Evidence for Evolution

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Evidence for Evolution

• Reading Freeman, Chapter 23, 26

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• 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 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, 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.

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• The existence of evolution has been proposed
  several times in history. For instance, the
  ancient Greek scientist, 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 contrary 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 scientific 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.

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Examples of the clinical significance of
evolutionary biology to medicine

• HIV. HIV is a retrovirus of enormous 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.
• This knowledge has alerted us to the danger of
  emergent diseases from other animal hosts, a
  reason for our concern about SARS and bird flu.
• In addition, it is an understanding of
  evolutionary biology that has enabled us to
  develop a therapy for HIV.
• 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 quickly, 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.

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• Antibiotic resistance is an evolutionary
  phenomenon of tremendous clinical significance.
• Early in 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 competitors.
• Humans have co-opted them for our own purposes.
• Since 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 strains of Neisseria
  gonorrheae have evolved resistance to
  penicillins, tetracyclines, spectinomycin and
  floroquinolones.

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• Natural 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 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.
• Natural Selection was not immediately accepted-it
  took until the1930s for Darwins ideas to be
  synthesized with a modern understanding of
  genetics for widespread acceptance.

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Intellectual stepping-stones to developing a
theory of evolution

• Linnaeus and Taxonomy
• Malthus and the Principle of Population
• Lyell and Uniformitarianism
• Lamarck and the fist comprehensive theory of
  evolution
• The Voyage of the Beagle
• Wallace and Darwin

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Linneus and Taxonomy

• Carolus Linnaeus was a sixteenth-century Swedish
  physician and Botanist.
• He founded the science of taxonomy, the branch of
  biology concerned with naming and classifying
  living things.
• He developed the two part system of binomial
  nomenclature we use today.
• 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 thinking about evolution from a common
  ancestor.

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Malthus

• Thomas Malthus, an eighteenth century economist,
  published An Essay on the Principle of
  Population in 1798.
• This 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 strongly influenced he noted that every
species has more offspring than can be expected
to survive.
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How Old is the Earth?

• From a scientific standpoint, the age of the
  Earth was essentially unknown until the 19th
  century.
• Early ideas varied greatly, some cultures, such
  as classical Hindu society, thought of the Earth
  as incredibly old.
• Christian 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..
• Based upon the old testament, the Archbishop
  James Usher calculated that God created the Earth
  in 4004BC. This left little time for incredibly
  slow, gradual processes like evolution...

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Hutton, Lyell and Uniformitarianism

• 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.
• Canyons 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.

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• The English geologist, Charles Lyell was a
  contemporary of Darwins.
• He was a proponent of Hutton, 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.
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Jean Baptiste Lamarck

• Jean Baptiste de Lamark developed the first
  comprehensive model of evolution.
• Lamarck was a French Zoologist, curator of the
  invertebrate collection at the Paris museum.
• Lamarck saw many different lines of descent among
  the fossil invertebrates he encountered instead
  of Aristotles single scala natura, there were
  many.
• He proposed that organisms increased in
  complexity through time because of an innate
  tendency.

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• Lamarck proposed that interactions of organisms
  and environment drove the process of evolution.
• He followed the widely accepted notion that
  characteristics acquired during an individuals
  lifetime could be passed to ones offspring.
• He 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.

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• According to Lamarck, every organism was
  continually striving for greater complexity, a
  clam strove to be a better clam, etc.
• Lamarckian 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 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.

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The Voyage of the Beagle

• Much of Charles Darwins inspiration for his
  theory of evolution by natural selection came
  from his voyage on the HMS Beagle, in 1831.
• He saw an incredible diversity of species, with
  adaptations to a wide variety of environments
  Brazilian rainforests, Chilean deserts, oceanic
  islands, etc.
• The Galapagos islands particularly impressed him
  most of the species there live nowhere else in
  the world, yet their closest living relative is
  on the mainland a few hundred miles away.
• He was to spend the next 27 years developing a
  theory to explain what he saw.

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Darwin and Wallace

• Alfred Russell Wallace, a nineteenth century
  naturalist and explorer, an expert on collecting
  specimens for resale in Europe, developed
  essentially the same theory of 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.
• The two shared credit for the discovery, a rare
  example of diplomacy in 19th century science.
• Darwin is better known today, because he amassed
  a considerable amount of evidence to support his
  ideas. Wallaces arguments were more intuitive
  and contained a less-extensive battery of
  examples.

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• 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 he
  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 supernatural, 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.

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1859 The Origin of Species

• Darwins manuscript contained several new ideas,
  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.

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• The Origin 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 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
  points
• Darwin concluded, based upon intuitive grounds,
  that, over vast spans of time, present day
  species have descended from a common ancestor.
  The book contained no mechanism for speciation,
  however.

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Evidence for Evolution

• 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 explain how
  this occurred.
• 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.

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Some of the original evidence for evolution

• Embryology
• Vestigial and Homologous structures
• Biogeography
• The Fossil Record

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Embryology

• Closely related species go through similar stages
  of development, although the adults may not
  resemble each other very closely.
• For instance, all vertebrate embryos develop gill
  pouches at some stage, even though in many
  species, they are lost later. This is suggestive
  of a common origin for vertebrates.
• Embryological development is often suggestive of
  evolution birds have many developmental features
  in common with reptilian ancestors, land
  vertebrate embryos have many features suggestive
  of an aquatic existence (gill pouches, a
  notochord, blocks of segmented muscle).

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Snake Chicken Possum Cat Bat Human
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Vestigial Structures
Many 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, in whom they were important to
survival or reproduction
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Comparative Development and Embryology

• If members of a taxonomic unit share a common
  ancestry, it is reflected in their development
• Two of the many examples
• limb bud development in whales
• extraembryonic membranes of the amniote egg

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Homologous Structures

• Closely 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 whale, a human, a dog, etc. all
  contain the same bones, despite their different
  uses.
• This suggests that common ancestry, rather than
  design, plays a role in the construction of
  species.

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The Fossil Record

• The succession of forms in the fossil record
  clearly suggests that organisms change through
  time, and have descended from a common ancestor.
• Different 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.
• Many forms have gone extinct, another observation
  that is at odds with the view that each species
  was specially created for a purpose.

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• In 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 evolution over
  millions of years.
• Since 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.

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Example-Whales have an excellent fossil
record-showing transitional forms
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Biogeography

• The 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 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 recognizable
  ancestor from the coast of Ecuador, but species
  there had numerous adaptations specific to the
  climate of the Islands.
• Wallace observed the same pattern in many
  different parts of the world.

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Modern Evidence

• Since Darwins time, there have been hundreds of
  studies of evolution.
• Natural selection has been measured in many
  organisms in the field, and in laboratory
  populations.
• An understanding of evolution has also become
  important to combating disease.

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Example-DDT resistance in mosquitoes

• The misuse of DDT, and the re-emergence of
  malaria as an important human pathogen, is
  perhaps one of the greatest public health
  failures of the century
• it could possibly have been prevented if the
  evolution of mosquitoes had been taken into
  account

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• In nonresistant insects, DDT is a very effective
  insecticide-causing massive mortality and very
  strong selective pressure in favor of any
  mutation that might lead to resistance
• 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.

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• Resistance takes many forms-some of this genetic
  variation was probably present in the mosquito
  population before the use of DDT, but in the
  absence of DDT, these variants are selected
  against.
• 1) Chemical adaptation enzymes evolve that break
  down the pesticide.
• 2) Behavioral adaptation They evolved to move
  from inner, sprayed walls to outer, unsprayed
  walls. They evolved sensitivity and avoid the
  pesticide.

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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
spraying
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Adaptation

• Natural Selection as the mechanism for adaptation
  was Darwins most important contribution.
• There are other forces of evolution (most of
  which were discovered after Darwin), but natural
  selection is the only evolutionary mechanism that
  can produce adaptation.
• Some examples of adaptation are very impressive.

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Find the mantis in this picture
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The Variation Problem

• For Natural Selection to be effective, there must
  be genetic variation upon which selection acts.
• Darwin discussed the origin of variations
  extensively in On the Origin of Species.., but
  he did not know how variation persists.
• Although he was a contemporary of Mendels,
  Darwin did not know Mendelian genetics (his work
  was not well understood at the time).
• The current theory of genetics, blending
  inheritance, suggested that useful adaptations
  would blend into the population and become
  diluted.

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• The synthesis of Darwins theory with Mendelian
  genetics led to our modern understanding of
  Evolution.
• Several early twentieth century evolutionary
  biologists are widely credited with developing
  our modern understanding
• R.A. Fisher
• J.B.S. Haldane
• Sewall Wright
• Theodosius Dobzhanski
• Thomas Hunt Morgan