Download •The Earth has millions of organisms that display different

•The Earth has millions of organisms that display different
characteristics and traits. This variety of living things is called
biological diversity. How did all of these different organisms
arise? How are they related? The Evolutionary Theory explains
these questions by using observations, scientific facts, and
hypotheses.
Evolution, or change over time, is the process by
which modern organisms have descended from ancient
organism. A theory is a well supported testable explanation of
phenomena that have occurred in the natural world.
Charles Darwin
Charles Darwin was one of the first scientists to develop a theory
of evolution. His theory was based on observations he made while
sailing around the world in 1831 on the H.M.S Beagle. One of the
focal points of his journey was the Galapagos Islands.
The Galapagos
Islands at the time
were fairly isolated
from the mainland of
Ecuador and Peru.
Within the island
chain there was
significant
differences between
each island. Hood
Island, for example,
had sparse
vegetation and was
extremely hot.
Isabela Island had
rich vegetation that
animals could easily
reach.
Darwin noticed that animals on each of the Islands were well
suited to survive and flourish according to the conditions on that
specific island. These observations started his thinking toward
how these animals became so well adapted.
The central theme in Darwin’s theory of evolution is the
concept of natural selection. Natural selection deals with
an organism struggle for existence, meaning that members of
each species compete regularly to obtain food, living space, and
other necessities. The ability of an individual to survive and
reproduce in its specific environment is called fitness. An
adaptation is any characteristic that increases an organism’s
chance of survival.
The struggle for existence, fitness, and adaptation lead to the
concept of survival of the fittest. Over time, natural
selection results in changes in the inherited
characteristics of a species’ fitness in its environment.
Great White Shark
Giant Anteater
How have these animal adapted to their environment, or changed
over time to become successful?
Darwin’s Theory of Evolution can be summarized.
1.
Individual organisms in nature differ from one
another. Some of this variation is inherited.
2. Organisms in nature produce more offspring than can
survive, and many of those that survive do not reproduce.
3. Because more organisms are produced that can survive,
members of each species must compete for limited
resources.
4. Because each organism is unique, each has different
advantages and disadvantages in the struggle for
existence.
5. Individuals best suited to their environment survive
and reproduce most successfully. The characteristics
that make them best suited to their environment are
passed down to the offspring.
6. Species change over time. Over long periods of time
natural selection causes changes in the characteristics of
species.
7. Species alive today have descended with modifications
from species that lived in the past.
8. All organisms on Earth are united into a single tree of
life by common descent.
•The formation of a new species is called speciation. Recall that
biologists define a species as a group of organisms that breed
with one another and produce fertile offspring. This means that
individuals in the same species share common traits. If those
traits increase fitness, than the traits will eventually be found
throughout the population.
•As new species evolve, populations become reproductively
isolated from each other. When the members of two
populations cannot interbreed and produce fertile offspring,
reproductive isolation has occurred. The sharks above are
related but unable to mate and produce offspring therefore they
are members of two different species.
Reproductive isolation can develop in a variety of ways including
behavioral isolation, geographic isolation, and temporal
isolation.
Behavioral isolation: occurs when two populations are capable of
interbreeding but have differences in courtship rituals or other
behaviors.
Geographic isolation: occurs when populations are separated by
geographic barriers such as rivers, mountains, or bodies of water.
Temporal isolation: occurs when two or more species reproduce
at different times. For example, three similar species of orchid
all live in the some rain forest, and they all reproduce at
different times of the year.
Speciation in Darwin’s Finches
•Speciation in the Galapagos finches occurred by founding of a
new population, geographic isolation, changes in the new
population’s genes, reproductive isolation, and ecological
competition.
1.) Founders Arrive: Many years
ago, a few finches from the
mainland of South America arrived
on one of the Galapagos Islands.
These birds may have gotten lost or
were blown off course by a storm
when flying. They managed to
survive on the island.
2.) Separation of Populations:
Later on, some birds from Species
A crossed to another island in the
Galapagos chain. These birds do
not usually fly over open water
therefore the birds on the two
islands were isolated from each
other.
4.) Reproductive isolation: The
two populations change some much
over time that if birds from island
B went back to island A they would
not mate with the island A birds.
. This is reproductive isolation, and
the formation of two different
species, A and B.
3.) Changes in the Genes: Over
time the two populations on the
different islands adapted to their
local environments. Each island
offered different food sources and
over this long period of time the
Finches evolved differently to deal
with this.
As the finches competed for food and space there was more
migration and speciation until multiple species of finches existed.
Today there 13 different species of finches on the Galapagos
Islands.