Phylogeny of dogs - NAU web server

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The nature of natural selection – 10
• “New” traits can evolve although natural selection acts on existing
– 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
The panda’s thumb
Oil content of corn
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
– Again, he didn’t know any genetics and therefore did
not have a mechanistic understanding of mutation
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
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
Objections to Darwinism – Origin of complex
adaptations (pt. 2)
• There are at least two problems with this
– 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
Variation in mollusc eyes
Change in function or addition of new
• 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
• 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
Eukaryotic cilium (flagellum) — structure
• Components are: atubulin, b-tubulin,
dyneins, nexin, spoke
protein, bridge protein
Eukaryotic cilium (flagellum) — “irreducibly
• “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.
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
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
• Gene duplication and modification or simply a
second function for the same gene product: e.g.
the e (epsilon) crystallin of chickens is also the
enzyme lactate dehydrogenase
Gene co-option in the crystallins of animal eye lenses
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
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)
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
• G.L. Stebbins. 1950. Variation and
Evolution in Plants.
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” (=
• 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.
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.