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22.2 Descent with
modification
Evolution through natural
selection
It was Darwin’s research
that was the catalyst that
would change the way in
which we understand
evolution.
All life is interconnected
in some way. It is
through observing both
the unity and diversity of
life that we begin to
understand how Darwin
arrived at his conclusions.
Unity and Diversity of
life?
The HMS Beagle
and Darwin’s Voyage
Darwin considered much about
geology in his travels. He was
well read on Lyell’s works on
geology and understood the
mechanics by which geologic
processes worked.
He noticed that along his travels
around Chile, he noticed that
after a violent earthquake, several
small crags were jetting out of the
water a few feet in the air. The
crags contained fossils of sea life
in them.
He also made this same
observation in the Andes
mountains, where he found the
same phenomena of sea shells on
tops of mountains. He
concluded that these mountains
were created by the same types of
earthquakes, which reinforced the
idea that the Earth was more
than just a couple thousand years
old.
continued...
Another important thing
Darwin noticed was in
the Galapagos islands.
A large and diverse
population of organisms
that had some similar
features of a mainland
South American bird
were unique and only
found on those islands.
Darwin’s Focus on Adaptations
Adaptations are inherited
characteristics that helps
an organism enhance its
chances of survival and
reproduction.
Natural Selection in the finch
populations
Natural selection is a
process by which
individual that have a
certain trait tend to
survive and reproduce at
higher rates than other
individuals because of
those traits.
Darwin still felt he did
not have enough evidence
and did not publish his
work right away.
Wallace and his theory on
natural selection
Alfred Russel Wallace, a
British naturalist sent a paper
to Darwin in 1858 that
outlined, nearly identically, all
of the points and conclusions
drawn by Darwin, asking him
if it were worthy of being
published.
Soon after, Darwin quickly
finished his book, On the
Origin of Species By Means of
Natural Selection.
Although Wallace published
first the credit still remains
Darwin’s.
On the Origin of Species
Evolution was not used in
the book’s first edition. The
words “Descent with
modification” were used
instead.
It was explained that all
organisms that share
common traits must have
some form of common
ancestor that through
migration to various places,
accumulated modifications,
or adaptations that fit them
to a certain way of life.
Darwin the concluded that
descent with modification
led to a rich diversity of life.
Artificial Selection, Natural
Selection, and Adaptation
Artificial Selection
Natural Selection
Also called selective
breeding, this has allowed
for desirable (chosen)
characteristics and even
animals to survive over
long periods of time.
Individual that have a
certain trait tend to survive
and reproduce at higher
rates than other individuals
because of those traits.
Processes like artificial
selection often lead to
organisms that are next to
nothing like their
ancestors.
Darwin concluded that
processes like artificial
selection mirror those of
natural selection, only by
different means of selection.
Darwin’s Observations and
Inferences about Natural selection
Observations
Members of populations
often vary in inherited
traits.
All species can produce
more offspring than their
environment can support,
and many of these
offspring fail to survive
and reproduce.
Inferences
Individuals whose inherited
traits give them a higher
probability of surviving and
reproducing tend to leave
more offspring.
The unequal ability of
individuals to survive and
reproduce will lead to the
accumulation of favorable
traits in the population over
generations.