Biology (Holt)

Formation of New Species

Species formation occurs in stages. Recall that natural selection
favors changes that increase reproductive success. Therefore, a
species molded by natural selection has an improved “fit” to its
environment. The accumulation of differences between groups is
called (die VUHR jehns). Divergence leads to the for-
mation of new species. Biologists call the process by which new
species form (spee see AY shun).

Forming Subspecies

Separate populations of a single species often live in several differ-
ent kinds of environments. In each environment, natural selection
acts on the population. Natural selection results in the evolution of
offspring that are better adapted to that environment. If their envi-
ronments differ enough, separate populations of the same species
can become very dissimilar. Over time, populations of the same
species that differ genetically because of adaptations to different
living conditions become what biologists call .The mem-
bers of newly formed subspecies have taken the first step toward
speciation. Eventually, the subspecies may become so different that
they can no longer interbreed successfully. Biologists then consider
them separate species.

Maintaining New Species

What keeps new species separate? Why are even closely related
species usually unable to interbreed? Once subspecies become
different enough, a barrier to reproduction, like the one shown in
Figure 14, usually prevents different groups from breeding with
each other.

subspecies

speciation

divergence

SECTION 3Examples of Evolution 291

Real Life

Why do we find certain

people pretty or

handsome?

Some evolutionary

biologists think that many

traits that contribute to a

person’s attractiveness

actually reveal the per-

son’s fitness as a mate.

Finding Information

Research and compare

hypotheses concerning

biological and cultural

reasons that people judge

others as attractive.

Figure 14 Mating activity

in various frogs.Though

they appear to be similar, pick-

erel frogs (Rana palustris) and

leopard frogs (Rana pipiens)

are different species. The

graph shows that the time of

peak mating activity varies

between four species of frogs.

Leopard frog

Tree frog

Pickerel frog

Bullfrog

March 1 April 1 May 1 June 1 July 1

Mating activity

Month

Pickerel frog Mating Activity in Frogs

Leopard frog

3A