The Ecology Book

28 EVOLUTION BY NATURAL SELECTION

helped an individual organism

live longer and reproduce more

successfully would be passed on

to more offspring, while those that

made the organism less successful

would be lost. Darwin called this

“natural selection”—a process

that, over generations, enabled

a population of any given species

to adapt better and thrive in its

chosen habitat.

Sexual selection

Darwin also developed a theory

of sexual selection. First outlined in

On the Origin of Species, this was

developed further in The Descent

of Man, and Selection in Relation to

Sex (1871). This theory was distinct

from natural selection, as Darwin

recognized that animals select

mates based on characteristics that

do not simply favor survival. For

example, when Darwin considered

the spectacular but cumbersome

tails of male peafowl (peacocks), he

could not imagine the tail playing

any role in helping the individual

bird to survive. He concluded that

they were designed to boost an

individual’s chance of reproductive

success. Peahens choose males

with the brightest tails, so the

genetic material of these showy

males is passed to the next

generation. Bright tail feathers

indicate that the bird is healthy, so

choosing a mate with a bright tail

is a good strategy for the peahen.

However, Darwin’s idea that

females choose a mate came under

fire; 19th-century society could

accept that males competed to

reproduce (intrasexual selection),

but intersexual selection, where

one sex (usually the female) makes

the choice, was ridiculed.

Reproductive success is clearly

essential for the future of a species.

Natural selection is often described

as “survival of the fittest,” but

longevity alone is not particularly

Natural selection

There is variation

in traits.

For example, some

beetles are pale and

others dark.

There is differential reproduction.

No environment can support unlimited

population growth, so some individuals

lose out. Here, birds eat the pale

beetles, so fewer of them reproduce.

There is heredity.

The dark beetles have

more dark offspring

because this trait has a

genetic basis.

End result:

If darkness is the

winning trait, producing

more offspring, in time,

all beetles will be dark.

ideas. Papers by Darwin and

Wallace were jointly presented

at a meeting of the Linnean

Society of London in July 1858,

under the title “On the Tendency

of Species to form Varieties; and

on the Perpetuation of Varieties

and Species by Natural Means

of Selection..

The following year, Darwin

published the theory in On the

Origin of Species by Means of

Natural Selection. It offended some

scientists because it differed from

Lamarck’s ideas of transmutation,

and also upset creationists who

argued that it undermined a literal

interpretation of the Bible. Others

felt that the theory did not account

for the huge range of characteristics

in species and called it “unguided”

and “nonprogressive.”

Darwin was confident. He knew

that all individual organisms in a

species show a degree of natural

variation; some have longer

whiskers, or shorter legs, or brighter

colors, for instance. Because

members of all species compete for

limited resources, he deduced that

those whose traits are best suited

to their environment are more likely

to survive and reproduce. He also

argued that characteristics that

I see no good reasons why

the views given in this

volume should shock the

religious views of anyone.

Charles Darwin

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