Theory of Evolution: Theory, Trees, Predictions

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Scientific Theories
  • scientific theory is an axiom system
    • designed to explain certain kinds of phenomena
    • defined by its postulates
    • supported or disproved by its predictions
Big Picture
Common Descent with Modification
  • For every two species, except the root, there’s a most recent ancestor species from which the species evolved, by a combination of:
    • Reproductive isolation
    • Change in gene frequency due to:
      • Migration
      • Genetic Drift
      • Natural Selection
Mechanisms of CDM
  • Natural Selection
    • Individuals of a population compete for limited resources, such as food, water, space, and sexual partners. The more successful individuals (those better adapted to the environment) survive and reproduce. Many of the traits that contribute to the success of these individuals are passed on to their offspring. There is thus a gradual change of the gene frequency of population over time in response to changes in the environment. Over long periods of time the accumulated changes in the population can be extensive enough to yield a species different from that of the original population.
  • Migration
    • The change in gene frequency of a population because of the migration of outsiders, with slightly different genes, into the population
  • Genetic Drift
    • The change in gene frequency of a small population due to pure chance.
  • Reproductive Isolation
    • Two subgroups of a population are reproductively isolated if members of one group are unable to reproduce with members of the other, e.g. because of geographic separation.
  • Speciation (Species Branching)
    • One species becoming two
  • Using Phylogenetic Trees (Evolutionary Trees)
    • The evolution of species can be represented as a time-directed tree, where:
      • Time flows upwards
      • Vertical lines represent species, the longer the line the longer-lived the species.
      • Horizontal lines represent branching
    • For every two species, except the root, there’s a most recent ancestor species from which the species evolved.
      • For example, the Red and Green species evolved from the Blue species, their most recent common ancestor.
Prediction: Speciation (Species Branching)
How one species becomes two
Language Branching
  • Question A: Why did Vulgar Latin split into French and Spanish?
  • Question B: Why are French and Spanish more alike than French and German? 
  • Answer to A: Because
    • Languages change over time
    • Groups of speakers of Vulgar Latin became geographically separated
  • Answer to B: Because French and Spanish diverged from their nearest common ancestor later than French and German diverged from theirs.
    • In general, the more two languages are alike, the more recently they diverged from a common ancestor
Species Branching
  • Question A: Why did their nearest common ancestor split into humans and chimps?
  • Question B: Why are humans and chimps more alike than humans and horses?
  • Answer to A: Because
    • Species change over time due to
      • migration
      • genetic drift
      • natural selection
    • Populations of the nearest common ancestor of humans and chimps became reproductively isolated.
  • Answer to B: Because humans and chimps diverged from their nearest common ancestor later than humans and horses diverged from theirs.
    • In general, the more two species are alike, the more recently they diverged from a common ancestor
Prediction using Phylogenetic Trees (Evolutionary Trees)

The Theory of Evolution makes predictions using Phylogenetic Trees generated from the postulate of Common Descent and facts about species.

Prediction of Ancestral Whale’s Ankle
  • Using trees to make predictions about fossils: The whale’s ankle
    • Camels, pigs, giraffes, cows, and hippos have double pulley ankle.  Whales don’t have ankles. But a hypothesized phylogeny postulated that whales evolved from the same ancestor. The phylogeny correctly predicted that an ancestral whale had ankles that were double pulley.
Prediction of Tiktaalik, the “Fishapod”
  • Neil Shubin about the discovery of the Tiktaalik (tick-TAH-lick):
    • “Looking at the evolutionary tree, and knowing something about evolution and Earth history, we predicted there would be a Tiktaalik-like creature up there”
  • Tiktaalik, which lived 380 MYA, is a transitional species between fish and tetrapods (the first land vertebrates)
  • The discovery was made in the Canadian Arctic in 2004 by Neil Shubin, Edward Daeschler, and Farish A. Jenkins,
Phylogenetic Trees

A phylogenetic tree is a rooted, directed tree of the evolution of species.

The only diagram in the Origin of Species
Understanding Phylogenies (from Understanding Evolution)

View Understanding Phylogenies (from Understanding Evolution)

Trees as Hypotheses (from Understanding Evolution)

View Trees as Hypotheses (Understanding Evolution)

The Tree of Life (Understanding Evolution)

View The Tree of Life (Understanding Evolution)

Circle of Life

View All 2.3 Million Species Are Mapped into a Single Circle of Life, Scientific American, March 1, 2016

Evolution of the Theory
  • Intelligent Design
    • Before 1859 the only plausible explanation of the apparent design of functional organs was that an intelligent being designed them.
      • For example, eyes for seeing, hearts for pumping blood, and fins for swimming
  • Darwin and Wallace
    • 1831-1836 Darwin sails around the world on the HMS Beagle
    • 1837-1844 Darwin develops the theory evolution and writes a 189-page manuscript, which he keeps private
    • 1846-1854 Darwin studies barnacles, publishing two books on the subject
    • 1858 Darwin receives a paper on evolution from Alfred Russel Wallace, who’s in Indonesia
    • 1858 Darwin and Wallace jointly publish papers on evolution in the Proceedings of the Linnean Society
    • 1859 Darwin publishes On the Origin of Species
  • Eclipse of Darwinism
    • From 1880s to 1920s biologists accepted Common Descent but either rejected natural selection, or supplemented it, in favor of other mechanisms
      • Lamarckism: Inheritance of acquired characteristics
      • Saltationism: Instant creation of a new species by mutation
      • Orthogenesis: Propensity of evolution toward greater perfection
  • Synthetic Theory of Evolution
    • In the 1920’s and 1930s the synthetic theory of evolution was developed, combining Darwinian natural selection and Mendelian genetics.
  • Extension to Molecular Structure
    • Species are now compared not only regarding morphology, embryology, and behavior but also with respect to genes and proteins.
    • The hemoglobin protein, for example, is virtually identical in humans and chimpanzees.
    • Molecular clocks supplement stratigraphic and radiometric dating of species
How Darwin Ruined Paley’s Argument
  • Teleological Argument
    • An organ of a plant or animal has a specific function: eyes to see, hearts to pump blood, and white blood cells to fight infection and disease.  The only plausible explanation for the functional nature of such organs is that an intelligent being designed them for their purpose. The most likely theory is that the designer is God.
  • This is the famous Teleological Argument or Argument from Design, given its classic formulation by William Paley in 1802:
    • “In crossing a heath, suppose I pitched my foot against a stone and were asked how the stone came to be there, I might possibly answer that for anything I knew to the contrary it had lain there forever… But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place. I should hardly think of the answer which I had before given, that for anything I knew the watch might have always been there. When we come to inspect the watch, we perceive—what we could not discover in the stone—that its several parts are framed and put together for a purpose. The inference we think is inevitable, that the watch must have had a maker.”
  • The same reasoning, Paley argued, applies to functional organs like eyes, hearts, and white blood cells.
  • The Teleological Argument was a powerful argument for the existence of God until 1859, when On the Origin of Species provided a natural explanation for the origin of functional organs.  As Darwin wrote:
    • “The old argument of design in nature, as given by Paley, which formerly seemed to me so conclusive, fails, now that the law of natural selection had been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows. Everything in nature is the result of fixed laws.” (Darwin’s autobiography 1887)
Best Online Resource for Evolution

Understanding Evolution at