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The nature of natural selection – 10

• “New” traits can evolve although natural selection acts on existing traits– the Panda’s “thumb”– feathers– bird wings

• Long term directional selection can produce phenotypes that are far outside the phenotypic range of an ancestral population– Beak length in soapberry bugs– Oil content in corn (Illinois corn oil experiment)– These responses are due to the accumulation of “favorable” alleles at

several to many loci in the same individuals and to the occurrence of “favorable” chance mutations during the course of selection

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The panda’s thumb

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Oil content of corn

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Objections to Darwinism – Blending inheritance

• Natural selection can’t produce evolutionary change because new favorable variations will be “blended out” when individuals that carry them mate with individuals that don’t– This objection was raised at least as early as 1867. It

reflects the fact that nobody (except Mendel) had an understanding of genetics

• Darwin could not explain how new variations arise– Again, he didn’t know any genetics and therefore did

not have a mechanistic understanding of mutation

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Objections to Darwinism – Age of the earth

• The earth isn’t old enough and the “window of time” during which temperatures on the earth have been suitable for life is too narrow– This objection goes back to calculations by Lord

Kelvin (a prominent 19th British physicist) that placed the age of the earth at 15 - 20 Myr. Kelvin assumed that the earth was simply gradually losing its initial heat by radiation into space

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Objections to Darwinism – Origin of complex adaptations (pt. 1)

• The problem of “incipient adaptations” or “unfit intermediate stages”– The vertebrate eye is a complex highly integrated structure: variable

focal length image-forming lens, retina, iris, cornea, muscles for moving eye, etc. If any of those is missing or imperfectly formed, vision may be absent or seriously impaired. If eyes evolved gradually, then the intermediate stages could not have functioned as today’s eyes do and would not have conferred a fitness advantage, and would not, therefore, have been produced by natural selection

– This same argument is used by advocates of intelligent design when they refer to “irreducibly complex structures” that will not work unless all parts are present, and which therefore could not have evolved gradually because the intermediate, incomplete stages would not have worked and could not have conferred a selective advantage, e.g., the eukaryotic flagellum or cilium

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Objections to Darwinism – Origin of complex adaptations (pt. 2)

• There are at least two problems with this argument:– The assumption that intermediate and “imperfect”

stages did not “work” or were not adaptive, and did not, therefore, confer a selective advantage

– The assumption that the current use of a structure or organ is the same as that which has always governed its evolution, and that the structure arose for its current use

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Variation in mollusc eyes

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Change in function or addition of new functions

• Argument: birds can’t fly without feathers. Therefore feathers are an adaptation for flying. Since rudimentary “proto-feathers” could not have enabled their possessors to fly they would not have been adaptive and could not have been naturally selected. Feathers could not have evolved by Darwinian mechanisms.

• Assumption: the only adaptive value of feathers is for flight

• But feathers are also important for insulation/thermoregulation. In which case, even rudimentary feathers might be better than no feathers at all and conferred a selective advantage.

• Structures can be co-opted for uses that are unrelated to their evolutionary origin

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Eukaryotic cilium (flagellum) — structure

• Components are: -tubulin, -tubulin, dyneins, nexin, spoke protein, bridge protein

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Eukaryotic cilium (flagellum) — “irreducibly complex?”

• “Cilia are composed of at least half a dozen proteins …[that] must be present for the cilium to function. If the tubulins are absent there are no filaments to slide; if the dynein is missing then the cilium remains rigid and motionless; if nexin or the other connecting proteins are missing, then the axoneme falls apart.”

• “[S]ince the complexity of the cilium is irreducible, then it cannot have functional precursors. Since the irreducibly complex cilium can not have functional precursors it can not be produced by natural selection, which requires a continuum of function to work. Natural selection is powerless when there is no function to select.”

• Michael Behe. 1998.

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Eukaryotic cilium (flagellum) — eels and cellular proteins

• The eukaryotic flagellum is not irreducibly complex because the flagellum of eel sperm can function even though they do not have the central pair of tubules

– Therefore, Behe is wrong in his claim of “irreducible complexity”

• The microtubules dyneins of cilia and flagella are similar to components of the spindle apparatus of eukaryotic cells that functions during cell division

– Behe is wrong in his implicit assumption that the various components of cilia and flagella could not have originated for other purposes and then been co-opted

• It is extremely unlikely that the first cilia were as complex as those of current eukaryotes. However, any cilium, no matter how rudimentary, might confer a selective advantage over, say, no cilium at all. There is no logical barrier to the idea that the complex cilium that we see today could not have evolved gradually, by natural selection

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Gene co-option: crystallin proteins of animal eye lenses

• Water-soluble proteins that form densely-packed , transparent, light-refracting arrays, and that make up about 1/3 the mass of the lens of the eye

• All are derived from proteins that have other functions

• Gene duplication and modification or simply a second function for the same gene product: e.g. the (epsilon) crystallin of chickens is also the enzyme lactate dehydrogenase

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Gene co-option in the crystallins of animal eye lenses

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The modern synthesis – 1

• ca. 1930 - 1950• Incorporation of population genetics theory

into evolutionary biology• Geneticists/Statisticians:

– R. A. Fisher, Sewall Wright, J.B.S. Haldane

• Evolutionary biologists:– Th. Dobzhansky, George Gaylord Simpson,

Ernst Mayr, G. Ledyard Stebbins

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The modern synthesis – 2

• Gradual evolution results from small genetic changes that are acted upon by natural selection (and random genetic drift)

• The origin of species and higher taxa (macroevolution) can be understood as selection acting on individuals (microevolution) over long periods of time (i.e., Darwin was essentially correct in his Uniformitarian argument)

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Founding books of the modern synthesis

• R.A. Fisher. 1930. The Genetical Theory of Natural Selection.

• Th. Dobzhansky. 1937. Genetics and the Origin of Species.

• Ernst Mayr. 1942. Systematics and the Origin of Species.

• G.G. Simpson. 1944. Tempo and Mode in Evolution

• G.L. Stebbins. 1950. Variation and Evolution in Plants.

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The argument from design• William Paley, 1802. Natural theology: or,

Evidences of the existence and attributes of the Deity, collected from the appearances of nature.

• Organisms are so well-adapted and have such complex organs, for example, the vertebrate eye, that they have to be the work of a “designer” (= God)

• An analogy between a complex instrument that we know is designed by a human (watch, telescope), and a similarly complex organ (vertebrate eye) that begs also for a designer.

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The Darwinian revolution• Evolution results from materialistic phenomena

and processes that can be observed and studied by scientific methodology: phenotypic variation, inheritance, natural selection

• Adaptation, which is apparent design, arises by material processes – natural selection and evolution. There is no requirement for a supernatural designer.

• Evolutionary theory is the core of biology as a natural science. “Nothing in biology makes sense except in the light of evolution” – Dobzhansky.