Aim: Why do living things evolve?

I. Early Theories

A. Theory of Use and Disuse (Jean Baptiste Lamarck – French biologist)

Your thoughts on this Theory?

1. Use and Disuse - Lamarck said that if a living thing used an organ, that organ became more highly developed. On the other hand, an unused organ tends to wither away, or atrophy.

1809

2. Inheritance of Acquired Characteristics – characteristics developed by an organism during its lifetime are acquired characteristics. Lamarck said that acquired characteristics are passed on to the next generation. Ex: Snakes lost their legs because these structures hampered crawling through low and narrow openings.

Your thoughts?

B. August Weismann – German Biologist who designed an experiment to test Lamarcks’s assumption that characteristics were acquired.

1. Weismann cut off the tails of mice and then mated the mice. When the offspring were born, he cut off the tails of the second generation and mated these mice. He did this for 20 generations of mice.

What did the results show?

2. You are correct! Weismann found that the mice tails did not shorten or disappear disproving Lamarck’s theory of use and disuse.

What did Lamarck really discover?

ADAPTATION

II. The Theory of Natural Selection

H.M.S. Beagle

Galapagos Islands – 600 miles west of South America

1831 at age of 22

A. 5 Distinct Ideas

1. Overproduction – a species tends to produce too many offspring for the available food and other necessities of life.

2. Struggle for Existence – Overproduction causes a struggle for existence. Competition occurs between members of the same species for life needs.

3. Variation – no two individuals are exactly alike. Darwin assumed that most variations are passed on to succeeding generations.

4. Natural Selection

a. Some organisms have variations that give them an advantage over others.

b. Due to these advantages, they are better able to compete in their environment, to survive, and to reproduce their kind.

c. In nature, the fittest survive, others die off without leaving offspring.

5. Origin of New Species – new species arise by accumulation of variations among the members of a population. When the population is sufficiently different from the original population, it is a new species.

B. Weakness of Darwin’s Theory

1. Did not explain how variations arise

2. Did not know the science of genetics to explain the origin of inheritable changes. (Gregor Mendel – 1860’s)

How could the evolution of the giraffe’s neck be explained by the Theories of Darwin and Lamarck?

Lamarck: Ancestors of the modern giraffe had a short neck. When grass was scarce, they ate the leaves from lower branches of trees, stripping bare these branches. The animals of one generation stretched their necks while reaching for higher branches. The acquired characteristic of a slightly longer neck was passed by heredity to the next generation. These animals also stretched their necks. After many generations, the short neck descendents had long necks.

Darwin: Among the population of short necked giraffes more were born than the land could support. These animals competed with each other for food that was on the branches of trees. Some giraffes had longer necks than others of their species. Those with the longer necks survived to reproduce and pass on this heredity to their offspring. After many generations of this procedure, the giraffes with short necks disappeared and the entire giraffe population had long necks.