Evolution by Natural Selection

LaMarck

• Individual organisms adapted to their environments by acquiring traits– change in their life timechange in their life time

• DisuseDisuseorganisms lost parts because they did not use them — like the organisms lost parts because they did not use them — like the missing eyes & digestive system of the tapewormmissing eyes & digestive system of the tapeworm

• Perfection with Use & NeedPerfection with Use & Needthe constant use of an organ leads that organ to increase in the constant use of an organ leads that organ to increase in size — like the muscles of a blacksmith or the large ears of a size — like the muscles of a blacksmith or the large ears of a night-flying batnight-flying bat

– transmit acquired characteristics to next generation

Charles Darwin

• 1809-1882• British naturalist• Proposed the idea of

evolution by natural selection

• Collected clear evidence to support his ideas– Galapagos Islands

Darwin studied unique species on the Galapagos Islands

Finch? Sparrow?

Woodpecker? Warbler?

Darwin found… birds

Finch? Sparrow?

Woodpecker? Warbler?

Collected many different Collected many different birds on the Galapagos birds on the Galapagos Islands. Islands.

Thought he found Thought he found very different kinds…very different kinds…

Darwin was amazed to find out: All species of birds were finches…

Sparrow?

But Darwin found… a lot of finches

Large Ground Finch

Small Ground Finch

Warbler Finch Veg. Tree Finch

But there is only one species of finch on the mainland!

How didone species of finches becomeso many differentspecies now?

Tree Thinking

Large-seed eater? Small-seed eater?

Warbler? Leaf-browser?

Large GroundFinch

Small GroundFinch

Warbler Finch Veg. Tree Finch

Ancestralspecies—1 finch from the mainland

Descendantspecies—many finch species on the islands

Correlation of finch species to food source

Adaptive radiation

SeedSeedeaterseaters

FlowerFlowereaterseaters

InsectInsecteaterseaters

Rapid speciation:new species filling new niches,because they inheritedsuccessful adaptations.

Warbler finch

Woodpecker finch

Small insectivoroustree finch

Largeinsectivorous

tree finch

Vegetariantree finch

Cactus finch

Sharp-beaked finch

Small groundfinch

Mediumground finch

Large ground finch

Insect eaters

Bud eater

Seed eaters

Cactuseater

Warbler

finch

Tree

finc

hes Ground finches

Darwin’s finches• Differences in beaks – associated with eating different foods– survival & reproduction of beneficial beak

adaptations to foods available on islands

Darwin’s finches• Darwin’s conclusions – small populations of original South American finches

landed on islands• variation in beaks enabled individuals to gather food

successfully in the different environments

– over many generations, the populations of finches changed anatomically & behaviorally• accumulation of advantageous traits in population• emergence of different species

Seeing this gradation & diversity of structure in one small, intimately related group of birds, one might really fancy that from the original gene pool of birds in this archipelago, one species has been taken & modified for different ends.

• Differences in beaks allowed some finches to…– successfully compete – successfully feed• Live longer

– successfully reproduce• pass successful traits

onto their offspring

Darwin’s finches

Correlation of species to food source

Whoa,Turtles, too!

More observations…

Evolution by Natural Selection• There exists variation within a POPULATION’S

gene pool– Some traits are more frequent (common) than

others– Those with the most advantageous traits survive

(best camouflage, best way to obtain food, etc)• These organisms outcompete competitors• These organisms live longer = have more offspring• These organisms pass on their advantageous traits,

changing the gene frequency of that trait

• A change in the gene frequency within a population’s gene pool = EVOLUTION

This is not just a process of the past…

It is all around us today

Artificial selection

Selective breeding

the raw genetic material (variation) is hidden there

Selective breedingHidden variation can be exposed through selection!

A Reluctant Revolutionary • Darwin returned to England in 1836– wrote papers describing his collections &

observations– long treatise on barnacles– draft of his theory of

species formation in 1844• instructed his wife to

publish this essay upon his death• reluctant to publish but

didn’t want ideas to die with him

Voyage: 1831-1836

November 24, 1859, Darwin published

“On the Origin of Species by Means of Natural Selection”

Essence of Darwin’s ideas• Natural selection– variation exists in populations– over-production of offspring

• more offspring than the environment can support– competition

• for food, mates, nesting sites, escape predators– differential survival

• successful traits survive = adaptations (camouflage, etc)– differential reproduction

• Successful organisms live longer, Successful organisms live longer, have more offspringhave more offspring• adaptations become more adaptations become more

common in population, changingcommon in population, changing the gene frequencies within that the gene frequencies within that population’s gene poolpopulation’s gene pool

(5)(5) Over long periods of time, and given a steady input of new variation Over long periods of time, and given a steady input of new variation into a population, these processes lead to the into a population, these processes lead to the emergence of new emergence of new speciesspecies

Restated…(1)(1) VariationVariation exists in natural populations exists in natural populations

(2)(2) Many Many more offspringmore offspring are born each season than can are born each season than can possibly survive to maturitypossibly survive to maturity

(3)(3) As a result, there is a As a result, there is a struggle for existencestruggle for existence

- - competitioncompetition

(4)(4) Characteristics beneficialCharacteristics beneficial in the struggle in the struggle for existence will tend to become more common in the for existence will tend to become more common in the population, changing the average characteristics of the population, changing the average characteristics of the populationpopulation

- - adaptationsadaptations

LaMarckian vs. Darwinian view• LaMarck– in reaching higher

vegetation giraffes stretch their necks & transmits the acquired longer neck to offspring

• Darwin– giraffes born with longer

necks survive better & leave more offspring who inherit their long necks