GEOL 102 Historical Geology

Spring Semester 2020

Detail of the cover page of On the Origin of Species by Means of Natural Selection by Charles Darwin (1859)

I. Descent with Modification
What is Evolution?

Darwin (and Wallace) did not discover evolution, nor did its study stop with his work. At least some of the evidence for evolution was long known before his time (although we've added a LOT, even to these lines!)

  • Why does comparative anatomy work (i.e., where are their homologies: the same anatomical structures in different species of organism)?
  • Why are there adaptations (specialized structures or behaviors that allow organisms to interact with their environment in certain specific ways)?
  • Why is Life so effectively organized under a nested hierarchy of groups?
  • Why should the Principle of Fossil Succession work?
  • Where should vestigial organs exist?
  • Why should groups closely related to each other have embryos similar throughout most of their development, while those of more distantly related forms have fewer shared stages?
  • Why does the fossil record hold evidence of creatures intermediate in form between distinct modern groups?
  • Why is there a biogeography (non-random distribution of living things)?

    Historically have been two primary competing views about life:

    Both ideas can be found in ancient Greek writing, and might have been even older.

    Traditionally, most people accepted the fixity of species just as they accepted that the world today is pretty much the same now as in the past.

    Theological argument for fixity under the Biblical concept of the Plenum ("fullness"):

    Many early naturalists accepted the Plenum, but evidence of extinction (man-made, as in the dodo, and natural, as in fossils) showed that things could be removed from Creation. What about adding to it?

    The discoveries of the early (18th and 19th Century) geologists put paid to the idea that the surface of the Earth was unchanging:


    While some thinkers once thought that life as we see it now is the way it has always been, the discovery of the fossil record showed that strange creatures once roamed the Earth that are no longer there. Naturalist John Herschel (in an 1836 letter to Charles Lyell) wrote:

    How to explain these observations? Two main possibilities:

    Transmutationism, a set of early evolutionary models, accepted by several prominent scientists by the late 1700s. Among them were Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (normally known as Jean Baptiste de Lamarck) and Erasmus Darwin (doctor, scientist, surgeon, abolitionist, and INCREDIBLY rich).

    Fossils demonstrated that the living component of the Earth changed through time, sometimes linking what are now distinct groups; shared anatomical features showed connections between groups; adaptations showed organisms "fit" to their environment; the nested hierarchy of classification showed structure to the distribution of anatomical features among organisms; natural hybrids demonstrated that species were not distinct inseparable kinds; embryology revealed organized similarities between distinct groups; the distribution of species over the surface of the Earth and through time showed similar groups in close regions. Together, all this showed that life had a history. The Transmutationists thus already accepted the central tenets of Evolutionary Theory:

    But what caused the modifications?

    II. On the Origin of Species by Means of Natural Selection

    The discovery of the primary mechanism of evolution was the work of two English naturalists:

    These two had similar backgrounds: The two made the same sets of important observations independently, and independently came up with the same mechanism to explain evolution. Darwin (older than Wallace) had developed his ideas earlier, but kept them secret. In 1858 when Wallace asked Darwin for advice about his ideas, Darwin went to other scientists to present both his and Wallace's ideas at the same time, so that they both got credit for their independent discovery. (However, Darwin's book On the Origin of Species by Means of Natural Selection sold extremely well, so more people then and now know Darwin's name.)

    This was the independent discovery of Charles Robert Darwin and Alfred Russel Wallace. Their model was called Natural Selection, and was analogous to "artificial selection" (e.g., domestication). Here is Natural Selection as described in Darwin's own words:

    Their model was called Natural Selection, and was analogous to "artificial selection" (e.g., domestication). Darwin and Wallace's observations:

    Thus, IF some variation gives the individual a slight advantage (bigger, stronger, smaller, smarter, less tasty, whatever) at surviving; and IF that variation is heritable; THEN there is a somewhat better than average chance that organisms with that variation will survive to bear the next generation. Over the long expanse of geologic time, the accumulation of these variations will change the population from one form to another: the origin of species.


    * Natural Selection is the differential survival and reproduction of variants in a population resulting in a net change in phenotype of the descendants. *

    (Short form: "Natural selection is the differential survival and reproduction of variants in a population.")

    (Even shorter form with a 20th Century slant: MUTATION PROPOSES, SELECTION DISPOSES)

    If Evolution can be summarized as "no one is identical to their parents", then Natural Selection can be summarized as "no one is identical to their siblings, either; plus, life's hard!"

    Key points of Natural Selection:

    "Survival of the Fittest"?: Not as such. Phrase not in the earlier editions of the Origin, nor was it coined by Darwin. Comes from economist/philosopher Herbert Spencer:

    Evolution by natural selection explained a lot:

    From Darwin and Wallace, we get the beginnings of modern evolutionary theory. It has five major components:

    III. Patterns of Evolution

    With the discovery of evolution by natural selection, biologists from Darwin and Wallace's time onward have documented many different patterns and processes in evolution. Sometimes they refer to "microevolution" (changes within an species) and "macroevolution" (patterns on the larger scale; changes from one species to another, or between different lineages of ancestors and descendants). It is important to remember that "micro-" vs "macro-" is just a matter of scale and perception: at the level of individuals and populations, there is just variability, heritability, and superfecundity.

    The most important pattern: the Tree of Life. Darwin and Wallace demonstrated the reality of Divergence through Time and Common Ancestry:

    Thus, the basic pattern of the history of living things is a Tree of Life, where the trunk and stems are lineages of ancestors, the branching points representing divergences between lineages, and the tips of the branches living species (or extinct species that died without descendants).

    Other important patterns and processes:

    A relevant video:

    Here is Carl Sagan's summary of Darwinian evolution and the Tree of Life, from the TV series Cosmos:

    And here is another summary of evolution and how it works (and how it ISN'T like the parody-version of evolution which Creationists claim scientists believe):

    And yet another:

    And its sequels:

    To Lecture Notes.

    Last modified: 23 January 2020

    Evolution of smaller body size on the way to birds, by Davide Bonadonna, from Lee et al. (2014) doi: 10.1126/science.1252243