HONR 259C "Fearfully Great Lizards": Topics in Dinosaur Research

Fall Semester 2017
Species, Evolution & Macroevolution

Diversity of horned ceratopsids (and related dinosaurs), by Julius Csotonyi

Key Points:
•Taxonomy (biological nomenclature) is a way of having a universal set of names for groups of living things.
•A formal set of rules exists for naming and organizing taxa (named groups of organisms). Among these are that taxa are named in Latin (or at least in Latinate style) and that they are organized as a nested hierarchy.
•Species represent a fundamental unit in taxonomy. Species are grouped into genera.
•There is no single universally recognized method of identifying when two individuals are in teh same species This is even more problematic for fossils, where some relevant information (such as interbreeding, DNA, and so on) is not available.
•Taxonomic differences are not the only reason that two individuals might be different: ontogenetic, sexual, geographic & individual variations have to be considered as well.
•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.

Taxon (pl. taxa): a named group of organisms.

Traditionally, each culture had its own name for the animals, plants, and other organisms in their region. But EACH culture had its own set of names, so the same type of animal might have many different names. During the 1600s and 1700s, methods were proposed for a formal scientific set of names.

Carlos Linnaeus developed a universal set of rules in the Systema Naturae ("System of Nature") in 1758; later workers added and modified the system (primarily with the addition of new "ranks").

Some of the Linnaean rules:

Linnaean taxonomy has its own special set of grammatical rules:

Type Specimens and Type Species: Another aspect of Linnaean taxonomy is that each species must have a particular type specimen. This is a particular individual preserved specimen (extant animal) or fossil (extinct animal) that is the "name holder" for that species. A type specimen is specifically referred to in the original description and diagnosis of the species. It need not be the most complete specimen known at the time (although that helps, as the more complete it is, the better the chance a less-complete individual can be compared to it!). The type specimen plus all the additional (referred specimens) are collectively called the hypodigm. Ultimately, if a species is regarded as being "valid" (that is, representing a real species in Nature), the type specimen is the only individual that is absolutely certain to belong that that species.

Similarly, each genus has a particular type species. This is the particular species to which the genus name is linked. If a genus is valid, the type species is the only species that is absolutely guaranteed to be within that genus.

As an example, CM 9380 (in the collections of the Carnegie Museum of Natural History) is the type specimen of Tyrannosaurus rex, and Tyrannosaurus rex is the type species of the genus Tyrannosaurus.

Because there is disagreement about the features used to define a particular species or genus, different biologists and paleontologists will sometimes disagree about which specimens belong in a particular species, and which species belong in a particular genus (and so forth).

For those interested in a website concerning some unusual Linnaean species names, click here.

What is a species? Above we see the rules for these names, but it doesn't tell us about what it is being named.

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

Darwin did not regard species as a distinct "kind" of biological entity. Instead, he considered them as essentially the same thing as geographic or stratigraphic variations (see these below), but ones in which extinction has removed the intermediate forms that otherwise would blend into the closest living relative group.

20th Century biologist Ernst Mayr (and most contemporary biologists) formalized their definition of a species as a "naturally occurring populations that interbreed and produce viable fertile offspring".

But there are some problems with this. For one: hybrids (crosses between two separate species) do occur naturally, and many of these are actually fertile! And for paleontologists: we can't test interfertility between populations because they are dead!

So we are stuck looking only at shapes (and in fact, only the shapes of those hard parts that survive fossilization).

The question then becomes: how different do two individuals, or two populations, have to be for us to consider them different species? This is actually a terribly difficult question even with living organisms!! There are several sources of variation:

In fact, the recognition that species were NOT absolute kinds, but instead have "fuzzy" boundaries that blend into each other, is one of the main clues to the discovery of evolution.

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:


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: Macroevolution
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:

Speciation is the process of the origin of a species. It doesn't happen immediately or instantaneously: it is indeed a process rather than an instantaneous event. (In fact, except in rare cases, it is unlikely that it you there during it that you would recognize it as such.)

Some aspects of the origin of species to consider:

During the 20th Century (especially during the first half), evolutionary biologists assumed the dominant trends were sympatry and anagenesis. However, as a better understanding of genetics was developed, some (including Mayr) argued that allopatry, peripatry, and parapatry (which all require cladogenesis) were actually more common.

The problem, of course, is that speciation takes time, and field biologists are unlikely to observe it. If only there were some sort of record of changes over time. Say, for example, a fossil record...

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: 25 September 2017

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