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A Brief and Idiosyncratic A Brief and Idiosyncratic History of the Concept of History of the Concept of

EvolutionEvolutionor why we think we know how we or why we think we know how we

got heregot here

PrePre--scientific Beliefsscientific Beliefs

Understanding of the universe was Understanding of the universe was based on the interpretation of based on the interpretation of supernatural conceptssupernatural concepts

There were two longThere were two long--standing notions standing notions that impeded the development of the that impeded the development of the concept of evolution:concept of evolution:––Fixity of speciesFixity of species––Relatively short duration since the creation Relatively short duration since the creation

of the earthof the earth

PtolemyPtolemy(2nd century A.D.)(2nd century A.D.)

Earth as center of universeEarth as center of universe Inorganic and organic world fixed by Inorganic and organic world fixed by

creationcreationGreat Scale of Being: simple to complex Great Scale of Being: simple to complex

(from Aristotle)(from Aristotle)No new species since creationNo new species since creationEndorsed by Christian churchEndorsed by Christian church

Archbishop James UssherArchbishop James Ussher(1581(1581--1656)1656)

A scholar in the preA scholar in the pre--scientific tradition, scientific tradition, Ussher used a literal interpretation of Ussher used a literal interpretation of the bible to estimate the time of creationthe bible to estimate the time of creation––By evaluating how many years individuals By evaluating how many years individuals lived and how long from one “begat” to the lived and how long from one “begat” to the next, Ussher’s reading of Genesis led him next, Ussher’s reading of Genesis led him to estimate that the earth was created in to estimate that the earth was created in 4,004 B.C.4,004 B.C.––I.e., the earth was less than 6,000 years I.e., the earth was less than 6,000 years

oldold

Scientific RevolutionScientific Revolution

Science provided a framework to Science provided a framework to attempt to discover the Natural Laws or attempt to discover the Natural Laws or fundamental principles that govern the fundamental principles that govern the universeuniverse

The procedure includesThe procedure includes––The formulation of testable hypothesesThe formulation of testable hypotheses––The application of empiricism, the The application of empiricism, the

collection of data to test the hypothesescollection of data to test the hypotheses

Copernicus (1473Copernicus (1473--1543) 1543) An early proponent of the scientific An early proponent of the scientific

method, Copernicus postulated that the method, Copernicus postulated that the sun (not the earth) was center of universe, sun (not the earth) was center of universe, and the earth rotated around the sunand the earth rotated around the sun

He espoused the removal of a He espoused the removal of a supernatural interpretation of the cosmossupernatural interpretation of the cosmos

His predictions were subject to His predictions were subject to observationobservation

The planets obeyed Natural LawsThe planets obeyed Natural Laws

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Carolus Linnaeus (1707Carolus Linnaeus (1707--1778)1778)Proposed a hierarchical classification of Proposed a hierarchical classification of

organisms into four levelsorganisms into four levels––C, O, G, SC, O, G, S

Attempted to discover the grand design Attempted to discover the grand design of the Scale of Beingof the Scale of Being

Accepted fixity of species, as created Accepted fixity of species, as created Binomial Nomenclature: Genus and Binomial Nomenclature: Genus and speciesspecies––e.g., e.g., Homo sapiensHomo sapiens for manfor man

Comte de Buffon, 1707 Comte de Buffon, 1707 -- 17881788

While denying the existence of While denying the existence of macroevolution, believing that all macroevolution, believing that all species continue as created, he species continue as created, he espoused microevolutionespoused microevolution––In his view, the differences between In his view, the differences between

populations of a species were the result of populations of a species were the result of their accommodation to local their accommodation to local environmental conditionsenvironmental conditions

Erasmus Darwin (1731Erasmus Darwin (1731--1802)1802)

Charles’ GrandfatherCharles’ GrandfatherDiscussed Natural Laws in his poetryDiscussed Natural Laws in his poetryPublished several ideas that presaged Published several ideas that presaged

evolution in hisevolution in his ZoonomiaZoonomia––Competition for resources between Competition for resources between

members of a speciesmembers of a species––He was aware that species change through He was aware that species change through

time but did not have a mechanismtime but did not have a mechanism

Chevalier de Lamarck (1744Chevalier de Lamarck (1744--1829)1829)Evolution of species according to fixed Evolution of species according to fixed

lawslaws––Species Species adaptadapt to changing environment by to changing environment by

use/disuse of organsuse/disuse of organs––Systems develop according to wants & Systems develop according to wants &

needs of organismneeds of organism••e.g., fish in drying lake develop lungs as e.g., fish in drying lake develop lungs as

needed for oxygen needed for oxygen

––Proposed mechanism: Inheritance of Proposed mechanism: Inheritance of acquired characteristicsacquired characteristics

Georges Cuvier (1769Georges Cuvier (1769--1832)1832)Pope of BonesPope of Bones----Father of PaleontologyFather of Paleontology––Skills in comparative anatomy gained from Skills in comparative anatomy gained from

the reassembling of disarticulated the reassembling of disarticulated skeletons from slaughter houses made him skeletons from slaughter houses made him an expert for the reconstruction of fossilsan expert for the reconstruction of fossils

Believed in the fixity of speciesBelieved in the fixity of speciesProposed Catastrophism from natural Proposed Catastrophism from natural

causes as a mechanism to produce causes as a mechanism to produce fossilsfossils

Charles Lyell (1797Charles Lyell (1797--1875)1875)

Amplified and popularized work of Amplified and popularized work of earlier scholars (e.g., Hutton) proposing earlier scholars (e.g., Hutton) proposing uniformitarianism as an explanation for uniformitarianism as an explanation for the fossils and geologic depositsthe fossils and geologic deposits

UniformitarianismUniformitarianism––Geological processes same in past as they Geological processes same in past as they

are todayare today––Slow, gradual process of earth's formationSlow, gradual process of earth's formation

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Thomas Malthus (1766Thomas Malthus (1766--1834)1834)In his 1798 “An Essay on the Principle of In his 1798 “An Essay on the Principle of

Population as It Affects the Future Population as It Affects the Future Improvement of Society” he noted:Improvement of Society” he noted:––Population growth potentially exceeds increases Population growth potentially exceeds increases

in food resourcesin food resources––In nature, overpopulation checked by struggle In nature, overpopulation checked by struggle

for existencefor existence––Not all organisms secure adequate food every Not all organisms secure adequate food every

generationgeneration––Most die, only a few survive to perpetuate the Most die, only a few survive to perpetuate the

speciesspecies

Charles Darwin (1809Charles Darwin (1809--1882)1882)

Born in a wellBorn in a well--toto--do family, father a do family, father a physicianphysician

Had an early interest in nature, Had an early interest in nature, collecting eggs, pebbles, and the likecollecting eggs, pebbles, and the like

Did not do well in schoolDid not do well in schoolCould not stomach the cruelty of Could not stomach the cruelty of

medicine when he went to the medicine when he went to the University of Edinburgh to studyUniversity of Edinburgh to study

Charles Darwin (1809Charles Darwin (1809--1882)1882)

Transferred to Cambridge for a degree Transferred to Cambridge for a degree in divinityin divinity––Read Lamarck and his grandfather Read Lamarck and his grandfather Erasmus’ writings at this timeErasmus’ writings at this time

As he was about to become a As he was about to become a clergyman he was invited to join the clergyman he was invited to join the Honor Expedition as the ship’s naturalist Honor Expedition as the ship’s naturalist aboard the H.M.S. Beagleaboard the H.M.S. Beagle

Charles DarwinCharles Darwin

During the 5 year cruise of the Beagle, During the 5 year cruise of the Beagle, Darwin collected specimens, read Darwin collected specimens, read Lyell’s Lyell’s Principles of GeologyPrinciples of Geology, and , and became convinced of the transmutability became convinced of the transmutability of speciesof species

After returning home, a reading of After returning home, a reading of Malthus in 1838 finally triggered the Malthus in 1838 finally triggered the notion of natural selection as the notion of natural selection as the mechanism driving evolutionmechanism driving evolution

Charles DarwinCharles Darwin

In 1838, after reading Malthus, Darwin In 1838, after reading Malthus, Darwin realized that through the struggle for realized that through the struggle for existence, “existence, “favourable favourable variations would variations would tend to be preserved and tend to be preserved and unfavourable unfavourable ones to be destroyed. The result of this ones to be destroyed. The result of this would be the formation of new species.” would be the formation of new species.” (Darwin 1892:43).(Darwin 1892:43).

Alfred R. Wallace (1823Alfred R. Wallace (1823--1913)1913)

Came to the same conclusions about Came to the same conclusions about evolution by natural selection as a result evolution by natural selection as a result of his work in the Malay Archipelagoof his work in the Malay Archipelago

He is supposed to have arrived at his He is supposed to have arrived at his conclusions about natural selection conclusions about natural selection while lying delirious from a malarial while lying delirious from a malarial fever fever

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Alfred Russell WallaceAlfred Russell Wallace

1858 sent essay to Darwin1858 sent essay to Darwin––On the Tendency of Varieties To Depart On the Tendency of Varieties To Depart Indefinitely from the Original Type”Indefinitely from the Original Type”

Wallace became a fervent selectionist, Wallace became a fervent selectionist, ending up not believing in human ending up not believing in human evolution because he could not envision evolution because he could not envision the adaptive purpose for the human the adaptive purpose for the human mindmind

PublicationPublicationLyell helped to persuade Darwin to publish Lyell helped to persuade Darwin to publish an extract of his book along with Wallace’s an extract of his book along with Wallace’s essay.essay.––Appeared in 1858 Journal of the Appeared in 1858 Journal of the Linnean Linnean

SocietySociety

1859 On the Origin of Species by Means of 1859 On the Origin of Species by Means of Natural Selection or The Preservation of Natural Selection or The Preservation of Favoured Races in the Struggle for LifeFavoured Races in the Struggle for Life

Thomas Huxley (1825 Thomas Huxley (1825 -- 1895)1895)Acquainted with both Darwin and Acquainted with both Darwin and

WallaceWallaceUpon reading Darwin’s Upon reading Darwin’s On the Origin of On the Origin of

Species...Species... is reported to have said, is reported to have said, “How obvious. How stupid of me not to “How obvious. How stupid of me not to have thought of it myself.”have thought of it myself.”

Became the “Darwin’s Bulldog”, the Became the “Darwin’s Bulldog”, the fierce proponent of evolution by natural fierce proponent of evolution by natural selection in the public forumselection in the public forum

Evolution by Natural SelectionEvolution by Natural SelectionOffspring produced faster than food supply Offspring produced faster than food supply

increasesincreasesAll living organisms vary (especially within All living organisms vary (especially within

species)species)Because more individuals are born than Because more individuals are born than

survive, there is a survive, there is a strugglestruggle forfor existenceexistence––individuals with favorable adaptations are most individuals with favorable adaptations are most

likely to survive and leave offspring, sometimes likely to survive and leave offspring, sometimes known as known as survivalsurvival ofof thethe fittestfittest

Evolution by Natural SelectionEvolution by Natural Selection

As a result, individuals with favorable As a result, individuals with favorable adaptations increase in relative number adaptations increase in relative number from generation to generation, so from generation to generation, so greater and greater numbers within the greater and greater numbers within the species have the adaptationspecies have the adaptation

Over long periods of time, such Over long periods of time, such successful variations (favorable successful variations (favorable adaptations) produce differences that adaptations) produce differences that result in the formation of new speciesresult in the formation of new species

Darwin's EvidenceDarwin's EvidenceChanges bred into domesticated Changes bred into domesticated

organisms, especially pigeonsorganisms, especially pigeonsGeographic distributions of species such as Geographic distributions of species such as

the turtles of the Galapagos Islandsthe turtles of the Galapagos IslandsThe geological and paleontological recordThe geological and paleontological recordComparative anatomy (e.g. the Comparative anatomy (e.g. the

resemblance between apes and man) and resemblance between apes and man) and embryology (e.g., the possession of gills by embryology (e.g., the possession of gills by mammalian embryos)mammalian embryos)

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Darwin's FailuresDarwin's Failures

Never understood the source of the Never understood the source of the variation that is so key for the operation variation that is so key for the operation of natural selectionof natural selection

Never understood the particulate nature Never understood the particulate nature of inheritance, in spite of having a of inheritance, in spite of having a reprint of Mendel’s 1865 publicationreprint of Mendel’s 1865 publication

Darwin's view of how speciation occurs, circa 1870

Variation between individuals occurs in all species and is caused by the variability of the environment to which a population is exposed.

Individual variation is inheritable since, as my provisional hypothesis of pangenesis explains, each part of the body emits invisible gemmules that pass through the bloodstream to collect in the germ cells, carrying with them information on the exact nature of the body part from which they came.

This slight variation that benefits an individual in its struggle for existence enables it to survive longer and reproduce more successfully than others.

Conversely, individuals with injurious variations perish. This principle I have called natural selection.

Due to the inheritance of beneficial variations, natural selection also acts on further variation in successive generations, so that initially small beneficial traits may in time become major through continued descent with modification. Since the individuals whose favorable variations diverge most markedly from the parent stock receive the most benefit, those most divergent variations are preserved and accumulated by natural selection.

After much variation has accumulated, fairly well marked varieties and then species may be noted, since species are only strongly marked and well defined varieties.

In this way, transmutation of species occurs and new species originate.

The process resulting in new species, repeated all over the earth and over great spans of time, has produced the present diversity of life, the clear relationships between species alive and extinct, and the descent of all life from some primordial species.

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EvolutionEvolution

The Modern SynthesisThe Modern Synthesis

“Nothing in biology makes “Nothing in biology makes sense except in the light of sense except in the light of

evolution”evolution”

Theodosius DobzhanskyTheodosius Dobzhansky

“The alternative to thinking in “The alternative to thinking in evolutionary terms is not to evolutionary terms is not to

think at all”think at all”

Sir Peter MedawarSir Peter Medawar

Simultaneous Rediscovery of Simultaneous Rediscovery of Mendel (1900)Mendel (1900)

Hugo deHugo de VriesVries, Holland, HollandErich vonErich von TschermakTschermak, Austria, AustriaCarlCarl CorrensCorrens, Germany , Germany

Basis of population geneticsBasis of population genetics----equilibrium modelequilibrium model

William E. Castle, 1903, American William E. Castle, 1903, American animal breederanimal breeder

Godfrey H. Hardy, 1908, British Godfrey H. Hardy, 1908, British mathematicianmathematician

Wilhelm Weinberg, 1908, German Wilhelm Weinberg, 1908, German physicianphysician

Development of Population Development of Population Genetics from HGenetics from H--W ModelW Model

R.A. Fisher, 1930, The Genetical R.A. Fisher, 1930, The Genetical Theory of Natural SelectionTheory of Natural Selection

J.B.S.J.B.S. HaldaneHaldane, 1932, The Causes of , 1932, The Causes of EvolutionEvolution

SewallSewall Wright, 1931, Evolution in Wright, 1931, Evolution in Mendelian PopulationsMendelian Populations

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Synthesis of genetics and Synthesis of genetics and DarwinismDarwinism

Theodosius Dobzhansky, 1937, Theodosius Dobzhansky, 1937, Genetics and the Origin of SpeciesGenetics and the Origin of Species

Julian S. Huxley, 1942, Evolution. The Julian S. Huxley, 1942, Evolution. The Modern SynthesisModern Synthesis

Ernst Mayr, 1942, Systematics and the Ernst Mayr, 1942, Systematics and the Origin of SpeciesOrigin of Species

George Gaylord Simpson, 1944, Tempo George Gaylord Simpson, 1944, Tempo and Mode in Evolutionand Mode in Evolution

Species ConceptsSpecies Concepts

Species is derived from the LatinSpecies is derived from the Latinspecerespecere, “to look at”, referring to the , “to look at”, referring to the outward appearance as the basis of outward appearance as the basis of speciesspecies

Aristotle conceived of species as Aristotle conceived of species as unstable and highly changeableunstable and highly changeable––believed in spontaneous generation and all believed in spontaneous generation and all

kinds of crosses between different specieskinds of crosses between different species

Species ConceptsSpecies Concepts

Typological species Typological species ––Discreteness of speciesDiscreteness of species––Fixity of speciesFixity of species––One member of each species was all that One member of each species was all that

was necessary to serve as a was necessary to serve as a typetypespecimen representing the morphology of specimen representing the morphology of the entire speciesthe entire species––Think of a dog show where the judge Think of a dog show where the judge

evaluates the dog against a notion of the evaluates the dog against a notion of the perfect type for the breedperfect type for the breed

Darwin’s Species ConceptDarwin’s Species Concept

On the view that species are only strongly On the view that species are only strongly marked and permanent varieties, and that marked and permanent varieties, and that each species first existed as a variety, we can each species first existed as a variety, we can see why it is that no line of demarcation can see why it is that no line of demarcation can be drawn between species, commonly be drawn between species, commonly supposed to have been produced by special supposed to have been produced by special acts of creation, and varieties which are acts of creation, and varieties which are acknowledged to have been produced by acknowledged to have been produced by secondary laws (Darwin, 1872:171 sixth secondary laws (Darwin, 1872:171 sixth edition of “Origin of Species”).edition of “Origin of Species”).

Biological Species ConceptBiological Species Concept

Species are groups of actually or Species are groups of actually or potentially potentially interbreedinginterbreeding natural natural populations that are populations that are reproductively reproductively isolatedisolated from other such groups (Mayr, from other such groups (Mayr, 1963:19)1963:19)––Defines species in terms of a gene pool, Defines species in terms of a gene pool,

isolated from other similar gene poolsisolated from other similar gene pools

LimitationsLimitations

Restricted to sexually reproducing Restricted to sexually reproducing organisms, so does not apply to single organisms, so does not apply to single cell organisms that reproduce by simple cell organisms that reproduce by simple cell division (mitosis)cell division (mitosis)

Also does not fit many plants speciesAlso does not fit many plants speciesNo clear application to the fossil record, No clear application to the fossil record,

since reproductive isolation does not since reproductive isolation does not show up in fossilized materialsshow up in fossilized materials

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Where does evolution take place?Where does evolution take place?

Biological HierarchyBiological HierarchyBiosphereBiosphere––CommunityCommunity••PopulationPopulation––IndividualIndividual••Organs, tissues, cells, moleculesOrgans, tissues, cells, molecules

Where is the species?Where is the species?––Our taxonomy doesn’t reflect ecological Our taxonomy doesn’t reflect ecological

realityreality

Reproductive IsolationReproductive Isolation

PrePre--matingmating––Mechanisms that prevent the transfer of Mechanisms that prevent the transfer of

gametes between members of different gametes between members of different speciesspecies

PostPost--matingmating––Mechanisms that reduce the viability or Mechanisms that reduce the viability or

fertility of hybrid offspringfertility of hybrid offspring

PrePre--mating Mechanismsmating Mechanisms

Mechanisms causing a failure to meet Mechanisms causing a failure to meet potential mates in space or timepotential mates in space or time––Habitat isolationHabitat isolation••Species occupying different segments of the Species occupying different segments of the

habitat, e.g., smallhabitat, e.g., small--mouthed salamander,mouthed salamander,Ambystoma texanumAmbystoma texanum, breeds in ponds, but , breeds in ponds, but A.A.barbouribarbouri breeds in streamsbreeds in streams

––Seasonal isolationSeasonal isolation••Plant species which release pollen at different Plant species which release pollen at different

timestimes

PrePre--mating Mechanismsmating MechanismsMechanisms causing a failure to mate Mechanisms causing a failure to mate

between individuals who meetbetween individuals who meet––Mechanical isolationMechanical isolation••Copulation is attempted but no transfer of Copulation is attempted but no transfer of

sperm take place. Many insect species require sperm take place. Many insect species require a lock and key link for sperm transfera lock and key link for sperm transfer••Behavioral or ethological isolationBehavioral or ethological isolation••Potential mates meet but do not matePotential mates meet but do not mate––Coloration, movement, song, or odor may not elicit Coloration, movement, song, or odor may not elicit

readiness for sperm transferreadiness for sperm transfer

PostPost--mating Mechanismsmating MechanismsMechanisms reducing the viability of Mechanisms reducing the viability of

hybrid offspringhybrid offspring––GameticGametic mortalitymortality••Sperm transfer takes place but the egg is not Sperm transfer takes place but the egg is not

fertilized, e.g.,fertilized, e.g., SonoranSonoran topminnow and topminnow and nematodesnematodes

––Zygotic mortalityZygotic mortality••Egg is fertilized but zygote dies. Goat by sheep Egg is fertilized but zygote dies. Goat by sheep

cross results in early zygotic death, as do some cross results in early zygotic death, as do some leopard frog hybrids (leopard frog hybrids (Rana pipiensRana pipiens))

PostPost--mating Mechanismsmating MechanismsMechanisms reducing the fertility of Mechanisms reducing the fertility of

hybrid offspringhybrid offspring––HybridHybrid inviabilityinviability••Zygote produces offspring of reduced viability, Zygote produces offspring of reduced viability,

e.g., some frogs in the genuse.g., some frogs in the genus PseudophrynePseudophryne

––Reduced fertilityReduced fertility••Horse by donkey cross produces a sterile muleHorse by donkey cross produces a sterile mule••Papio hamadryasPapio hamadryas male by male by Papio anubisPapio anubis female female

results in fertile female and infertile malesresults in fertile female and infertile males

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PaleospeciesPaleospecies

A paleospecies or chronospecies is a A paleospecies or chronospecies is a morphologically or anatomically defined morphologically or anatomically defined fossil speciesfossil species––Since interbreeding is not determinable for Since interbreeding is not determinable for

extinct species, species boundaries are extinct species, species boundaries are determined by similarity of formdetermined by similarity of form

SpeciationSpeciation

The process by which new species are The process by which new species are produced from earlier ones. This is the produced from earlier ones. This is the central process of macroevolution, or central process of macroevolution, or evolution at the level of the species or evolution at the level of the species or aboveabove

Microevolution involves evolutionary Microevolution involves evolutionary change at the level of the population, change at the level of the population, and is defined by changes in allele and is defined by changes in allele frequencies within the populationfrequencies within the population

Origin of SpeciesOrigin of Species

Speciation occurs when populations of Speciation occurs when populations of a species become reproductively a species become reproductively isolated so that they can no longer isolated so that they can no longer interbreed with full fertilityinterbreed with full fertility

Geographic isolation of populations Geographic isolation of populations usually initiates the processusually initiates the process––Selection operates to produce isolating Selection operates to produce isolating

mechanisms of anatomy, behavior, genesmechanisms of anatomy, behavior, genes

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Darwin’s FinchesDarwin’s Finches

Synthetic Evolution, circa 1950sSynthetic Evolution, circa 1950s

New variants of genes are continually being added to populations through mutation. In addition, as explained by Mendel's two laws, recombination of genes increases the variation initially produced by mutation, since segregation and independent assortment of genes result in almost endless novelty in gene combinations and thus an almost infinite variety of possible genotypes in a population.

Because of the particulate nature of inheritance, no loss of variation occurs from parent to progeny. Thus, each population has an enormous store of variation on which natural selection can act. The fittest individuals, defined as those which leave the most viable progeny that mature and reproduce, contribute the most genes to the next generation, and thus their genes become most common in the gene pool of the population.

New mutations and new combinations of genes continually produce some individuals that are better adapted than others; natural selection favors these and selects out less fit types. Gradually, the population evolves through the differential perpetuation of genes.

If a population should become divided into two by a geographic barrier, evolution of each new population continues independently. Differences between the two, including differences in reproductive processes, gradually accumulate such that reproductive isolating mechanisms become more and more effective over prolonged periods of time.

Ultimately, on secondary contact, the two populations are reproductively isolated and two new species have evolved by the process of geographic speciation. Therefore, species can be defined as groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups.

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Speciation events lead to multiplication and diversification of species into higher taxathat retain clear phylogenetic links with parental stock, so that all species can be traced to the origin of life itself.

Modes of EvolutionModes of Evolution

Cladogenesis is branching evolution, Cladogenesis is branching evolution, the production of two or more daughter the production of two or more daughter species from a single ancestral speciesspecies from a single ancestral species

Anagenesis is linear evolution, the Anagenesis is linear evolution, the production of a single daughter species production of a single daughter species from a single ancestral speciesfrom a single ancestral species

Tempo of EvolutionTempo of Evolution

Gradualism views evolutionary change Gradualism views evolutionary change as occurring at a slow, steady rate over as occurring at a slow, steady rate over timetime

Punctuated Equilibrium views the tempo Punctuated Equilibrium views the tempo of evolution as consisting of long of evolution as consisting of long periods of stability (equilibrium), periods of stability (equilibrium), followed by relatively short periods of followed by relatively short periods of rapid evolutionary change rapid evolutionary change (punctuations)(punctuations)

Tempo of EvolutionTempo of Evolution

a. Gradual change through timea. Gradual change through timeb. Increasing rate of change through timeb. Increasing rate of change through timec. Punctuated equilibrium: stasis and bursts of c. Punctuated equilibrium: stasis and bursts of

changechange

SourcesSources

Darwin, C. 1872 On the Origin of Darwin, C. 1872 On the Origin of Species by Means of Natural Selection, Species by Means of Natural Selection, or The Preservation of “Favoured Races or The Preservation of “Favoured Races in the Struggle for Life. 6th Edition. in the Struggle for Life. 6th Edition. London: Murray.London: Murray.

Mayr, E. 1963 Animal Species and Mayr, E. 1963 Animal Species and Evolution. Cambridge, MA:Evolution. Cambridge, MA: BelknapBelknapPressPress

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SourcesSources

Price, P.W. 1996 Biological Evolution. Price, P.W. 1996 Biological Evolution. Philadelphia: Saunders.Philadelphia: Saunders.

Relethford, J.H. 1997 The Human Relethford, J.H. 1997 The Human Species: An Introduction to Biological Species: An Introduction to Biological Anthropology. Third Edition. Mountain Anthropology. Third Edition. Mountain View, CA: Mayfield Press.View, CA: Mayfield Press.