Download Reading: Charles Darwin and the Process of Natural Selection

Evolution, Natural Selection,
Cladograms and Evidence of
Evolution
Evolution
• Evolution is change within a population over time.
• Charles Darwin (a scientist and a
naturalist) proposed the theory that
evolution happens by NATURAL
SELECTION – populations CHANGE
over time in response to changes in the
environment.
Natural Selection
• What does natural selection mean? The environment changes and
acts on variations (differences in characteristics – controlled/
determined by genes) within populations – only those individuals
with the “best” variations (the best genes) for the new environment
survive and pass on their genes. Over many generations, more and
more of the members of the population have these genes/
characteristics. These characteristics are called favorable variations,
or ADAPTATIONS!
• What are adaptations? Adaptations
are characteristics that increase an
organism’s chances of surviving and
reproducing in their environment.
Methods of Evolution
Evolution can create new species (speciation), and it can
happen quickly or slowly
• Gradualism – small changes in the environment lead to
small changes in populations over LONG periods of time
(population changes SLOWLY)
• Punctuated Equilibrium –
Fast/ large-scale changes
in the environment cause
one species to become two
(or more) different species
(they can no longer
reproduce)
Natural Selection and Artificial Selection
•
• Natural selection/ breeding: Individuals within populations mate
on their own, with NO external (human) influence and are subject
to natural selection
Artificial selection/ breeding:
Individuals within populations
are forced to mate/ reproduce with
others in the population by humans
Why artificial breeding?
– Better/ healthier crops and/ or livestock
– More desirable traits (dogs, horses, flowers etc.)
• Why not artificially breed?
– It decreases genetic variation – there are not as
many differences in the gene pool (the total
number of genes from every individual in a
population) so the population is more fragile (vulnerable/ susceptible)
to changes in environment
Peppered Moths
• Industrial melanism is the
darkening of populations
over time in response to
pollution in the environment.
• Peppered moths have TWO color variations – light and
dark (two different alleles)
• Around the 1850’s (the industrial revolution) the darker
moths became more frequent in
the forests around industrial
centers. WHY?
How do scientists show how different species are connected?
A cladogram is a branched diagram which shows how
organisms are related.
At each branch, or "Y”, characteristics of evolutionary
origin are used to separate one group from another.
Cladograms can also be used for
organisms very closely related
Comparing organisms
Which are more similar: a rabbit and a crocodile
or a bird and a crocodile.
Explain your
answer including
specific traits.
What evidence supports the Theory of
Evolution?
• Molecular Evidence (DNA)
– Scientists can analyze DNA from
different organisms. The more similar
the DNA, the more closely related
they are!
• Biogeography
• The study of the distribution
(where they are found) of species.
• Organisms in places that are near
each other will be more similar/
closely related.
• When land masses separate
(islands, continental drift etc.)
species will evolve differently to
“fit” the different environments,
and eventually become different
species (no longer able to
reproduce). They are still similar,
though, because they share a
common ancestor.
Comparative Embryology
• An embryo is an unborn organism in the
earliest stages of life.
• The embryos of very different organisms
are very similar, suggesting that the
basic body plan of organisms is formed
based on similar DNA instructions that
came from a common ancestor.
– Example: Fish embryos and human embryos both
have gill slits!
Comparative Anatomy
• Similar body structures are found in
different organisms. They are “built/ put
together” with the same basic parts.
This shows “variation on a theme.” A
similar, basic body plan has adapted to
“fit” different environments.
• Example: forelimbs (bat wings, whale front fins,
human arms, dolphin fins, horse forelimbs, cat
forelimbs)
– The Fossil Record
• Fossils are the remains of once-living
organisms. They show that once-living
organisms were similar to those living today
(those fossils in younger rocks) or very
different (those fossils found in older rocks).
– Artificial Selection/ Breeding
• Humans can choose/ select which organisms
to mate. Over many generations, the genes
in these domesticated populations are very
different than their wild counterparts. This
shows that the gene pool CAN change over
time due to selection (even though it is
artificial selection).
Evidence for Evolution Video (Paul Anderson)
http://www.youtube.com/watch?v=cC8k2Sb1o
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