148 CHAPTER 6 Ecosystems and Evolution
selection: The best-adapted individuals are those that
reproduce most successfully, whereas less-fit indi-
viduals die prematurely or produce fewer or inferior
offspring. Over time, enough changes may accumu-
late in geographically separated populations (often
with slightly different environments) to produce new
species (Figure 6.18).
One premise on which Darwin based his theory of
evolution by natural selection is that individuals transmit
traits to the next generation. However, Darwin could not
explain how this occurs or why individuals within a popu-
lation vary. Beginning in the 1930s and 1940s, biologists
combined the principles of genetics with Darwin’s theory
of natural selection. The resulting unified explanation of
evolution is known as the modern synthesis (where synthe-
sis refers to a combination of parts of previous theories).
The modern synthesis explains Darwin’s observa-
tion of variation among offspring in terms of mutation,
Natural Selection
Evolution occurs through the process of natural selection.
As favorable traits increase in frequency in successive
generations, and as unfavorable
traits decrease or disappear, the
collection of characteristics of a
given population changes. Natu-
ral selection is the process by
which successful traits are passed
on to the next generation and un-
successful ones are weeded out. It
consists of four phenomena that
occur in the natural world , which
can be considered conditions
necessary for natural selection to
take place:
- High reproductive capacity. Each species produces more
offspring than will survive to maturity. Natural popu-
lations have the reproductive potential to increase
their numbers continuously over time (Figure 6.17). - Limits on population growth, or a struggle for existence.
Only so much food, water, light, growing space, and
so on are available to a population, and organisms
compete with one another for the limited resources
available to them. Because there are more individu-
als than the environment can support, not all of an
organism’s offspring will survive to reproductive age,
including many of the fish yet to hatch in Figure 6.17.
Other limits on population growth include predators
and diseases. - Heritable variation. The individuals in a population
exhibit variation. Each individual has a unique com-
bination of traits, such as size, color, and ability to
tolerate harsh environments. Some traits improve the
chances of an individual’s survival and repro ductive
success, whereas others do not. It is important to
remember that the variation necessary for evolution
by natural selection must be inherited so that it can
be passed to offspring. - Differential reproductive success. Individuals that possess
the most favorable combination of characteristics
(those that make individuals better adapted to their
environment) are more likely than others to sur-
vive, reproduce, and pass their traits to the next gen-
eration. Sexual reproduction is the key to natural
High reproductive capacity and limits to
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A jawfish incubates eggs in his mouth. If all offspring of a jawfish
pair survived and in turn reproduced, reefs would be choked
with jawfish. Yet this fish species has not overrun the ocean,
because individuals must avoid predation and compete for
limited resources. Photographed at Dimakya Island, Philippines.
natural selection
The tendency of
better-adapted
individuals—those
with a combination
of genetic traits
best suited to
environmental
conditions—to
survive and
reproduce, increasing
their proportion in the
population.
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