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

18591874

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