Environmental Engineering Reference
In-Depth Information
Figure 4-2
Fossilized skeleton of an ichthyo-
saur, a marine reptile that lived more than
200 million years ago.
Evidence about the earth's early history also
comes from chemical analysis and measurements of
the half-lives of radioactive elements in primitive
rocks and fossils. Scientists also drill cores from glacial
ice and examine the kinds of life found at different lay-
ers. They also compare the DNA of past and current
organisms.
species arise from ancestral species and other species
are lost through extinction.
How Does Microevolution Work?
A population's gene pool changes over time when
beneficial changes or mutations in its DNA molecules
are passed on to offspring.
A population's
gene pool
consists of all of the genes
(Figure 2-4, p. 25) in its individuals.
Microevolution
is a
change in a population's gene pool over time.
The first step in microevolution is the develop-
ment of
genetic variability
in a population. Genetic vari-
ability in a population originates through
mutations:
random changes in the structure or number of DNA
molecules in a cell. Mutations can occur in two ways.
First,
DNA may be exposed to external agents such as
radioactivity, X rays, and natural and human-made
chemicals (called
mutagens
).
Second,
random mistakes
sometimes occur in coded genetic instructions when
DNA molecules are copied each time a cell divides and
whenever an organism reproduces.
Mutations can occur in any cells, but only those in
reproductive cells are passed on to offspring. Although
some mutations are harmless, most are lethal.
Occasion-
ally,
a mutation results in beneficial genetic traits may
give that individual and its offspring better chances for
survival and reproduction under existing environmen-
tal conditions or when such conditions change.
4-2
EVOLUTION AND ADAPTATION
Biological Evolution
Biological evolution is the change in a population's
genetic makeup over time.
According to scientific evidence, populations of organ-
isms adapt to changes in environmental conditions
through
biological evolution,
known more simply as
evolution.
Evolution involves the change in a popula-
tion's genetic makeup through successive generations.
Populations—not individuals—evolve by becoming geneti-
cally different.
According to the
theory of evolution,
all species
descended from earlier, ancestral species. In other
words, life comes from life. This widely accepted sci-
entific theory explains how life has changed over the
past 3.7 billion years (Figure 4-3, p. 66) and why it is so
diverse today. Religious and other groups may offer
other explanations, but evolution is the accepted scien-
tific explanation.
Natural Selection
Some members of a population may have genetic
traits that enhance their ability to survive and produce
offspring with these traits.
Natural selection
occurs when some individuals of a
population have genetically based traits that increase
their chances of survival and their ability to produce
offspring with the same traits.
Get a detailed look at early evolution—the roots of the tree
of life—at Environmental ScienceNow.
Biologists use the term
microevolution
to describe
small genetic changes that occur in a population. They
use the term
macroevolution
to describe long-term,
large-scale evolutionary changes through which new
