CHAPTER 22DARWINISM

Two points in The Origin of Species:

Published November 24, 1859

Natural selection provided a mechanism for evolutionary change in populations.

Today’s organisms descended from ancestral species.

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.

On the Origin of Species by Means of Natural Selection

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 Aristotle

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.

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).

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.

Major taxonomic categories; kingdom > phylum > class > order > family > genus > species

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.

Old

New

From stratum to stratum, new species appear and others disappear.

Cuvier even recognized that extinction had been a common occurrence in the history of life.

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

FloodsVolcanism

Competing with Cuvier’s theory of catastrophism was a very different idea of how geologic processes had shapedEarth’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.

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.

Second, very slow and subtle processes persisting over a long period of time can add up to substantial change.

HuttonLyell

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.

MichaelJordan

NolanRyan

LanceArmstrong

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).

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.

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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.

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.

Sunday Service at Sea by Augustus Earle (ship’s artist)

DarwinFitzRoy

(1831-1836)

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.

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Three-toed Sloth (South America)

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.

Drawing of Toxodon platensis skull that Darwin collected on the voyage

Toxodon platensis skull London Natural History Museum

South American Ostrich (Rhea) and young

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.

Sally Light Foot Crab

Giant Tortoise

Blue Footed Booby Finch

Penguin

“Flightless Cormorant”

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”

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’sPigeons

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. Joseph Hooker

Naturalist

Lyell

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, 1882Buried in Westminster Abbey

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 appearin many cartoons

Descent with Modification

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.

K = AnimaliaP = ChordataC = MammaliaO = CarnivoraF = FelidaeG = PantheraS = leo

K = AnimaliaP = ChordataC = MammaliaO = CarnivoraF = FelidaeG = PantheraS = tigris

differ

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 = AnimaliaP = ChordataC = MammaliaO = ProboscidaeF = ElephantidaeG = LoxodontaS = africanus

K = AnimaliaP = ChordataC = MammaliaO = ProboscidaeF = ElephantidaeG = ElephasS = maximus

differ

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

A few of the color variations in a population of Asian lady beetles.

OBSERVATION #5: Much of this variation is heritable.

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.

Wild Mustard

Humans have modified other species over many generations by selecting individuals with the desired traits as breeding stock.

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 familiaris

Canis lupus

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.

Victoria

Tanganyika

Malawi

The Great Rift Valley

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

Natural selection and the adaptive evolution it causes are observable phenomena.

Natural Selection in Action: The Evolution of Insecticide-Resistant Insects

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.

The historical constraints of this retrofitting are evident in anatomical imperfections.

Comparative anatomy, the comparison of body structures between species, confirms that evolution is a remodeling process.

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.

Boa and Python Families

Whales and Dolphins

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.

Ostrich wing

Mole-rat eye

Embryological HomologiesSometimes, 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.

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.

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.

Anatomical homology cannot help us link such distantly related organisms as plants and animals, which have no anatomy in common.

Molecular Homologies

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

Tim

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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.)

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.

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.

Table 22.1 compares the amino acid sequence of human hemoglobin, the oxygen-transporting protein of blood, with the hemoglobin of other vertebrates.

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 AnimalsTintoretto, c. 1551

The Creation of the AnimalsRaffaello, 1518-19

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

Coelacanth

Transitions from fishes to first amphibians (tetrapods)

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 thefossil 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.