Chapter 18 Fossils and Geologic Time

420 CHAPTER 18: FOSSILS AND GEOLOGIC TIME
internal hard parts appeared about 545 million years ago.
These organisms eventually gave rise to the complex lifeforms
that exist today. The gradual change in living organisms
from generation to generation is known as evolution.
Although it is called a theory, organic evolution is one of the
most fundamental ideas of science.
The Work of Darwin
Organic evolution became accepted by most scientists after
Charles Darwin published On the Origin of Species by Means
of Natural Selection” in 1859. A species can be defined as a
group of organisms so similar they can breed to produce fertile
offspring. Humans are a good example of a biological
species. Darwin based his book on scientific observations and
extensive travel. He spent many years organizing his ideas
about the development of life before he published his book. In
fact, Alfred Russell Wallace independently arrived at similar
conclusions at about the same time.
Four Principles of Evolution
Organic evolution is based on four principles. First, variations
exist among individuals within a species. Besides differences
in size and shape, individual organisms have different abilities
to find food, resist disease, and reproduce effectively. Second,
organisms usually produce more offspring than the
environment can support. Think of the thousands of seeds
many trees produce or the number of eggs one salmon can lay.
If all these offspring survived, the world would be overrun
with just trees and salmon. The third principle is the effect of
competition among individuals of a given generation. Organisms
die for a variety of reasons, but those best suited to their
environment are most likely to survive. Fourth, the individuals
best suited to the environment will live long enough to reproduce
and pass their traits to following generations. This is
what Darwin meant by natural selection.