How does Evolution Occur.pdf - Mrs Stovel

How Does Evolution Occur?
Darwin's Theory of Natural Selection has been divided into five parts to
explain clearly how evolution happens in nature.
1. Organisms produce more offspring than actually survive.
Organisms can die from many causes: disease, starvation, and being eaten,
among other things. The environment can't support every organism that is born.
Many die before they are able to reproduce.
2. Every organism must struggle to survive.
One reason that not all organisms survive is that there are not enough resources,
things that they need, to go around. Organisms must struggle to get what they
need to survive, competing against other organisms that want the same things
they do. They also have to struggle to get away from predators and to overcome
disease. For example, a fox struggling to catch a rabbit, which struggles to
escape. As explained in part one, not all of them make it.
3. There is variation within a species.
Not all of the individuals in a species are exactly the same. There are variations,
differences, among members of a species. If you look at the spots on several
different ladybugs, or the stripes on zebras, you will notice that they don't all
have the same number or arrangement of spots or of stripes. In addition to these
easily visible variations, there are differences in skill and behavior, such as
differences in how fast the zebras can run. If organisms were all the same, none
would be better suited than any other, and selection could not occur.
4. Some variations allow members of a species to survive and reproduce better
than others.
If an organism has a trait that helps it survive or reproduce, it is more likely to
survive and be able to reproduce. A faster cheetah is more likely to catch a
gazelle and survive, and a faster gazelle is more likely to escape the cheetah
and survive. A showier flower is more likely to be noticed by a bee, which
enables it to reproduce. A thornier cactus is more likely than other cactuses to
be left alone by animals, rather than be eaten and die.

5. Organisms that survive and reproduce pass their traits to their offspring, and
the helpful traits gradually appear in more and more of the population.
Most of an organism's traits are passed on to its offspring. If more of the
organisms with the helpful trait survive, then in the following generations,
more and more of the population have that trait. If there are some faster
cheetahs and some slower cheetahs, the faster cheetahs will be better able to
catch food and survive. With more of the slower ones dying before they can
reproduce, and more of the faster ones surviving and reproducing, over
generations the population on the whole will gradually become made up of
faster cheetahs.
Here is an example demonstrating each of these points, told as the story of a
population of brown hares in a polar region becoming white hares:
1. There are hares living in a cold, snowy, polar region. All of the hares are
brown* and many of them are killed and eaten by other animals each year, so
more are born than survive.
2. These hares are competing for life and struggling to survive, partly by trying
to avoid predators, to "not be the one who gets eaten."
3. Each hare is a little bit different from other hares, but one time a few hares
are born albino, white, because of a mutation in the genes.
4. This variation in color helps the albino hares to survive. Against the white
snow, predators have a harder time seeing them, so more of the albino hares are
able to survive and therefore to reproduce.
5. In the next generation there are more white hares, because the white hair
gene is passed on. In this generation, too, more white hares survive and
reproduce. They pass the albino trait on to their offspring, who also survive and
reproduce more than do the brown hares. Gradually, there are more and more
white hares in the population, until the entire population is made of white hares.
* You might ask: What were brown hares doing in a polar region? The brown
color in an Arctic region was caused by one of two things, both of which are
major causes of change and the arising of new species. One possibility was that
a group of hares migrated from a warmer region and became isolated, the other
is that the region where the hares lived became colder.

Lamarck vs. Darwin
Jean-Baptiste Lamarck (1744-1829) and Charles Darwin (1809-1882) both thought and
had ideas about how life on earth got to be the way it is now. They had some similar and
some very different ideas.
How They Agreed
Unlike most other people at that time, Darwin and Lamarck both thought that life had
changed gradually over time and was still changing, that living things change to be
better suited and adapted to their environments, and that all organisms are related.
Darwin and Lamarck also agreed that life evolved from fewer, simpler organisms to
many, more complex organisms.
What Lamarck Believed
Lamarck is best known for his Theory oflnheritance ofAcquired Characteristics, first
presented in 1801 (Darwin 's first book dealing with natural selection was published in
1859): If an organism changes during life in order to adapt to its environment, those
changes are passed on to its offspring. He said that change is made by what the
organisms want or need. For example, Lamarck believed that elephants all used to have
short trunks. When there was no food or water that they could reach with their short
trunks, they stretched their trunks to reach the water and branches, and their offspring
inherited long trunks. Lamarck also said that body parts that are not being used, such as
the human appendix and little toes are gradually disappearing. Eventually, people will be
born without these parts. Lamarck also believed that evolution happens according to a
predetermined plan and that the results have already been decided.
What Darwin Believed
Darwin believed that the desires of animals have nothing to do with how they evolve,
and that changes in an organism during its life do not affect the evolution of the species.
He said that organisms, even of the same species, are all different and that those which
happen to have variations that help them to survive in their environments, get to
reproduce and have more offspring. The offspring are born with their parents` helpful
traits, and as they reproduce, individuals with that trait make up more of the population.
Other individuals, that are not so well adapted, die off For ex., elephants used to have
short trunks, but some had longer trunks. When there was no food or water that they
could reach with their short trunks, the ones with short trunks died off, and the ones with
long trunks survived and reproduced. Eventually, all of the elephants had long trunks.
Darwin also believed that evolution does not happen according to any sort of plan.

Change and Isolation
Environmental change and isolation of groups of organisms play an important
role in evolution.
Environmental change is any change in an environment to which an organism
must adapt. Change can be gradual, such as when mountains or deserts form,
other species die out, or new species evolve. These things can take millions of
years, Change to an environment can also be quick, such as floods, volcanoes,
or earthquakes. It can also be caused not by change to the environment itself,
but by the organism's movement to a different environment.
Change in an organism's environment forces the organism to adapt to fit the
new environment, eventually causing it to evolve into a new species. For
example, if a species of animal is mostly limited to eating one kind of leaf, and
a change occurs: a fungus attacks and kills most of that kind of plant, the
animal has to evolve either to fight the fungus or to eat something else.
Isolation means that organisms of the same species are separated, and happens
when there is something between the organisms that they can't cross.
Organisms become isolated as a result of environmental change. The cause of
isolation can be gradual, like when mountains or deserts form, or continents
split apart. It can also be quick, such as organisms being blown to different
places by a storm or tsunami (tidal waves).
When organisms become isolated the two groups are also not able to reproduce
together, so variations and mutations that occur in one group are not necessarily
found in the other group. The longer the groups are isolated, the more different
they are. They eventually become different species. Moreover, if there is a
change in the environment of one group it does not necessarily occur in the
environment of the other. So they will evolve and adapt differently.
The finches and other organisms that Darwin found on the Galapagos Islands
are examples that demonstrate the effect that environmental change and
isolation can have on a species.

Sexual Selection
Up until now, we have discussed reproduction of an organism as resulting
from traits that enable it to survive. For example, in a cold climate, lots of fur
enables survival, which enables reproduction (which leads to these traits being
passed on and becoming more common). This can be called "reproduction
through survival." The ability to reproduce, however, also results from traits
that are directly related to the ability to reproduce, but play no role in the
survival of the individual organism. This is "reproduction through
reproduction."
In evolution, having traits that help one survive is very important, but it is only
important so that one can reproduce and pass those traits to the next generation.
For this reason, traits that enable an organism to reproduce, without necessarily
helping them to survive are selected for: the ones that reproduce pass the traits
that helped them to reproduce on to their offspring and the ones that don't
reproduce don't have offspring, so their traits disappear from the population.
Darwin called this idea "sexual selection".
Here are two examples:
The male peacock has a lot of showy feathers. One would think that these
would not appear in evolution; they would be noticeable to predators and would
get the peacock stuck so it would not be able to escape from predators or get
food. However, peacocks use their feathers to attract peahens (female
peacocks). The peacocks with the showier feathers are able to attract mates, so
they are the ones that have offspring, and pass on the fancy-feather genes to the
next generation. However, the organism must also be able to survive. Peacocks
can fold up their tails, which lessens the danger of being noticed by predators
or getting caught in the bushes. In addition, as with most species where the
males are colorful or fancy to attract the females, the peahens are much duller
and more camouflaged, in a large part because they are the ones who guard the
eggs and chicks. Survival of the next generation is very important.
Many plants have flowers that are pretty and brightly colored. This is because
the brightest colors attract bees, butterflies, and other pollinators, which enable
the plant to reproduce. If a flower is dully colored, it does not reproduce, and
the dull color genes dissappear from the population, whereas the plants with
brightly colored flowers reproduce, so they gradually make up more and more
of the population.

Co-Evolution
One of the most important parts of an organism's environment is other organisms. Coevolution
occurs when, in adapting to their environments, two or more organisms evolve
together. To "make the best of' where they live, organisms make use of other organisms
by eating them, living on or in them, and/or building a "partnership" with them.
Organisms co-evolve with many species at the same time, because an environment
includes many different types of organisms.
Organisms have to adapt to form these relationships just as they have to adapt to any
other part of their environments. These adaptations cause so much change that
descendants, who are better suited to take advantage of the relationship, are so different
that they may be an entirely different species than their ancestors.
It is important to note that species that are closely related did not always appear at the
same time. For example, although the first mammals appeared approximately 225
million to 180 million years ago, not all mammal species appeared at that time. This is
especially important to realize in the case of co-evolution. Organisms have co-evolved
whose families first appeared very far apart on evolution's time scale. For example, the
first segmented flatworms originated millions of years before the first mammals.
However, the tapeworm, a segmented flatworm that is a parasite of humans, cows, and
other mammals, could not have evolved before the first large mammals, because it
adapted so much to the parasitic relationship that its ancestors before the relationship
were not of the same species of worm.
Three types of co-evolution are predator-prey relationships, symbiotic relationships, and
parasitic relationships.