Unit: Evolution
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“evolution” : the gradual change in the genotypes and phenotypes of a species over time
I.
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History of Evolutionary Theory
Up to a certain point, the scientific community in Europe generally believed that all living
organisms were created exactly as they were found (ie: there had been no changes in the bodies
of a species since they had been created)
This belief was k now as “Creationism”
Some scientists didn’t feel that the Creationist idea could be correct because they knew that
animal characteristics could be changed over time using selective breeding techniques
Ex: horse breeders would cross their fastest male horses with their fastest female horses. The
resulting offspring would usually be faster than their parents
In the 1760’s, a scientist named Leclerc began to study several things that he felt could only be
explained if present animals had evolved from different ancestors
Ex: pigs often have a hind toe that serves no purpose (called vestigial appendage). Leclerc felt
that the pigs couldn’t have been created with a body part that served no purpose. He thought
that pigs must have changed over time and somehow lost the use of this toe
Ex: the limb bones of mammals are very similar in design, even though they have very different
functions. A human’s arm, a bat’s wing and a whale’s fluke all have a single upper arm bone, two
forearm bones and several bones in five fingers
Leclerc was sure that some animals had evolved, sometimes losing useless traits, sometimes
keeping useless traits and sometimes gaining new traits
He couldn’t support his theory because he was unable to explain how it was that animals could
evolve
Many scientists tried to come up with a theory of how evolution worked
In the 1770’s, Lamarck (a student of Leclerc) came up with a hypothesis
His idea was that animals would change their phenotypes to suit their environment, and then
their offspring would inherit these new beneficial characteristics
Ex: the giraffes of today ended up with long necks because the original short necked giraffes
would stretch their necks to reach leaves and their offspring would be born with longer necks as
a result
Lamarck called this a “God inspired drive towards perfection”
Lamarck’s explanation for how evolution worked was completely wrong. A change to an
organism’s phenotype while it’s alive does not get passed on to its offspring
Ex: if you get a tattoo (a change to your phenotype) what are the chances that your children will
inherit the tattoo when they are born?
In the 1850’s, a scientist named Charles Darwin came up with an explanation for how organisms
could evolve
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Darwin had worked as a naturalist on a ship that sailed around the world. While collecting
specimens he noticed two things:
1) Most organism of a species are not identical (there is a variation in a species)
2) There is some relationship between the way parents look and the way their offspring look
(characteristics do get inherited)
Darwin knew that every species struggled to survive (to keep the birth rate above the death
rate). He felt that the sligh variations between the individuals in a species would help some
organisms survive and reproduce better than others
Those that were able to reproduce would pass on their useful characteristics to their offspring
Over time, a species would evolve because the organisms carrying the useful traits would breed
and survive, and the organisms that lacked the useful traits would be killed
Ex: all giraffes now have long necks because environmental conditions (the height of the trees)
did not favour giraffes with short necks. Giraffes with long necks were able to reach their food
and live. Giraffes with short necks starved to death.
Darwin called this process “Natural Selection”
Darwin knew that the characteristics were inherited but he couldn’t explain how. Mendel’s work
was still undiscovered
Because he couldn’t explain inheritance, Darwin waited before he published his book On the
Origin of the Species
While he waited, another biologist named Alfred Wallace came up with a similar theory
As a result, Wallace is co-credited with the theory of natural selection
When Mendel’s research was rediscovered in the 1900’s, it was quickly matched up to the
theory of natural selection. Scientists saw that traits were inherited in an organized fashion
II. Fossil Evidence and Earth History
 Most scientists realized that in order for evolution to have proceeded according to natural
selection, it must have taken place over a very long period of time. They began to look at fossils
in order to better understand what types of organisms existed at one time.
 A fossil is a preserved specimen of an organism. If an organism dies in certain conditions (so that
it is not eaten and it doesn’t decomposed too quickly) then it may form a fossil in one of three
ways:
1. Petrification : were minerals fill the hard tissues (bones, scales, teeth) and form a rocky
framework of the original organism
Ex: petrified wood, fossil skeletons
2. Moulds: this is when a cavity is made by the imprint of an organism or its tracks
Ex: Dinosaur tracks, leaf imprints
3. Casts: when sediment fills a mould and makes a 3D impression