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Chapter 15
Important People
• James Hutton and Charles Lyell – geologists who
helped scientists recognize that Earth is many millions
of years old, and that the processes that changed Earth
in the past are the same processes that operate in the
present.
• Jean-Baptiste Lamarck – recognized that living things
have changed over time and that all species were
descended from other species. He proposed that by
selective use or disuse of organs, organisms acquired
or lost certain traits during their lifetime.
– Theses traits could be passed off to offspring and lead to
change in a species.
– We now know that part of his hypothesis was incorrect.
• Thomas Malthus – economist who reasoned
that if the human population continued to
grow unchecked, sooner or later there would
not be enough living space and food for
everyone.
– He said that the only factors working against this
growth were war, famine, and disease.
• Charles Darwin – (duh!!!) 25 years after his
trip, he published On the Origin of Species
because another naturalist, Alfred Russel
Wallace was about to publish the same idea!
– The book came out in 1859 and natural selection
was presented as the mechanism for evolution.
Evolution
• The central ideas of evolution are that life has
a history — it has changed over time — and
that different species share common
ancestors.
Darwin’s Ideas
• Struggle for existence
– Members of each species compete regularly to
obtain food, living space, and other necessities of
life.
• Ex: Predators who are faster will catch more prey, but
prey that are faster, better camouflaged, or better
protected can avoid being caught.
• Fitness
– The ability of an individual to survive and
reproduce in its specific environment.
– Fitness is the result of ADAPTATION.
• Adaptation
– An inherited feature that is common in a
population because it provides some improved
function and increases survival.
– Adaptations can take many forms: a behavior that
allows better evasion of predators, a protein that
functions better at body temperature, or an
anatomical feature that allows the organism to
access a valuable new resource — all of these
might be adaptations.
• Natural Selection (“Survival of the Fittest”)
– Individuals that are better suited for their
environment , with adaptations that enable fitness,
survive and reproduce most successfully.
• Individuals with characteristics that are not well suite for
their environment , with low levels of fitness, either die or
leave few offspring.
– Over time, natural selection results in changes in the
inherited characteristics of a population.
– These changes increase a species’ fitness in its
environment.
– Natural selection takes place without human control
or direction.
• If humans controlled it, this would be called artificial
selection.
• Descent with Modification
– Over long periods, natural selection produces
organisms that have different structures, establish
different niches, or occupy different habitats.
– This is why species today look different from their
ancestors.
– COMMON DESCENT: All species, living and extinct,
were derived from common ancestors.
• “Tree of Life” links all living things.
Evidence of Evolution
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Fossil record
Geographic Distribution
Homologous Structures
Vestigial Organs
Embryology
DNA
Fossil Record
• Darwin saw fossils as a record of the history of
life on Earth.
• By comparing fossils from older rock layers
with fossils from younger rock layers,
scientists could document the fact that life on
Earth has changed over time.
Geographic Distribution
• Species now living on different continents had
each descended from different ancestors.
• However, because some animals on each
continent were living under similar ecological
conditions, they were exposed to similar
pressures of natural selection.
• Because of these similar selection pressures,
different animals ended up evolving certain
features in common. SEE PAGE 383
Homologous Structures
• We use homologous characters — characters
in different organisms that are similar because
they were inherited from a common ancestor
that also had that character.
– An example of homologous characters is the four
limbs of tetrapods.
• Birds, bats, mice, and crocodiles all have four limbs.
Sharks and bony fish do not.
– The ancestor of tetrapods evolved four limbs, and
its descendents have inherited that feature — so
the presence of four limbs is a homology.
• Not all characters are homologies.
– For example, birds and bats both have wings, while mice
and crocodiles do not. Does that mean that birds and bats
are more closely related to one another than to mice and
crocodiles? No. When we examine bird wings and bat
wings closely, we see that there are some major
differences.
• Bat wings consist of flaps of skin stretched between the bones of
the fingers and arm.
• Bird wings consist of feathers extending all along the arm.
– These structural differences suggest that bird wings and
bat wings were not inherited from a common ancestor
with wings.
– Bird and bat wings are analogous — that is, they have
separate evolutionary origins, but look similar because
they evolved to serve the same function.
Vestigial Organs
• A vestigial structure is a feature that was an
adaptation for the organism's ancestor, but that
evolved to be non-functional because the
organism's environment changed.
– Fish species that live in completely dark caves have
vestigial, non-functional eyes.
– When their sighted ancestors ended up living in caves,
there was no longer any natural selection that
maintained the function of the fishes' eyes.
– So, fish with better sight no longer out-competed fish
with worse sight.
– Today, these fish still have eyes — but they are not
functional and are not an adaptation; they are just the
by-products of the fishes' evolutionary history.
Embryology
• The early stages, or embryos, of many animal
with backbones are very similar.
• The same groups of embryonic cells develop
in the same order and in similar patterns to
produce the tissues and organs of all
vertebrates.
– This is how homologous structures are produced.
DNA Evidence
• Similar species share similar genetics like genes and
proteins.
• The fact that organisms have evolved successively from
relatively simple ancestors implies that a record of
evolutionary change is present in the cells of each of us, in
our DNA.
– When an ancestral species gives rise to two or more
descendants, those descendants will initially have fairly high
overall similarity in their DNA. However, as the descendants
evolve independently, they will accumulate more and more
differences in their DNA.
• Comparison of the DNA of different species provides strong
evidence for evolution. Species deduced from the fossil
record to be closely related are more similar in their DNA
than are species thought to be more distantly related.