Download Unit 1 Evolution Chp 22 Darwinism PPT

CHAPTER 22
DARWINISM
On the Origin of Species by Means of Natural Selection
Published November 24, 1859
Two points in The Origin of Species:
Today’s organisms descended
from ancestral species.
Natural selection provided a
mechanism for evolutionary change
in populations.
The basic idea of natural selection is that a
population of organisms can change over the
generations if individuals having certain
heritable traits leave more offspring than
other individuals.
The result of natural selection is evolutionary adaptation, a prevalence of inherited characteristics
that enhance organisms’ survival and reproduction in specific environments.
In modern terms, we would say that the genetic composition of the population had changed over
time, and that is one way of defining evolution.
The Origin of Species was truly radical
for its time;
not only did it challenge prevailing
scientific views, but it also shook the
deepest roots of Western culture.
Darwin’s view of life contrasted sharply with
the conventional paradigm of an Earth only
a few thousand years old, populated by
unchanging forms of life that had been
individually made during the single week in
which the Creator formed the entire universe.
Darwin’s book challenged a worldview
that had been taught for centuries.
Plato (427-347 B.C.) and his student
Aristotle (384-322 B.C.), held opinions
that opposed any concept of evolution.
Plato believed in two worlds: a real world
that is ideal and eternal and an illusory world
of imperfection that we perceive through our
senses.
Evolution would be counterproductive in
a world where ideal organisms were already
perfectly adapted to their environments.
Plato
Aristotle
The School of Athens - Raphael
Aristotle believed that all living forms could be
arranged on a scale, or ladder, of increasing
complexity, later called the scala naturae
("scale of nature").
Each form of life had its allotted rung on this ladder,
and every rung was taken
In this view of life, which prevailed for over 2,000
years, species are permanent, are perfect, and
do not evolve.
In Judeo-Christian culture, the Old Testament
account of creation fortified the idea that
species were individually designed and
nonevolving.
In the 1700s, biology in Europe and America
was dominated by natural theology, a
philosophy dedicated to discovering the
Creator’s plan by studying nature.
Natural theologians saw the adaptations of
organisms as evidence that the Creator had
designed each and every species for a
particular purpose.
A major objective of natural theology was to
classify species in order to reveal the steps
of the scale of life that God had created.
Carolus Linnaeus (1707-1778), a Swedish physician
and botanist, sought to discover order in the diversity
of life "for the greater glory of God."
Linnaeus specialized in taxonomy, the branch of
biology concerned with naming and classifying the
diverse forms of life.
He developed the two-part, or binomial, system of
naming organisms according to genus and species
that is still used today.
In addition, Linnaeus adopted a system for grouping
similar species into a hierarchy of increasingly
general categories (phylogeny).
Major taxonomic categories;
kingdom > phylum > class > order > family > genus > species
For example, similar species are grouped in
the same genus, similar genera (plural of genus)
are grouped in the same family, and so on.
To Linnaeus, clustering similar species together
implied no evolutionary kinship, but a century
later his taxonomic system would become a
focal point in Darwin’s arguments for evolution.
differ
The study of fossils also helped lay
the groundwork for Darwin’s ideas.
Fossils are relics or impressions of
organisms from the past, preserved
in rock
Most fossils are found in sedimentary rocks formed from the sand and mud that
settle to the bottom of seas, lakes, and marshes.
New layers of sediment cover older ones and compress them into superimposed
layers of rock called strata.
Later, erosion may scrape or carve through upper (younger) strata and reveal more
ancient strata that had been buried.
Fossils within the layers show that a succession of organisms has populated Earth
throughout time.
Paleontology, the study of fossils, was largely developed
by French anatomist Georges Cuvier (1769-1832).
Realizing that the history of life is recorded in strata
containing fossils, he documented the succession of
fossil species in the Paris Basin.
He noted that each stratum is characterized by a unique
group of fossil species and that the deeper (older) the
stratum, the more dissimilar the fossils are from modern life.
New
Old
Cuvier even recognized that extinction
had been a common occurrence in the
history of life.
From stratum to stratum, new species
appear and others disappear.
Yet Cuvier was a staunch opponent of
the evolutionists of his day.
Instead, he advocated catastrophism,
speculating that each boundary between
strata corresponded in time to a
catastrophe, such as a flood or drought,
that had destroyed many of the species
living there at that time.
He proposed that these periodic
catastrophes were usually confined to local
geographic regions and that the ravaged
region was repopulated by species
immigrating from other areas.
Landslides
Floods
Volcanism
Competing with Cuvier’s theory of catastrophism was a very
different idea of how geologic processes had shaped
Earth’s crust.
In 1795, Scottish geologist James Hutton (1726-1797)
proposed that it was possible to explain the various landforms
by looking at mechanisms currently operating in the world.
For example, he suggested that canyons were formed by
rivers cutting down through rocks and that sedimentary rocks
with marine fossils were built of particles that had eroded from
the land and been carried by rivers to the sea
Hutton explained Earth’s geologic
features by the theory of
gradualism, which holds that
profound change is the cumulative
product of slow but continuous
processes.
The leading geologist of Darwin’s era, a Scot named
Charles Lyell (1797-1875), incorporated Hutton’s
gradualism into a theory known as uniformitarianism.
The term refers to Lyell’s idea that geologic processes
have not changed throughout Earth’s history.
Thus, for example, the forces that build mountains
and erode mountains and the rates at which these
forces operate are the same today as in the past.
Darwin was strongly influenced
by two conclusions that followed
directly from the observations
of Hutton and Lyell.
Hutton
First, if geologic change results from
slow, continuous actions rather than
sudden events, then Earth must be
very old, certainly much older than the
6,000 years assigned by many
theologians on the basis of biblical
inference.
Lyell
Second, very slow and subtle
processes persisting over a
long period of time can add up
to substantial change.
Darwin was not the first to apply the principle
of gradualism to biological evolution, however.
Toward the end of the 18th century, several
naturalists, including Erasmus Darwin,
Charles Darwin’s grandfather, suggested that
life had evolved as environments changed.
One of Charles Darwin’s predecessors developed
a comprehensive model that attempted to explain
how life evolves: Jean Baptiste Lamarck
Lamarck published his theory of evolution in
1809, the year Darwin was born.
Lamarck was in charge of the invertebrate collection
at the Natural History Museum in Paris.
By comparing current species
with fossil forms, Lamarck could
see what appeared to be several
lines of descent, each a chronological
series of older to younger fossils
leading to a modern species.
Lamarck is remembered most for the mechanism he proposed to explain how specific
adaptations evolve.
It incorporates two ideas that were popular during Lamarck’s era.
The first was use and disuse, the idea that those parts of the body
used extensively to cope with the environment become larger and
stronger while those that are not used deteriorate.
Among the examples Lamarck cited were a blacksmith developing a bigger bicep
in the arm that wields the hammer and a giraffe stretching its neck to reach leaves
on high branches.
The second idea Lamarck adopted was called the inheritance of acquired characteristics.
In this concept of heredity, the modifications an organism acquires during its lifetime
can be passed along to its offspring.
The long neck of the giraffe, Lamarck reasoned, evolved gradually as the cumulative
product of a great many generations of ancestors stretching ever higher.
There is, however, no evidence that acquired characteristics can be inherited.
Blacksmiths may increase strength and stamina by a lifetime of pounding
with a heavy hammer, but these acquired traits do not change genes transmitted
by gametes to offspring.
Even though the Lamarckian theory of evolution is ridiculed often today because of its
erroneous assumption that acquired characteristics are inherited, in Lamarck’s time
that concept of inheritance was generally accepted (and, indeed, Darwin could
offer no acceptable alternative).
Lance
Armstrong
Michael
Jordan
Nolan
Ryan
To most of Lamarck’s contemporaries, however, the mechanism of evolution was an
irrelevant issue because they firmly believed that species were fixed and that no theory
of evolution could be taken seriously.
Lamarck was vilified, especially by Cuvier, who denied that species ever evolve.
In retrospect, Lamarck deserves much credit for his theory, which was visionary in
many respects: in its claim that evolution is the best explanation for both the fossil
record and the current diversity of life; in its recognition of the great age of Earth;
and especially in its emphasis on adaptation to the environment as a primary
product of evolution.
X
The Darwinian Revolution
Natural theology still dominated the intellectual climate as the 19th century dawned.
Charles Darwin (1809-1882) was born in Shrewsbury in western England.
Even as a boy he had a consuming interest in nature.
When he was not reading nature books, he was
fishing, hunting, and collecting insects.
Darwin’s father, an eminent physician, could see no future for a naturalist and sent
Charles to the University of Edinburgh to study medicine.
Only 16 years old at the time, Charles found medical school boring and distasteful.
He left Edinburgh without a degree and shortly thereafter enrolled at
Christ College at Cambridge University, with the intent of becoming a clergyman.
At that time in Great Britain, most naturalists and other scientists belonged to the
clergy, and nearly all saw the world in the context of natural theology.
Darwin became the protégé of the Reverend John Henslow,
professor of botany at Cambridge.
Soon after Darwin received his B.A. degree in 1831,
Professor Henslow recommended the young graduate to
Captain Robert FitzRoy, who was preparing the survey ship
Beagle for a voyage around the world.
Darwin would pay his own way and serve as a conversation companion to the
young captain.
FitzRoy chose Darwin because of his education and because he was of the
same social class and about the same age as the captain.
(1831-1836)
FitzRoy
Darwin
Sunday Service at Sea by Augustus Earle (ship’s artist)
Darwin was 22 years old when he sailed from Great Britain aboard HMS Beagle in
December 1831
The primary mission of the voyage was to chart poorly known stretches of the
South American coastline.
While the ship’s crew surveyed the coast, Darwin spent most of his time on shore,
observing and collecting thousands of specimens of South American plants and animals.
As the ship worked its way around the continent, Darwin observed the various
adaptations of plants and animals that inhabited such diverse environments as the
Brazilian jungles, the expansive grasslands of the Argentine pampas, the desolate
lands of Tierra del Fuego near Antarctica, and the towering heights of the
Andes Mountains.
Brazilian
Jungle
Darwin noted that plants and animals he studied had definite South American
characteristics, very distinct from those of Europe.
That in itself may not have been surprising.
But Darwin also noted that the plants and animals in temperate regions of
South America were more closely related to species living in tropical regions of that
continent than to species in temperate regions of Europe.
x
=
Furthermore, the South American fossils that Darwin found, though clearly different
from modern species, were distinctly South American in their resemblance to the
living plants and animals of that continent.
South American Ostrich
(Rhea) and young
Toxodon platensis skull
London Natural
History Museum
Drawing of Toxodon platensis skull
that Darwin collected on the voyage
Three-toed Sloth (South America)
The geographic distribution of species interested Darwin.
For example, he was curious about the fauna of the Galápagos, islands of relatively
recent volcanic origin that lie on the equator about 900 km west of the South American
coast.
He learned that most of the animal species on the Galápagos live nowhere
else in the world, although they resemble species living on the South American mainland.
“indigenous”
“Flightless Cormorant”
Penguin
Giant Tortoise
Sally Light Foot Crab
Blue Footed
Booby
Finch
Marine Iguana
It was as though the islands had been colonized by plants and animals that strayed
from the South American mainland and then diversified on the different islands.



Among the birds Darwin collected on the Galápagos were several types of finches
that, although quite similar, seemed to be different species. Some were unique to
individual islands, while other species were distributed on two or more islands that
were close together.
Darwin read Lyell’s Principles of Geology while on board the Beagle .
Lyell’s ideas, together with his own experiences on the Galápagos, had Darwin
doubting the church’s position that Earth was static and had been created only a
few thousand years ago.
By acknowledging that Earth was very old and constantly changing, Darwin took an
important step toward recognizing that life on Earth had also evolved.
Soon after returning to Great Britain in 1836, Darwin started reassessing all that he
had observed during the voyage of the Beagle .
He began to perceive the origin of new species and
adaptation to the environment as closely related processes.
Could a new species arise from an ancestral form by the
gradual accumulation of adaptations to a different environment?
Darwin’s
Pigeons
From studies made years after Darwin’s voyage, biologists have concluded that this
is what happened to the Galápagos finches.
Among the differences between the finches are their beaks, which are adapted to the
specific foods available on their home islands.
Darwin anticipated that explaining how such adaptations
arise was essential to understanding evolution.
By the early 1840s, Darwin had worked out the major features of his theory of
natural selection as the mechanism of evolution.
The basic idea of natural selection is that a
population of organisms can change over the
generations if individuals having certain heritable
traits leave more offspring than other individuals.
However, he had not yet published his ideas.
He was in poor health, and he rarely left home.
Despite his reclusiveness, Darwin was not isolated from the
scientific community.
Already famous as a naturalist
because of the letters and
specimens he sent to Great Britain
during the voyage of the Beagle,
Darwin had frequent correspondence
and visits from Lyell, Henslow,
and other scientists.
Lyell
Joseph Hooker
Naturalist
In 1844, Darwin wrote a long essay on the origin of species and natural selection.
However, Darwin was reluctant to introduce his theory publicly, apparently because
he anticipated the uproar it would cause.
While he procrastinated, he continued to compile evidence in support of his theory.
Lyell, not himself yet convinced of evolution, nevertheless advised Darwin to
publish on the subject before someone else came to the same conclusions and
published first.
In June 1858, Lyell’s prediction came true.
Darwin received a letter from Alfred Wallace (1823-1913), a young British naturalist
working in the East Indies.
The letter was accompanied by a manuscript in which Wallace developed a theory
of natural selection essentially identical to Darwin’s.
Wallace asked Darwin to evaluate the paper and forward it to Lyell if it merited
publication.
Darwin complied, writing to Lyell: "Your words have come true with a vengeance ... .
I never saw a more striking coincidence ... so all my originality, whatever it may
amount to, will be smashed."
Lyell and a colleague presented Wallace’s paper, along with extracts from
Darwin’s unpublished 1844 essay, to the Linnaean Society of London on July 1, 1858.
Darwin quickly finished The Origin of Species and published it the next year.
Although Wallace wrote up his ideas for publication first, Darwin developed and
supported the theory of natural selection so much more extensively that he is
known as its main architect.
And Darwin’s notebooks prove that he formulated his theory of natural selection
15 years before reading Wallace’s manuscript.
Within a decade, Darwin’s book and its proponents had convinced the majority of
biologists that biological diversity was the product of evolution.
Darwin succeeded where previous evolutionists had failed, partly because science
was beginning to shift away from natural theology, but mainly because he convinced
his readers with immaculate logic and an avalanche of evidence in support of evolution.
1831- 1858 = 27 years of study
Died April 19, 1882
Buried in Westminster Abbey
Descent with Modification
The Origin of Species developed two main points:
the occurrence of evolution and natural selection as its mechanism
“Darwinism” has a dual meaning. It refers to evolution as the explanation for life’s unity
and diversity, and it also refers to the Darwinian concept of natural selection as the
cause of adaptive evolution.
In the first edition of The Origin of Species , Darwin
did not use the word evolution until the last paragraph,
referring instead to descent with modification, a
phrase that condensed his view of life.
Caricatures of Darwin would appear
in many cartoons
Darwin perceived unity in life, with all organisms related through descent from some
unknown ancestor that lived in the remote past.
As the descendants of that ancestral organism spilled into various habitats over millions
of years, they accumulated diverse modifications, or adaptations, that fit them to specific
ways of life.
In the Darwinian view, the history of life is like a tree, with multiple branching and
rebranching from a common trunk all the way to the tips of the youngest twigs,
symbolic of the diversity of living organisms.
At each fork of the evolutionary tree is an ancestor common to all lines of evolution
branching from that fork.
Closely related species, such as the Asian elephant and the African elephant, are
very similar because they share the same line of descent until a relatively recent
divergence from a common ancestor
Most branches of evolution, even some major ones, are dead ends; about 99% of
all species that have ever lived are extinct.
Descent with modification.
This evolutionary tree of the
elephant family is based
mainly on evidence from
fossils--their anatomy, their
order of appearance in
geologic time, and their
geographic distribution.
Ironically, Linnaeus, who apparently believed that species are fixed, provided Darwin
with a connection to evolution by recognizing that the great diversity of organisms
could be ordered into "groups subordinate to groups" (Darwin’s phrase).
To Darwin, the natural hierarchy of the Linnaean
scheme reflected the branching history of the
tree of life, with organisms at the different
taxonomic levels related through descent
from common ancestors.
Two species, such as lions and tigers, that are grouped in the same family
(family Felidae) share a more recent common ancestor than two species, such as
lions and elephants, that belong to different families within the same class
(class Mammalia).
K = Animalia
P = Chordata
C = Mammalia
O = Carnivora
F = Felidae
G = Panthera
S = leo
differ
K = Animalia
P = Chordata
C = Mammalia
O = Carnivora
F = Felidae
G = Panthera
S = tigris
K = Animalia
P = Chordata
C = Mammalia
O = Proboscidae
F = Elephantidae
G = Loxodonta
S = africanus
differ
K = Animalia
P = Chordata
C = Mammalia
O = Proboscidae
F = Elephantidae
G = Elephas
S = maximus
Natural Selection and Adaptation
How does natural selection work?
And how does natural selection explain adaptation?
Evolutionary biologist Ernst Mayr has dissected the logic of
Darwin’s theory of natural selection into three inferences
based on five observations:
OBSERVATION #1: All species have such great
potential fertility that their population size would
increase exponentially if all individuals that are
born reproduced successfully.
Overproduction of offspring.
A cloud of millions of spores is
exploding from this puffball, a type
of fungus. The wind will disperse the
spores far and wide. Only a tiny
fraction of the spores will actually
give rise to offspring that survive
and reproduce.
OBSERVATION #2: Populations tend to remain stable in size, except for seasonal
fluctuations.
OBSERVATION #3: Environmental resources are limited.
INFERENCE #1: Production of more individuals than the environment can support
leads to a struggle for existence among individuals of a population, with only a
fraction of offspring surviving each generation.
OBSERVATION #4: Individuals of a population vary extensively in their characteristics;
no two individuals are exactly alike
OBSERVATION #5: Much of this variation is heritable.
A few of the color variations
in a population of Asian lady
beetles.
INFERENCE #2: Survival in the struggle for existence is not random, but depends in
part on the hereditary constitution of the individuals. Those individuals whose
inherited traits best fit them to their environment are likely to leave more offspring
than less fit individuals.
INFERENCE #3: This unequal ability of individuals to survive and reproduce will lead
to a gradual change in a population, with favorable characteristics accumulating over
the generations.
We can summarize Darwin’s main ideas as follows:
Natural selection is differential success in reproduction
(unequal ability of individuals to survive and reproduce).
Natural selection occurs through an interaction between
the environment and the variability inherent among the
individual organisms making up a population .
The product of natural selection is the adaptation of
populations of organisms to their environment.
Let’s elaborate on the important connections Darwin perceived between natural selection,
the struggle for existence, and the capacity of organisms to "overreproduce."
Darwin appparently began to recognize the struggle for existence after he read an
essay on human population written in 1798 by Thomas Malthus.
Malthus contended that much of human suffering--disease, famine, homelessness,
and war--was the inescapable consequence of the potential for the human population
to increase faster than food supplies and other resources.
The capacity to overproduce seems to be characteristic of all species.
Of the many eggs laid, young born, and seeds spread, only a tiny fraction complete
their development and leave offspring of their own.
The rest are eaten, frozen, starved, diseased, unmated, or unable to reproduce for
some other reason.
In each generation, environmental factors filter heritable variations, favoring some
over others.
Differential reproduction--whereby organisms with traits favored by the environment
produce more offspring than do organisms without those traits--results in the favored
traits being disproportionately represented in the next generation.
This increasing frequency of the favored traits in a population is evolution.
Darwin illustrated the power of selection as a force in evolution with examples from
artificial selection, the breeding of domesticated plants and animals.
Humans have modified other species over
many generations by selecting individuals with
the desired traits as breeding stock.
Wild Mustard
The plants and animals we grow for food often bear little resemblance to their
wild ancestors
The power of selective breeding is especially apparent in our pets, which have been
bred more for fancy than for utility.
Canis lupus
Canis familiaris
If so much change can be achieved by artificial selection in a relatively short period
of time, Darwin reasoned, then natural selection should be capable of considerable
modification of species over hundreds or thousands of generations.
Even if the advantages of some heritable traits over others are slight, the advantageous
variations will accumulate in the population after many generations of natural
selection, eliminating less favorable variations.
Darwin incorporated gradualism, a concept so important in Lyell’s geology, into
evolutionary theory.
He envisioned life as evolving by a gradual accumulation of minute changes, and he
postulated that natural selection operating in varying contexts over vast spans of time
could account for the entire diversity of life.
We can now summarize the two main features of the Darwinian view of life:
(1) The diverse forms of life have arisen by descent with modification from ancestral species.
(2) the mechanism of modification has been natural selection working over enormous tracts of time.
Some Subtleties of Natural Selection
One subtlety is the importance of populations in evolution.
Population = a group of interbreeding individuals belonging to a particular
species and sharing a common geographic area.
A population is the smallest unit that can evolve.
Natural selection occurs through interactions between individual organisms and their
environment, but individuals do not evolve.
Evolution can be measured only as changes in relative proportions of heritable
variations in a population over a succession of generations.
Another key point about natural selection is that it can amplify or diminish only
heritable variations.
As we have seen, an organism may become
modified through its own experiences during
its lifetime, and such acquired characteristics
may even adapt the organism
to its environment, but there is no evidence
that characteristics acquired during a
lifetime can be inherited.
We must distinguish between adaptations
an organism acquires by its own actions
and inherited adaptations that evolve in a
population over many generations as a
result of natural selection.
It must also be emphasized that the specifics of natural selection are situational;
environmental factors vary from place to place and from time to time
An adaptation in one situation may be useless or even detrimental in different
circumstances.
Some examples will reinforce this situational quality of natural selection.
Cichlids in Lake Victoria
The
Victoria
Great Rift
Tanganyika
Valley
Malawi
Natural Selection in Action: The Evolution of Insecticide-Resistant Insects
Natural selection and the adaptive evolution it causes are observable phenomena.
A classic and unsettling example of natural selection is the evolution of insecticide
resistance in hundreds of insect species.
Whenever a new type of insecticide is used to control agricultural pests, the story is
usually the same.
Early results are encouraging.
A relatively small amount of the poison dusted onto a crop may kill 99% of the insects.
But subsequent sprayings are less and less effective.
It is natural selection that causes the evolution of resistance to insecticides.
The relatively few survivors of the first insecticide
wave are insects with genes that somehow
enable them to resist the chemical attack.
In some cases, the lucky few carry genes coding
for enzymes that destroy the insecticide.
The poison kills most members of the insect
population, leaving the resistant individuals to reproduce.
And their offspring inherit the genes for insecticide
resistance. In each generation, the proportion of
insecticide-resistant individuals in the insect
population increases.
The population has adapted to a
change in its environment.
This example of insect adaptation to insecticides highlights two key points about
natural selection.
First, notice that natural selection is more a process of editing than it is a creative
mechanism.
An insecticide does not create resistant individuals, but selects for resistant insects
that were already present in the population.
Second, note again that natural selection is contingent on time and place.
It favors those characteristics in a varying population that fit the current, local
environment.
What is adaptive in one situation may be useless or even detrimental in different
circumstances.
For example, some genetic mutations that endow houseflies with
resistance to the insecticide DDT also reduce a fly’s growth rate.
Before DDT was introduced to environments, those particular
genes were a handicap. But the appearance of DDT changed the
environmental arena and favored insecticide-resistant individuals.
Natural Selection in Action: The Evolution of Drug-Resistant HIV
Researchers have developed numerous drugs to combat the human immunodeficiency
virus (HIV), the pathogen that causes AIDS.
In every case, resistance to a drug evolves rapidly in the HIV population of an individual
patient soon after treatment with that drug begins.
For example, this graph illustrates the
evolution of HIV resistance to a drug
named 3TC.
Notice that the 3TC-resistant forms of
HIV begin to increase in number almost
immediately and make up 100% of the
total HIV population in each patient after
just a few weeks.
Scientists designed the drug 3TC to interfere with reverse transcriptase, the enzyme
HIV uses to copy its RNA genome into the DNA of the human host cell
DNA, remember, is a polymer of four kinds of nucleotides, abbreviated A, G, T, and C
The drug 3TC mimics the C (cytosine) nucleotide of DNA. The HIV’s reverse
transcriptase will pick up a 3TC molecule instead of C and insert it into a growing
DNA chain. This error terminates further elongation of the DNA and thus blocks
reproduction of the HIV.
The 3TC-resistant variety of HIV has a slightly different version of reverse transcriptase
that is able to discriminate between the drug and the normal C nucleotide.
Members of the HIV population that inherit the gene for this form of the enzyme have
no advantage in the absence of 3TC; in fact, they replicate their DNA more slowly than
the "normal" variety of HIV.
But once 3TC is added to the environment of these viruses, it becomes a potent force
in natural selection, favoring reproduction of the resistant individuals.
Other evidence of evolution pervades biology
We have examined cases of evolution by natural selection that occur rapidly enough to
be directly observed. However, the much grander changes of biological diversity
documented by the fossil record occur on a time scale spanning hundreds of millions
of years.
Evidence that the diversity of life is a product of evolution pervades every research field
of biology. And, as biology progresses, new discoveries, including the revelations of
molecular biology, continue to validate the Darwinian view of life.
Homology
Descent with modification, Darwin’s term for evolution, means that new species
descend from ancestral species by the accumulation of modifications as populations
adapt to new environments.
The novel features that characterize a new species are not entirely new, but are altered
versions of ancestral features.
Species with common ancestry should display underlying similarities, even in features
that no longer match in function. Similarity in characteristics resulting from common
ancestry is known as homology.
Anatomical Homologies
Descent with modification is indeed evident in anatomical similarities between species
grouped in the same taxonomic category.
For example, many of the same skeletal elements make up the forelimbs of humans,
cats, whales, bats, and all other mammals, although these appendages have very
different functions
The basic similarity of these
forelimbs is the consequence
of the descent of all mammals
from a common ancestor.
Variations on a common structural theme.
In taking on different functions in each species, the basic structures were modified.
Such anatomical signs of evolution are called homologous structures.
Comparative anatomy, the comparison of body structures between species, confirms
that evolution is a remodeling process.
The historical constraints of this retrofitting are evident in anatomical imperfections.
For example, the human knee joint and spine were derived from ancestral structures that
supported four-legged mammals. Almost none of us will reach old age without experiencing
knee or back problems.
If these structures had first taken form specifically to support our bipedal posture, we would expect
them to be less subject to injury. The anatomical remodeling that stood us up was apparently
constrained by our evolutionary history.
Some of the most interesting homologous structures are vestigial organs, structures of
marginal, if any, importance to the organism.
Vestigial organs are historical remnants of structures that had important functions in ancestors.
For instance, the skeletons of some snakes retain vestiges of the pelvis and leg bones of
walking ancestors. We would not expect to see these structures if snakes had an origin separate
from other vertebrate animals.
Boa and Python Families
Whales and Dolphins
Ostrich wing
Mole-rat eye
Embryological Homologies
Sometimes, homologies that are not obvious in adult organisms become evident when
we look at embryonic development.
For example, all vertebrate embryos have structures called pharyngeal pouches in their throat
regions at some stage in their development.
These embryonic structures develop into homologous structures with very different functions, such
as the gills of fish or the Eustachian tubes that connect the middle ear with the throat in humans
and other mammals.
A radiogram of the
sacral region of a
six-year old girl
with an atavistic tail.
Molecular Homologies
Anatomical homology cannot help us link such distantly related organisms as plants
and animals, which have no anatomy in common.
However, plants and animals, along with all other organisms, do share certain characteristics at the
molecular level: For example, all species of life use the same basic genetic machinery of DNA and
RNA, and the genetic code is essentially universal.
Evidently, the language of the genetic code has been passed along through all branches of the
tree of life ever since the code’s inception in an early life-form. Molecular biology provides new
tools for exploring evolutionary relationships in the diversity of life.
Homologies and the Tree of Life
Homologies mirror the taxonomic hierarchy of the tree of life.
Some homologies, such as the genetic code, are shared by all life because they date
back to the deep ancestral past.
Homologies that evolved more recently are shared only by smaller branches of the
tree of life.
For example, all tetrapods (from the Greek tetra , "four," and pod , "foot"), the vertebrate
branch consisting of amphibians, reptiles, birds, and mammals, share the same
basic five-digit limb structure
Time -----------------------------------------------------
Thus, homologies form a layered pattern,
with all life sharing the deepest layer and
each smaller group adding fresh
homologies to those they share with
larger groups.
This hierarchical pattern is exactly what
we would expect if life evolved and
diversified from a common ancestor,
but not what we would see if each
species arose separately.
If homologies reflect evolutionary history, we should expect to find similar patterns
whether we are comparing molecules or bones or any other characteristics.
The new tools of molecular biology have generally corroborated rather than contradicted
evolutionary trees based on comparative anatomy and other methods.
Evolutionary relationships among species are
documented in their DNA and proteins--in their
genes and gene products.
If two species have libraries of genes and proteins
with sequences that match closely, the sequences
have probably been copied from a common
ancestor.
(If two long paragraphs match except for the
substitution of a letter here and there, we would
surely attribute them both to a single source.)
Table 22.1 compares the amino acid sequence
of human hemoglobin, the oxygen-transporting
protein of blood, with the hemoglobin of other
vertebrates.
The data show the same pattern of evolutionary
relationships that researchers find when they
compare other proteins or assess relationships
based on nonmolecular methods, such as
skeletal anatomy.
The Darwinian view of life predicts that different
kinds of homologies--anatomical, embryological,
and molecular--will fall into the same hierarchical
pattern because they have all evolved during the
same branching pattern of evolutionary history.
Biogeography
The geographic distribution of species--biogeography--first suggested evolution to Darwin.
Species tend to be more closely related to other species from the same area than to other species
with the same way of life but living in different areas.
For example, Australia is the home of a group of mammals--the marsupials--that are distinct from
another group of mammals--the eutherians--that live elsewhere on Earth.
(Eutherians are mammals that complete their embryonic development in the uterus, while
marsupials are born as embryos and complete their development in an external pouch.)
Some Australian marsupials have eutherian look-alikes with similar adaptations living
on other continents.
For example, a forest-dwelling marsupial
called the sugar glider is superficially very
similar to flying squirrels, eutherians that
live in North American forests
These two mammals have adapted to the
same way of life, but they evolved
independently from different ancestors.
The sugar glider is distinctly marsupial,
much more closely related to kangaroos
and other Australian marsupials than to
flying squirrels or any other eutherian
mammals.
The sugar glider is a marsupial not because
that is a requirement for its gliding lifestyle
but simply because its ancestors were
marsupials.
The unique fauna of Australia diversified on that island continent after it became
isolated from the landmasses on which placental mammals diversified.
The resemblance between sugar gliders and
flying squirrels is an example of what biologists
call convergent evolution
Sugar Glider
Flying Squirrel
Islands are showcases of biogeographic evidence for evolution.
They generally have many species of plants and animals that are endemic, which
means they are found nowhere else in the world.
And yet, as Darwin observed when he reassessed his collections from the voyage of the Beagle ,
most island species are closely related to species from the nearest mainland or neighboring island.
This explains why two islands with similar environments in different parts of the world
are populated not by closely related species but by species taxonomically affiliated with
the plants and animals of the nearest mainland, where the environment is often quite
different.
Island chains, or archipelagos, are especially interesting in their biogeography.
If a species that disperses from a mainland to an island succeeds in its new environment,
it may give rise to several new species as populations spread to other islands in the
archipelago.
The example of finches on the Galápagos archipelago came up earlier in the chapter.
The evolution of fruit fly (Drosophila) species on the Hawaiian archipelago
Geologists have determined the ages of these volcanic islands, which are progressively younger from Kauai (the oldest) to the big
island of Hawaii (the youngest, still growing as active volcanoes add lava rock to the shoreline). The islands have about 500 endemic
species of the fruit fly genus Drosophila , all descended from a common ancestor that managed to reach Kauai over 5 million years
ago. The arrows trace the history of just a few of the species in one evolutionary branch. The vintage of each species closely matches
the age of its island home.
The Fossil Record
The succession of fossil forms is compatible with what is known from other types of
evidence about the major branches of descent in the tree of life.
For instance, evidence from biochemistry, molecular biology, and cell biology places prokaryotes
as the ancestors of all life and predicts that prokaryotes should precede all eukaryotic life in the
fossil record. Indeed, the oldest known fossils are prokaryotes.
Another example is the chronological appearance of the different classes of vertebrate
animals in the fossil record.
Fossil fishes predate all other vertebrates, with amphibians next, followed by reptiles,
then mammals and birds.
This sequence is consistent with the history of vertebrate descent as revealed by many
other types of evidence.
In contrast, the idea that all species were individually created at about the same time
predicts that all vertebrate classes would make their first appearance in the fossil
record in rocks of the same age, a prediction at odds with what paleontologists
actually observe.
The Creation of the Animals
Raffaello, 1518-19
The Creation of the Animals
Tintoretto, c. 1551
The Darwinian view of life also predicts that evolutionary transitions should leave signs
in the fossil record.
Paleontologists have discovered fossils of many transitional forms that link even older
fossils to modern species.
For example, a series of fossils documents the changes in skull shape and size that
occurred as mammals evolved from reptiles.
“apsid” – extra holes
Transitions from fishes
to first amphibians (tetrapods)
Coelacanth
Every year, paleontologists turn up other important links between modern forms and
their ancestors. In the past few years, for instance, researchers have found fossilized
whales that link these aquatic mammals to their terrestrial predecessors
The hypothesis that whales evolved from terrestrial
(land-dwelling) ancestors predicts a four-limbed
beginning for whales. Paleontologists digging in
Egypt and Pakistan have identified extinct whales
that had hind limbs. Shown here are the fossilized
leg bones of Basilosaurus , one of those ancient
whales. These whales were already aquatic animals
that no longer used their legs to support their weight.
The leg bones of an even older fossilized whale
named Ambulocetus are heftier. Ambulocetus may
have split its time between living on land and in water.
(A) Pan troglodytes, modern chimpanzee;
(B) Australopithecus africanus, 2.6 My;
(C) Australopithecus africanus, 2.5 My;
(D) Homo habilis, 1.9 My;
(E) Homo habilis, 1.8 My;
(F) Homo rudolfensis, 1.8 My;
(G) primitive Homo erectus, Dmanisi cranium, 1.75 My;
(H) Homo ergaster (late H. erectus), 1.75 My;
(I) Homo heidelbergensis, "Rhodesia man," 300,000 - 125,000 y;
(J) Homo sapiens neanderthalensis, 70,000 y;
(K) Homo sapiens neanderthalensis, 60,000 y;
(L) Homo sapiens neanderthalensis, 45,000 y;
(M) Homo sapiens sapiens, Cro-Magnon, 30,000 y;
(N) modern Homo sapiens sapiens.
Homo sapiens evolution
Thus, the Darwinian view of life endures in biology because it is supported by
independent types of evidence: evolutionary patterns of homology that match patterns
in space (biogeography) and time (the fossil record)
Some people dismiss Darwinism as "just a theory”. This tactic for nullifying the evolutionary view of
life has two flaws. First, it fails to separate Darwin’s two claims: that modern species evolved from
ancestral forms and that natural selection is the main mechanism for this evolution. The conclusion
that life has evolved is based on historical evidence--the signs of evolution discussed in the
previous section.
What, then, is theoretical about evolution? Theories are our attempts to explain facts and integrate
them with overarching concepts. To biologists, Darwin’s theory of evolution is natural selection--the
mechanism Darwin proposed to explain the historical facts of evolution documented by fossils,
biogeography, and other types of evidence.
So the "just a theory" argument concerns only Darwin’s second point, his theory of natural
selection. This brings us to the second flaw in the "just a theory" case. The term theory has a
very different meaning in science than in everyday use. The colloquial use of the word theory
comes close to what scientists mean by a hypothesis. In science, a theory is more comprehensive
than a hypothesis. A theory, such as Newton’s theory of gravitation or Darwin’s theory of natural
selection, accounts for many facts and attempts to explain a great variety of phenomena. Such a
unifying theory does not become widely accepted in science unless its predictions stand up to
thorough and continual testing by experiments and observations (see Chapter 1). Even then, good
scientists do not allow theories to become dogma. For example, many evolutionary biologists now
question whether natural selection alone accounts for the evolutionary history observed in the
fossil record.
The study of evolution is livelier than ever, and we will evaluate some of the current debates in the
next three chapters. But these questions about how life evolves in no way imply that most biologists
consider evolution itself to be "just a theory." Debates about evolutionary theory are like arguments
over competing theories about gravity; we know that objects keep right on falling while we debate
the cause.
By attributing the diversity of life to natural causes rather than to supernatural creation, Darwin gave
biology a sound, scientific basis (FIGURE 22.18). Nevertheless, the diverse products of evolution
are elegant and inspiring. As Darwin said in the closing paragraph of The Origin of Species:
"There is grandeur in this view of life."
“There is grandeur in this view of life; with its several
powers having been originally breathed by the Creator
into a few forms or into one; and that, whilst this planet
has gone cycling on according to the fixed law of gravity,
from so simple a beginning endless forms most wonderful
and most beautiful have been, and are being evolved.”
Charles Darwin in 1859, the year
The Origin of Species was published.