Chapter 15 “The Theory of Evolution”

• The central thought behind evolution is that organisms must survive and produce healthy offspring in order to continue their species.

• In 1859, Charles Darwin published evidence that species do evolve.

• Almost all scientists accept that evolution is the basis for the diversity of life on Earth.

Four Principles of Natural Selection

1. Variation exists within the genes of every species as a result of random mutations.

2. In particular environments, some individuals within a population are better suited to survive & have more offspring

3. Favorable traits will increase in a population.

4. Evidence that living species evolved from organisms that are extinct.

Lamarck’s Law of Use and Disuse

• Before Darwin’s proposal of evolution, a French scientist Jean Baptiste Lamarck, proposed a mechanism for evolution.

• He said that evolution occurs through the use and disuse of physical features by individual members of a species.

• He believed that in the lifetime of an individual, structures increase in size because of use or reduce in size because of disuse.

• He then reasoned that the modified part is then passed on to the organism’s offspring.

• If Lamarck was correct, the offspring of bodybuilders would be born with enormously developed muscles.

Natural Selection

1. “Struggle for Existence”: individuals compete for limited resources (water, living space, light, food).

• In this struggle, the predators that are faster or have a particular way of catching other organisms can catch more prey.

• Those prey that are faster, better camouflaged, or better protected can avoid being caught.

2. “Survival of the Fittest”: Some inherited traits give individuals an advantage in coping with environmental challenges, allowing them to survive longer & produce more offspring Individuals differ in their traits.

• They may differ in size, coloration, running speed, resistance to disease, etc. this variation must be inherited for it to influence natural selection

• Individuals with characteristics that are not suited for their environment, either die or leave few offspring

• Individuals that are better suited to their environment, survive and reproduce most successfully. Because individuals with advantageous traits have more offspring, each new generation contains a greater proportion of the offspring with these traits than did the previous generation.

Descent with Modification

• Over long periods of time, natural selection produces organisms that have different structures, establish different niches or occupy different habitats.

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

Adaptation

• Organisms differ from place to place because their habitats present different challenges

• Each species has evolved in response to its particular environment

• The changing of a species that results in it being better suited to its environment is called adaptation

Isolation of Species

• Populations of the same species living in different locations tend to evolve in different directions

• As 2 isolated populations of the same species become more different over time, they may be unable to breed with another

• Generally, when individuals of 2 related populations can no longer breed, the 2 populations are considered different species.

Isolation leads to species formation

• The Kaibab squirrel and the Abert squirrel populations became isolated from each other 10,000 years ago.

• The Kaibab squirrel lives on the North Rim of the Grand Canyon

• The Abert squirrel lives on the South Rim• Over time, the Kaibab squirrel evolved a

black belly & other characteristics• The Abert squirrel evolved a white belly.

Abert squirrel

Kaibab squirrel

Evidence of Evolution

• Fossils offer the most direct evidence that evolution takes place

• Darwin’s theory is accepted by scientists as the best available explanation for the biological diversity on Earth

Most scientists agree on the following three points:

1. Earth is about 4.5 billion years old

2. Organisms have lived on Earth for most of its history

3. All organisms living today evolved from earlier, simpler life-forms

Fossil Record

Reasons for an incomplete fossil record:

1. Many species have lived in environments where fossils do not form.

2. Their bodies are eaten and/or scattered by scavengers.

3. It is difficult to form a fossil from a soft bodied animal as opposed to a hard bodied animal.

Anatomy & Development Support for Evolution

• Bones and other structures that are present in organisms but are reduced in size and either have no use or have a less important function are vestigial structures

• As different vertebrates evolved, particular sets of bones evolved differently

• Similarities in bone structures suggest that all vertebrates share a common ancestor

• Such structures are called homologous structures

• The forelimbs of all vertebrates are made from the same basic groups of bones

• Homologous structures are structures that share a common ancestry

Embryonic Development

DNA Contains a Record of Evolution

• The more similarities found in two DNA sequences, the more closely related are the organisms

Species Amino Acid differences

Gorilla 1

Rhesus monkey

8

Mouse 27

Chicken 45

Frog 67

Lamprey 125

Hemoglobin Comparison