Population Responses to Changing Conditions over Time: Evolution 147- Which environmental factors shape flowing-
water ecosystems? standing-water ecosystems? - How do the characteristics of a freshwater
wetland differ from those of an estuary? How
does a mangrove swamp differ from a salt marsh?
Both salt marsh and mangrove forest ecosystems have
experienced significant losses due to coastal development.
Salt marshes have been polluted—by countless ongoing
sources as well as oil spills—and turned into dumping
grounds; mangrove forests have been logged unsustain-
ably and used as aquaculture sites. Some countries, such
as the Philippines, Bangladesh, and Guinea-Bissau, have
lost 70 percent or more of their mangrove forests.- Since that time, scientists have accumulated an
enormous body of observations and experiments that
support Darwin’s theory. Although biologists still do not
agree completely on some aspects of the evolutionary
process, the concept that evolution by natural selec-
tion has taken place and is still occurring is now well
documented.
Population Responses to Changing
Conditions over Time: Evolution
LEARNING OBJECTIVES
- Define evolution.
- Explain the four conditions necessary for
evolution by natural selection to occur. - Describe various types of evidence that
supports evolution.
S
cientists think all of Earth’s remarkable variety
of organisms descended from earlier species
by a process known as evolution. The concept
of evolution dates back to the time of Aristotle
(384–322 b.c.e.), but Charles Darwin (1809–1882), a 19th-
century naturalist, proposed the mechanism of evolution
that today’s scientific community
still accepts (Figure 6.16). As you
will see, the environment plays a
crucial role in Darwin’s theory of
evolution.
It occurred to Darwin that in
a population, inherited traits fa-
vorable to survival in a given environment tended to be
preserved over successive generations, whereas unfavor-
able traits were eliminated. The result is adaptation, an
evolutionary modification that improves the chance of
survival and reproductive success of a species in a given
environment. Eventually the accumulation of many adap-
tive modifications might result in a new species.
Darwin proposed the theory of evolution by natu-
ral selection in his monumental book The Origin of Spe-
cies by Means of Natural Selection, which was published inevolution
The cumulative
genetic changes in
populations that occur
during successive
generations.Portrait of a young Charles Darwin
UÊ}ÕÀiÊÈ°£ÈÊJAMES L. STANFIELD/NG Image Collection