Darwin’s Theory of Evolution

Chapter 15

The Puzzle of Life’s Diversity15-1

• In 1831, at age 22, Charles Darwin joined the crew of the H.M.S. Beagle as a naturalist for a 5 year voyage around the world.

• Darwin went ashore and collected plant and animal specimens for his collection.

• He studied the specimens, read the latest scientific books, and filled many notebooks with his observations and thoughts.

• This led him to propose a revolutionary hypothesis about the way life changes.– EVOLUTION!

What is Evolution?

• Any change in the heritable traits within a population across generations

• Results in new species

Darwin’s Observations

1. Life is Diverse:– Darwin observed many different kinds of

plants and animals – All were well suited to the environments

they inhabited – He was impressed by the ways in which

organisms survived and produced offspring

2. Fossils:– The preserved

remains of once living organisms

– Some of those fossils resembled organisms that were still alive

– Others looked completely unlike any creature he had ever seen

3. The Galapagos Islands:– The islands are very close together but

have very different climates.• Some are hot and dry with little vegetation

while others are rich in rainfall and vegetation

– Each island had its own unique assortment of plant and animal species.

Pinta IslandIntermediate shell

Pinta

Isabela IslandDome-shaped shell

Hood IslandSaddle-backed shellHoodFloreana

Santa Fe

Santa Cruz

James

Marchena

Fernandina

Isabela

Tower

Giant Tortoises of the Galápagos Islands

On the desert-like Hood Island,tortoises had long necks…

…while on the lush rainforest of Isabela Island,tortoises had short necks…

and on Pinta Island, tortoise neckswere somewhere in between

• After his voyage, Darwin spent a great deal of time thinking about his findings.– Had animals living on different islands

once been members of the same ancestral species that developed different characteristics after becoming isolated from one another in different habitats?

Ideas that Shaped Darwin’s Thinking

15-2• James Hutton

– 1785– Proposes that the earth

was shaped by geological forces which occurred over very long periods of time and is millions of years old.

• Charles Lyell– 1833– Proposes that the

geological process that shaped the earth are still occurring now.

• How did Hutton and Lyell’s work influence Darwin’s theory of evolution?

• This understanding of geology led Darwin to conclude…– If the earth could change over time, so

could life.– The earth must be older that was

previously thought

• Thomas Malthus– 1798– Observed that birth rate

far surpassed death rate.

– Proposed that famine, disease, and war were necessary and unavoidable population controls.

– Darwin is influenced because he sees that there will be a struggle for existence within species.

• Jean-Bapiste Lamarck– 1809– First to propose a

scientific theory for the way life changed over time

– First to suggest that all life descended from a common ancestor

– First to realize that species were specially adapted to their environments

Lamarck’s Theory

• Lamarck proposed that through selective use and disuse of organs, organisms could gain or loose traits.

• Further, he believed that those acquired characteristics could be inherited by offspring.

• He says that this is how species changed over time

The male fiddler crab uses its front claw to attract mates and fight off predators.

Through repeated use, the front claw becomes larger.

The fiddler passes on this acquired characteristic to its offspring

• What’s RIGHT about Lamarck’s theory?

– First to develop a scientific hypothesis about evolution and recognize that organisms are adapted to their environments.

• What’s WRONG about Lamarck’s theory?– Didn’t know about

genes and how traits are inherited

– Acquired traits CAN NOT be passed on!

• Ex) if you lifted weights your whole adult life, and then had children, your children would not be born more muscular!

Darwin Presents his Case15-3

• After Darwin returned to England in 1836 he filled notebooks with his ideas and the process that he would later call evolution.

• He did not rush to publish his ideas because they disagreed with the fundamental religious and scientific ideas of the day.

• He asked his wife to publish his ideas after he died.

• 1858—Alfred Russel Wallace, another naturalist, publishes an essay describing his work in Malaysia and outlining the same ideas that Darwin had for 25 years!– This provided the incentive that Darwin

needed to publish his own ideas.

• 1859—Darwin publishes On the Origin of Species – Presented his

evidence for evolution

– Darwin proposes a mechanism for evolution called NATURAL SELECTION

Inherited Variation & Artificial Selection

• Darwin noticed that plant and animal breeders would breed only the largest hogs, fastest horses, or the cows that produced the most milk– He called this, Artificial

Selection Selection by humans

for breeding of useful traits from the natural variation among different organisms.

Breeds of Dogs

Chihuahua – bred from Techichi of Mexicoby Mayans, had religious significance

Saint Bernard – bred by monks around 1050 A.D. to rescue travelers of mountain passes in the

Swiss Alps between Italy and Switzerland

Irish Wolfhound – bred in Ireland to hunt wolves and elk

Dachshund – bred in Germany as earlyas the 15th century to hunt badgers

Evolution by Natural Selection

• Darwin compared processes in nature to artificial selection. He thought that nature could “select” organisms based on their fitness.

Natural Selection1. Genetic variation is found naturally

in all populations2. The environment presents certain

challenges– Struggle for existence: members of

each species must compete for food, space, and other resources.

3. Some variation is more favorable; some is not favorable

Natural Selection (cont.)

4. Individuals with the favorable traits (adaptations) will survive and reproduce more than those individuals without the favorable trait.

= Survival of the fittest

– Ability to survive and reproduce = fitness– Any inherited characteristic that increases

an organisms chance of survival = adaptation

5. The favorable variation will become more common in the population.

Descent with Modification• Suggests that

each species has descended with changes from other species over time.

• Further suggests that all living things are related to each other and that all species, living and extinct, share a common ancestor.

Evidence for Evolution

1. The Fossil Record– Fossils = the remains of ancient

organisms found in layers of rock– Fossils are thought to be the same age

as the rock they are found in– Rock tells the history of earth while

fossils tell the history of life.

Transitional Fossils

• TIKTAALIK!• FishTetrapod• Shows the

transition between 2 species

Evidence (cont.)2. Geographic Distribution

– Darwin thought that the Galápagos finches could have descended with modification from a common mainland ancestor

Evidence (cont.)3. Homologous Body

Structures– these are structures

that have different mature forms, but develop from the same embryonic tissue

– similarities and differences help biologists group animals according to how recently they shared a common ancestor

Turtle Alligator Bird Mammal

Ancient lobe-finned fish

Homologous Body Structures

• Not all homologous structures serve important functions.

• The organs of many animals are so reduced in size that they are just vestiges, or traces, of homologous organs in other species.

• These organs are called vestigial organs.

Evidence (cont.)4. Embryology

– The early stages, or embryos, of many animals with backbones are very similar.

– Similarities of embryos provides evidence of a common ancestor.

Patterns of Evolution17-4

• Macroevolution—refers to large scale evolutionary patterns and processes that occur over long periods of time

• Five important topics in macroevolution are:– extinction– adaptive radiation– convergent evolution– coevolution– gradualism & punctuated equilibrium

Extinction• More than 99% of all species that

have ever lived are now extinct!• Mass Extinction = when large

numbers of species disappear• Result is the remaining species now

have new niches (jobs) to fill, and may then thrive causing bursts of evolution that produces many new species.

Adaptive Radiation• Process by which a single

species or a small group of species evolves into several different forms that live in different ways– Ex) Darwin’s Finches—more

than a dozen species evolved (due to different habitats) from a single species

• Can also happen on a much larger scale– Extinction of dinosaurs =

adaptive radiation of mammals

Convergent Evolution• Process by which unrelated organisms come to

resemble one another, due to living in similar environments.

– They develop structures that look and function similarly, but are made up of different parts = analogous structures

– EX: A bat’s wing (mammal) and a bird’s wing are analogous structures

Coevolution

• When organisms that are closely connected to one another by ecological interactions evolve together.

• The two species evolve in response to changes in each other over time.

Gradualism vs. Punctuated Equilibrium

• Evolution has often proceeded at different rates for different organisms at different times during the history of life on Earth.

• Gradualism:– Darwin felt that

biological change was slow and steady, an idea known as gradualism.

Gradualism vs. Punctuated Equilibrium

• Punctuated equilibrium: a pattern of evolution in which long stable periods are interrupted by brief periods of rapid change.