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1、Evidence for EvolutionReading:Freeman,Chapter 23,26n The 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
2、is 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”,
3、the 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 sci
4、entist,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 cont
5、rary 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 scienti
6、fic 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 medicinen HIV.HIV is a retrovirus of eno
7、rmous 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.n This knowledge has alerted us to the danger of emergent diseases from other animal hosts,a reason for our concern a
8、bout SARS and bird flu.n In addition,it is an understanding of evolutionary biology that has enabled us to develop a therapy for HIV.n The 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 quickl
9、y,it 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.n Natural Selection as the Mechanism of Evolution:Scientific understanding of evolution came
10、out of its infancy in 1859,when theories of evolution by natural selection by Charles Darwin and 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.Nat
11、ural Selection was not immediately accepted-it took until the1930s for Darwins ideas to be synthesized with a modern understanding of genetics for widespread acceptance.Intellectual stepping-stones to developing a theory of evolutionn Linnaeus and Taxonomyn Malthus and the Principle of Populationn L
12、yell and Uniformitarianismn Lamarck and the fist comprehensive theory of evolutionn The Voyage of the Beaglen Wallace and DarwinLinneus and Taxonomyn Carolus Linnaeus was a sixteenth-century Swedish physician and Botanist.n He founded the science of taxonomy,the branch of biology concerned with nami
13、ng and classifying living things.n He developed the two part system of binomial nomenclature we use today.n His 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
14、thinking about evolution from a common ancestor.How Old is the Earth?n From a scientific standpoint,the age of the Earth was essentially unknown until the 19th century.n Early ideas varied greatly,some cultures,such as classical Hindu society,thought of the Earth as incredibly old.n Christian theolo
15、gy 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.n Based upon the old testament,the Archbishop James Usher calculated that God created the Earth in 4004BC.This left little tim
16、e for incredibly slow,gradual processes like evolution.Hutton,Lyell and Uniformitarianismn The 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 sedimentation.n Canyons were cut by the e
17、rosion 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.Jean Baptiste Lamarckn Jean Baptiste de Lamark developed the first comprehensive model of evolution.n Lamarck was a French Zoologist,c
18、urator of the invertebrate collection at the Paris museum.n Lamarck saw many different lines of descent among the fossil invertebrates he encountered:instead of Aristotles single scala natura,there were many.n He proposed that organisms increased in complexity through time because of an innate tende
19、ncy.n According to Lamarck,every organism was continually striving for greater complexity,a clam strove to be a better clam,etc.n Lamarckian evolution can be disproved by experiment,specifically,we now know that acquired characters cannot be passed to offspring,also,evolution carries no innate tende
20、ncy toward increasing complexity,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 V oyage of the Beaglen Much of Charles Darwins inspiration for his theory of evolution by natural selection came from h
21、is voyage on the HMS Beagle,in 1831.n He saw an incredible diversity of species,with adaptations to a wide variety of environments;Brazilian rainforests,Chilean deserts,oceanic islands,etc.n The Galapagos islands particularly impressed him;most of the species there live nowhere else in the world,yet
22、 their closest living relative is on the mainland a few hundred miles away.n He was to spend the next 27 years developing a theory to explain what he saw.n Darwin had spent much of his life amassing the evidence he needed to support his model of evolution.He was finally goaded into publishing when h
23、e came across 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 supernat
24、ural,it removed 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 Speciesn Darwins manuscript contained several new idea
25、s,ideas not found 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.n The Origi
26、n of Species makes 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
27、 of this variation 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 sp
28、ecies have descended from a common ancestor.The book contained no mechanism for speciation,however.Evidence for Evolutionn The 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
29、explain how this 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
30、 the original evidence for evolution:n Embryologyn Vestigial and Homologous structuresn Biogeographyn The Fossil RecordSnake Chicken Possum Cat Bat HumanVestigial StructuresMany species retain structures that only make sense in light of their ancestry.These structures are typically reduced and nonfu
31、nctional,but they are inherited from ancestors,in whom they were important to survival or reproductionHomologous Structuresn Closely related species frequently have homologous structures:structures that are similar in their fundamental layout and construction,although they may serve very different p
32、urposes.For example,the forelimbs of mammals are constructed from the same skeletal elements:The wings of a bat,a whale,a human,a dog,etc.all contain the same bones,despite their different uses.n This suggests that common ancestry,rather than design,plays a role in the construction of species.The Fo
33、ssil Recordn The succession of forms in the fossil record clearly suggests that organisms change through time,and have descended from a common ancestor.n Different groups appear in the fossil record at different times,with a general trend toward the simplest organisms appearing the earliest.this is
34、at odds with the view that they were all created at the same time.n Many forms have gone extinct,another observation that is at odds with the view that each species was specially created for a purpose.n In some cases,a direct line of descent,and change through time,can be observed in fossils.Foramin
35、ifera,small oceanic protozoans,leave a continuous fossil record in oceanic sediments.It is possible to trace their gradual evolution over millions of years.n Since Darwins day,our knowledge fossil record has improved tremendously,we can trace the evolution of many different groups through fossils:ho
36、rses,for instance,have a superb fossil record,showing many instances of speciation and many intervals of evolutionary change.Example-Whales havean excellent fossil record-showingtransitional formsBiogeographyn The distribution of living plants and animals suggests that organisms adapted to one envir
37、onment can invade a new environment,and develop specific adaptations to the new conditions.On the HMS Beagle,Darwin noted that 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 rec
38、ognizable ancestor from the coast of Ecuador,but species there had numerous adaptations specific to the climate of the Islands.n Wallace observed the same pattern in many different parts of the world.Modern Evidencen Since Darwins time,there have been hundreds of studies of evolution.n Natural selec
39、tion has been measured in many organisms in the field,and in laboratory populations.n An understanding of evolution has also become important to combating disease.Example-DDT resistance in mosquitoesn The misuse of DDT,and the re-emergence of malaria as an important human pathogen,is perhaps one of
40、the greatest public health failures of the century it could possibly have been prevented if the evolution of mosquitoes had been taken into account n In nonresistant insects,DDT is a very effective insecticide-causing massive mortality and very strong selective pressure in favor of any mutation that
41、 might lead to resistancen Indiscriminate spraying(when there was no particular need to control the organism)led to the rapid evolution of pesticide resistance.Five Anopheles species were resistant by 1956 and 38 by 1968.n Resistance takes many forms-some of this genetic variation was probably prese
42、nt in the mosquito population before the use of DDT,but in the absence of DDT,these variants are selected against.n 1)Chemical adaptation:enzymes evolve that break down the pesticide.n 2)Behavioral adaptation:They evolved to move from inner,sprayed walls to outer,unsprayed walls.They evolved sensiti
43、vity and avoid the pesticide.These data are from Bangkok-the R allele is resistant,the+allele is not.Note that the+allele becomes more common in the absence of DDT sprayingAdaptationn Natural Selection as the mechanism for adaptation was Darwins most important contribution.n There are other forces o
44、f evolution(most of which were discovered after Darwin),but natural selection is the only evolutionary mechanism that can produce adaptation.n Some examples of adaptation are very impressive.Find the mantis in this pictureThe Variation Problemn For Natural Selection to be effective,there must be gen
45、etic variation upon which selection acts.n Darwin discussed the origin of variations extensively in“On the Origin of Species.”,but he did not know how variation persists.n Although he was a contemporary of Mendels,Darwin did not know Mendelian genetics(his work was not well understood at the time).n
46、 The current theory of genetics,blending inheritance,suggested that useful adaptations would blend into the population and become diluted.n The synthesis of Darwins theory with Mendelian genetics led to our modern understanding of Evolution.n Several early twentieth century evolutionary biologists are widely credited with developing our modern understanding:n R.A.Fishern J.B.S.Haldanen Sewall Wrightn Theodosius Dobzhanskin Thomas Hunt Morgan