20.1.
History of the Theory of Evolution
A. Darwin's Voyage
1. In 1831, at the age of 22, Charles Darwin accepted a naturalist position
aboard the ship HMS Beagle.
2. HMS Beagle began a five-year
voyage around the world; it provided
3. The pre-Darwinian world-view was
different from the post Darwinian world-view.
a.
Pre-Darwinian world-view was determined by deep-seated beliefs held to be
intractable truths.
1) The earth is young.
2) Each species was specially created and did not change over time.
3) Variations are imperfections varying from a perfectly-adapted creation.
4) Observations are to substantiate the prevailing view.
b. However,
c.
B. Mid-Eighteenth-Century Contributions
1. Carolus Linneaus and Taxonomy
a. Taxonomy
is the science of classifying organisms; taxonomy had been a main concern of
biology.
b. Carolus
Linneaus (1707-1778) was a Swedish naturalist in the field of taxonomy:
1. Linneaus developed a binomial system of nomenclature (two-part names for
each species
[e.g., Homo sapiens]).
2. He developed a system of classification for all known plants.
3. Like other taxonomists of his time, Linnaeus believed in the ideas of
a. special creation -- each species had an "ideal"
structure and function; and
b. fixity of species -- each species had a place in the scala
naturae, a sequential ladder of life.
c. Linnaeus thought that classification should describe the fixed features of
species and reveal
God's divine plan.
d. His ideas reflected the ideas of Plato and Aristotle: the ideal form can be
deduced, and organisms
can be arranged in order of increasing complexity.
e. His later work with hybridization suggested species might change with time.
2. Georges Louis Leclerc
a. Georges
Louis Leclerc, known by his title, Count Buffon (1707 - 1788), was a French
naturalist.
b. He wrote
a 44-volume natural history of all known plants and animals.
c. He also
provided evidence of descent with modification.
d. His
writings speculated on: influences of the environment, migration, geographical
isolation, overcrowding,
and the struggle for existence.
e. Buffon
vacillated on whether he believed in evolutionary descent and he professed to
believe in special
creation and the fixity of species.
3. Erasmus Darwin
a. Erasmus
Darwin (1731-1802) was Charles Darwin's grandfather.
b. He was a
physician and a naturalist whose writings on both botany and zoology contained
many comments
that suggested the possibility of common descent.
c. He based
his conclusions on:
1) changes undergone by animals during development,
2) artificial selection by humans, and
3) the presence of vestigial organs (organs that are believed to have been
functional in an ancestor but
are reduced and nonfunctional in a descendant).
d. Erasmus
Darwin offered no mechanism by which evolutionary descent might occur.
C. Late Eighteenth Century Contributions
1. Cuvier and Catastrophism
a. George Cuvier (1769-1832), a distinguished French vertebrate zoologist, was
the first to use
comparative anatomy to develop a system of classifying animals.
b. He founded the science of paleontology, the study of fossils, and suggested
that a single fossil bone
was all he needed to deduce the entire anatomy of an animal.
c. To explain the fossil record, Cuvier proposed that a whole series of
catastrophes (extinctions) and
re-populations from other regions had occurred.
d. Cuvier was also a staunch advocate of special creation and fixity of
species; this presented him with
a serious problem when geological evidence of a particular region showed a
succession of life forms
in the earth's strata.
e. Catastrophism is the term applied to Cuvier's explanation of
fossil history, the belief held by Cuvier
that catastrophic extinctions occurred, after which repopulation of surviving
species took place,
giving an appearance of change through time.
2. Lamarck's Theory of Evolution
a. Lamarck
(1744-1829) was first to state that descent with modification occurs and that
organisms
become adapted to their environments.
b. Lamarck
was an invertebrate zoologist and held ideas different from Cuvier.
c.
Unfortunately, he saw the drive for perfection as inherent in all living
things.
d. Inheritance
of acquired characteristics was the Lamarckian belief that organisms
become adapted to
their environment during their lifetime and pass on these adaptations to their
offspring.
e. He
believed the increasing complexity of life forms in strata is the result of a
natural tendency toward
perfection inherent in all living things.
f.
Experiments fail to uphold Lamarck's inheritance of acquired characteristics;
molecular mechanism of
inheritance show phenotypic changes do not result in genetic changes that can
be passed on.
20.2.
Darwin's Theory of Evolution
A. Darwin's Background
1. His nature was too sensitive to pursue medicine; he attended divinity school
at Cambridge.
2. He attended biology and geology
lectures and was tutored by the Reverend John Henslow.
3. Henslow arranged his trip on the
HMS Beagle; Darwin was an observant student of nature.
B. Geology and Fossils
1. His study of geology and fossils caused him to concur with Lyell that the
observed massive geological
changes were
caused by slow, continuous processes.
a. In his
book Principles of Geology, Charles Lyell presented arguments to support a
theory of geological
change proposed by James Hutton.
b. In
contrast to catastrophists, Hutton proposed that the earth was subject to slow
but continuous
geological processes (e.g., erosion and uplifting) that occur at a uniform rate.
c. Darwin
took Lyell's book on the voyage of the HMS Beagle.
2. Fossil Evidence
a. The
Argentina coast had raised beaches; he witnessed earthquakes raising the earth
several feet.
b. Marine
shells occurred far inland and at great heights in the Andes.
c. Fossils
of huge sloths and armadillo-like animals suggested modern forms were descended
from
extinct forms with change over time.
C. Biogeography
1. Biogeography is the study of the geographic distribution of
life forms on earth.
2. Darwin's comparison of the
animals of South America and the Galapagos Islands caused him to conclude
that
adaptation to the environment can cause diversification, including origin of
new species.
3. Patagonian hares replaced rabbits
in the South American grasslands.
4. The greater rhea found in the
north was replaced by the lesser rhea in the south.
5. The Galapagos Islands
a. These
volcanic islands off the South American coast had fewer types of organisms.
b. Island
species varied from the mainland species, and from island-to-island.
c. Each
island had a variation of tortoise; long and short necked tortoises correlated
with different vegetation.
d. Darwin's
Finches
1) Finches on the Galapagos Islands resembled a mainland finch but there were
more types.
2) Galapagos finch species varied by nesting site, beak size, and eating
habits.
3) One unusual finch used a twig or thorn to pry out insects, a job normally
done by a woodpecker.
4) The finches posed questions to Darwin: did they descend from one mainland
ancestor, did islands
allow isolated populations to evolve independently, and could present-day
species have resulted
from changes occurring in each isolated population?
D. Natural Selection and Adaptation
1. Darwin decided adaptations develop over time; he sought a mechanism by which
adaptations might arise.
2. 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.
3. Because the environment is always
changing, there is no perfectly-adapted organism.
4. Preconditions for natural
selection
a. The
members of a population have random but heritable variations.
b. In a
population, many more individuals are produced each generation than an
environment can support.
c. Some
individuals have adaptive characteristics that enable them to survive and
reproduce better.
5. Consequences of natural selection
a. An
increasing proportion of individuals in succeeding generations have the
adaptive characteristics.
b. The
result of natural selection is a population adapted to its local environment.
6. Natural selection can only
utilize variations that are randomly provided; therefore, there is no
directedness
or
anticipation of future needs.
7. Extinction occurs when previous
adaptations are no longer suitable to a changed environment.
E. Organisms
Have Variations
1. In contrast to the previous world-view, variations are highly significant.
2. Darwin suspected, but did not
have today's evidence, that variation is completely random.
3. New variations are as likely to
be harmful as helpful.
4. Variations that make adaptation
possible are those that are passed on generation to generation.
5. Darwin could not state the cause
of variations because genetics was not yet established.
F. Organisms Struggle to Exist
1. Darwin and Wallace both read an essay by Thomas Malthus, a clergyman and
socio-economist.
2. Malthus proposed that human
populations outgrow resources and death and famine were inevitable.
3. Darwin applied this to all
organisms; resources were not sufficient for all members to survive.
4. Therefore, there is a constant
struggle for existence; only certain members survive and reproduce.
G. Organisms Differ in Fitness
1. Organisms whose traits enable them to reproduce to a greater degree have a
greater fitness.
a. Fitness
is a measure of an organism's reproductive success.
b. Black
western diamondback rattlesnakes are more likely to survive on lava flows;
lighter-colored
rattlesnakes are more likely to survive on desert soil.
2. Darwin noted that humans carry
out artificial selection.
a. Early
humans likely selected wolf variants; consequently, desirable traits increase
in frequency in
subsequent generations and produced the varieties of domestic dogs.
b. Many crop
plant varieties can be traced to a single ancestor.
c. In
nature, interactions with the environment determine which members reproduce
more.
d. Evolution
by artificial or natural selection occurs when more fit organisms reproduce and
leave more
offspring that the less fit.
H. Organisms Become Adapted
1. An adaptation is a trait that helps an organism be more suited
to its environment.
2. Unrelated organisms living in the
same environment often display similar characteristics.
3. Because of differential
reproduction, adaptive traits increase in each succeeding generation.
I. On Origin of Species by Darwin
1. After the HMS Beagle returned to England in 1836, Darwin waited over 20
years to publish.
2. He used the time to test his
hypothesis that life forms arose by descent from a common ancestor and
that natural
selection is a mechanism by which species can change and new species arise.
3. Darwin was forced to publish Origin
of Species after reading a similar hypothesis by Alfred Russel Wallace.
20.3.
Evidence for Evolution
A. Common Descent Adapted
1. The hypothesis of common descent is supported by many lines of evidence.
2. The more varied the evidence, the
more certain it becomes.
3. Darwin synthesized much of the
current data but biochemical research was yet to come.
B. Fossils Evidence
1. The fossil record is the history of life recorded by remains
from the past.
2. Fossils are at least 10,000 years
old and include skeletons, shells, seeds, insects trapped in amber,
and imprints
of leaves.
3. The fossil record traces history
of life and allows us to study history of particular organisms.
4. Fossil evidence supports the
common descent hypothesis; fossils can be linked over time because they
reveal a
similarity in form, despite observed changes.
5. Transitional forms reveal links
between groups.
a. Caudipteryx
is between dinosaurs and birds.
1) This Chinese fossil shows some dinosaurs had feathers on arms, tail and
probably body.
2) Advantages during running and escape gave rise to birds once lift-off
occurred.
b. Eustheopteron
is an amphibious fish.
c. Seymouria
is a reptile-like amphibian.
d.
Therapsids were mammal-like reptiles.
6. The fossil record allows us to
trace the history of the modern-day horse Equus.
a. Earliest
fossils show an ancestral Hyracotherium the size of a dog, with
cusped low-crowned molars,
four toes on each front foot, three on each hind foot -- all adaptations for
forest living.
b. When
forests were replaced by grasslands, the intermediates were selected for
durable grinding teeth,
speed, etc. with an increase in size and decrease in toes.
c. Living
organisms resemble most recent fossils in the line of descent; underlying
similarities allow us
to trace a line of descent over time.
C. Biogeographical Evidence
1. Biogeography studies the distribution of plants and animals
worldwide.
2. Distribution of organisms is
explained by related forms of evolving in one locale and spreading to
other
accessible areas.
a. Darwin
observed South America had no rabbits; he concluded rabbits originated
elsewhere.
b.
Biogeography explains the many finch species on the Galapagos Islands but not
the mainland.
3. Physical factors, such as the
location of continents, determine where a population can spread.
a. Cacti are
restricted to North American deserts and euphorbia grow in African deserts.
b.
Marsupials arose when South America, Antarctica, and Australia were joined;
Australia separated
before placental mammals arose, so only marsupials diversified in Australia.
D. Anatomical Evidence
1. Organisms have anatomical similarities when they are closely related because
of common descent.
a. Homologous
structures in different organisms are inherited from a common ancestor.
b. Analogous
structures are inherited from a unique ancestors and have come to
resemble each other
because they serve a similar function.
c.
Vertebrate forelimbs contain the same sets of bones organized in similar ways,
despite their dissimilar functions.
2. Vestigial Structures
are remains of a structure that was functional in some ancestor but is no longer
functional in the organism in question.
a. Most
birds have well-developed wings; some bird species have reduced wings and do
not fly.
b. Humans
have a tailbone but no tail.
c. Presence
of vestigial structures is explained by the common descent hypothesis; these
are traces of
an organism's evolutionary history.
3. Embryological development
reveals a unity of plan.
a. During
development, all vertebrates have a post-anal tail and paired pharyngeal
pouches.
1) In fishes and amphibian larvae, the pouches become gills.
2) In humans, first pair of pouches becomes a cavity of middle ear and auditory
tube; second pair
becomes tonsil, while third and fourth pairs become thymus and parathyroid
glands.
3) Above features are explained if fishes are ancestral to other vertebrate
groups.
E. Biochemical Evidence
1. Almost all living organisms use the same basic biochemical molecules, e.g.,
DNA, ATP, and many
identical or
nearly identical enzymes.
2. Organisms utilize the same DNA
triplet code and the same 20 amino acids in their proteins.
3. Many organisms share same introns
and types of repeats, which is remarkable since there is no obvious
functional
reason why these components need to be so similar.
4. This is substantiated by analysis
of degree of similarity in amino acids for cytochrome c among organisms.
5. These similarities can be
explained by descent from a common ancestor.
6. Life's vast diversity has come
about by only a slight difference in the same genes.
F. Because it is supported by so many lines of evidence, evolution is no
longer considered a hypothesis.
1. Evolution is one of the great unifying theories of biology.
2. In science, theory
is reserved for those conceptual schemes that are supported by a large number
of
observations
or a large amount of experimental evidence and have not been found lacking.