Chapter 4 – Evolution and Extinction
4.1 Introduction
 Charles Darwin laid the foundation of the modern theory of evolution
o Supported (and was supported by) the theory of deep time of Hutton and
Lyell
o Darwin recognized change in life through time rather than strict
equilibrium
 Earth’s physical systems have also changed
o Natural systems are open systems
o They can exhibit directionality or secular change
4.2 Early Theories of Evolution
 Lamarck – accepted the notion of spontaneous generation (origin of life from
nonliving matter)
o Believed in a “Chain of Being” – all organisms can be arranged in a
hierarchy from simple to complex (humans)
 Evolution as progressive (toward more perfect forms)
o Thought that organisms could evolve organs they needed in response to
environmental conditions
 “Theory of the inheritance of acquired characteristics”
 Evolution seen as teleological (goal-oriented or purposeful)
o Could not answer the question as to how characteristics are passed to
offspring
 Cuvier – renowned for knowledge of comparative anatomy
o Based on anatomic relationships, concluded that species were so complex
that they must be fixed and unchangeable
 Different groups viewed as separate branches of a tree
 No branch was more advanced than any other (unlike “chain of
being”)
o Based on fossils, realized that a series of extinctions had occurred
 Related these to movements of the land and sea
 Lyell thus viewed Cuvier as a catastrophist
 Cuvier did not accept evolution of species – suggested that
populations repopulated the Earth from refuges after extinction
4.3 Charles Darwin and the Beginnings of the Modern Theory of Evolution
 On the voyage of the H.M.S. Beagle (1831-1836), Darwin observed the great
variety of plants and animals in nature and their biogeographic distribution
o Questioned whether divine creation would have produced this variety and
distribution
o In the Galapagos Islands, observed variations of finches and tortoises
between islands
 Was greatly influenced by Lyell’s Principles of Geology
 Hesitant to publish his theory of evolution until learning of a similar theory
developed by Alfred Russell Wallace based on studies of animals in southeast Asia
o Darwin’s On the Origin of Species was finally published in 1859
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o Refrained from addressing human evolution (later included in The Descent
of Man)
4.4 Basic Premises of Darwinian Evolution
 Darwin reasoned that tiny differences occurred in the natural variations of
organisms
o These variations could be passed from one generation to the next
 Plant and animal populations do not grow unchecked
o A “struggle for existence” eliminates unfit individuals (those with less
suitable variations)
 Darwin called this “natural selection” based on the artificial
selection used by plant and animal breeders
 Differential reproduction occurs – survivors live long enough to breed and pass on
favorable traits to offspring
o Thus there is descent with modification, and there is natural selection for
more fit individuals over long intervals of time
o Natural selection acts as negative feedback on unfit individuals (and as
positive feedback on more fit ones)
 Natural selection could account for trends seen in the fossil record
o Similar to Lyell’s view of slow, gradual change
o However, results in directional change (initially rejected by Lyell)
4.5 Inheritance and Variation
 Darwin explained inheritance through his theory of “pangenesis”
o He suggested that each organ of an individual’s body develops particles he
called “gemmules”
o Gemmules could take on characteristics in the manner Lamarck had
described
o Each offspring would be a blend of gemmules from both parents
 Gregor Mendel discovered the true basic mechanism of inheritance
o A monk who tended the monastery garden, Mendel conducted
experiments in which he crossed varieties of peas
o He concluded that genetic traits occur in pairs (alleles), which behaved
like particles
o His theory is known as the particulate theory of inheritance
o Mendel published the work in an obscure journal (and sent a copy to
Darwin, which he apparently never read);
 Later work revealed the existence of chromosomes
o Mendel’s work was rediscovered and its significance for heredity realized
o Genetic recombination produced the variety on which natural selection
could act
4.6 Genetic Code and Mutation
 Breeding experiments established that changes in certain portions of
chromosomes corresponded to alterations in traits
o Thus, variation also resulted from mutations (changes to genes)
o Mutations are now recognized as producing the “raw material” for natural
selection
 DNA (deoxyribonucleic acid) was recognized to serve as the hereditary code
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o Molecular structure of DNA was determined by Watson and Crick (1950s)
 Natural selection acts on mutations, coupled with genetic recombination, to
produce evolutionary change that results in new species
4.7 Evidence for Natural Selection
 Since Darwin’s time a number of well-documented cases of natural selection have
helped to substantiate his theory
o Industrial melanism – soot-blackened trees led to the darkening of moths;
with the decline of coal use, the original coloration has returned
o Other examples are related to the role of medicine and agriculture in
society
4.8 Speciation
 Mutations are spread through populations through interbreeding
o Speciation could occur through geographic isolation of populations that
prevents interbreeding (allopatric speciation)
o Another mechanism of speciation is genetic drift, which involves some
genetic traits being lost simply by chance, whereas others are passed on
o Populations that have partly diverged (but not developed into full-fledged
species) are called subspecies
4.9 Evolution and the Fossil Record
 Comparative Anatomy
o Homologous structures (common evolutionary ancestry but dissimilar
function) – evolved for different functions (divergent evolution)
o Analogous structures (similar function but dissimilar structure) – arise
through similar selective pressures (convergent evolution)
 Cladistics
o Classifies taxa according to whether or not they share the same traits
o Shared traits indicate common ancestry, indicated in a cladogram
o Traits are considered to be primitive (appearing first) or derived
(appearing later in succession)
 Microevolution
o Involves slow, gradual processes
o Thought to result from short-term genetic processes like those documented
in genetic experiments and biogeographic studies of species distribution
o The resulting pattern was called phyletic gradualism
o Was thought to produce pseudoextinction of species (anagenesis, the
gradual transition of one species to another)
o Lack of observed transitions was dismissed as an artifact of the geologic
record (due to nonpreservation or erosion)
o Punctuated equilibrium arose as an alternative mode of speciation; argued
that long intervals of slow, nondirectional change were punctuated by the
abrupt appearance of new species
o Punctuated equilibrium is basically a record of allopatric speciation
preserved in the fossil record
 Macroevolution
o New species (or taxa at higher categories) may eolve because of rapid
genetic change in small populations
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o The modern synthesis (post-WWII) coupled Darwinian evolution with
modern genetics; it dismissed macroevolution based on genetic
experiments on human time scales
o Haeckel’s biogenetic law (“ontogeny recapitulates phylogeny”) states that
an individual’s development reflects its evolutionary history
o Scientists recently discovered regulatory genes called Homeobox (“Hox”)
genes that control the early development of certain body regions
o Rapid evolution of new structures and possibly whole new taxa may take
place through mutations in these master control genes (Hox genes)
4.10 Mass Extinction
 Extinction is normal and has occurred through geologic time as a kind of
background extinction
o 99.9% of all species that have ever existed have become extinct
 Many species may die out when the biosphere is decimated by global cooling,
massive volcanism, decreased oxygen in the ocean, and meteor impacts
o Such events are mass extinctions
 Mass extinctions may have acted to increase Earth’s biodiversity by creating new
evolutionary opportunities
o New species evolving to fill the niches of previously existing species is
called ecologic replacement
o Ecologic replacement occurring among a particular taxon is an adaptive
radiation
 The survival of a species during a mass extinction depends on its tolerance to
environmental change
o Eurytopic species are more likely to survive than stenotopic ones
 After extinction (during the recovery phase), any and all mechanisms of
microevolution and macroevolution may give rise to new taxa through adaptive radiation
4.11 Biodiversity Through the Phanerozoic
 The fossil record reflects major changes in the evolution of marine and terrestrial
communities through the Phanerozoic
o Three evolutionary faunas can be recognized: the Cambrian Fauna, the
Paleozoic Fauna, and the Modern Fauna
o Biodiversity appears to have increased through the Phanerozoic
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