Biology 2

Chapter 20 NOTES

Evolution, the unifying theme woven throughout the text and course, refers to the processes that have

transformed life on earth from its earliest forms to the enormous diversity that characterizes it today.

The first convincing case for evolution was published in a book by Charles Darwin on November 24,

1859. In this book, On the Origin of Species by Means of Natural Selection, Darwin:

• Synthesized seemingly unrelated facts into a conceptual framework that accounts for both the unity

and diversity of life.

Discussed important biological issues about organisms, such as why there are so many kinds of

organisms, their origins and relationships, similarities and differences, geographic distribution, and

adaptations to their environment.

• Made two major points:

1. Species evolved from ancestral species and were not specially created.

2. Natural selection is a mechanism that could result in this evolutionary change.

Western culture resisted evolutionary views of life

The impact of Darwin's ideas partially depended upon historical and social context. (See

Campbell, Figure 20. 1)

Darwin's view of life contrasted sharply with the accepted viewpoint: the Earth was only a

few thousand years old and was populated by unchanging life forms made by the Creator

during a single week.

Thus, On the Origin of Species by Means of Natural Selection not only challenged

prevailing scientific views, but also challenged the roots of Western culture.

A. The Scale of Life and Natural Theology

Many Greek philosophers believed in the gradual evolution of life. However, the two that

influenced Western culture most, Plato (427 ‑ 347 B.C.) and his student Aristotle (384 ‑

322 B.C.), held opinions which were inconsistent with a concept of evolution.

Plato, whose philosophy is known as idealism (essentialism), believed that there

were two coexisting worlds: an ideal eternal real world and an illusionary imperfect

world that humans perceive with their senses. To Plato,

=> Variations in plant and animal populations were merely imperfect

representatives of ideal forms; only these perfect ideal forms are real.

Evolution would be counterproductive in a world where ideal organisms are

already perfectly adapted to their environments.

Aristot 'le questioned the Platonic philosophy of dual worlds, but his beliefs also

excluded evolution.

Recognizing that organisms range from simple to complex, he believed that they

could be placed on a scale of increasing complexity (scala naturae); on this

ladder of life, each form had its allotted rung and each rung was occupied.

In this view of life, species are fixed and do not evolve.

The scala naturae view of life prevailed for over 2000 years

Descent With Modification: A Darwinian View of Life 357

The creationist‑essentialist dogma that species were individually created and fixed

became embedded in Western thought as the Old Testament account of creation from the

Judeo‑Christian culture fortified prejudice against evolution.

Natural Theology, a philosophy that the Creator's plan could be revealed by studying

nature, dominated European and American biology even as Darwinism emerged.

For natural theologians, adaptations of organisms were evidence that the Creator had

designed every species for a particular purpose.

Natural theology's major objective was to classify species revealing God's create

steps on the ladder of life.

Carolus Linnaeus (1707 ‑ 1778), a Swedish physician and botanist, sought order in the

diversity of life ad majorem Dei gloriam (for the greater glory of God).

• Known as the father of taxonomy ‑ the naming and classifying of organisms ‑ he

developed the system of binomial nomenclature still used today.

• He adopted a system for grouping species into categories and ranking the categories

into a hierarchy. For example, similar species are grouped into a genus; similar

genera are grouped into the same order; etc.

Linnaeus found order in the diversity of life with his hierarchy of taxonomic categories.

The clustering of species in taxonomic groups did not imply evolutionary

relationships to Linnaeus, since he believed that species were permanent creations.

Linnaeus, a natural theologian, developed his classification scheme only to reveal

God's plan and even stated Deus creavit, Linnaeus disposuit ("God creates, Linnaeus

arranges").

B. Cuvier, Fossils, and Catastrophism

Fossils = Relics or impressions of organisms from the past, preserved in rock.

Most fossils are found in sedimentary rocks, which:

Form when new layers of sand and mud settle to the bottom of seas, lakes, and

marshes, covering and compressing older layers into rock (e.g. sandstone and

shale).

May be deposited in many layers (strata) in places where shorelines repeatedly

advance and retreat.

Later erosion can wear away the upper (younger) strata, revealing older strata

which had been buried.

The fossil record thus provides evidence that Earth has had a succession of flora and

fauna.

The study of fossils, paleontology, was founded by the French anatomist Georges Cuvier

(1769‑1832) who:

Realized life's history was recorded in fossil‑containing strata and documented the

succession of fossil species in the Paris Basin.

3 5 8 Descent With Modification: A Darwinian View of Life

• Noted each stratum was characterized by a unique set of fossil species and that the

older (deeper) the stratum, the more dissimilar the flora and fauna from modem life

forms.

• Understood that extinction had been a common occurrence in the history of life

since, from stratum to stratum, new species appeared and others disappeared.

Even with paleontological evidence, Cuvier was an effective opponent to the evolutionists

of his day.

• He reconciled the fossil evidence with his belief in the fixity of species by

speculating that boundaries between fossil strata corresponded in time to

catastrophic events, such as floods or droughts.

• This view of Earth's history is known as catastrophism.

Catastrophism = Theory that major changes in the Earth's crust are the result of

catastrophic events rather than from gradual processes of change.

Cuvier explained the appearance of new species in younger rock that were absent from

older rock by proposing that:

• Periodic localized catastrophes resulted in mass extinctions.

• After the local flora and fauna had become extinct, the region would be repopulated

by foreign species immigrating from other areas.

11. Theories of geological gradualism helped clear the path for evolutionary biologists

In the late 18th century, a new theory of geological gradualism gained popularity among

geologists that would greatly influence Darwin.

Gradualism = Principle that profound change is the cumulative product of slow, continuous

processes.

• Competed with Cuvier's theory of catastrophism.

• Proposed by James Hutton (1975), a Scottish geologist. He proposed that it was possible

to explain the various land forms by looking at mechanisms currently operating in the

world.

• For example, canyons form by erosion from rivers, and fossil‑bearing sedimentary rocks

form from particles eroded from the land and carried by rivers to the sea.

Charles Lyell, a leading geologist of Darwin's time expanded Hutton's gradualism into the theory

known as uniformitarianism.

Uniformitarianism = Theory that geological processes are uniform and have operated from the

origin of the Earth to the present.

Was Lyell's extreme idea that geological processes are so uniform that their rates and

effects must balance out through time.

For example, processes that build mountains are eventually balanced by the erosion of

mountains.

Descent With Modification: A Darwinian View of Life 359

Darwin rejected uniformitarianism, but was greatly influenced by conclusions that followed

directly from the observations of Hutton and Lyell:

• The Earth must be ancient. If geological change results from slow, gradual processes

rather than sudden events, then the Earth must be much older than the 6000 years indicated

by many theologians on the basis of biblical inference.

• Very slow and subtle processes persisting over a great length of time can cause substantial

change.

111. Lamarck placed fossils in an evolutionary context

Several 18th century naturalists suggested that life had evolved along with Earth's changes. Only

Jean Baptiste Lamarck (1744‑1829) developed and published (1809) a comprehensive model

which attempted to explain how life evolved.

In charge of the invertebrate collection at the Natural History Museum in Paris, Lamarck:

Compared modem species to fossil forms and, in the process, identified several lines of

descent composed of a chronological series of older fossils to younger fossils to modem

species.

Envisioned many ladders of life which organisms could climb (as opposed to Aristotle s

single ladder without movement).

=> The bottom rungs were occupied by microscopic organisms which were continually

generated spontaneously from inanimate material.

=> At the top of the ladders were the most complex plants and animals.

Lamarck believed that evolution was driven by an innate tendency toward increasing complexity,

which he equated with perfection.

As organisms attained perfection, they became better and better adapted to their

environments.

Thus, Lamarck believed that evolution responded to organisms' sentiments interieurs ("felt

needs").

Lamarck proposed a mechanism by which specific adaptations evolve, which included two

related principles:

Use and disuse. Those body organs used extensively to cope with the environment

become larger and stronger while those not used deteriorate.

2. Inheritance of acquired characteristics. The modifications an organism acquired during

its lifetime could be passed along to its offspring.

Although his mechanism of evolution was in error, Lamarck deserves credit for proposing that:

Evolution is the best explanation for both the fossil record and the extant diversity of life.

The Earth is ancient.

Adaptation to the environment is a primary product of evolution.

360 Descent With Modification: A Darwinian View of Life

TV. Field research helped Darwin frame his view of life: science as a process

At the beginning of the 19th century, natural theology still dominated the European and

American intellectual climate. In 1809, the same year Lamarck published his theory of

evolution, Charles Darwin was born in Shrewsbury, England.

• Though interested in nature, Charles (at 16) was sent by his physician father to the

University of Edinburgh to study medicine, which he found boring and distasteful.

• He left Edinburgh without a degree and enrolled at Christ College, Cambridge University

to prepare for the clergy.

=> Nearly all naturalists and other scientists were clergymen, and a majority held to the

philosophy of natural theology.

Charles studied under the Reverend John Henslow, a botany professor at Cambridge,

and received his B.A. degree in 183 1.

Professor Henslow recommended him to Captain Robert FitzRoy who was preparing

the survey ship HMS Beagle for an around the world voyage.

A. The Voyage of the Beagle

The HMS Beagle, with Darwin aboard, sailed from England in December 183 1.

• The voyage's mission was to chart the poorly known South American coastline.

• While the ship's crew surveyed the coast, Darwin spent most of his time ashore

collecting specimens of the exotic and diverse flora and fauna.

While the ship worked its way around the continent, Darwin observed the various

adaptations of plants and animals that inhabited the diverse environments of South

America: Brazilian jungles, grasslands of the Argentine pampas, desolate islands of Tierra

del Fuego, and the Andes Mountains. Darwin noted the following:

The South American flora and fauna from different regions were distinct from the

flora and fauna of Europe.

Temperate species were taxonomically closer to species living in tropical regions of

South America than to temperate species of Europe.

o The South American fossils he found (while differing from modem species) were

distinctly South American in their resemblance to the living plants and animals of

that continent.

Geographical distribution was particularly confusing in the case of the fauna of the

Galapagos, recently formed volcanic islands which lie on the equator about 900 km west

of South America.

Most animal species on the Galapagos are unique to those islands, but resemble

species living on the South American mainland.

Darwin collected 13 types of finches from the Galapagos, and although they were

similar, they seemed to be different species.

=:> Some were unique to individual islands.

ð Others were found on two or more islands that were close together.

Descent With Modification: A Darwinian View of Life 36t

By the time the Beagle left the Galapagos, Darwin had read Lyell's Principles of Geology,

and was influenced by Lyell's ideas.

• Darwin had begun to doubt the church's position that the Earth was static and had

been created only a few thousand years before.

• When Darwin acknowledged that the Earth was ancient and constantly changing, he

had taken an important step toward recognizing that life on Earth had also evolved.

B. Darwin Focuses on Adaptation

Darwin was not sure whether the 13 types of finches he collected on the Galapagos were

different species or varieties of the same species.

• After he returned to England in 1836, an ornithologist indicated that they were

actually different species.

• He reassessed observations made during the voyage and in 1837 began the first

notebook on the origin of species.

Darwin perceived the origin of new species and adaptation as closely related processes;

new species could arise from an ancestral population by gradually accumulating

adaptations to a different environment. For example,

• Two populations of a species could be isolated in different environments and diverge

as each adapted to local conditions.

• Over many generations, the two populations could become dissimilar enough to be

designated separate species.

This is apparently what happened to the Galapagos finches; their different beaks are

adaptations to specific foods available on their home islands. (See Campbell, Figure

20.4)

By the early 1840's, Darwin had formed his theory of natural selection as the mechanism

of adaptive evolution, but delayed publishing it.

• Reclusive and in poor health, Darwin was well known as a naturalist from the

specimens and letters he had sent to Britain from the voyage on the Beagle.

• He frequently corresponded and met with Lyell, Henslow, and other scientists.

In 1844, Darwin wrote a long essay on the origin of species and natural selection.

• He realized the importance and subversive nature of his work, but did not publish the

information because he wished to gather more evidence in support of his theory.

• Evolutionary thinking was emerging at this time, and Lyell admonished Darwin to

publish on the subject before someone else published it first.

In June 1858, Darwin received a letter from Alfred Wallace who was working as a

specimen collector in the East Indies.

• Accompanying the letter was a manuscript detailing Wallace's own theory of natural

selection which was almost identical to Darwin's.

• The letter asked Darwin to evaluate the theory and forward the manuscript to Lyell if

it was thought worthy of publication.

Darwin did so, although he felt that his own originality would be "smashed."

362 Descent With Modification: A Darwinian View of Life

Lyell and a colleague presented Wallace's paper along with excerpts from Darwin's

unpublished 1844 essay to the Linnaean Society of London on July 1, 1858.

Darwin finished The Origin ofSpecies and published it the next year.

• Darwin is considered the main author of the idea since he developed and supported

natural selection much more extensively than Wallace.

• Darwin's book and its proponents quickly convinced the majority of biologists that

biodiversity is a product of evolution.

• Darwin succeeded where previous evolutionists had failed not only because science

was moving away from natural theology, but because he convinced his readers with

logic and evidence.

V. The Origin of Species developed two main points: the occurrence of evolution and

natural selection as its mechanism

A. Descent with Modification

Darwin used the phrase "descent with modification," not evolution, in the first edition of

The Origin of Species.

He perceived a unity in life with all organisms related through descent from some

unknown ancestral population that lived in the remote past.

Diverse modifications (adaptations) accumulated over millions of years, as

descendants from this common ancestor moved into various habitats.

Darwin's metaphor for the history of life was a branching tree with multiple branching

from a common trunk to the tips of living twigs, symbolic of the diversity of contemporary

organisms.

At each fork or branch point is an ancestral population common to all evolutionary

lines of descent branching from that fork.

Species that are very similar share a common ancestor at a recent branch point on the

phylogenetic tree.

Less closely related organisms share a more ancient common ancestor at an earlier

branch point.

Most branches of evolution are dead ends since about 99% of all species that ever

lived are extinct.

To Darwin, Linnaeus' taxonomic scheme reflected the branching genealogy of the tree of

life.

It recognized that the diversity of organisms could be ordered into "groups

subordinate to groups" with organisms at the different taxonomic levels related

through descent from common ancestors.

Classification alone does not confirm the principle of common descent, but when

combined with other lines of evidence, the relationships are clear.

For example, genetic analysis of species that are thought to be closely related on the

basis of anatomical features and other criteria reveals a common hereditary

background.

Descent With Modification: A Darwinian View of Life 363

B. Natural Selection and Adaptation

Darwin's book focused on the role of natural selection in adaptation. Ernst Mayr of

Harvard University dissected the logic of Darwin's theory into three inferences based on

five observations:

Observation 1: All species have such great fertility that their population size would

increase exponentially if all individuals that are born would reproduce successfully.

Observation 2: Most populations are normally stable in size except for seasonal

fluctuations.

Observation 3: Natural 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.

Inference 2: Survival in the struggle for existence is not random, but depends

in part on the hereditary constitution of the surviving individuals. Those

individuals whose inherited characteristics fit them best 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.

Natural selection is this differential success in reproduction, and its product is adaptation

of organisms to their environment.

Natural selection occurs from the interaction between the environment and the

inherent variability in a population.

Variations in a population arise by chance, but natural selection is not a chance

phenomenon, since environmental factors set definite criteria for reproductive

success.

Darwin was already aware of the struggle for existence caused by overproduction, when he

read an essay on human population written by the Reverend Thomas Malthus (179 8).

• In this essay, Malthus held that much of human suffering was a consequence of

human populations growing faster than the food supply.

• This capacity for overproduction is common to all species, and only a fraction of

new individuals complete development and leave offspring of their own; the rest die

or are unable to reproduce.

Variation and overproduction in populations make natural selection possible.

On the average, the most fit individuals pass their genes on to more offspring than

less fit individuals.

This results from environmental editing, which favors some variations over others.

364 Descent With Modification: A Darwinian View of Life

From his experiences with artificial selection, Darwin inferred that natural selection could

cause substantial change in populations.

• Through the breeding of domesticated plants and animals, humans have modified

species over many generations by selecting individuals with desired traits as

breeding stock.

• The plants and animals we grow for food show little resemblance to their wild

ancestors.

• Darwin reasoned that if such change could be achieved by artificial selection in a

relatively short period of time, then natural selection should be capable of

considerable modifications of species over hundreds of thousands of generations.

Gradualism is fundamental to the Darwinian view of evolution. Darwin reasoned that:

• Life did not evolve suddenly by quantum leaps, but instead by a gradual

accumulation of small changes.

• Natural selection operating in differing contexts over vast spans of time could

account for the diversity of life.

Summarizing Darwin's view of evolution:

• The diverse forms of life have arisen by descent with modification from ancestral

species.

• The mechanism of modification has been natural selection working gradually over

long periods of time.

1. Some Subtleties of Natural Selection

Populations are important in evolutionary theory, since a population is the smallest

unit that can evolve.

Population = A group of interbreeding individuals belonging to a particular species

and sharing a common geographic area.

Natural selection is a consequence of interactions between individual organisms and

their environment, but individuals do not evolve.

Evolution can only be measured as change in relative proportions of variations

in a population over several generations.

Natural selection can only amplify or diminish heritable variations.

Organisms can adapt to changes in their immediate environment and can be

.otherwise modified by life experiences, but these acquired characteristics cannot

be inherited.

Evolutionists must distinguish between adaptations an organism acquires during

its lifetime and those inherited adaptations that evolve in a population over many

generations as a result of natural selection.

Specifics of natural selection are situational.

Environmental factors vary from area to area and from time to time.

An adaptation under one set of conditions may be useless or detrimental in

different circumstances.

Descent With Modification: A Darwinian View of Life 365

Natural Selection in Action: Two Examples

In an effort to test Darwin's hypothesis that the beaks of Galapagos finches are

evolutionary adaptations to different food sources, Peter and Rosemary Grant of

Princeton University have been conducting a long‑term study on medium ground

finches (Geospiza fortis) on Daphne Major, a tiny Galapagos island. They have

discovered that:

* Average beak depth (an inherited trait) oscillates with rainfall.

In wet years, birds preferentially feed on small seeds, and average beak

depth decreases.

in dry years, small seeds are less plentiful, so survival depends on the

finches being able to crack the less preferred larger seeds. Average beak

depth increases during dry years.

It can be inferred that the change in beak depth is an adaptive response to the

relative availability of small seeds from year to year.

This study illustrates some important points about adaptive change:

Natural selection is situational. What works in one environmental context may

not work in another.

Beak evolution on Daphne Major does not resultftom inheritance of acquired

characteristics. The environment did not create beaks specialized for large or

small seeds, but only acted on inherited variations already present in the

population. The proportion of thicker‑beaked finches increased during dry

periods because, on average, thicker‑beaked birds transmitted their genes to

more offspring than did thinner‑beaked birds.

Michael Singer and Camille Parmesan of the University of Texas, have documented

rapid evolutionary adaptation in a butterfly population (Edith's checkerspot) living in a

meadow near Carson City, Nevada.

In only a decade, this butterfly population apparently adapted to changing

vegetation by inherited changes in reproductive behavior.

Females lay eggs preferentially on certain plants which provide food for the

larvae after they hatch. In 1983, checkerspots laid about 80% of their eggs on a

native plant, Collinsiaparviflora.

By 1993, the butterflies were laying about 70% of their eggs on Plantago

lanceolata, an invading weed from surrounding cattle ranches.

The researchers demonstrated that the switch in plant preference is genetic;

daughters of butterflies that deposited eggs on Plantago inherited the taste for

that plant, choosing it over Collinsia when they laid their eggs.

There are hundreds of examples of natural selection in laboratory populations of

Drosophila. A few other examples of natural selection in action include:

• Antibiotic resistance in bacteria.

• Body size of guppies exposed to different predators. (See Campbell, Chapter 1)

Descent With Modification: A Darwinian View of Life 367

C. Comparative Anatomy

Anatomical similarities among species grouped in the same taxonomic category are a

reflection of their common descent.

The skeletal components of mammalian forelimbs are a good example. (See

Campbell, Figure 20. 10)

Although the limbs are used for different functions, it is obvious that the same

skeletal elements are present.

it is logical that whether the forelimb is a foreleg, wing, flipper, or arm, the basic

similarity is the consequence of descent from a common ancestor and that the

limbs have been modified for different functions. They are homologous

structures.

Homologous structures = Structures that are similar because of common ancestry.

other evidence from comparative anatomy supports that evolution is a remodeling

process in which ancestral structures that functioned in one capacity have become

modified as they take on new functions.

Some homologous structures are vestigial organs.

Vestigial organs = Rudimentary structures of marginal or no use to an organism.

Vestigial organs are remnants of structures that had important functions in ancestral

forms but are no longer essential.

An example of vestigial organs are the remnants of pelvic and leg bones in snakes.

They show descent from a walking ancestor, but have no function in the snake.

D. Comparative Embryology

Closely related organisms go through similar stages in their embryonic development.

Vertebrate embryos (fishes, amphibians, reptiles, birds, mammals) go through an

embryonic stage in which they possess gill slits on the sides of their throats.

• As development progresses, the gill slits develop into divergent structures

characteristic of each vertebrate class.?

• In fish, the gill slits form gills; in humans, they form the eustachian tubes that

connect the middle ear with the throat.

Development of diverse organs with different functions from the gill slits common

to all vertebrate embryos supports the conclusion that all vertebrates descended from

aquatic ancestors with gills.

In the late nineteenth century, embryologists developed the view that "ontogeny

recapitulates phylogeny."

This view held that the embryonic development of an individual organism

(ontogeny) is a replay of the evolutionary history of the species (phylogeny).

This is an extreme view; what does occur is a series of similar embryonic stages that

exhibit the same characteristics, not a sequence of adult‑like stages.

These characteristics then become modified, possibly by natural selection, since

even embryonic processes can ultimately affect the fitness of the adult organism.

368 Descent With Modification: A Darwinian View of Life

Comparative embryology can often establish homology among structures that are so

altered in later development that their common origin can not be determined by comparing

their fully developed forms.

E. Molecular Biology

An organism's hereditary background is reflected in its genes and their protein products.

• Siblings have greater similarity in their DNA and proteins than do two unrelated

organisms of the same species.

• Likewise, two species considered to be closely related by other criteria should have a

greater proportion of their DNA and proteins in common than more distantly related

species.

Molecular taxonomists use a variety of modem techniques to measure the degree of

similarity among DNA nucleotide sequences of different species.

The closer two species are taxonomically, the higher the percentage of common

DNA; this evidence supports common descent.

• Common descent is also supported by the fact that closely related species also have

proteins of similar amino acid sequence (resulting from inherited genes).

• If two species have many genes and proteins with sequences of monomers that

match closely, the sequences must have been copied from a common ancestor.

Molecular biology has also substantiated Darwin's idea that all forms of life are related to

some extent through branching descent from the earliest organisms.

Even taxonomically distant organisms (bacteria and mammals) have some proteins

in common.

For example, cytochrome c (the respiratory protein) is found in all aerobic species.

Cytochrome c molecules of all species are very similar in structure and function

even though mutations have substituted amino acids in some areas of the protein

during the course of evolution.

Additional evidence for the unity of life is the common genetic code. This

mechanism has been passed through all branches of evolution since its beginning in

an early form of life.

V11. What is theoretical about the Darwinian view of life?

Dismissing Darwinism as "just a theory" is flawed because:

9 Darwin made two claims:

1. Modem species evolved from ancestral forms.

2. The mechanism for evolution is natural selection.

The conclusion that species change or evolve is based on historical fact.

Descent With Modification: A Darwinian View of Life 369

What then is theoretical about evolution?

• Theories are conceptual frameworks with great explanatory power used to interpret facts.

• That species can evolve is fact, but the mechanism Darwin proposed for that change ‑

natural selection ‑ is a theory. Darwin used this theory of natural selection to explain facts

of evolution documented by fossils, biogeography, and other historical evidence.

In science, "theory" is very different from the colloquial use of the word, which comes closer to

what scientists mean by a hypothesis, or educated guess.

• Unifying concepts do not become scientific theories, unless their predictions stand up to

thorough and continuous testing by experiment and observation.

• Good scientists, however, do not allow theories to become dogma; many evolutionary

biologists now question whether natural selection alone can account for evolutionary

history observed in the fossil record.