollution, light allele color dominant (higher frequency) –After change, dark allele dominant

Gene Frequencies

Hardy-Weinberg Equilibrium Frequency of alleles in a stable population will not change over time –Very large population –Population is isolated –Mutations don’t alter gene pool –Random mating –All individuals are equal in reproductive success In reality, this never happens

Agents of Change Genetic Drift –Bottle neck affect

Bottle Neck Affect

Agents of Change Genetic Drift –Bottle neck affect –Founder affect Gene Flow

Agents of Change Genetic Drift –Bottle neck affect –Founder affect Gene Flow Mutation

Agents of Change Genetic Drift –Bottle neck affect –Founder affect Gene Flow Mutation Non Random Mating Natural Selection

Variation Traits Morphs -Polymorphism Geographic variation -Clines Natural Selection acts on variation

Variation Traits Morphs -Polymorphism Geographic variation -Clines Natural Selection acts on variation

Polymorphism in Snakes

Types of Selection

Speciation Speciation = evolution of a new species –Isolation often leads to speciation Barriers between organisms or populations Division is often not precise –Line gets blurry –Where do you draw the line when populations overlap?

Isolation Geogrphic isolation = Physical barriers –Valley, mountain range, river, ocean, etc. Reproductive Barriers –Separated by when they happen Prezygotic barriers = before fertilization Post zygotic = after fertilization

Geographic Isolation

Prezygotic Barriers Temporal isolation Habitat isolation Behavioral isolation Mechanical isolation Gametic isolation

Booby courtship behavior

Postzygotic Barriers Hybrid inviability Hybrid sterility Hybrid breakdown

Horse Donkey Mule

Types of Speciation Allopatric speciation –Population becomes cut off from other populations Becomes an isolated gene pool Often the initial event is geographic event –Isolated population often evolves in different direction

Types of Speciation Adaptive radiation –Development of many new species from a common ancestor introduced into a new and diverse environment –Very common on islands Islands must be colonized from the outside Darwins finches Hawai’ian honeycreapers

Honeycreapers

Adaptive Radiation

Types of Speciation Sympatric speciation –Speciation within a continuous range or area –Enough genetic change on either end of a large population range –Accidents in meiosis Primarily in plants

Selection & Speciation

Evolutionary Theories