CHAPTER 22 DESCENT WITH MODIFICATION: A …lhsteacher.lexingtonma.org/Pohlman/22B2-DarwinianRevolution.pdf · Section B2: The Darwinian Revolution (continued) 3. ... CHAPTER 22 DESCENT

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section B2: The Darwinian Revolution (continued)

3. Examples of natural selection provide evidence of evolution4. Other evidence of evolution pervades biology5. What is theoretical about the Darwinian view of life?

CHAPTER 22DESCENT WITH MODIFICATION:

A DARWINIAN VIEW OF LIFE

3. Natural selection in action: theevolution of insecticide-resistance


• The evolution of resistance to insecticides inhundreds of insect species is a classic example ofnatural selection in action.

• Insecticides are poisons that kill insects that arepests in crops, swamps, backyards, and homes.

• The results of application of new insecticide aretypically encouraging, killing 99% of the insects.

• However, the effectiveness of the insecticidebecomes less effective in subsequent applications.


• The few survivors from the early applications ofthe insecticide are those insects with genes thatenable them to resist the chemical attack.

• Only these resistant individuals reproduce, passingon their resistance to their offspring.

• In each generation the percentage of insecticide-resistant individuals increases.


Fig. 22.12


• In general, natural selection operates not to createvariation, but to edit existing variation.• For example, resistant insects are favored and non-

resistant individuals are not when insecticides areapplied.

• Natural selection favors those characteristics in avariable population that fit the current, localenvironment.

4. Natural selection in action: the evolutionof drug-resistant HIV


• While researchers have developed many drugs tocombat the human immunodeficiency virus (HIV),drug-resistant strains evolve rapidly in the HIVpopulation infecting each patient.

• Natural selection favors those characteristics in avariable population that fit the current, localenvironment.

• The evolution of drug resistance or pesticideresistance differ only in speed, not in basicmechanism, from other cases of natural selection.


• For patients treated with the drug 3TC, whichinterferes with genome replication in HIV, 3TC-resistant strains become 100% of the population ofHIV in just a few weeks.

Fig. 22.13

• In addition to those cases in which we can observeevolution directly, we see evidence of evolution bynatural selection in the much grander changes inbiological diversity documented by the fossilrecord.• Evidence that the diversity of life is a produce of

evolution pervades every research field of biology.

• As biology progresses, new discoveries, including therevelations of molecular biology, continue to validatethe Darwinian view of life.

4. Other evidence of evolution pervadesbiology


• In descent with modification, new species descendfrom ancestral species by the accumulation ofmodifications as populations adapt to newenvironments.• The novel features that characterize a new species are

not entirely new, but are altered versions of ancestralfeatures.

• Similarities in characteristics resulting from commonancestry is known as homology.


• Descent with modification is indeed evident inanatomical similarities between species grouped inthe same taxonomic category.

• For example, the forelimbs of human, cats, whales,and bats share the same skeletal elements, butdifferent functions because they divergedfrom the ancestraltetrapod forelimb.

• They arehomologousstructures.


Fig. 22.14

• Comparative anatomy confirms that evolution is aremodeling process via alteration of existingstructures, rather than uniquely engineered for theirexisting function.• Historical constraints on this retrofitting are evident in

anatomical imperfections.

• For example, the back and knee problems of bipedalhumans are an unsurprising outcome of adaptingstructures originally evolved to support four-leggedmammals.


• Some of the most interesting homologousstructures are vestigial organs, structures ofmarginal, if any importance to a current organism,but which had important functions in ancestors.• For example, the skeletons of some snakes and of fossil

whales retain vestiges of the pelvis and leg bones ofwalking ancestors.


• Sometimes, homologies that are not obvious inadult organisms become evident when we look atembryonic development.• For example, all vertebrate embryos have structures

called pharyngeal pouches in their throat at some stagein their development.

• These embryonic structures develop into very different,but still homologous, adult structures, such as the gillsof fish or the Eustacean tubes that connect the middleear with the throat in mammals.


• The concept of homology also applies at themolecular level (molecular homology) and allowslinks between organisms that have no macroscopicanatomy in common (e.g., plants and animals).• For example, all species of life have the same basic

genetic machinery of RNA and DNA and the geneticcode is essentially universal.

• Evidently, the language of the genetic code has beenpassed along through all the branches of the tree of liveeve since the code’s inception in an early life-form.


• Homologies mirror the taxonomic hierarchy of thetree of life.• Some homologies, such as the genetic code, are shared

by all life because they date to the deep ancestral past.

• Other homologies that evolved more recently are sharedonly by smaller branches of the tree of life.

• For example, only tetrapods (amphibians, reptiles,birds, and mammals) share the same five-digit limbstructure.

• This hierarchical pattern of homology is exactly whatwe would expect if life evolved and diversified from acommon ancestor, but not what we would see if eachspecies arose separately.


• If hierarchies of homology reflect evolutionaryhistory, then we should expect to find similarpatterns whether we are comparing molecules orbones or any other characteristics.• In practice, the new tools of molecular biology have

generally corroborated rather than contradictedevolutionary trees based on comparative anatomy andother methods.

• Evolutionary relationships among species aredocumented in their DNA and proteins - in their genesand gene products.


• If two species have libraries of genes and proteinswith sequences that match closely, the sequenceshave probably been copied from a commonancestor.• For example, the number of amino acid differences in

human hemoglobin when compared to other vertebratesshow the same patterns of evolutionary relationshipsthat researchers find based on other proteins or othertypes of data.



• The geographical 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 sameway of life, but living in different areas.

• For example, even though some marsupial mammals(those that complete their development in an externalpouch) of Australia have look-alikes among the eutherianmammals (those that complete their development in theuterus) that live on other continents, all the marsupialmammals are still more closely related to each other thanthey are to any eutherian mammal.


• For example, while the sugar glider and flyingsquirrel have adapted to the same mode of life,they are not closely related.

• Instead, the sugar glider from Australia is more closelyrelated to other marsupial mammals from Australia than tothe flying squirrel, aplacental mammalfrom North America.

• The resemblancebetween them is anexample ofconvergentevolution.


Fig. 22.15

• Island and island archipelagos have providedstrong evidence of evolution.• Often islands have many species of plants and animals

that are found nowhere else in the world, calledendemics.

• As Darwin observed when he reassessed his collectionsfrom the Beagle’s voyage, these endemic species aretypically related more closely to species living on thenearest mainland (despite different environments) thanthose from other island groups.


• In island chains, or archipelagos, individual islandsmay have different, but related species as the firstmainland invaders reached one island and thenevolved into several new species as they colonizedother islands in the archipelago.• Several well-investigated examples of this phenomenon

include the diversification of finches on the GalapagosIslands and fruit flies (Drosophila) on the HawaiianArchipelago.



Fig. 22.16

• All of the 500 or so endemic species of Drosophila in theHawaiian archipelago descended from a common ancestorthat reached Kauai over 5 million years ago.

• The succession of fossil forms is compatible withwhat is known from other types of evidence aboutthe major branches of descent in the tree of life.• For example, fossil fishes predate all other vertebrates,

with amphibians next, followed by reptiles, thenmammals and birds.• This 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 allvertebrate classes would make their first appearance inthe fossil record in rocks of the same age.• This is not what paleontologists actually observe.


• The Darwinian view of life also predicts thatevolutionary transitions should leave signs in thefossil record.• For example, a series of fossils documents the changes

in skull shape and size that occurred as mammalsevolved from reptiles.

• Recent discoveriesinclude fossilizedwhales that linkthese aquaticmammals totheir terrestrial ancestors.


Fig. 22.17

• Arguments by individuals dismissing the Darwinianview as “just a theory” suffer from two flaws.• First, it fails to separate Darwin’s two claims: that

modern species evolved from ancestral forms and thatnatural selection is the main mechanism for thisevolution.

• The conclusion that life has evolved is supported by anabundance of historical evidence.

• To biologists, Darwin’s theory of evolution is naturalselection - the mechanism that Darwin proposed toexplain the historical facts of evolution documented byfossils, biogeography, and other types of evidence.

4. What is theoretical about the Darwinianview of life?


• The “just a theory” arguments concerns onlyDarwin’s second point, his theory of naturalselection.• Here lies the second flaw, as the term theory in

colloquial use is closer to the concept of a “hypothesis”in science.

• In science, a theory is more comprehensive than ahypothesis.

• A theory, such as Newton’s theory of gravitation orDarwin’s theory of natural selection, accounts for manyfacts and attempts to explain a great variety ofphenomena.


• Natural selection is widely accepted in sciencebecause its predictions have withstood thorough,continual testing by experiments and observations.• However, science is not static and arguments exist

among evolutionary biologists concerning whethernatural selection alone accounts for the history of life asobserved in the fossil record.

• The study of evolution is livelier than ever, butthese questions of how life evolves in no wayimply that most biologists consider evolution itselfto be “just a theory.”


• By attributing the diversity of life to natural causesrather thanto supernaturalcreation, Darwingave biology asound, scientificbasis.

• As Darwin said,“There is grandeurin this view of life.”


Fig. 22.18

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CHAPTER 22 DESCENT WITH MODIFICATION: A …lhsteacher.lexingtonma.org/Pohlman/22B2-DarwinianRevolution.pdf · Section B2: The Darwinian Revolution (continued) 3. ... CHAPTER 22 DESCENT