LECTURE 2: DARWINIAN EVOLUTION

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What is What is Evolution?Evolution?

• Evolution is the slow , gradual change in a population of organisms over time

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What was the Voyage of the What was the Voyage of the Beagle?Beagle?

Charles DarwinCharles Darwin• Joined Crew of HMS Beagle,

1831• Naturalist• 5 Year Voyage around world• Collected specimens of South

American plants and animals• Observed adaptations of plants

and animals that inhabited many diverse environments

• Main focus of geographic distribution of species = Galápagos Islands near the equator west of South America

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Darwin Left England in 1831Darwin Left England in 1831

Darwin returned 5 years later in 1836Darwin returned 5 years later in 1836

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• Both Living Both Living Organisms & Organisms & Fossils collectedFossils collected

• Fossils included:Fossils included:•TrilobitesTrilobites•Giant Ground Giant Ground Sloth of South Sloth of South AmericaAmerica

This species NO longer existed.This species NO longer existed. What had happened to them?What had happened to them?

• Noted flora and fauna on islands off of Noted flora and fauna on islands off of South America were more like continental South America were more like continental species than those of other islands with species than those of other islands with similar climate and habitatssimilar climate and habitats

• Contributions of Lyell and Malthus along Contributions of Lyell and Malthus along with his observations lead him to his with his observations lead him to his mechanism for evolution mechanism for evolution

• He spent the next 28 years researching He spent the next 28 years researching more evidence.more evidence.

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What are The Galapagos What are The Galapagos Islands?Islands?

• Small Group of Volcanic Islands 1000 Small Group of Volcanic Islands 1000 km West of South Americakm West of South America

• Very Different ClimatesVery Different Climates

• Animals On Islands UniqueAnimals On Islands Unique• TortoisesTortoises

• IguanasIguanas

• FinchesFinches

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What are The Galapagos What are The Galapagos Islands?Islands?

• Island species varied Island species varied from mainland from mainland species & from species & from island-to-island island-to-island speciesspecies

• Each island had long Each island had long or short neck or short neck tortoisestortoises

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What are Characteristics of What are Characteristics of Finches?Finches?

• Finches on the islands Finches on the islands resembled a resembled a mainland finchmainland finch

• More types of finches appeared on the More types of finches appeared on the islandsislands where the available food was where the available food was different (seeds, nuts, berries, insects…)different (seeds, nuts, berries, insects…)

• Finches had Finches had different types of beaksdifferent types of beaks adapted to their adapted to their type of food gatheringtype of food gathering

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What were Darwin’s What were Darwin’s Observations?Observations?

• Left unchecked, the number of organisms of each species will increase exponentially, generation to generation

• In nature, populations tend to remain stable in size

• Environmental resources are limited

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What were Darwin’s What were Darwin’s Observations?Observations?

• Individuals of a population vary extensively in their characteristics with no two individuals being exactly alike

• Much of this variation between individuals is inheritable

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What were Darwin’s What were Darwin’s Conclusions?Conclusions?

• Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals

• Only a fraction of offspring survive each generation• Survival of the Fittest

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What were Darwin’s What were Darwin’s Conclusions?Conclusions?

• Individuals who inherit characteristics most fit for their environment are likely to leave more offspring than less fit individuals

• Called Natural Selection

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•The The unequal ability of individuals unequal ability of individuals to survive and reproduceto survive and reproduce leads to leads to a gradual change in a population, a gradual change in a population, with favorable characteristics with favorable characteristics accumulating over generations accumulating over generations (natural selection)(natural selection)

•Eventually, New species evolveEventually, New species evolve

What is What is Darwin’s Darwin’s Theory Theory

of of EvolutioEvolutio

n?n?

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What are Natural Selection What are Natural Selection Concepts?Concepts?

• The Struggle for ExistenceThe Struggle for Existence – compete for food, mates, space, water, etc.compete for food, mates, space, water, etc.

• Survival of the FittestSurvival of the Fittest – better adapted able to survive and better adapted able to survive and

reproducereproduce

• Descent with Modification Descent with Modification – new species arise from common ancestor new species arise from common ancestor

replacing less fit speciesreplacing less fit species

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What is Fitness?What is Fitness?

• Ability of an Individual To Survive & Reproduce

• An individual’s contribution of genes to the next generation (i.e. number of offspring)

– Fitness Is Central To The Process Of Evolution Is Central To The Process Of Evolution

– Individuals With Low FitnessIndividuals With Low Fitness• DieDie• Produce Few OffspringProduce Few Offspring

Survival of the FittestAKA AKA Natural Selection

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What are Adaptations?What are Adaptations?

Inherited Characteristic That Increases Inherited Characteristic That Increases an Organisms Chance for Survivalan Organisms Chance for Survival

• Adaptations Can Be:–Physical

• Speed, Camouflage, Claws, Quills, Speed, Camouflage, Claws, Quills, etc.etc.

–BehavioralBehavioral• Solitary, Herds, Packs, Activity, etc.Solitary, Herds, Packs, Activity, etc.

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What is Survival of the Fittest?What is Survival of the Fittest?

Key ConceptOver Time, Natural Selection Over Time, Natural Selection

Results In Changes In The Results In Changes In The Inherited Characteristics Of A Inherited Characteristics Of A Population (adaptations). These Population (adaptations). These Changes Increase A Species Changes Increase A Species Fitness In Its EnvironmentFitness In Its Environment

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Natural SelectionNatural Selection

• Cannot Be Seen DirectlyCannot Be Seen Directly

• It Can Only Be Observed As It Can Only Be Observed As Changes In A Population Over Changes In A Population Over Many Successive GenerationsMany Successive Generations– RadiationRadiation

– Fossil RecordFossil Record

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What is Common Descent with What is Common Descent with Modification?Modification?

• Darwin proposed Darwin proposed that organisms that organisms descended from descended from common ancestorscommon ancestors

• Idea that organisms Idea that organisms change with timechange with time, , diverging from a diverging from a common formcommon form

• Caused Caused evolution of evolution of new speciesnew species

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What is Descent With What is Descent With Modification?Modification?

• Takes Place Over Takes Place Over Long Periods of of TimeTime

• Species Species Today Look Different From From Their AncestorsTheir Ancestors– Each Living Species HasEach Living Species Has

• Descended With Changes From Other Descended With Changes From Other Species Over TimeSpecies Over Time

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Descent With ModificationDescent With Modification

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What is Descent With What is Descent With Modification?Modification?

• ImpliesImplies

– All Living Organisms Are RelatedAll Living Organisms Are Related

– Single Tree of LifeSingle Tree of Life

• DNA, Body Structures, Energy DNA, Body Structures, Energy SourcesSources

• Common Descent

– All Species, Living & Extinct, Were All Species, Living & Extinct, Were Derived From Common AncestorsDerived From Common Ancestors

• #1#1: : Variation is the raw Variation is the raw material for natural material for natural selection.selection.

• #2 Some Variations are #2 Some Variations are better than othersbetter than others

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 22.8

What are the 5 main points of Darwin’s Theory of Evolution by

Natural Selection?

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

#3#3: Living things face a constant struggle for : Living things face a constant struggle for existence. “Survival of the fittest” existence. “Survival of the fittest” Organisms produce more offspring than can Organisms produce more offspring than can

survive.survive.

#4#4: Only some individuals survive and reproduce. : Only some individuals survive and reproduce. Some individuals survive the challenges of life Some individuals survive the challenges of life

better than others. They have favorable traitsbetter than others. They have favorable traits

#5: Natural Selection causes #5: Natural Selection causes genetic changegenetic change

• The ones with the The ones with the favorable traitfavorable trait survive, survive, reproduce and pass reproduce and pass on this favorable trait on this favorable trait to their offspring.to their offspring.

• Over time the Over time the favorable trait favorable trait increasesincreases in the in the population.population.

What are the 5 main points of Darwin’s Theory of Evolution by

Natural Selection?

• Population has variations.

• Some variations are favorable.

• More offspring are produced than survive

• Those that survive have favorable traits.

• A population will change over time.

Natural Selection and Adaptation

• Evolutionary biologist Ernst Mayr has dissected the logic of Darwin’s theory into 3 inferences based on five observations

• Observation #1• For any species, population sizes would

increase exponentially if all individuals that are born reproduced successfully

• Observation #2• Populations tend to be stable in size, except

for seasonal fluctuations• Observation #3

• Resources are limited• Inference #1:

• Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

• Observation #4• Members of a population vary in their

characteristics; no two are exactly alike• Observation #5

• Much of this variation is heritable• Inference #2

• Survival depends in part on inherited traits; individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals

•Inference #3 • This unequal ability of

individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations

A flower mantidin Malaysia

A stick mantidin Africa

Species adapt to their Species adapt to their environmentenvironment

• Natural Selection tends to make a Natural Selection tends to make a population population better suitedbetter suited to its to its environment.environment.

• The environment determines the The environment determines the directiondirection of genetic change. of genetic change.

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