Jemma, this is a fantastic question.
People for hundreds of years have recognized
that living things do change form and abilities
over time – that living things evolve.
While evidence like dinosaur bones and the
domestication of animals by humans suggested that
living things must evolve in some way, people
in the 19th century did not agree on how evolution
The inadequacy of previous explanations and
numerous insightful observations about the world
led Charles Darwin to develop a superior
explanation, evolution by natural selection.
Darwin’s idea combines a few claims about the world:
(1) all organisms are in competition over
limited resources, especially opportunities to
(2) organisms exhibit individual
differences, few are totally alike;
(3) differences can be passed down in
genes, for instance children resemble their
(4) some individuals had traits that enhanced
their ability to reproduce, better eyesight
(5) those traits that enhanced an organism’s
ability to reproduce then get passed down to
offspring more often, and eventually that
trait will become standard among the
species –evolution has then occurred.
Evolution by natural selection best explains the
diversity and complexity of life on Earth. The
environment acts as a filter, determining
which genes are successfully transmitted to the
next generation: animals evolved and continue to
evolve by descent with modification.
Thanks for the great question,
Good question. There are four different ways
that evolution can happen. These are:
(1) mutation, by which alleles (copies of
genes) get transformed into new alleles by
ultraviolet, chemical alteration, etc.
(2) genetic drift, which is random
differences in survival between individuals that
happen to have different alleles,
natural selection, which is
individuals with some alleles surviving more
because of their alleles and is not random, and
(4) gene flow, which is populations
containing different alleles intermingling and so
causing their alleles to move between populations.
All four of these processes exist in
nature. For example, in humans, the ancestors
of our species all had brown eyes. Ten thousand
years ago or so, a mutation occurred in the Black
Sea region that created a human strain that had
blue eyes. In Europe and northern Asia,
this blue-eyed strain survived more because
there is less ultraviolet light far from the
equator, so it became common among Europeans
and Russians by natural selection, but it did not
survive well closer to the equator, which is why
Iranians and other southern peoples still have
Peoples around the Mediterranean traded and
intermarried, which is how brown-eyed people are
present in places like England, and European
colonists traveled to America and took their blue
eyes with them, an example of gene flow.
Finally, Europeans and Americans have largely
stable birth rates thanks to societal changes
which is not true in Africa and southern Asia
where almost everybody is brown eyed, but these
changes are not a consequence of eye color, making
this rapid expansion of brown-eyed peoples an
example of genetic drift.
Because natural selection is the only way that
evolution can happen that is not random, all
complex adaptations had to have come about or
proliferated as a result of natural
selection, but the true story is almost
certainly more complicated than just natural
selection in the vast majority of cases. In many
cases, the details are still very murky. Blue eye
color in humans is recently evolved and simple, so
the story is (relatively) obvious. We do not know
yet, for example, how intelligence evolved in
humans, and the same is true of a great many
other traits in other animals (and plants and
fungi and bacteria and viruses and everything else
that can evolve).
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