Chapter 22 Descent with Modification: Darwinian View of Life
Overview: Darwin Introduces a Revolutionary Theory
• On November 24, 1859, Charles Darwin published On the Origin of
Species by Means of Natural Selection.
• Darwin’s book drew a cohesive picture of life by connecting what had
once seemed a bewildering array of unrelated facts.
• Darwin made two major points in The Origin of Species:
1. Today’s organisms descended from ancestral species that were
different from modern species.
2. Natural selection provided a mechanism for this evolutionary
• The basic idea of natural selection is that a population can
change over time if individuals that possess certain heritable
traits leave more offspring than other individuals.
• Natural selection results in evolutionary adaptation, an
accumulation of inherited characteristics that increase the
ability of an organism to survive and reproduce in its
• Eventually, a population may accumulate enough change that it
constitutes a new species.
• In modern terms, we can define evolution as a change over time in
the genetic composition of a population.
• Evolution also refers to the gradual appearance of all biological
• Evolution is such a fundamental concept that its study is relevant to
biology at every level, from molecules to ecosystems.
• Evolutionary perspectives continue to transform medicine,
agriculture, biotechnology, and conservation biology. Concept 22.1 The Darwinian revolution challenged traditional views of
a young Earth inhabited by unchanging species
Western culture resisted evolutionary views of life.
• Darwin’s view of life contrasted with the traditional view of an Earth
that was a few thousand years old, populated by life forms that were
created at the beginning and had remained fundamentally
• The Origin of Species challenged a worldview that had been
• The Greek philosopher Aristotle (384–322 B.C.E.) opposed any
concept of evolution and viewed species as fixed and unchanging.
• Aristotle believed that all living forms could be arranged on a
ladder of increasing complexity (scala naturae) with perfect,
permanent species on every rung.
• The Old Testament account of creation held that species were
individually designed by God and, therefore, perfect.
• In the 1700s, natural theology viewed the adaptations of organisms
as evidence that the Creator had designed each species for a
• Carolus Linnaeus (1707–1778), a Swedish physician and botanist,
founded taxonomy, a system for naming species and classifying
species into a hierarchy of increasingly complex categories.
• Linnaeus developed the binomial system of naming organisms
according to genus and species.
• In contrast to the linear hierarchy of the scala naturae, Linnaeus
adopted a nested classification system, grouping similar
species into increasingly general categories.
• For Linnaeus, similarity between species did not imply
evolutionary kinship but rather the pattern of their creation.
• Darwin’s views were influenced by fossils, remains or traces of
organisms from the past mineralized in sedimentary rocks. • Sedimentary rocks form when mud and sand settle to the
bottom of seas, lakes, and marshes.
• New layers of sediment cover older ones, creating layers of
rock called strata.
• Erosion may later carve through sedimentary rock to expose
older strata at the surface.
• Fossils within layers of sedimentary rock show that a
succession of organisms have populated Earth throughout time.
• Paleontology, the study of fossils, was largely developed by the
French anatomist Georges Cuvier (1769–1832).
• In examining rock strata in the Paris Basin, Cuvier noted that the
older the strata, the more dissimilar the fossils from modern life.
• Cuvier recognized that extinction had been a common
occurrence in the history of life.
• Instead of evolution, Cuvier advocated catastrophism,
speculating that boundaries between strata were due to local
floods or droughts that destroyed the species then present.
• He suggested that the denuded areas were later repopulated
by species immigrating from unaffected areas.
Theories of geologic gradualism prepared the path for evolutionary
• In contrast to Cuvier’s catastrophism, Scottish geologist James
Hutton (1726–1797) proposed a theory of gradualism that held that
profound geological changes took place through the cumulative effect
of slow but continuous processes identical to those currently
• Thus, valleys were formed by rivers flowing through rocks and
sedimentary rocks were formed from soil particles that eroded
from land and were carried by rivers to the sea. • Later, geologist Charles Lyell (1797–1875) proposed a theory of
uniformitarianism, which held that geological processes had not
changed throughout Earth’s history.
• Hutton’s and Lyell’s observations and theories had a strong influence
• First, if geologic changes result from slow, continuous
processes rather than sudden events, then the Earth must be
far older than the 6,000 years estimated by theologians from
• Second, slow and subtle processes persisting for long periods
of time can also act on living organisms, producing substantial
change over a long period of time.
Lamarck placed fossils in an evolutionary context.
• In 1809, French biologist Jean-Baptiste de Lamarck (1744–1829)
published a theory of evolution based on his observations of fossil
invertebrates in the collections of the Natural History Museum of
• By comparing fossils and current species, Lamarck found what
appeared to be several lines of descent.
• Each was a chronological series of older to younger fossils,
leading to a modern species.
• He explained his observations with two principles: use and disuse of
parts and the inheritance of acquired characteristics.
• Use and disuse was the concept that body parts that are used
extensively become larger and stronger, while those that are
not used deteriorate.
• The inheritance of acquired characteristics stated that
modifications acquired during the life of an organism could be
passed to offspring.
• A classic example is the long neck of the giraffe. Lamarck
reasoned that the long, muscular neck of the modern giraffe
evolved over many generations as the ancestors of giraffes reached for leaves on higher branches and passed this
characteristic to their offspring.
• Lamarck thought that evolutionary change was driven by the innate
drive of organisms to increasing complexity.
• Lamarck’s theory was a visionary attempt to explain the fossil record
and the current diversity of life with recognition of gradual
• However, modern genetics has provided no evidence that
acquired characteristics can be inherited.
• Acquired traits such as a body builder’s bigger biceps do not
change the genes transmitted through gametes to offspring.
Concept 22.2 In The Origin of Species, Darwin proposed that species
change through natural selection
• Charles Darwin (1809–1882) was born in western England.
• As a boy, he developed a consuming interest in nature.
• When Darwin was 16, his father sent him to the University of
Edinburgh to study medicine.
• Darwin left Edinburgh without a degree and enrolled at Cambridge
University with the intent of becoming a clergyman.
• At that time, most naturalists and scientists belonged to the
clergy and viewed the world in the context of natural theology.
• Darwin received his B.A. in 1831.
• After graduation Darwin joined the survey ship HMS Beagle as ship
naturalist and conversation companion to Captain Robert FitzRoy.
• FitzRoy chose Darwin because of his education, and because
his age and social class were similar to that of the captain.
Field research helped Darwin frame his view of life.
• The primary mission of the five-year voyage of the Beagle was to
chart poorly known stretches of the South American coastline. • Darwin had the freedom to explore extensively on shore while the
crew surveyed the coast.
• He collected thousands of specimens of the exotic and diverse flora
and fauna of South America.
• Darwin explored the Brazilian jungles, the grasslands of the
Argentine pampas, the desolation of Tierra del Fuego near
Antarctica, and the heights of the Andes.
• Darwin noted that the plants and animals of South America were very
distinct from those of Europe.
• Organisms from temperate regions of South America more
closely resembled those from the tropics of South America than
those from temperate regions of Europe.
• Further, South American fossils, though different from modern
species, more closely resembled modern species from South
America than those from Europe.
• While on the Beagle, Darwin read Lyell’s Principles of Geology.
• He experienced geological change firsthand when a violent
earthquake rocked the coast of Chile, causing the coastline to
rise by several feet.
• He found fossils of ocean organisms high in the Andes and
inferred that the rocks containing the fossils had been raised
there by a series of similar earthquakes.
• These observations reinforced Darwin’s acceptance of Lyell’s
ideas and led him to doubt the traditional view of a young and
• Darwin’s interest in the geographic distribution of species was further
stimulated by the Beagle’s visit to the Galapagos, a group of young
volcanic islands 900 km west of the South American coast.
• Darwin was fascinated by the unusual organisms found there. • After his return to England, Darwin noted that while most of the
animal species on the Galapagos lived nowhere else, they
resembled species living on the South American mainland.
• He hypothesized that the islands had been colonized by plants
and animals from the mainland that had subsequently
diversified on the different islands.
• After his return to Great Britain in 1836, Darwin began to perceive
that the origin of new species and adaptation of species to their
environment were closely related processes.
• For example, clear differences in the beaks among the 13
species of finches that Darwin collected in the Galapagos are
adaptations to the specific foods available on their home
• By the early 1840s, Darwin had developed the major features of his
theory of natural selection as the mechanism for evolution.
• In 1844, he wrote a long essay on the origin of species and natural
selection, but he was reluctant to publish and continued to compile
evidence to support his theory.
• In June 1858, Alfred Russel Wallace (1823–1913), a young naturalist
working in the East Indies, sent Darwin a manuscript containing a
theory of natural selection essentially identical to Darwin’s.
• Later that year, both Wallace’s paper and extracts of Darwin’s essay
were presented to the Linnaean Society of London.
• Darwin quickly finished The Origin of Species and published it the
• While both Darwin and Wallace developed similar ideas
independently, the theory of evolution by natural selection is
attributed to Darwin because he developed his ideas earlier and
supported the theory much more extensively.
• The theory of evolution by natural selection was presented in
The Origin of Species with immaculate logic and an avalanche
of supporting evidence. • Within a decade, The Origin of Species had convinced most
biologists that biological diversity was the product of evolution.
The Origin of Species developed two main ideas: that evolution
explains life’s unity and diversity and that natural selection is the
mechanism of adaptive evolution.
• Darwin scarcely used the word evolution in The Origin of Species.
• Instead he used the phrase descent with modification.
• All organisms are related through descent from a
common ancestor that lived in the remote past.
• Over evolutionary time, the descendents of that common
ancestor have accumulated diverse modifications, or
adaptations, that allow them to survive and reproduce in
• Viewed from the perspective of descent with modification, the history
of life is like a tree with multiple branches from a common trunk.
• Closely related species, the twigs on a common branch of the
tree, shared the same line of descent until their recent
divergence from a common ancestor.
• Linnaeus recognized that some organisms resemble each other more
closely than others, but he did not explain these similarities by
• However, his taxonomic scheme fit well with Darwin’s theory.
• To Darwin, the Linnaean hierarchy reflected the branching
history of the tree of life.
• Organisms at various taxonomic levels are united through
descent from common ancestors.
• How does natural selection work, and how does it explain
• Evolutionary biologist Ernst Mayr has dissected the logic of Darwin’s
theory 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.
• 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 the individuals of a population, with only a fraction
of the 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 inherited traits. Those
individuals whose inherited traits are best suited for
survival and reproduction in 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
• A 1798 essay on human population by Thomas Malthus heavily
influenced Darwin’s views on “overreproduction.”
• Malthus contended that much human suffering—disease,
famine, homelessness, war—was the inescapable
consequence of the potential for human populations to increase
faster than food supplies and other resources.
• The capacity to overproduce seems to be a characteristic of all
• Only a tiny fraction of offspring produced complete their development
and reproduce successfully to leave offspring of their own. • In each generation, enviro