Lecture 3: Natural Selection

EEES 3050

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Darwin’s Five Theories 1 – The non-constancy of species

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.

Review

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Review

Evidence Fossil Record, Biogeography, Morphology, Vestigial

Structures, Molecular Evidence

Evolutionary Ecology: World changes and so do the phenomena studied in

ecology Sex ratios, feeding preference, life cycles

Human impacts Antibiotic resistance, epidemiology, conservation biology

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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.

Darwin’s Model of Natural Selection:

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)

Darwin’s Model – 5 facts, 3 inferences

Selection is a process of chance.

Selection is deterministic.

Natural selection is both!

Two criticisms of natural selection

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The 2 steps of Natural Selection Random production of variation

Mutation Recombination Random mate choice

Non-random aspects of survival and reproduction Superior success of certain phenotypes Non-random mate choice.

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Natural Selection

What drives selection? Usually an emphasis on survival or struggle for existence. Natural selection is actually a process of elimination

“Survival of the fittest” – coined by Sociologist Herbert Spencer.

What else drives selection? All factors leading to an increase in reproductive success. Sexual Selection or “Selection for reproductive success”

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Fitness

What is fitness? “…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

Directional Selection

Stabilizing Selection

Disruptive Selection

Lower rate of survival

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Lower rate of survival

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Lower rate of survival

Types of Natural Selection Directional selection

Industrial Melanism Galapagos ground finch. Other cases of anthropogenic selection

Stabilizing Selection Infant Mortality Hatch date in lesser snow geese Clutch size in birds

Disruptive Selection Black-bellied seed crackers in Africa

Species has two sizes of beak – no intermediates.

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Planned Tangent

Throughout this semester, there will be a common framework of how to think about conducting ecology.

I will ask (again and again and again) 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.

Industrial Melanism Background: The species: Peppered Moth (Biston betularia)The place: Industrial Europe (also USA)The time: last 100 yearsScenario: adults rest on tree trunks

natural state: light colored, lichen covered trunksindustrial state: soot-darkened trunks

Observation: proportions of light:dark moths natural state: mostly light colored (>90%) industrial state: increasingly dark (>90%)

Industrial Melanism

Natural state: light colored, left Industrial state: dark colored, right

Observation: proportions of light:dark moths natural state: mostly light colored (>90%) industrial state: increasingly dark (>90%)

Industrial MelanismHypothesis:

Birds eat moths they can see.birds are visual predators

Experiment: release marked adults of both color

Resultsrural: more light adults survivedurban: more dark adults survived

Visual verification of bird predation

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

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Darwin’s ground finch

The Beagle visited 4 islands, Darwin merely labeled all specimens as “Galapagos Islands”.

Recommended reading: Darwin’s Finches by D. Lack 1947

•Change in percent survival as related to bill depth during a drought.

Anthropogenic Selection

Every domesticated plant and animal

vs.

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Pest species

Antibiotic resistance.

Anthropogenic Selection

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Stabilizing Selection

Human infant mortality

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Stabilizing Selection: Clutch Size in Birds Theory:

Natural selection should favor birds with most descendants. Hypothesis:

Birds should lay as many eggs as possible. Determinate vs. Indeterminate Indeterminate – continue to lay eggs.

Experiment remove eggs

Results 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 Cost-benefit Analysis

No organism has an infinite amount of energy to spend on its activities

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Stabilizing Selection

Blue tit experiment Observation:

Normal brood size is 9-11 eggs. Hypothesis:

Changing number of eggs will reduce fitness, i.e. offspring survival

Experiment: Add and remove chicks

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Coevolution: the evolutionary “Arms Race” Reciprocal evolutionary influences.

Occurs when a trait of species A has evolved in response to a trait of species B.

Example: Cowbirds (See Essay 2.1 in book) Flowers and pollinators Defense chemicals of plants:

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!’

The species problem…What is a species? Mayr (2001) “Even at present there is not yet

unanimity on the definition of the species”. Major problem:

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.

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Types of speciation

Allopatric Speciation Sympatric Speciation Instantaneous Speciation

Polyploidy Species by hybridization

Only 8 cases known Speciation by distance (Circular overlap)

Allopatric Speciation

Geographic/Reproductive IsolationDichopatric speciation

New geologic barrierplate techtonics (e.g.nothofagus, ratites)uplift (e.g., Hawaiian land snails)

a

ba

b

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Peripatric speciation Founder populations beyond the periphery of the

current range.

Distribution of Nothofagus•Genus of about 35 species the “southern” beeches.

Ratite Distribution

Brown & Lomolino, 1998

Ratites

Sympatric Speciation

Speciation occurring without geographic separationMore difficult to explain

Insects Plant specificity

FishesSimultaneous habitat preference among certain males and females.

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Speciation by distance (Circular overlap): Ensatina salamanders

Rates of Speciation

Highly variable – The less gene flow between populations the faster the rate of speciationOpposite extremes

Skunk Cabbage in Eastern U.S. and Asia Isolated for 6-8 million years.

Lake Victoria – 400 species of cichlids Basin was dry 12,000 years ago.

Extinction5 major extinctions

End of Ordovician - 444 Million years agoMain theory – onset of a long ice age

Late Devonian -364 myaMain theory - multiple causes and a series of distinct extinction pulses

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…

Triassic-Jurassic Extinction – 200 myaOpened the door for the dinosaursMain theories – climate change, asteroid, volcanoes

Cretaceous-tertiary Extinction – 65.5 mya50–80% of all plant and animal families Main theory – asteroid impact.

Extinction rates

Background rate (poorly understood): Mammals: 1 species in 400 yearsBirds: 1 species in 200 years

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

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)

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