How Populations Evolve
Historical Theories
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Anaximander (~2500 yrs ago)
Aristotle
Georges Buffon (1700’s)
Jean Baptist Lemark (late 1700’s early1800’s)
• Erasmus Darwin
Charles Darwin
1874
1859
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
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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
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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
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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