Download Chapter 10: Natural Selection

EVOLUTION
Artificial and Natural
Selection
Natural Variation
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Darwin observed that there is natural
variation among species.
He also believed that because more
organisms were born than could possibly
survive, the struggle for survival kept Earth
from being overpopulated by plants and
animals.
Natural Variation
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Natural selection is the process that allows
only the organisms with the best fitness to
survive.
In nature, the environmental conditions
choose the adaptations that are most
beneficial, so that only those organisms with
the best traits are reproducing.
Selective Breeding
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Darwin noticed that when farmers and
breeders choose which animals or plants to
mate they are relying on the fact that there
are different traits in the population
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No two individuals are exactly alike, and
variation is inheritable, meaning it can be
passed on to future generations
Artificial Selection
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Farmers chose best characteristics
Nature provided variation, humans controlled
the breeding
This is called artificial selection
Has resulted in a wide range of plants and
animals that are very different from their
ancestors
Selection in Nature
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Inheritable variation exists in nature as well
Because each organism is slightly different,
each has slightly different advantages and
disadvantages
Some individuals are more fit than others
(better suited to survive and reproduce, or
survival of the fittest)
Selection in Nature
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Generation after generation, the struggle for
existence selects the fittest individuals to
survive and reproduce
Species in general become better suited to
their environments as they respond to
selection pressures
Where does variation come from?
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Variations in traits are variations in
phenotypes
Traits are controlled by genes
Inheritable variation comes from differences
in genes (genetic diversity)
Differences in genes come from mutations
and gene shuffling
Mutations and Variation
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Mutation rates vary by species
– Humans have a low rate, bacteria have a
high rate
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Can be positive or negative
Gene Shuffling and Variation
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Sexual reproduction is an important source
of variation in nature
Genes are shuffled during meiosis when
gametes are formed
Even the same genes combined in different
ways produce different results
Principles of Natural Selection
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Individuals in a population show variations
among others of the same species.
Variations are inherited.
Animals have more young than can survive
on available resources.
Variations that increase reproductive success
will be more common in the next generation.
Inheritable Variation
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Variation occurs at all levels (physical and
biochemical)
Many traits are also polygenic, creating a
range of phenotypes
Natural selection does not act on
genotype – affects variations in
phenotype only!