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Chapter 15 Darwin's Theory of Evolution

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15–1 The Puzzle of Life's Diversity

This variety of living things is called biological diversity.

How did all these different organisms arise? How are they related?

Evolution, or change over time, is the process by which modern organisms have descended from ancient organisms.

A scientific theory is a well-supported testable explanation of phenomena that have occurred in the natural world.

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Voyage of the Beagle Charles Darwin (born February 12, 1809 in

England) 1831 – set sail on the H.M.S. Beagle

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During his travels, Darwin made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time.

That hypothesis, now supported by a huge body of evidence, has become the theory of evolution.

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On The Voyage

Collected plants & animals on land At sea – studied specimens, read,

recorded observations Saw great diversity in the organisms –

enormous numbers of species!

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Patterns of Diversity Plants and animals seemed so

well suited to their environments

Impressed by the many ways organisms survive and reproduce

Similar areas on different continents were inhabited by very different animals. Ex: why are there no rabbits on

the Australian grasslands? No kangaroos in England?

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Living Organisms and Fossils

Darwin also collected fossils Some fossils looked like organisms that were

still alive Other fossils looked completely different! Darwin wondered:

Why had so many of these species disappeared? How were they related to living species?

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The Galápagos Islands 1000 km west of South America (Ecuador) Islands are close together but very different

climates Darwin was fascinated in particular by the land

tortoises and marine iguanas

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The shape of a tortoise's shell could be used to identify which island a particular tortoise inhabited.

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The Journey Home

Darwin began to wonder if animals living on different islands had once been members of the same species.

these separate species would have evolved from an original South American ancestor species after becoming isolated from one another

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Massive, rich fossil record – much older than the current age of the earth beliefs.

James Hutton (1795) – old earth – great earth changes take time

Charles Lyell (1831) – the processes shaping the Earth now are the same ones that shaped the earth in the past Could life change as earth has changed?

15-2 Ideas that shaped Darwin’s thinking

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Jean Baptiste Lamarck 1809 “living things have changed over time” Species descended from other species Lamarck’s theory of evolution:

Tendency toward perfection Use and Disuse Inheritance of Acquired Traits

Why was he wrong? Behavior doesn’t affect the DNA

He was WRONG.

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Thomas Malthus

1798 Population growth (if gone unchecked) would

quickly over run the earth Why don’t maple trees cover the earth since

they each release 1000s of seeds? Darwin’s questions:

What causes the deaths of so many? What factors determine which survive and

reproduce?

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All species tend to produce more offspring than they can support

Frog eggs

Turtle eggs

Insect eggs

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15-3 Darwin presents his case

Worked for years but scared to publish because of religious backlash

1858 – sent an essay by Alfred Russell Wallace summarizing Darwin’s theory of evolution

1859 – Darwin publishes “On the origin of species by means of natural selection”

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What did Darwin say?

Inherited Variation & Artificial Selection Members of a species vary

Artificial Selection = nature provides the variation and humans select the variations they find useful Examples: fat hog, fast horse, high milk producing

cow Does something in nature select the same way?

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Variation within a species

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Artificial Selection

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Evolution by Natural Selection

Compared nature to artificial selection

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Struggle for Existence

Members of each species compete for food, space, mates, and other necessities of life

Predators (that are better/faster) catch more prey

Prey (faster, better camouflage) avoid being caught Examples: fast cheetah, small rabbit, stronger

lion, more colorful butterfly

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Preying Mantis sunflower

Producing more offspring than can possibly survive thus creating a struggle for existing

resources

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Survival of the Fittest

Fitness = being able to survive and reproduce in its specific environment

Fitness results from adaptations Adaptations = inherited characteristics that

improve an organisms chance of survival Successful adaptation better suited to the

environment more fitness

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Adaptations – Anatomical/structural (quills on a porcupine) Physiological (photosynthesis) Behavioral (live/hunt in groups)

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Low Fitness organisms die off High Fitness survive + reproduce

Survival of the Fittest!

Natural Selection of most fit organisms!

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Nature (the environment) will determine which organism is the

fittest to Survive

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Natural Selection

Results from NO human intervention, control, or direction

Results in changes in the inherited characteristics of a population

Increases a species’ fitness in the environment (over time!)

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Descent with Modification

Over large amounts of time… natural selection produces organisms: with different structures establishes different niches occupies different habitats

organisms look different than their ancestors Descent with modification = each living thing

has descended with changes, from other species over time.

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Common Ancestors Implies that organisms are all related to one

another. Tiger, panther, cheetah – related Horses, dogs, cats – related farther back Mammals, birds, reptiles, fish – related even

farther back All living things – related!

Single Tree of Life!

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Evidence of Evolution

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Living things have been evolving on Earth for millions of years…

Evidence.. Fossil Record Geographic distribution of living species Homologous body structures

body part with the same basic structure and embryonic origin as that of another organism, though not necessarily sharing the same function

Similarities in early development (embryology)

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The Fossil Record

Fossils = remains of ancient life Different layers of rock were formed at

different times during Earth’s history. Darwin proposed that organisms had

coming into being, lived, and vanished. Life had changed over time!

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Fossil Cephalopods  

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Fossil record of the modern horse

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Fossilrecord of the camel

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Since Darwin… The number of known fossils has gone up

dramatically! Researchers have discovered many hundreds of

transitional fossils that document various intermediate stages in the evolution of modern species from organisms that are now extinct.

Gaps remain in the fossil records These gaps do not indicate weakness in the

theory of evolution itself. Rather, they point out uncertainties in our understanding of exactly how some species evolved.

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Geographic Distribution of Living Species 

Darwin wondered about the finches he found in the Galapagos Islands.

Where were they similar but still different? Also.. Slightly different from S. American mainland

species Could the birds have changed as they adapted

to local environments?

=> descended with modification from a common mainland ancestor

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Convergent Evolution

Darwin found entirely different species of animals on the continents of South America and Australia.

Similar environments on those continents he sometimes saw different animals that had similar anatomies and behaviors.

The existence of similar but unrelated species was a puzzle to Darwin.

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Similar Species in Similar Environments

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Convergent Evolution

The adaptive evolution of superficially similar structures, such as the wings of birds and insects, in unrelated species subjected to similar environments.

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Unrelated organisms come to resemble one another

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Analogous Structures

Structures that serve the same function in different species but they evolved independently

same function, different structure Ex: bat wing, butterfly wing

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Divergent Evolution

two or more related species becoming more and more dissimilar

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Homologous Body Structures 

striking anatomical similarities among the body parts of animals with backbones

of reptiles, birds, and mammals—arms, wings, legs, and flippers—vary greatly in form and function.

all constructed from the same basic bones limbs has adapted in ways that enable

organisms to survive in different environments

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Homologous Body Structures 

structures that have different mature forms in different organisms but develop from the same embryonic tissues

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Vestigial Organs organ that serves no useful function in an

organism Why would an organism possess organs with

little or no function? One possibility = presence of a vestigial organ may

not affect an organism's ability to survive and reproduce natural selection would not cause the elimination of that organ.

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The human appendix

has no function in

humans but is very

important in cows and

other animals

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Human tailbone is a vestigial vertebrate tail

Human tail bone consists of fused vertebrae that nolonger functionas a tail

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Similarities in Embryology  embryos of many animals with backbones are

very similar many embryos look especially similar during

early stages of development

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chickens, turtles, and rats

Pharyngeal slits

Dolphin

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Summary of Darwin's Theory

Individual organisms differ, and some of this variation is heritable.

Organisms produce more offspring than can survive, and many that do survive do not reproduce.

Because more organisms are produced than can survive, they compete for limited resources.

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Each unique organism has different advantages and disadvantages in the struggle for existence. Individuals best suited to their environment survive and reproduce most successfully. These organisms pass their heritable traits to their offspring. Other individuals die or leave fewer offspring. This process of natural selection causes species to change over time.

Species alive today are descended with modification from ancestral species that lived in the distant past. This process, by which diverse species evolved from common ancestors, unites all organisms on Earth into a single tree of life.