Lecture 3: Natural Selection
EEES 3050
Review
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Darwin’s Five Theories
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1 – The non-constancy of species
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The idea of a changing world can be considered the fact
of evolution.
2 – The descent of all organisms from common
ancestors
3 – Gradualism (no saltations, no discontinuities)
4 – Speciation by populations
5 – Natural selection

This is the “theory” of the process of evolution.
2
Review

Evidence
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Fossil Record, Biogeography, Morphology, Vestigial
Structures, Molecular Evidence
Evolutionary Ecology:
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World changes and so do the phenomena studied in
ecology
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Sex ratios, feeding preference, life cycles
Human impacts
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Antibiotic resistance, epidemiology, conservation biology
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Darwin’s Model of Natural Selection:
5 facts, 3 inferences
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Fact 1: Every population has such high
fertility that its size would increase
exponentially if not constrained. (Malthus)
Fact 2: The size of populations, except for
temporary annual fluctuations, remains stable
over time. (Natural History)
Fact 3: The resources available to every
species are limited. (Malthus)
 Inference 1: There is intense competition
among the members of a species.
4
Darwin’s Model – 5 facts, 3 inferences
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Fact 4: No two individuals of a population are
exactly the same. (Animal breeders and
taxonomists)
 Inference 2: Individuals of a population differ from
each other in the probability of survival. (Darwin)
Fact 5: Many of the differences among the
individuals of a population are, at least in part,
heritable. (Animal breeders)
 Inference 3: Natural selection, continued over
many generations results in evolution. (Darwin)
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Two criticisms of natural selection

Selection is a process of chance.
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Selection is deterministic.
Natural selection is both!
The 2 steps of Natural Selection
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Random production of variation
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Mutation
Recombination
Random mate choice
Non-random aspects of survival and
reproduction
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Superior success of certain phenotypes
Non-random mate choice.
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Natural Selection

What drives selection?
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Usually an emphasis on survival or struggle for existence.
Natural selection is actually a process of elimination
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“Survival of the fittest” – coined by Sociologist Herbert
Spencer.
What else drives selection?
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All factors leading to an increase in reproductive success.
Sexual Selection or “Selection for reproductive success”
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Fitness

What is fitness?
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“…a measure of the contribution of an individual to
future generations and can also be called adaptive
value”.
A relative term
Not absolute
Cannot compare across species
Not only reproductive success
Not a short-term measure
Not about individual traits.
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Three types of selection
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Directional Selection
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Stabilizing Selection
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Disruptive Selection
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Lower rate
of survival
Lower rate
of survival
12
Lower rate
of survival
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Types of Natural Selection
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Directional selection
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Stabilizing Selection
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Industrial Melanism
Galapagos ground finch.
Other cases of anthropogenic selection
Infant Mortality
Hatch date in lesser snow geese
Clutch size in birds
Disruptive Selection
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Black-bellied seed crackers in Africa
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Species has two sizes of beak – no intermediates.
Planned Tangent
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Throughout this semester, there will be a
common framework of how to think about
conducting ecology.
I will ask (again and again and again)
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What are the stated observations or theory in
question?
Develop an hypothesis based on the
observations?
How would you develop an experiment to test
your hypothesis?
Summarize the reported results.
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Industrial Melanism
Background:
The species: Peppered Moth (Biston betularia)
The place: Industrial Europe (also USA)
The time: last 100 years
Scenario: adults rest on tree trunks
natural state: light colored, lichen covered trunks
industrial state: soot-darkened trunks
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Observation:
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proportions of light:dark moths
natural state: mostly light colored (>90%)
industrial state: increasingly dark (>90%)
Industrial Melanism
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Natural state: light colored, left
Industrial state: dark colored, right
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Observation: proportions of light:dark moths
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natural state: mostly light colored (>90%)
industrial state: increasingly dark (>90%)
Industrial Melanism
Hypothesis:
Birds eat moths they can see.
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birds
are visual predators
Experiment:
release marked adults of both color
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Results
rural: more light adults survived
urban: more dark adults survived
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Visual verification of bird predation
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Footnote
However ... other factors also are important
• differences in physiology
• differences in dispersal rates
But, melanism is decreasing with cleaner air!
Results: Pepper Moth Frequencies
Darwin’s ground finch
•Change in percent
survival as related to bill
depth during a drought.

The Beagle visited 4 islands, Darwin merely labeled all specimens as
“Galapagos Islands”.
Recommended reading: Darwin’s Finches by D. Lack 1947
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Anthropogenic Selection
Every
domesticated plant and animal
vs.
Anthropogenic Selection
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Pest species

Antibiotic resistance.
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Stabilizing Selection
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Human infant mortality
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Stabilizing Selection: Clutch Size in Birds
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Theory:
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Natural selection should favor birds with most descendants.
Hypothesis:
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Birds should lay as many eggs as possible.
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Experiment
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remove eggs
Results
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Determinate vs. Indeterminate
Indeterminate – continue to lay eggs.
Most birds under normal circumstances do not lay their
physiological limit of eggs.
One mallard female laid an egg a day for 100 days.
Lack – 1947: clutch size in birds is determined ultimately
by the number of young that parents can provide with
food.
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Stabilizing Selection
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Cost-benefit Analysis
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No organism has an infinite amount of energy to
spend on its activities
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Stabilizing Selection
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Blue tit experiment
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Observation:
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Hypothesis:
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Normal brood size is 9-11 eggs.
Changing number of eggs will reduce fitness, i.e.
offspring survival
Experiment:
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Add and remove chicks
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Coevolution: the evolutionary “Arms Race”
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Reciprocal evolutionary influences.
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Occurs when a trait of species A has evolved in
response to a trait of species B.
Example:
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Cowbirds (See Essay 2.1 in book)
Flowers and pollinators
Defense chemicals of plants:
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Plants develop toxins that protect against herbivory
Herbivores develop detoxifying enzymes to enable them to
eat the plant.
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Red Queen Hypothesis
‘Now! Now!’ cried the Queen. ‘Faster! Faster!’ And they went so fast that at last they
seemed to skim through the air, hardly touching the ground with their feet, till suddenly,
just as Alice was getting quite exhausted, they stopped, and she found herself sitting on
the ground, breathless and giddy.
The Queen propped her up against a tree, and said kindly, ‘You may rest a little now.’
Alice looked round her in great surprise. ‘Why, I do believe we've been under this tree the
whole time! Everything's just as it was!’
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The species problem…What is a species?
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Mayr (2001) “Even at present there is not yet
unanimity on the definition of the species”.
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Major problem:
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Species concept vs. species as taxon.
Species concept = the meaning of species in
nature and to their role in the household of
nature.
Species taxon = a zoological/botanical
object.
Types of speciation
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Allopatric Speciation
Sympatric Speciation
Instantaneous Speciation
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Species by hybridization
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Polyploidy
Only 8 cases known
Speciation by distance (Circular overlap)
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Allopatric Speciation
Geographic/Reproductive
Dichopatric
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Isolation
speciation
New geologic barrier
plate
techtonics (e.g.nothofagus, ratites)
uplift (e.g., Hawaiian land snails)
b
b
a
a
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Peripatric speciation
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Founder populations beyond the periphery of the
current range.
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Distribution of Nothofagus
•Genus of about 35 species the “southern” beeches.
Ratite Distribution
Brown & Lomolino, 1998
Ratites
Sympatric Speciation
Speciation
occurring without geographic separation
More difficult to explain
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Insects
Plant specificity
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Fishes
Simultaneous habitat preference among certain males and
females.
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Speciation by distance (Circular overlap):
Ensatina salamanders
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Rates of Speciation
variable –
The less gene flow between populations the
faster the rate of speciation
Opposite extremes
Highly
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Skunk Cabbage in Eastern U.S. and Asia
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Isolated for 6-8 million years.
Lake Victoria – 400 species of cichlids
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Basin was dry 12,000 years ago.
Extinction
5
major extinctions
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End of Ordovician - 444 Million years ago
Main
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theory – onset of a long ice age
Late Devonian -364 mya
Main
theory - multiple causes and a series of distinct extinction
pulses
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Permian (The Great Dying) – 251 mya
~96
% of all marine species and 70 % of terrestrial vertebrate
species becoming extinct
Many theories - plate tectonics, an impact event, a supernova,
extreme volcanism…
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Triassic-Jurassic Extinction – 200 mya
Opened
the door for the dinosaurs
Main theories – climate change, asteroid, volcanoes
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Cretaceous-tertiary Extinction – 65.5 mya
50–80%
of all plant and animal families
Main theory – asteroid impact.
Extinction rates
Background
rate (poorly understood):
Mammals: 1 species in 400 years
Birds: 1 species in 200 years
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The
process is natural, the current rate is not!
Generalists
tend to survive better than
specialists.
Rats, coyotes, cockroaches and humans
Relationship to Life History
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Rarity: geographic range, habitat breadth, local
density
Dispersal Ability ~ isolation
Specialization: especially nutritional requirements
Population Variability: boom and bust?
Trophic Status: Eltonian Pyramid
Longevity: long-lived may survive variations
Intrinsic Rate of Increase: quick recovery
Threats (mostly human)
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Habitat loss or modification
Overexploitation
Introduced species
Persecution, such as predator "control"
Incidental take: fishing "by-catch"
Introduced disease
Combinations of the above: multiple threats
Recommended readings: Guns, Germs and Steel and Collapse by Jared Diamond