Chapter 15

Darwin’s Theory of Evolution

Section 15-1 – Puzzle of Life’s Diversity

Biological Diversity – variety of living things• How did all these organisms come to be?• How are they related?

Evolution - change over time; the process by which modern organisms have descended from ancient organisms• Change in a population of organisms over

generations

I. Voyage of the Beagle

A. Charles Darwin1. Born 18092. Attended Cambridge - Studied natural

history, geology3. After college, joined the crew of the H.M.S.

Beagle.• His observations & collected data led to him

proposing evolutionary theory

II. Darwin’s Observations

A. Patterns of Diversity1. Plants & animals seemed remarkable well

suited to their environment 2. Types of organisms are found in only

certain areas of the world• Argentina & Australia have grasslands. So why

don’t they have the same organisms living there?

B. Living Organisms & Fossils1. Darwin also collected fossils (remains of

ancient organisms)• What are some different types/examples of

fossils?

2. Some fossils resembled current, living organisms, while others looked completely different• Why had some species disappeared?• How are they related to living species?

C. The Galapagos Islands1. Found 13 species of finches that were similar

except for their beaks2. Each Island had different food sources, requiring

different beaks!3. Also noticed different species of land tortoises

on different islands.4. The shape of a tortoise’s shell differed from

island to island. See pg. 371, Figure 15-3

Figure 22.6

(a) Cactus-eater (b) Insect-eater

(c) Seed-eater

Section 15-2: Ideas that shaped Darwin’s Thinking

I. The Beliefs of the TimeA. Earth & all life had only been created a few

thousand years ago1. No organisms had changed, but the robin had

always been a robin

B. As fossils record grew, adjustments to beliefs began

1. Periods of creation occurred following catastrophic events that killed off many forms of life

II. An Ancient Changing Earth

A. James Hutton1. Proposed theory of Gradualism where by Earth’s

features were created (& changed) gradually by natural processes over time

2. Mountain building, valleys, canyons, oceanic ridges, etc. occur over millions of years

3. Proposed Earth was much older than a few thousand years.

B. Charles Lyell1. Before his journey, Darwin was given Lyell’s book,

Principles of Geology2. Incorporated gradualism into uniformitarianism,

which states that geologic processes operate today just as they did for most of Earth’s history

3. Influenced Darwin!• If the Earth could change over time, might life

change too?• If so, life would change slowly, over millions of

years, just like the Earth.

C. Jean Babtiste de Lamark – 1st to hypothesize on evolution

1. Similar species descended from a common ancestor. Meaning all similar species come from one ancient species.

2. New species develop through the passing of acquired traits, which are traits not determined by genes, but by the environment.

3. According to Lamark, species acquire traits because/by:• All organisms acquire traits that help them live

more successfully in their environment• Using/not using certain parts of their body to

change the size/shape of organs

– Ex: A water acquired webbed-feet by stretching the membrane between its toes. Every offspring of this bird will have webbed feet.

– Ex: A person is blinded in an accident. This acquired trait will be passed on to offspring.

Does this make sense? NOAre acquired traits passed on? NO

C. Lamark’s theories Disproved/Proved1. Acquired Traits are not passed on to offspring,

only genetic traits are!2. Similar species do tend to share a common

ancestor and organisms do change over time.

D. Thomas Malthus – observed that babies were born faster than people or dying

1. If the human population grows unchecked, living space & food will become insufficient.

2. Darwin applied this concept to plants/animals.• 1 maple tree produces 1000’s of seeds a year• Not all survive = many offspring die before

reproducing• What factor(s) determines who survives & who

doesn’t?

Section 15-3: Darwin’s Theory of Evolution

I. The Race to publish1. Alfred Wallace also proposed a theory on

evolutionary change, sent the essay to Darwin

2. Drove Darwin to write out and publish his theory ……. before Wallace.

3. 1859, published On the Origin of Species: By Means of Natural Selection

II. Natural Variation & Artificial SelectionA. Natural variation is differences among

individuals of a species in nature1. This variation could be inherited.2. Darwin noted that breeders used variation to

improve their crops, livestock through a technique called selective breeding.

B. Darwin termed this process as artificial selection 1. Nature provides the variation, and humans

select those variations that they find useful.

III. Evolution by Natural SelectionA. Struggle for Existence

A. Members of each species struggle to exist by competing for food, living space, etc.

B. Survival of the Fittest1. Fitness: ability of an individual to survive and

reproduce in its specific environment2. The fitness is the result of adaptations,

inherited characteristic that increases an organism's chance of survival.• Anatomical/structural, characteristics,

organism’s physiological processes or functions, or complex features (behaviors).

3. Darwin referred to the survival of the fittest as Natural Selection• Successful adaptations = organisms become

better suited to their environment = better able to survive & reproduce.

• Unsuccessful characteristics = organisms not well suited to their environment (low levels of fitness) = death or few offspring.

4. Natural Selection• The traits being selected increase over time &

contribute to an organism's fitness in its environment.

• Takes place without human control or direction.• Cannot be seen directly; it can only be observed

as changes in a population over generations.

C. Descent with Modification – Each living species has descended, with changes, from other species over time

1. As a result, species today look different from their ancestors.

2. Also implies that all living organisms are related to one another.

3. Common Descent: All species—living & extinct—were derived from common ancestors

4. a single “tree of life” links all living things.

Darwin’s Tree of Life

IV. Evidence of Evolution

A. Fossil Record1. Fossils formed in different rock layers showed

that organisms have changed over time.2. Fossils from sequential rock layers could show

how a species changed and produced a new species!

B. Geographic Distribution of Living Species1. Darwin’s finches in the Galápagos were similar,

yet different from one another & also different from a similar species on the mainland.

2. Darwin’s finches could have descended with modification from a common mainland ancestor.

3. Entirely different species of animals living in similar environments can have similar structures/behaviors • descended from different ancestor. • Living under similar ecological

conditions, being exposed to similar pressures of natural selection, they ended up evolving common features.

4. Regardless of appearance, two species are closely related when they share a common ancestry.

C. Anatomical Evidence1. Homologous structures: Body parts with same

basic structure but different function• Structures have different mature forms but

develop from same embryonic tissues • Indicates common ancestry• Foreleg – same set of bones for vertebrate.

C. Anatomical Evidence2. Analogous Structures - Body parts with similar

function but different structure• Indicates NO common ancestry• e.g. wings if insect and wings of bat

C. Anatomical Evidence3. Vestigial Structure: structure reduced in size• No known function• Left over from other structure• e.g. tailbone for humans

D. Embryological Evidence – Compares development of embryos

1. Similar embryologycommon ancestry.2. Embryo looks similar for longer period = more

closely related species3. Embryonic cells develop in same order and pattern

to produce tissues and organs which produce homologous structures

E. Biochemical Evidence1. The sequence of amino acids that make up

proteins are similar similar DNA strandsorganism show evolutionary relationship.

2. e.g. Humans and Chimpanzees (97% similar)