Darwin’s Theory of Evolution

Chapter 15

What is evolution?

• Evolution is a theory, meaning it is widely approved by the scientific community and heavily supported with evidence.

• Evolution describes when a GROUP of organisms changes over time. This can occur over long periods of time or short periods of time.

How does an idea become a theory?

Contributors to the Theory of Evolution

James Hutton • Claimed that rocks move up

from bottom of crust and are then eroded away

• Earth is several millions of years old

Thomas Malthus • Said that as the human

population grows, we will eventually run out of space and resources

Contributors to the Theory of Evolution (cont)

Jean Baptiste Lamarck • Believed that acquired traits

could be passed on– E.g. If the parents lift weights

and have large muscles, their children will also inherit these acquired muscles

Charles Lyell • Said that Earth has changed

over time and we can only understand Earth’s features by learning about the geological processes

Contributors to the Theory of Evolution (cont)

Charles Darwin• Considered the founder of

evolutionary theory

• Developed most of his ideas in the 1830s while traveling the world on the ship The Beagle

Alfred Wallace • Wrote to Darwin about

natural selection ONE YEAR before The Origin of Species was published

Darwin’s Journey on The Beagle

Darwin’s Observations

• Argentina and Australia had similar organisms even though they are very far apart. This was caused by similar climates.

• Fossils of extinct living things are similar to current living things.

Darwin’s Observations

• Darwin spent a lot of time studying the tortoises of the Galapagos Islands. He noticed that each island had a different type of tortoise.

• Darwin noticed a similar phenomenon in the finches he studied.

Darwin’s Findings• Although Darwin traveled the world in the 1830s, he did not publish his

findings until the late 1850s with The Origin of Species.

1. Living things are different, and some of these differences are inherited. Mutations and sexual reproduction cause variation.

2. Not all offspring survive and/or reproduce.

3. Living things compete for resources in a struggle for existence.

4. The living things with inheritable adaptations that are able to reproduce the most successfully will survive. This process is called natural selection and facilitates evolution.

5. Species today have descended with modification from other organisms. All living things have a single common ancestor, ergo the “Tree of Life.”

How does natural selection work?• First, you need to know a few definitions:

• Fitness – ability to survive and reproduce

• Adaptation – inherited trait that increases chance of survival

• Natural selection – survival of the fittest, nature changes and only a select few survive

• Descent with modification - living things pass on inherited changes

• Common descent – all living things have a common ancestor

How does natural selection work?• Organisms that are more suited to the environment have advantageous

adaptations that allow them to survive. If these adaptations are inheritable, then they are passed on.

• These adaptable organisms have a high degree of fitness, which means they are able to survive and reproduce successfully.

• When they compete for sources with other individuals, the more adaptable organisms will win this “survival of the fittest” due to natural selection.

• Over time, the entire group will change to favor the more adaptable organisms. This shift in the entire group is called evolution.

How does natural selection work?

What evidence of evolution do we have?

• Fossil Record

• Embryology

• Geographic Distribution

• Anatomical Structures

Fossils

• Fossils are physical remains of living things. They include rocks, bones, imprints, materials trapped in amber, etc.

• Some fossils are called transitional fossils because they fill in the “gaps” of evolution.

Dating Fossils

• Fossils can be found in layers of rocks called strata. The oldest layers are closest to the bottom. Paleontologists can determine the relative age of a fossil based on its location in the rock and comparing it to index fossils.

Dating Fossils (cont)

• Scientists can also used radioactive dating to determine a more accurate age of a fossil. They calculate the age using the half-life of a radioactive isotope. – A half-life is how long it

takes for half of the material to decay.

Fossil Formation

• Although there are many types of fossils, most fossils are formed with the presence of water.

1. Water carries small rock particles to lakes and seas.

2. Dead organisms are buried by layers of sediment, which forms new rock.

3. The preserved remains may later be discovered and studied.

Geographic Distribution• Sometimes very distant places have similar species because they

have adapted to a similar climate. Climate refers to regular weather of an area and is influenced by temperature, rainfall, latitudes, etc.

Geographic Distribution (cont)• At one time, all the land on Earth formed

a “supercontinent” called Pangaea. According to Plate Tectonics (aka Continental Drift Theory), the continents have slowly drifted away from each other.

• The movement of these plates separates species which must adapt to changing environments.• E.g. Ferns evolved early and appear on all

continents whereas some types of mammals on exist in certain countries.

Geographic Distribution (cont)

Geographic Distribution (cont)

• The drift of continental plates still happens at a very slow rate, but sudden movements can cause natural disasters and/or change the landscape.• Earthquakes• Tsunamis • Mountain ranges

(Himalayas)

Embryology

• Embryology is the study of embryos, which are fertilized eggs after they have started to develop.

• The embryos of chordates (living things with a backbone) develop in the same order and are virtually indistinguishable in early development.

Anatomical Structures

• Scientists can also compare the anatomical (body) structures of organisms.

• Organisms are said to have homologous structures if they have similar anatomical features. This may imply that they have a common ancestor. – E.g. Forelimbs in chordates follow

the same pattern of bones.

Anatomical Structures (cont)

• Analogous structures describe when two organisms evolved separately BUT have similar structures because they lived in similar environments.– E.g. Bats and birds both have

wings, but they are not related. Bat wings have bones more like humans, and birds have a completely different bone structure.

Anatomical Structures (cont)

• Occasionally an anatomical structure loses its purpose. Since it is neither advantageous nor disadvantageous, it remains in the body. This is called a vestigial organ.– E.g. Humans do not use their

appendix, but it has been suggested that it was used to assist the digestive system.