What about fitness?

What about fitness?

Biologists use the word fitness to
describe how good a particular genotype is
at leaving offspring in the next generation relative to how good
other genotypes are at it. So if brown beetles consistently leave
more offspring than green beetles because of their color, you’d say
that the brown beetles had a higher fitness.

How to determine fitness in our beetle population
The brown beetles have a greater fitness relative to the green beetles.

Of course, fitness is a relative thing. A genotype’s fitness depends on the
environment in which the organism lives. The fittest genotype during an ice
age, for example, is probably not the fittest genotype once the ice age is

Fitness is a handy concept because it lumps everything that matters
to natural selection (survival, mate-finding, reproduction) into one idea.
The fittest individual is not necessarily the strongest, fastest,
or biggest. A genotype’s fitness includes its ability to survive,
find a mate, produce offspring — and ultimately leave
its genes in the next generation.

peacock Caring
for your offspring (above left), and producing thousands of young — many
of whom won’t survive (above right), and sporting fancy feathers that attract
females (left) are a burden to the health and survival of the parent.
These strategies do, however, increase fitness because they help the parents
get more of their offspring into the next generation.

It might be tempting to think of natural selection acting exclusively
on survival ability — but, as the concept of fitness shows, that’s
only half the story. When natural selection acts on mate-finding
and reproductive behavior, biologists call it sexual selection.

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