natural selection - mr. ciardullo's class webpage€¦ · natural selection •the organisms...
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Natural Selection
Charles Darwin & Alfred Russell
Wallace
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Darwin’s Influences
• Darwin observed such variations in
species on his voyage as a naturalist on
the HMS Beagle
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Darwin’s Influences
• Kept vast diaries of the many organisms seen
and their tremendous variations
Ex. Galapagos Islands—finches, tortoises
Ex. Snakes with legs
Ex. Penguins use wings to swim
Ex. Fossils of extinct animals as well
as those that closely resemble
modern animals
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Darwin’s Influences
• J. B. de Lamarck
- Claimed that all organisms
had a desire to change
- The more an organism
used a structure, the larger
it became
- These acquired traits could
be inherited by their
offspring
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Darwin’s Influences
• Charles Lyell
• “Principles of Geology”
– Geological processes on Earth resulted from
continuous cycling over a long period of
time.
– This allowed Darwin to consider that the
Earth was much older than originally
believed and thus organisms could change
over a very long period of time
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Darwin’s Influences
• Artificial Selection
The process in which breeders choose the
variations to be used to produce the
following generations
– Darwin noted that breeders and farmers
create better (stronger/faster) livestock and
crops through selective breeding…
Would make sense that this could
occur in nature too….
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Artificial SelectionAll these veggies from wild mustard plants?!
http://evolution.berkeley.edu/evolibrary/article/evo_30
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Darwin’s Influences
Thomas Malthus’ theory
• Natural populations do not grow out of
control in nature, even though some
reproduce by the 1000’s
• Populations are kept in check by
predators, diseases, limitations in food,
water, and other resources that are
essential for survival, hence….there is a
“struggle for existence” (Darwin)
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Change is slow…
but it does happen!
The following circumstances are needed for
natural selection to occur..
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Population Size
• Natural populations do not grow out of
control in nature, even though some
reproduce by the 1000’s
• Populations are kept in check by
predators, diseases, limitations in food,
water, and other resources that are
essential for survival, hence….there is a
“struggle for existence” (Darwin)
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Variation
• Variation in genotype and phenotype
exists in populations (from sexual
reproduction)
• Some variations are more suitable for
environmental conditions
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Variation in Cuban tree snails…
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VARIATION
in flowers…
In lady
bugs…
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Fitness of Individuals
• Some individuals have genetic variations
that create more favourable traits in a
particular environment or habitat.
• These favourable traits may allow an
individual to live longer and produce more
offspring.
• This individual is more fit
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Natural Selection
• The organisms best suited for the
environment will survive
• The best suited individual will live longer
and leave more offspring
• These offspring will inherit the
advantageous genotypes and phenotypes
of their parents and will also be fit for
their environment.
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At the same time….
• Organisms with less favourable traits are
less likely to survive and reproduce
• Thus, they are less likely to pass on these
unfavourable genes to the next generation
• Over time, these genes may be “wiped
out” of a population.
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Struggle for Survival
• The fact that there is a constant struggle
for survival among individuals in an
overcrowded population means that some
of the individuals will die before
reproducing.
• The most fit individuals in a population are
the ones that are least likely to die of
starvation etc.
.
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NOTE: Natural selection will usually
only cause changes in a population if
the environment the population is in is
changing.
If the environment stays the same, it
will always be the same traits that are
favourable (fit).
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A summary of natural selection
1. No two members of a species are identical (there is variation)
2. Variation is inheritable (alleles of genes are passed from parents to offspring)
3. More organisms are born than reach adulthood
4. Organisms compete for resources (struggle to survive)
5. Survival of the fittest
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New Species
• Over time, this process can result in
adaptations that specialize populations
for particular ecological niches and may
eventually result in the emergence of new
species.
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Pepper Moths & Natural
Selection
In the beginning…
• Trees covered in off-
white lichens…moths
rest on trees
• Light coloured moths
have a survival
advantage
(camouflaged)
Industrial Revolution…
• Soot starts to cover
trees, darkening
them
• Dark coloured moths
have a survival
advantage
(camouflaged)
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Discuss with your partner how
you think the peppered moth
example demonstrates all the
conditions needed for natural
selection to occurs
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Summary
• Both colour variations exist in the moth
population BEFORE the industrial revol.
• Environmental changes select one colour
over the other as a survival advantage
• More of one colour moth survives longer,
reproduces more…thus, the population of
each colour increases or decreases
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