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Every plant or animal belongs to a species. A species is a population
of plants or animals that can breed to produce offspring that can then
produce offspring themselves. Biologists believe that new species evolve
from existing species by a process called natural selection. Here's how
it works. Genes are chemical structures in the cells of the organism.
The nature of the organism is determined by its genes. The organism inherits
the genes from its parents. Occasionally a gene changes accidentally.
That's called a mutation. The changed gene is passed on to the next generation.
Most mutations are bad, but some of them make the organism more successful
in its life. Organisms that inherit that favorable new gene are likely
to become more abundant than others of the species.
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Copyright © Bruce S. Grant
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This photograph shows two different moths against a tree trunk.
One is white with gray specks and one is black. In England
during the 1800s, most peppered moths were white with gray
specks. However , at the end of the century, industrial activity
in most cities polluted the air with soot and smog. The gray-speckled
moth was more visible agains the tree trunks and easier for
birds to prey upon. In contrast, the black-peppered moths
were less vulnerable to being eaten by birds because their
darker color camouflaged them agains the darker tree trunks
tainted by air pollution.
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Sometimes the population of a species becomes separated into two areas,
by geography or by climate. Then the two groups no longer breed with each
other. The two groups then slowly change by natural selection. Each group
changes in different ways. Eventually, the two groups are so different
that they can't breed to produce offspring any more. They have become
two different species. Species eventually become extinct. That means that
the population gets smaller and smaller, until no more organisms of that
species are left alive. Species become extinct for various reasons. If
the environment changes too fast, the species might not be able to adapt
fast enough. Also, a new species might evolve to compete with an existing
species. Biologists are sure that once a species becomes extinct it never
appears again.
In the modern world, biologists can identify species by seeing whether
the organisms can breed with one another. Paleontologists have much more
trouble with fossil species, because the organisms are no longer around
to breed! All that can be done is to match up shells or imprints that
look almost identical and then assume that they represent a species.
Paleontologists are sure that the fossil record is biased. That means
that some kinds of organisms are much scarcer as fossils than they were
when they were alive. Other kinds of organisms are much better represented
by fossils. Animals with hard shells and skeletons are represented well
in the fossil record. On the other hand, soft-bodied animals are probably
represented very poorly. It's likely that most soft-bodied species that
ever existed are gone forever without a trace. Land animals are probably
very poorly represented as well. For example, most animals that are now
alive, or ever have lived, are insects, but the fossil record of insects
is poor.
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