Download Theory of Natural Selection

Aim: Why do living things evolve?
I. Early Theories
A. Theory of Use and Disuse (Jean
Baptiste Lamarck – French biologist)
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.
Your thoughts on this Theory?
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
1831 at age of 22
Galapagos Islands – 600 miles west
of South America
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.