ficial because it allows the giraffe to reach leaves high in trees. Natural selection caused this
beneficial trait to become more common than short necks.
As new mutations (changes in the DNA sequence) are constantly being generated in a pop-
ulation’s gene pool, some of these mutations will be beneficial and result in traits that allow
adaptation and survival. Natural selection causes evolution through the genetic change of a
species as the beneficial traits become more common within a population.
Artificial selection is when humans select which plants or animals to breed to pass specific
traits on to the next generation. A farmer may choose to breed only the cows that produce
the best milk (the favored traits) and not breed cows that do not produce much milk (a
less desirable trait). Humans have also artificially breed dogs to create new breeds (Figure
7.26).
Figure 7.26: Artificial Selection: Humans used artificial selection to create these different
breeds. Both dog breeds are descended from the same wolves, and their genes are almost
identical. Yet there is at least one difference between their genes that determine size. ( 8 )
Reproductive Isolation
There are two main ways that speciation happens naturally. Both processes create new
species by isolating groups (populations) of the same species from each other. Organisms
can be reproductively isolated from each other either geographically or by some behavior.
Over long period of time (usually thousands of years), each population evolves in a different
direction. One way scientists test whether two populations are separate species is to bring