Transcript Darwin Presents His Case

Darwin Presents His Case
Darwin Presents His Case
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In 1859, Darwin published “On the Origin
of Species” – in it he proposed a
mechanism for evolution which he called
natural selection
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In addition, he presented evidence that
evolution has been taking place for millions
of years—and continues in all living things
Inherited Variation
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Members of each species vary from one
another in important ways
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Example: Some plants in a species may
produce larger fruit than others (lima beans),
some cows produce more milk than others,
birds have varying sized beaks
Darwin learned that some of this variation
is inherited → differences that are passed
from parents to offspring
Inherited Variation
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At the time, Darwin had no idea how
heredity works, but we now know that
heritable variation is caused by variation in
an organism’s genes → many people
viewed this variation as defects, but
Darwin knew that this variation (which we
now know as genetic variation) was
significant
Artificial Selection
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Darwin knew that plant and animal
breeders took advantage of this variation
to improve crops and livestock through
artificial selection
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artificial selection – nature provides the
variation and humans selected those
variations that they found useful
Artificial Selection
Evolution by Natural Selection
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Next, by comparing the processes in
nature to artificial selection, Darwin
developed a hypothesis to explain how
evolution occurs
The Struggle for Existence
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Darwin realized that high birth rates and a
shortage of life's basic needs would force
organisms to compete for resources
The Struggle for Existence means that
members of each species compete
regularly to obtain food, living space, and
other resources
The Struggle for Existence
Examples:
 Predators: individuals that are fast or have
a particular way to catch their prey will be
more efficient at collecting food
 Prey: individuals that are fast, well
camouflaged, or protected can avoid being
eaten
 Darwin’s Finches: certain beak
shapes/sizes are more efficient at eating
different types of food → led to speciation
Survival of the Fittest
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A key factor in the struggle for existence
is how well suited an organism is to its
environment
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fitness – the ability of an individual to survive
and reproduce
Darwin proposed that fitness is the result
of adaptations
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adaptation – any inherited characteristic that
increases an organism’s chance of survival
Survival of the Fittest
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Successful adaptations enable organisms
to be better suited to their environment,
and thus, better able to survive and
reproduce
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Adaptations can be structural (ex. beak shape
in finches), physiological processes (how living
things function; ex. photosynthesis), or they
can be behavioral (choosy females, mating
rituals, siblicide)
Adaptations
Survival of the Fittest
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Individuals with characteristics that are
not well suited to their environment
have low fitness → they either die or
leave few offspring
Individuals that are better suited to
their environment – that is to say they
have adaptations that enable fitness
Survival of the Fittest
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Darwin called this process survival of the
fittest → because of its similarity to
artificial selection, Darwin referred to
survival of the fittest as natural selection
Over time, natural selection results in
changes in the inherited characteristics of
a population. These changes increase a
species’ fitness in its environment
Descent with Modification
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Over long periods of time, natural
selection produces organisms that have
different structures and occupy different
habitats → as a result species today
look different from their ancestors
Descent with Modification
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Darwin referred to this principle as
descent with modification → over time,
each living species has descended, with
changes, from other species
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this implies that all living things are related to
one another
common descent – all species – living and
extinct – were derived (came from a
common ancestor