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GEOL 104 Dinosaurs: A Natural History

Fall Semester 2017
Evolution


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

Key Points:
•Evolution is the phenomenon where species are the product of descent with modification of older species.
•Many lines of evidence pointed to evolution, but it was the 19th Century naturalists Charles Darwin & Alfred Russel Wallace who discovered the primary mechanism of evolution: natural selection.
•Natural selection is the differential survival and reproduction of variants in a population, resulting in a change in the form of the descendants. It is the result of ecological sorting of the genetically-produced variation.
•Evolution produces changes in lineages over time. Some of these changes include divergence from common ancestors; convergence between different lineages due to similar life style; and more.

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!)

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:

and

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).

The Initial Evidence for Transmutationism/Evolution

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. Transmutationists already accepted the central tenets of Evolutionary Theory:

But what caused the modifications?

Transmutationist models:

Problems with these ideas, however:

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.)

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.

Hence, 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.")

Another way of thinking about this is paleontologist's Leigh Van Valen's observation: Natural Selection 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:

III. Patterns and Processes
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:

There are many more aspects to evolutionary biology, but these basics will help us study the history of dinosaurs and their place in the world.

Here is a summary of evolution and how it works:

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 sequel:

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Last modified: 22 June 2017

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