Visualizing Environmental Science

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:

1. 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).


2. 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.


3. 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.


4. 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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