Thanks for the great question.
You are right! Populations of living things
change over time because of evolution by
A scientist named Charles Darwin observed that
(1) have to compete to survive,
(2) exhibit variation (differences in size,
coloration, behavior, etc.), and
(3) pass down their traits to their offspring.
It was those living things that fit best in
their environment that were able to survive and
reproduce the most. The traits of the fittest
individuals then spread throughout a species,
changing the population.
Over time, natural selection results in the
evolution of adaptations, or functional
of a living thing that promote its survival and
reproduction. An elephant’s trunk or giraffe’s
long neck are examples of adaptations.
Thanks again for your question,
Populations can change due to a variety of
factors. Some of the major causes are
1) changes in the environment that require
the population to change and adapt to survive,
specifically, changes that alter the availability
of resources such as food, water, and shelter,
such as the climate changing from hot to cold (can
decrease amount of food and water available and
require certain types of shelters);
2) spontaneous mutations, which can change
a fraction of a population such that this fraction
can grow faster and stronger under existing
conditions and eventually out-compete the rest of
3) introduction of another species/population,
which can compete with, coexist with, or
benefit the existing population, and the effects
from these three different modes are very
different in terms of changes to the existing
This is, of course, not an exhaustive list; 1)
and 3) are examples of selection (though 3)
can be seen as a subset of 1)), and 2) is an
example of variation (assuming no outside
forces). Adaptation is the word for the
population responding to changes, so the
concept of adaptation would apply to any changes
in the population to better survive a given
Yes, to all those. I just read in Science
magazine about sea nomads - people who have
for hundreds of years on houseboats on the ocean
between Malaysia, the Philippines, and Indonesia
[Apr 20 issue, pg 245].
Researchers say they have
'evolved adaptations' for fishing
long periods of time, where oxygen is low.
'Evolved' would be natural selection of people who
have beneficial 'adaptations' that arise through
'variation' in genes among individuals. These
people have big spleens that seem to be due to
about 25 changes in their genes, compared with
people in the same region who live on land.
Evolution can be caused by a number of processes.
These include natural selection, artificial
selection, genetic drift, mutation, and gene flow.
Except for mutation, all of these require there
to be existing variation in a population,
and furthermore that variation must be inherited
from parent to offspring. Mutation is different
because it creates variation. All other mechanisms
of evolution destroy variation, which means that
without mutation, evolution would eventually
grind to a halt as all of the variation got used
Natural selection takes place when
individuals that possess traits that enable them
to survive and/or reproduce do so, in the process
passing on any heritable variation they may
possess. If there is no variation, then natural
selection will not occur, because all of the
individuals are the same and have the same chances
of surviving and reproducing. If there is
variation but that variation cannot be inherited,
then natural selection will occur, but evolution
will not: the offspring will be no different
from the previous generation of offspring, even if
the parents have been filtered by selection. Only
if there is heritable variation that affects
survival and reproduction can natural selection
result in evolution.
Artificial selection works just like
natural selection, except that it is humans who
decide who gets to survive and reproduce rather
than natural forces like finding food, avoiding
becoming prey, fighting off parasites, surviving
dangerous weather, etc. Like natural selection,
artificial selection also requires heritable
variation, for the same reason.
Genetic drift is what happens when some
individuals just happen to have more offspring
than others , and who also happen to have
different heritable traits. Evolution happens as a
result of some traits being passed on while others
do not, but those traits do not affect survival or
reproduction in any way. They winners were just
lucky, and the losers unlucky.
Gene flow takes place where individuals with
heritable traits move to another population with
different heritable traits. This still causes
evolution because the traits in a population are
changing, but there are no winners or losers, just
individuals moving around.
Notice that genetic drift, mutation, and
gene flow are all completely random. This
means that, while they cause evolution, they do
not cause adaptation; the direction of evolution
may help offspring cope with their environment, or
it may harm them, or it may have no effect. Of
the natural evolutionary forces, the only one that
can cause adaptation is natural selection.
Artificial selection can also cause adaptation, if
what the humans doing the selecting want is
adaptive, but it can also be mal-adaptive (the
opposite of adaptive) as well, if the humans want
to create something mal-adaptive instead.
There may be other forces of evolution besides
these, as well, but these are the ones that we
Finally, there are ways to make a population
look different without it evolving. For example,
limber pine (Pinus flexilis) grows tall at lower
altitudes and is stunted at higher altitudes.
Warming the climate will make it grow taller and
cooling the climate will make it grow shorter.
This is not evolution because it's not acting on
heritable variation. If the climate changes, the
trees will grow differently. If the climate
changes back to what it was before, and the trees
change back to how they were before.
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