1、Evidence for EvolutionReading:Freeman,Chapter 23,26nThe Fact of Evolution Evolution-the progressive change of organisms as they descend from ancestral species-is a fact.By now,the evidence for it is overwhelming and ubiquitous.It is of such obvious clinical significance in medicine that to deny it i
2、s irresponsible.That said,any explanation for its existence and mode of action is a scientific theory,which must be testable and,in theory,falsifiable.Darwins theory of natural selection,combined with other mechanisms of evolution discovered since Darwin,form what is known as the“modern synthesis”,t
3、he current scientific paradigm in the biological sciences.It provides a central explanation for phenomena in such diverse fields as paleontology and developmental biology,medicine and psychology.The existence of evolution has been proposed several times in history.For instance,the ancient Greek scie
4、ntist,Animaxander,proposed a theory of evolution.In terms of modern science,it was first advanced proposed in the late 1700s and early 1800s by several scientists including Compte de Buffon and Erasmus Darwin.The idea of evolution remained controversial for a long time,partially because it ran contr
5、ary to contemporary religious ideas and partially because no mechanism for evolution was known.Darwin and Wallaces theory of evolution by natural selection was the first plausible,widely-accepted mechanism for evolutionary change.By now it is well-tested,supported by hundreds of independent scientif
6、ic investigations.It is also falsifiable-aspects of Darwins theory of evolution have been successfully challenged,others supported.This is the case for the other mechanisms of evolution as well.Examples of the clinical significance of evolutionary biology to medicinenHIV.HIV is a retrovirus of enorm
7、ous medical concern.Because of evolutionary studies,we know that two separate lineages of this retrovirus passed into the human population from African Apes in the mid 20th century.nThis knowledge has alerted us to the danger of emergent diseases from other animal hosts,a reason for our concern abou
8、t SARS and bird flu.nIn addition,it is an understanding of evolutionary biology that has enabled us to develop a therapy for HIV.nThe so-called“triple therapy”HIV treatment is an example of evolutionary medicine.A single drug will not work against the disease because the virus evolves so quickly,it
9、attains resistance to every drug we have within a few months.By using three drugs simultaneously,we subvert the evolution of the virusevolving resistance to one drug means loosing resistance to another.nAntibiotic resistance is an evolutionary phenomenon of tremendous clinical significance.nEarly in
10、 the 20th century,a variety of antibiotics,used to treat bacterial diseases,were developed.An understanding of evolution is helpful to understand where these antibiotics come from to begin withmany,such as penicillin,were evolved by fungi,over millions of years,to kill off their bacterial competitor
11、s.Humans have co-opted them for our own purposes.nSince the 20th century,the bacterial pathogens have evolved resistance to our antibiotics,because extensive use of these drugs has caused very strong natural selection in favor of mutations which favor antibiotic resistance.For instance,various strai
12、ns of Neisseria gonorrheae have evolved resistance to penicillins,tetracyclines,spectinomycin and floroquinolones.nNatural Selection as the Mechanism of Evolution:Scientific understanding of evolution came out of its infancy in 1859,when theories of evolution by natural selection by Charles Darwin a
13、nd Alfred Russel Wallace became widely known.We now know of other mechanisms of evolution,including genetic drift and mutation,but natural selection is the only mechanism capable of producing adaptation.Natural Selection was not immediately accepted-it took until the1930s for Darwins ideas to be syn
14、thesized with a modern understanding of genetics for widespread acceptance.Intellectual stepping-stones to developing a theory of evolutionnLinnaeus and TaxonomynMalthus and the Principle of PopulationnLyell and UniformitarianismnLamarck and the fist comprehensive theory of evolutionnThe Voyage of t
15、he BeaglenWallace and DarwinLinneus and TaxonomynCarolus Linnaeus was a sixteenth-century Swedish physician and Botanist.nHe founded the science of taxonomy,the branch of biology concerned with naming and classifying living things.nHe developed the two part system of binomial nomenclature we use tod
16、ay.nHis genera were clustered into increasingly broader categories;families,classes,phyla,and kingdomsalthough he did not believe in evolution by descent,this pattern does provide a framework for thinking about evolution from a common ancestor.MalthusnThomas Malthus,an eighteenth century economist,p
17、ublished“An Essay on the Principle of Population”in 1798.nThis document had profound implications.Simply stated:people tend to have more children than can possibly survive,and human populations have historically been kept in check by famine,starvation,and disease.Darwin read this essay and was stron
18、gly influenced:he noted that every species has more offspring than can be expected to survive.How Old is the Earth?nFrom a scientific standpoint,the age of the Earth was essentially unknown until the 19th century.nEarly ideas varied greatly,some cultures,such as classical Hindu society,thought of th
19、e Earth as incredibly old.nChristian theology limited the age of the Earth to a few thousand years,because of the biblical account of creation as lasting seven days,and the geneologies included in the book of numbers.nBased upon the old testament,the Archbishop James Usher calculated that God create
20、d the Earth in 4004BC.This left little time for incredibly slow,gradual processes like evolution.Hutton,Lyell and UniformitarianismnThe English geologist,James Hutton proposed that it was possible to explain geological land formations by processes that are currently in operation,such as erosion and
21、sedimentation.nCanyons were cut by the erosion of streams,layers of sediment were deposited at the edge of river deltas,these processes occurred slowly over a very long time-this idea was called gradualism.nThe English geologist,Charles Lyell was a contemporary of Darwins.He was a proponent of Hutto
22、n,and went a bit farther,embracing the principal uniformitarianism-the idea that geological processes in operation now operated similarly in the past,at about the same rate.The Uniformitarian view of nature,requires vast amounts of time to explain the present state of the Earth.Jean Baptiste Lamarck
23、nJean Baptiste de Lamark developed the first comprehensive model of evolution.nLamarck was a French Zoologist,curator of the invertebrate collection at the Paris museum.nLamarck saw many different lines of descent among the fossil invertebrates he encountered:instead of Aristotles single scala natur
24、a,there were many.nHe proposed that organisms increased in complexity through time because of an innate tendency.nLamarck proposed that interactions of organisms and environment drove the process of evolution.nHe followed the widely accepted notion that characteristics acquired during an individuals
25、 lifetime could be passed to ones offspring.nHe proposed that patterns of use and disuse drove the evolution of adaptations.In stretching their necks to reach leaves high in the treetop,giraffes acquired slightly longer necks,and passed these longer necks to their offspring.nAccording to Lamarck,eve
26、ry organism was continually striving for greater complexity,a clam strove to be a better clam,etc.nLamarckian evolution can be disproved by experiment,specifically,we now know that acquired characters cannot be passed to offspring,also,evolution carries no innate tendency toward increasing complexit
27、y,but Lamarcks theory was an important prelude to Darwins,it opened the door to thinking that organisms can and do change over the course of time.The Voyage of the BeaglenMuch of Charles Darwins inspiration for his theory of evolution by natural selection came from his voyage on the HMS Beagle,in 18
28、31.nHe saw an incredible diversity of species,with adaptations to a wide variety of environments;Brazilian rainforests,Chilean deserts,oceanic islands,etc.nThe Galapagos islands particularly impressed him;most of the species there live nowhere else in the world,yet their closest living relative is o
29、n the mainland a few hundred miles away.nHe was to spend the next 27 years developing a theory to explain what he saw.Darwin and WallacenAlfred Russell Wallace,a nineteenth century naturalist and explorer,an expert on collecting specimens for resale in Europe,developed essentially the same theory of
30、 evolution by natural selection as Darwin.An active man,he sat down to write it recovering from a bout of malaria,when he was unable to go out and explore.nThe two shared credit for the discovery,a rare example of diplomacy in 19th century science.Darwin is better known today,because he amassed a co
31、nsiderable amount of evidence to support his ideas.Wallaces arguments were more intuitive and contained a less-extensive battery of examples.nDarwin had spent much of his life amassing the evidence he needed to support his model of evolution.He was finally goaded into publishing when he came across
32、a manuscript by Wallace which contained many of the same ideas.Both theories had very broad implications,forcing European intellectuals to re-examine their place in nature.By proposing a mechanism for the evolution of the human species,its mind,and its achievements,that is not supernatural,it remove
33、d the need for a divine“prime mover”from science.Such a creator,or“prime mover”had been an element of Western science,since Roman times or earlier,and had been removed from physics and astronomy centuries earlier.1859:The Origin of SpeciesnDarwins manuscript contained several new ideas,ideas not fou
34、nd in earlier notions of evolution;All species evolved from earlier species.The mechanism is natural selection;members of a species possessing more desirable traits will have more offspring and survive to reproductive maturity.Evolution occurs over a very long span of time.nThe Origin of Species mak
35、es this argument,structured logically All organisms produce more offspring than can possibly survive All organisms vary for a wide variety of different attributes and features-they also vary in reproductive success:some have more offspring than others.Some variation is heritable.Some of this variati
36、on must influence reproductive success Given that the above are truedesirable characteristics will thus be preferentially passed to offspring This is a logical conclusion of the first four pointsDarwin concluded,based upon intuitive grounds,that,over vast spans of time,present day species have desce
37、nded from a common ancestor.The book contained no mechanism for speciation,however.Evidence for EvolutionnThe gradual evolution of life on the planet,and their descent from a common ancestor,is a fact.Darwins theory of evolution is a comprehensive body of evolution that attempts to explain how this
38、occurred.n One of the hallmarks of a truly revolutionary scientific theory is that it brings together many previously unexplained patterns under a single body of theory.Like Newtons theory of universal gravitation,Darwins theory of evolution created a new scientific paradigm.Some of the original evi
39、dence for evolution:nEmbryologynVestigial and Homologous structuresnBiogeographynThe Fossil RecordEmbryologynClosely related species go through similar stages of development,although the adults may not resemble each other very closely.nFor instance,all vertebrate embryos develop gill pouches at some
40、 stage,even though in many species,they are lost later.This is suggestive of a common origin for vertebrates.nEmbryological development is often suggestive of evolution:birds have many developmental features in common with reptilian ancestors,land vertebrate embryos have many features suggestive of
41、an aquatic existence(gill pouches,a notochord,blocks of segmented muscle).Snake ChickenPossumCatBat HumanVestigial StructuresMany species retain structures that only make sense in light of their ancestry.These structures are typically reduced and nonfunctional,but they are inherited from ancestors,i
42、n whom they were important to survival or reproductionnIf members of a taxonomic unit share a common ancestry,it is reflected in their development:nTwo of the many examples:limb bud development in whales extraembryonic membranes of the amniote eggComparative Development and EmbryologyHomologous Stru
43、cturesnClosely related species frequently have homologous structures:structures that are similar in their fundamental layout and construction,although they may serve very different purposes.For example,the forelimbs of mammals are constructed from the same skeletal elements:The wings of a bat,a whal
44、e,a human,a dog,etc.all contain the same bones,despite their different uses.nThis suggests that common ancestry,rather than design,plays a role in the construction of species.The Fossil RecordnThe succession of forms in the fossil record clearly suggests that organisms change through time,and have d
45、escended from a common ancestor.nDifferent groups appear in the fossil record at different times,with a general trend toward the simplest organisms appearing the earliest.this is at odds with the view that they were all created at the same time.nMany forms have gone extinct,another observation that
46、is at odds with the view that each species was specially created for a purpose.nIn some cases,a direct line of descent,and change through time,can be observed in fossils.Foraminifera,small oceanic protozoans,leave a continuous fossil record in oceanic sediments.It is possible to trace their gradual
47、evolution over millions of years.nSince Darwins day,our knowledge fossil record has improved tremendously,we can trace the evolution of many different groups through fossils:horses,for instance,have a superb fossil record,showing many instances of speciation and many intervals of evolutionary change
48、.Example-Whales havean excellent fossil record-showingtransitional formsBiogeographynThe distribution of living plants and animals suggests that organisms adapted to one environment can invade a new environment,and develop specific adaptations to the new conditions.On the HMS Beagle,Darwin noted tha
49、t in South America,temperate species tended to resemble their South American tropical relatives,rather than temperate species in Europe.On the Galapagos,most species had a recognizable ancestor from the coast of Ecuador,but species there had numerous adaptations specific to the climate of the Island
50、s.nWallace observed the same pattern in many different parts of the world.Modern EvidencenSince Darwins time,there have been hundreds of studies of evolution.nNatural selection has been measured in many organisms in the field,and in laboratory populations.nAn understanding of evolution has also beco
