GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:
The Fossil Record of Vanished Worlds of the Prehistoric Past

Spring Semester 2017

On the Origin of Species by Means of Natural Selection: An Introduction to Evolution

What is "Evolution?"

Species and Life's Diversity
Naturalists have long noted that there exist units of natural diversity, species, in which the members share certain distinctive features with each other. Following the work of Carl von Linne' (Linnaeus) in the 1700s, species were recognized as one unit within a nested hierarchy of larger clusters of organisms: taxa (singular, taxon; literally, "named thing").

What is a species? That is, what criteria were used to justify whether a population belonged to one or another species.

Linnaeus' species were taxa like lions, tigers, black bears, etc. These were assemblages of individuals that share certain attributes:

Later biologists formalized this as "naturally occurring populations that interbreed and produce viable fertile offspring". (We'll come back to the issue of "what is a species" next week.)

Were species fixed: that is, unchanging? Many people thought so:

However, several sets of observations showed that this was not so:

Fixity vs. Transmutation
Traditionally, 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; shared homologies showed connections between groups; adaptations showed organisms "fit" to their environment. Already accepted the central tenets of Evolutionary Theory:

But what caused the modifications?

Transmutationist models:

Problems with these ideas, however:

  • Inheritance doesn't happen by use & disuse; transformations to adult are not passed onto offspring

    Darwin 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!)

    Initial Evidence for Evolution

    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.)

    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)

    (Or, a different emphasis, from a 1973 book review by paleontologist Leigh Van Valen: Evolution is the Control of Ecology on Development)

    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:

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

    Some of the things that come out of Darwin's work:

    Some things Evolution and Natural Selection are NOT:

    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:

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    Last modified: 26 February 2017