Transcript WHAT DOES “EVOLUTION” MEAN?

Population change over time
“Evolution” means change over time.
The “Theory of Evolution” says:
– Living things on Earth have changed over time.
– Many species that once existed are now extinct.
– Many species alive today were not here when life
on Earth first formed.
First thought of by the ancient Greeks (in 600 BC )
but they did not try to explain HOW it happened.
First scientific attempt to explain how evolution happened came
2400 years later! In 1802 Jean LAMARCK proposed model:
Evolution by Acquired Characteristics
Jean Baptiste Lamarck
1744-1829
1. More individuals are born than the
environment can support - there is a
struggle to survive.
2. Some individuals respond by
developing traits that give them a
better chance to survive.
3. When these individuals reproduce
their offspring inherit their newly
acquired, improved traits and
develop them even further.
4. Eventually ALL members of the
species have the favorable trait.
Lamarck’s explanation of how the giraffe got its long neck:
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1. ALL early giraffes had the same (short ) necks.
2. As giraffe population increased there wasn’t enough food
within reach for all so some began stretching their necks
to reach food higher in the trees.
– The more they stretched, the longer their necks grew.
3. When they reproduced they passed their longer necks on
to their offspring who stretched their necks even more.
– Eventually all giraffes had long necks.
Lamarck’s model was quickly discarded.
Scientists tried but could not find evidence to
support his main ideas.
1. All members of a species are NOT alike as
Lamarck said. Great variation normally and
naturally exists within a species.
2. Organisms cannot change most of their basic
physical traits at will, even if survival depends on
It. They cannot “adapt” because they need to. For
example giraffes cannot make their necks grow
longer by stretching them!
3. Organisms can change and improve some
things during their lifetimes (lifting weights to
increase strength etc.) but these traits CANNOT be
passed to offspring.
If this man wants his future children to have
small noses will it help to get a nose job??
Is there anything he could do that might help?
In 1831, Charles Darwin
(failed medical student, failed
divinity student, avid beetle
collector) began a 5 year
journey on a small ship, the
H.M.S. Beagle.
90’ long
75 people
5 years
Around
the world!
Charles Darwin
1809-1882
Purpose: survey coast of South America
Darwin’s job: observe geology & biology of the area.
– Kept meticulous records of observations. His observations from the
Galapagos Islands led to his model of how evolution occurred.
– Did NOT set out to develop a model, but couldn’t ignore what he saw. His
curiosity drove him to try to explain what he had observed.
Galapagos Islands are
600 miles off west
coast of Ecuador
(South America).
Volcanic first rose above the
ocean surface less
than 15 million years
ago.
Why is this special?
1) Life has been there a relatively short time (compared to billions of years
on the mainland!).
2) It developed in isolation from life on the mainland.
Darwin was
surprised to find
species that
clearly were
closely related but
that had distinct
differences on
different islands.
High
lush
island
Low,
desert
island
Darwin’s explanation of how evolution
occurred:
1. More individuals are
born in a species than
the environment can
support. There is a
struggle to survive.
2. Within every species
there is naturally variation.
3. Variations of some give them an
advantage over others in the struggle to
survive.
4. More of these individuals will survive
and reproduce than will those with less
favorable traits.
5. Many traits are passed
from parents to offspring.
6. More individuals with
favorable traits reproduce,
so the # of individuals with
favorable traits increases
in each generation.
The # with less favorable
traits decreases.
7. Eventually the favorable
trait becomes the most
common form and over many
generations (and usually
millions of years) such
differences accumulate until
a new species results.
Darwin called his model
“Natural Selection”
Does it sound
familiar???
How Darwin explained the giraffe’s long neck:
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1. Ancestral giraffes had necks of various lengths.
2. When food became scarce, longer-necked individuals could reach more
food sources so they survived in greater numbers than those with shorter
necks, many of whom starved and died.
– more long necks survived, so more reproduced and
passed the long-necked trait to their offspring.
– each generation had more long and fewer short necks
than the one before.
3. Eventually after many generations and millions of years ALL giraffes
had long necks.
CHECK YOUR UNDERSTANDING:
1. Would you expect individual organisms to “evolve”
in their lifetime? NO!
2. What does evolve over time? POPULATIONS EVOLVE!
3. What “force” shapes the characteristics of species in other words, what “selects” traits?
THE ENVIRONMENT
4. What might cause evolution to“speed up” - that is,
cause species to change more rapidly?
DRASTIC CHANGE IN ENVIRONMENT
5. What was the original cause of variation within
species?
MUTATION