Mechanisms of Evolution Darwinian Evolution

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Mechanisms of EvolutionDarwinian Evolution
Descent with modification by means of natural
selection All life has descended from a common
ancestor The mechanism of modification is
natural selection
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• Concept 22.3 Darwins theory explains a wide
  range of observations
• Darwins theory of evolution
• Continues to be tested by how effectively it can
  account for additional observations and
  experimental outcomes

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Differential Predation in Guppy Populations
Natural Selection in Action

• Two examples provide evidence for natural
  selection

• Researchers have observed natural selection
• Leading to adaptive evolution in guppy populations

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The Evolution of Drug-Resistant HIV

• In humans, the use of drugs
• Selects for pathogens that through chance
  mutations are resistant to the drugs effects
• Natural selection is a cause of adaptive evolution

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• Researchers have developed numerous drugs to
  combat HIV
• But using these medications selects for viruses
  resistant to the drugs

Patient No. 1
Patient No. 2
Percent of HIV resistant to 3TC
Patient No. 3
Weeks
Figure 22.13
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• Bacteria and viruses are able to evolve rapidly
  because they reproduce rapidly and can pass on
  advantageous mutations to vast numbers of new
  individuals very quickly.

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Selection for beak size in a Galápagos population
of the medium ground finch
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Lines of Evidence that Support Natural Selection
as a Mechanism of Evolution
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Homology, Biogeography, and the Fossil Record

• Evolutionary theory
• Provides a cohesive explanation for many kinds of
  observations

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Anatomical Homologies
Homology

• Homology is similarity resulting from common
  ancestry

• Homologous structures between organisms
• Are anatomical resemblances that represent
  variations on a structural theme that was present
  in a common ancestor

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• Vestigial organs
• Are some of the most intriguing homologous
  structures
• Are remnants of structures that served important
  functions in the organisms ancestors

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Embryological Homologies

• Comparative embryology
• Reveals additional anatomical homologies not
  visible in adult organisms

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Molecular Homologies

• Biologists also observe homologies among
  organisms at the molecular level
• Such as genes that are shared among organisms
  inherited from a common ancestor
• The more closely related two organisms are, the
  more sequences they share

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• Anatomical resemblances among species are
  generally reflected in their molecules, their
  genes, and their gene products

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• Molecular biology has substantiated Darwins idea
  that all forms of life are related to some extent
  through branching descent from the earliest
  organisms.
• Even taxonomically distant organisms have some
  proteins in common.
• The common genetic code has been passed through
  all branches of life since its beginning in an
  early form of life.

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DNA Sequence Cytochrome C

• Cytochrome C of different orders of mammals and
  birds differs in 2 - 17 amino acids, classes of
  vertebrates in 7 - 38, and vertebrates and
  insects in 23 - 41 and animals differ from
  yeasts and molds in 56 - 72 amino acids.
• We can calculate the minimum numbers of single
  mutations needed to change the cytochrome C of
  one species into that of another. of mutations
  between cytochrome C of humans versus other
  living beings are
• Monkey1, Chicken18, Dog13, Penguin18, Horse17,
  Turtle19, Donkey16, Rattlesnake20, Pig13,
  Fish(tuna)31, Rabbit12, Fly33, Kangaroo12,
  Moth36, Duck17, Mold63, Pigeon16, Yeast56

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Homologies and the Tree of Life

• The Darwinian concept of an evolutionary tree of
  life
• Can explain the homologies that researchers have
  observed

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Phylogeny of the major groups of extant
vertebrates
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Biogeography

• Darwins observations of the geographic
  distribution of species
• Formed an important part of his theory of
  evolution
• The biogeographical patterns he observed only
  made sense in the context of evolution
• Review the material from lecture one pertaining
  to species on the continent of South America and
  the Galapagos Islands

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Biogeography

• Is the study of the geographical distribution of
  species.
• Islands have many endemic species which are
  closely related to species on the nearest
  mainland
• The occurrence of identical late Paleozoic
  fossils of plants and animals in Africa and South
  America is explained by continental drift

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• Some similar mammals that have adapted to similar
  environments
• Have evolved independently from different
  ancestors

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The Fossil Record

• The succession of forms observed in the fossil
  record
• Is consistent with other inferences about the
  major branches of descent in the tree of life

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Figure 25.1 A gallery of fossils
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Fossil evidence supported by other lines of
evidence

• Prokaryotes are placed as the ancestors of all
  life by evidence from cell biology, biochemistry
  and molecular biology
• The fossil record shows the chronological
  appearance of vertebrates as sequential with fish
  first, followed by amphibians, reptiles and then
  mammals and birds

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• The Darwinian view of life
• Predicts that evolutionary transitions should
  leave signs in the fossil record
• Paleontologists
• Have discovered fossils of many such transitional
  forms

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Many lineages are largely complete

• Planktonic organism lineages spanning 100 million
  years, including such groups as foraminifera,
  radiolaria, diatoms and coccolithophorids
• Horses
• Whales
• Elephants

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Figure 24.24 The branched evolution of horses
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Systematics

• Taxonomy and Classification as used in the
  science of Systematics often reflect actual
  evolutionary relationships as verified by other
  lines of evidence
• Darwin realized the connection between his Tree
  of Life and Linnaeus classification hierarchy

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What Is Theoretical about the Darwinian View of
Life?

• Theory does not mean the same thing in science
  that it does in everyday language
• In science, a theory
• Is based on laws, hypotheses, observations and
  inferences
• Accounts for many observations and data and
  attempts to explain and integrate a great variety
  of phenomena
• Must be testable

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Examples of Scientific Theories

• Gravitational Theory
• Heliocentrism
• Cell Theory
• Germ Theory
• Electromagnetic Theory
• Quantum Mechanics
• Plate Tectonics
• The criterion of the scientific status of a
  theory is its falsifiability, or refutability, or
  testability.

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• Darwins theory of evolution by natural selection
• Integrates diverse areas of biological study and
  stimulates many new research questions
• Has been the reigning paradigm in biology for the
  past 150 years
• Every question in biology and medicine is an
  evolutionary question
• We may discover new mechanisms of evolution, but
  the fact that the diversity of life on Earth has
  evolved from a common ancestor will not change.
• In science "fact" can only mean "confirmed to
  such a degree that it would be perverse to
  withhold provisional consent."

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• View the video Isnt Evolution Just a Theory at
  the PBS Website below
• http//www.pbs.org/wgbh/evolution/educators/teachs
  tuds/svideos.html