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Evolution
Earlier Theories
WHY EVOLUTION?
• Evolution as a PROCESS is
a SETTLED THEORY accepted by
biologists all over the world.
• Evolution provides a framework for
understanding all aspects of biology
(classification e.g. simple to complex
organisms, adaptation e.g. function,
environment & structure)
WHY EVOLUTION?
Cont.
Evolutionary biology provides tools for
many studies in genetics/genomics
- Relationships between groups,
- Research,
- Resolving legal issues such as
DNA fingerprinting & Sources of AIDS
infections,
Evolution is NOT the process of
superior animals triumphing over
inferior ones, but is rather a
series of alternative strategies
that have arisen in response to
changing environments.
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No organism is better or worse than another.
Arthropods (crabs, spiders, insects etc.) are more
successful than vertebrates when one considers
the number of species, number of individuals,
and number of adaptations to different
environments and habitats.
In a similar way, humans are more successful
than other modern apes in having a large number
of individuals adapted to living in different
environments.
EARLY THEORIES
•
Describe the two “laws” of Lamarck
• History and observations of Darwin
ideas/theory
• Application of the theories with
examples
• Why Lamarck`s theory was rejected
• Differences
Jean-Baptiste Lamarck
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French Biologist (17441829)
Professor of “Worms and
Insects” in Paris
The first scientific theory
of evolution
Lamarck theory
Use and disuse of organs
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Changes in the environment create new needs
that cause organisms to modify their existing
organs to meet the need. Repeated use of the
organ would cause it to enlarge and become
more efficient. Disuse of a organ would cause it
to degenerate
Inheritance of acquired
characteristics
The modification an organism acquired during its
life time could be pass on to its offspring.
Explain that Lamarck’s theory is not accepted by
most life scientists today
Acquired characteristicsare not
inherited/do
not cause any change to the DNA of an organism's
gametes (sperms or ova)
OR
Organisms did not evolve because they
want
to evolve
OR
Lamarck believed in determinism (internal drive
of
organisms to change)
How did Lamarck explained the long necks of
giraffes?
Lamarck
- All giraffes had short necks
originally
- Giraffes frequently
stretched/used their necks to
reach
- for leaves of tall trees
- necks become longer
- The long necks acquired in
this way
could be passed on to the next
generation /were inherited
Snake
Jean Baptiste de Lamarck
• Two “Laws”
1. Law of use and disuse: Use or disuse
causes structures to enlarge or shrink.
DISUSE & USE
2. “Law” of inheritance of acquired
traits
• Acquired traits can be inherited
• All such changes are heritable
Misuse
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Legs of snake
disappeared because
it did not use them in
its gliding movement;
also their bodies
became thin and long
to allow them to crawl
through narrow
spaces
Lamarck`s “Law” rejected
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His idea was that there is a ladder of life with
superior forms (humans of course) near the top
and lower forms on lower rungs (plants near the
bottom).
Theory is deterministic. He imagined that all
organisms had an internal drive to ascend the
ladder during evolution. Those that started
earlier (or had a stronger drive) would be higher
than those that evolved later. He did not think
of evolution in a tree-like form.
Lamarck`s “Laws” rejected (Cont.)
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He imagined that when an organism tries to use
an organ for some purpose REPEATEDLY that
organ will grow and, moreover, its offspring
would start with an already enlarged organ. For
example a giraffe would stretch for high leaves
throughout its life and this would result in its
offspring being borne with a longer neck and
legs.
That there is no way that an event going on
during an organisms life can causes directed
changes in its HERITABLE MATERIAL.
Charles Darwin (1809 -1882)
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English Naturalist
College dropout
Traveled around the world
Best known for On the Origin
of Species (1859)
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Abundant evidence for evolution
Proposed a mechanism: natural
selection
Envisaged evolution as a tree
A radical and novel view
Summary of Charles Darwin’s
theory
1. More individuals are born than survive to reproduce.
2. Variety exists among individuals of a species.
3. ‘Fitter’ individuals are more likely to reproduce
= “Survival of the fittest”: Natural selection
eliminates less well-adapted individuals.
4. If characteristics are inherited, species slowly evolve.
= adaptation, or ‘microevolution’
eventually giving rise to new species
= speciation, or ‘macroevolution’.
Summary of Charles Darwin’s
theory
Evidence
1. More individuals are born than survive to reproduce.
Then
2. Variety exists among individuals of a species.
3. ‘Fitter’ individuals are more likely to reproduce
4. If characteristics are inherited, species slowly evolve
= adaptation, or ‘microevolution’
eventually even giving rise to new species
= speciation, or ‘macroevolution.
Now
= “Survival of the fittest”: Natural selection
eliminates less well-adapted individuals.
Summary of Darwin’s theory
Based on Natural Selection observations:
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Fraction of offspring survive to maturity
Natural resources are limited
Population size fluctuate around carrying
capacity
A population has variations of similar traits
Traits best fitted to environment are passed on
to next generation - unsuitable traits disappear
How would Darwin account for the long
necks of the giraffes
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As a result of genetic variationin the giraffe population
some giraffes have longer necks than others
Environmental change/competition for resources
occurred causing those with shorter necks to die
and those with longer necks to survive
This is natural selection/survival of the fittest
The genes/genotype for longer necks
were passed on to subsequent generations most of
which now have long necks
Differences: Theories of
evolution
Lamarck’s theory
1809
Darwin’s theory 1859
Tabulate differences
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Lamarck
Variation because
individual change
Individuals in
population change
Because they want
to adapt to the
environment
Change acquired
inherited from
parent to offspring
Darwin
• Offspring showed
variation when produced
• Change because of
environmental factors
• Natural selection – best
suited
• Best suited
characteristics are passed
inherited from parent to
offspring