Change in a
The Theory of Evolution
The process by which species of organisms change over time is called the theory of evolution.
A theory is a scientiﬁc statement that is supported by many years of observations and experi-
ments, like the cell theory. The theory of evolution proposes that species change through the
processes of mutation, adaptation, natural selection, and extinction.
likely to survive longer and reproduce. As a result,
their offspring may inherit the same beneficial traits
and pass them on to successive generations. Over a
period of time, more and more organisms within a
population will have the favorable traits. Organisms
that do not inherit the beneficial traits may not sur-
Evolution does not say man came from a
monkey, but that man and some ape spe- vive as well or live long enough to reproduce. This
cies may have a common ancestor process is called NATURAL SELECTION.
When a species does not have the traits it
needs to survive in its environment, or to survive a
In some cases, genetic variations are caused by a change in its environment, extinction may occur.
change in the genetic code, or DNA itself, called a Extinction occurs when all the members of a species
mutation. A mutation can result in offspring inherit- disappear from the Earth.
ing different traits or characteristics than their par-
ents. Unfavorable changes in offspring can reduce an Peppered Moths are a
classic example of natural
organism’s ability to exist or survive in its environ- selection at work. Before
ment. Favorable changes improve an organism’s the industrial revolution in
ability to survive in its environment. These favor- England, trees along the
able changes are called adaptations. Thames river were white.
ADAPTATIONS are inherited characteristics or
behaviors that enable a species to survive and repro-
duce. Structural adaptations include such things as The moths that blended
body color, body covering, and different types of best were white, and the
black mutation was rare,
body parts such as beaks and claws. Behavioral ad- because the black moths
aptations could include beavers slapping their tails were more likely to be
and squirrels chattering to warn when a predator is eaten. After factory smog
near. turned the trees black, the
population became mostly black moths because
When organisms inherit adaptations that help the white are more likely to be eaten.
them survive in their environment, they are more
These processes of species change have occurred over long periods of time on the Earth, which scientists
believe may be close to 4.6 billion years old. We have learned about the age of the Earth and its changing
inhabitants by studying rocks and the remains of plants and animals preserved in rocks. These remains are
Scientists also use evidence from genetic information to support the theory of evolution.
They compare the DNA of different species and can tell how closely they are related by the
number of similarities they find. Scientists believe that organisms with similar genes are de-
scended from a common ancestor. For example scientists believe that bears and raccoons
come from a common ancestor that lived over 40 million years ago.
Distribution of Organisms
The distribution of organisms on Earth is also used as evidence for the theory of evolution.
Scientists have found that there is an unequal distribution of organisms on Earth. A species that is found in a
particular ecosystem, such as cacti in the deserts of North America, is not found in a similar ecosystem such as
the deserts of Africa. Scientists believe that each species originated or began in one location and from this
point of origin spread out until it was stopped by a physical geographic barrier such an ocean, an environ-
mental barrier such as a colder climate, or an ecological barrier such as competition from another species.
Anatomical evidence is also used to support the theory of evolution. Scientists study the similarities in body
structure among different species such as whales and humans. Such similarities as skeletal structure provide
evidence that two very different organisms may have evolved from a common ancestor.
HOMOLOGOUS STRUCTURES are body parts that are structurally similar in related species; these provide
evidence for a common ancestor. These homologous structures are also linked to common DNA. Homologous
structures include a dolphins front flipper, a human arm, a bat’s wing and a rats foreleg. All have different
purposes and look different on the outside, but the underlying bone structure is very similar. This is different
from a bat’s wing and a bee’s wing - which have similar function (to fly) but very different structures.