Download Chapter 15 Darwin and Evolution

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Chapter 15
Darwin and
Evolution
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History of Evolutionary Thought
• Darwin used a variety of data to come to the
conclusion that the Earth is very old, not young, and
that biological evolution is the method by which
species arise and change.
• Evolution is now considered the unifying principle of
all the biological sciences. It explains both the unity
and diversity of life on Earth.
• Evolution illustrates that living things share like
characteristics because they have a common ancestry.
Mid-Eighteenth-Century Contributions
• Taxonomy, the science of classifying organisms, was
an important endeavor during the mid-eighteenth
century.
• Chief among the taxonomists was Carolus Linnaeus,
who developed the binomial system of nomenclature
and a system of classification of living things.
• Besides taxonomy, comparative anatomy, the
evaluation of similar structures across a variety of
species, was of interest to biologists prior to Darwin.
• Linnaeus, like other taxonomists of his time,
believed in the fixity of species.
– Each species had an “ideal” structure and function
and also a place in the scala naturae, a sequential
ladder of life.
– The simplest and most material being was on the
lowest rung of the ladder, and the most complex
and spiritual being was on the highest rung.
– Linnaeus and other taxonomists wanted to
determine the ideal characteristics of each species
and also wanted to discover the proper rank for
each species in the scala naturae. Therefore, for
most of his working life, Linnaeus did not even
consider the possibility of evolutionary change.
Late-Eighteenth-/Early-Nineteenth-Century
Contributions
• Linnaeus’ hierarchical method of classifying
organisms is consistent with modern
evolutionary thinking. This is the reason that
Linnaeus’ basic method of classification has
been modified even until today.
Cuvier and Catastrophism
• Baron Georges Cuvier used comparative
anatomy to develop a system of classifying
animals. He also founded the science of
paleontology, the study of fossils, and was quite
skilled at using fossil bones to deduce the
structure of an animal.
Lamarck and Acquired Characteristics
• Lamarck concluded, after studying the succession of
life-forms in strata, that more complex organisms are
descended from less complex organisms. He
mistakenly said, however, that increasing complexity is
the result of a natural force—a desire for perfection—
that is inherent in all living things.
• To explain the process of adaptation to the
environment, Lamarck supported the idea of
inheritance of acquired characteristics—that the
environment can bring about inherited change.
• One example that he gave is that the long neck
of a giraffe developed over time because
animals stretched their necks to reach food in
tall trees and then passed on a long neck to
their offspring.
• His hypothesis of inheritance of acquired
characteristics has never been substantiated by
experimentation.
• Phenotypic changes acquired during an
organism’s lifetime do not result in genetic
changes that can be passed to subsequent
generations.
Darwin’s Theory of Evolution
• Geology and Fossils
– Darwin’s study of geology and fossils caused him to
concur with Lyell that the observed massive
geological changes were caused by slow, continuous
processes.
– Fossils of huge sloths and armadillo-like animals
suggested modern forms were descended from
extinct forms with change over time; therefore
species were not fixed.
Fig. 15.4
• Biogeography
– Biogeography is the study of the range and
geographic distribution of life-forms on Earth.
– Darwin reasoned that related species could be
modified according to environmental differences.
– The Galápagos Islands
•Each island had a variation of tortoise; long and short
necked tortoises correlated with different vegetation.
•The finches of the Galápagos Islands seemed to Darwin
like mainland finches, but they exhibited significant
variety with regard to their beaks.
Natural Selection and Adaptation
• Natural selection was proposed by both Alfred
Russel Wallace and Darwin as a driving
mechanism
of
evolution
caused
by
environmental selection of organisms most fit to
reproduce, resulting in adaptation.
• Natural selection is a process consisting of these
conditions:
– The members of a population have random but
heritable variations.
– In a population, many more individuals are
produced each generation than the environment
can support.
– Some individuals have adaptive characteristics that
enable them to survive and reproduce better.
Darwin called the ability to have more offspring,
differential reproductive success.
– The result of natural selection is a population
adapted to its local environment.
• Extinction occurs when previous adaptations
are no longer suitable to a changed
environment.
Organisms Have Inheritable Variations
• Darwin emphasized that variations were essential to
the natural selection process.
• New variations are as likely to be harmful as helpful.
• The variations that make adaptation to the
environment possible are those that are passed on
from generation to generation.
• Darwin could not state the cause of variations because
genetics was not yet established.
• Natural selection only operates on variations that are
already available in a population’s gene pool.
Organisms Compete for Resources
• Darwin and Wallace both read an essay by
Thomas Malthus, a socioeconomist.
• Malthus proposed that human populations
outgrow food supply and death and famine
were inevitable.
• Darwin applied this to all organisms; resources
were not sufficient for all members to survive.
Organisms Differ in Reproductive Success
• In nature, interactions with the environment
determine which members of a population
reproduce to a greater degree than other
members.
• Natural selection occurs because certain
members of a population happen to have a
variation that allows them to survive and
reproduce to a greater extent than other
members.
• Artificial selection is only possible because the
original population exhibits a range of
characteristics, allowing humans to select which
traits they prefer to perpetuate. Therefore,
several varieties of vegetables can be traced to a
single ancestor.
• Darwin used artificial selection as a model when
he developed the concept of natural selection
• Chinese cabbage, brussel sprouts, and kohlrabi
are all derived from a single species, Brassica
oleracea.
Organisms Become Adapted
• Adaptations are especially recognizable when
unrelated organisms, living in particular
environment, display similar characteristics. For
example, manatees, penguins, and sea turtles all
have flippers, which help them move through
the water.
• Differential reproduction generation after
generation can cause adaptive traits to be
increasingly represented in each succeeding
generation.
Alfred Russel Wallace (Science Focus Box)
• Alfred Russel Wallace (1823-1913) was an English
naturalist who independently and simultaneously
proposed natural selection as a mechanism for
evolution.
• In 1858 Wallace concluded changes in species are due
to changes in the environment through natural
selection.
• Wallace wrote a manuscript of these findings and sent
it to Charles Darwin for review. Darwin was shocked
that Wallace had the same theories that he had.
Darwin told Wallace to publish his manuscript at
once.
• Darwin published Origin of the Species one year later
while Wallace was in the field.
Fig. 15B
Natural Selection Can Be Witnessed
• Darwin had formed his natural selection
hypothesis by observing the distribution of
tortoises and finches on the Galápagos Islands.
• Tortoises with domed shells and short necks
live on well-watered islands, where grass is
available. Those with shells that flare up in front
have long necks and are able to feed on cacti.
• Similarly, the islands are home to many
different types of finches.
– The heavy beak of the large, ground-dwelling finch is suited
to a diet of seeds.
– The thin, sharp beak of a warbler-finch is able to probe
vegetation for and spear insects.
– The decurved beak of a cactus-finch can find and feed on
cactus seeds.
• Scientists are currently witnessing natural selection on
the Galapagos Islands. Peter and Rosemary Grant
have been observing finch beak size change with
rainfall. During wet seasons, the offspring have smaller
beaks to eat the small seeds, however, in drier seasons,
offspring have larger beaks capable of breaking harder
seeds that survive the dry weather.
Fig. 15.11
• A common example of natural selection is
industrial melanism. Prior to the industrial
revolution in Great Britain, light-colored
peppered moths were more common (90%)
than the dark-colored peppered moths (10%).
– Following the industrial revolution and increase in
pollution, dark-colored peppered moths reached
80% of the peppered moth population.
– Once legislation regulated pollution reduction,
dark-colored peppered moths reduced to 19% in
one of the collecting sites.
Evidence of Evolution
• Fossil Evidence
– Fossils are the remains and traces of past life or any
other direct evidence of past life.
– Fossils include skeletons, shells, seeds, insects
trapped in amber, and imprints of leaves.
– Transitional fossils reveal links between groups.
• Archeopteryx is an intermediate between reptiles and
birds.
• Ambulocetus natans is a whale with legs.
Fig. 15.13
• Biogeographical Evidence
– Both cacti and spurges (Euphorbia) are plants
adapted to a hot, dry environment—both are
succulent, spiny, flowering plants. Why do cacti
grow in the American deserts and most Euphorbia
grow in African deserts when each would do well on
the other continent? It seems obvious that they just
happened to evolve on their respective continents.
– Why do so many species of finches live on the
Galápagos Island when these same species are not
on the mainland? The reasonable explanation is
that an ancestral finch originally inhabited the
different islands. Geographic isolation allowed the
ancestral finch to adapt and evolve into a different
species on each island.
• Anatomical Evidence
– Organisms have anatomical similarities when they
are closely related because of common descent.
•Homologous structures in different organisms are
inherited from a common ancestor.
•Analogous structures are inherited from unique
ancestors and have come to resemble each other because
they serve a similar function.
– Vestigial structures are remains of a structure that
was functional in some ancestors but is no longer
functional in the organism in question.
•Most birds have well-developed wings; some bird species
have reduced wings and do not fly.
•Humans have a tailbone but no tail.
•Presence of vestigial structures is explained by the
common descent hypothesis; these are traces of an
organism’s evolutionary history.
Fig. 15.15
– The homology shared by vertebrates extends to
their embryological development.
– At some time during development, all vertebrates
have a postanal tail and exhibit paired pharyngeal
pouches supported by cartilaginous arches.
Fig. 15.16
• Biochemical Evidence
– All living organisms use the same basic biochemical
molecules, e.g., DNA, ATP, and many identical or
nearly identical enzymes.
– Organisms utilize the same DNA triplet code and
the same 20 amino acids in their proteins.
– This is substantiated by the analysis of the degree of
similarity in amino acids for cytochrome c among
organisms.
– These similarities can be explained by descent from
a common ancestor.
Fig. 15.17
• Because it is supported by so many lines of
evidence, evolution is no longer considered a
hypothesis.
– Evolution is one of the great unifying theories
(GUTs) of biology, similar in status to the germ
theory of disease in medicine.
The modern synthesis combines
Darwin’s theory with genetics
• Beginning in the 1930s and 1940s, biologists
expressed a conceptual breakthrough when they
combine the principles of Mendelian
inheritance with Darwin’s theory of natural
selection. The result was a unified explanation
of evolution known as the modern synthesis, or
the synthesis theory of evolution. In this
context, synthesis refers to combining parts of
several theories to form a unified whole.
• Today, the modern synthesis incorporates our
expanding knowledge in genetics, systematics,
paleontology, developmental biology, behavior,
and ecology.
• The modern synthesis, which emphasizes the
genetics of populations as the central focus of
evolution, has held up well since it was
developed.