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GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2020
Variation and Fossil Individuals


Birds of a different color: Three major genes set feather hue in pigeons. 2014. Phys.org.

Key Points:
•The individual is the only true unit of variation among living things.
•The recognition of variation as an intrinsic part of nature was instrumental to the development of the Theory of Evolution.
•Variation results from a range of biological factors including: Individual, ontogenetic, sexual, generation-alternation, and ecophenotypic.
•Taphonomic issues impose their own sources of variation in fossils.
•Different taxa employ a range of growth strategies which impart different styles of ontogenetic variation
•Biomechanical constraints impart variation as organisms scale allometrically
•Heterochrony represents the linkage of ontogeny (growth) an evolutionary change.
•The grouping of variable individuals into species is problematic:
•Asexual "species," hybrids, hybridogens, morphoclines, sibling species, ring species, and closely related species clusters frustrate our understanding.
•Competing species criteria, including the Biological, Evolutionary, and Phylogenetic Species Concepts would divide up organismal diversity differently.
•Depending on the tempo and mode of evolution, the chronological duration of species is similarly uncertain.

"THE MANY slight differences which appear in the offspring from the same parents, or which it may be presumed have thus arisen, from being observed in the individuals of the same species inhabiting the same confined locality, may be called individual differences. No one supposes that all the individuals of the same species are cast in the same actual mould. These individual differences are of the highest importance for us, for they are often inherited, as must be familiar to every one; and they thus afford materials for natural selection to act on and accumulate, in the same manner as man accumulates in any given direction individual differences in his domesticated productions. (Charles Darwin, 1859. The Origin of Species)

Sources of Variation within Populations:

The ultimate goal of taxonomy is the description of the diversity of living things. Without this knowledge, we can barely begin to understand the fossil record, however before attempting to do taxonomy, we must consider the mechanisms that generate diversity among organisms.



A small sample of human variation

Growth Strategies:

After individual variation - an essential driver of evolution, ontogenetic variation is arguably the most significant source of diversity, so its interpretation merits detailed attention. Interpreting ontogenetic status requires first that the growth mode of hard parts be identified. The major growth strategies:

But note that many creatures combine strategies. E.G.:


Isometric scaling of a cube

Allometry:

A distinct ontogenetic issues is that of scaling. Typically, different parts of the organism grow at different rates - i.e. they experience allometric scaling. This results from a simple fact of geometry:

Comparative allometry:


White-tailed deer and cape buffalo - scaling with mechanical similarity
Allometric scaling is also evident in comparisons across taxa.


The "evolution" of Babylon 5's Jason Ironheart

Growth and evolution:

Remember, ontogeny pertains to the individual, evolution to the lineage. (TV sci-fi plots notwithstanding.) That doesn't mean they aren't connected in any way. Now that we know something about how organisms grow, let's consider how alterations in ontogeny over evolutionary time can drive evolutionary change through the process of heterochrony.

Heterchrony:

Subtle changes over evolutionary time in an organism's developmental timetable are a potent source of overall evolutionary change. This is an idea with a history as long as the study of evolution.

General patterns of heterochrony:


Gould added clarity to these terms by identifying the developmental parameters that could be tweaked to produce either major pattern, and naming the different pathways that resulted. In Gould's system, heterochronic change could result from changes in:

Combining these developmental parameters gives us:

Varieties of Paedomorphosis:


All of this has huge consequences for Morphometrics, the quantified statistical comparisions of shape.

But underlying all of this is a difficult philosophical issue:

Fossil Species: Species Concepts and Criteria


Species (pl. "species"; Latin for "kind"): generally considered the fundamental unit of biological diversity. Certainly is the primary entry in databases of diversity, abundance, occurence, etc. from modern and fossil assemblages, ecosystems, etc.

But, WHAT ARE SPECIES?!? We (sometimes) know them when we see them, but how do we recognize them? What is our species criterion? This is known in biology as the "species problem".

Difficulties with species delineation:

"Species" are our attempt scientifically to codify traditional "kinds," populations of interbreeding critters that are more or less morphologically uniform. Seems easy, but when you scrutinize living diversity in detail, a number of problems come up:


Offshore (left), transient (center), and resident (right) orcas.

It gets worse: Even without these complications, the delineation of "species" can be daunting. Consider the killer-whale Orcinus orca: Three morphological types are known from the Pacific Northwest (above) that are behaviorally distinct. and not observed to interbreed.

Nine such populations are known worldwide. Is Orcinus orca a species with distinct sub-populations, or a cluster of closely related species?

Species criteria:

The paradox of species criteria: We humans feel compelled to define species precisely when, in nature, their boundaries are fuzzy, indistinct, and best described probabilistically.

And yet, living things do seem to group into morphologically distinct populations rather than grading across wide ranges of morphospace without breaks, even those that reproduce asexually. Traditionally, species are morphospecies: "a diagnosible cluster of individuals within which there is a pattern of ancestry and descent, and beyond which there is not." However, individual variation is a basic attribute of ALL organisms; and geographic variations are very common as well. At what point are two different geographic populations different at "the species level"? Your instructors' personal rules of thumb:

But we're not the experts. Let's refer to the view of Charles Darwin, who felt that species were merely well delineated varieties whose distinctiveness from other organisms arose from the fact that intermediate forms were now extinct.

None of this has prevented biologists from attempting to develop hard and fast species concepts. Major attempts at species definitions that have gained significant traction include:


Liger from Ligerworld
Warning! Don't think that these criteria are simply different ways to reach the same underlying truth. In many cases, their practical application might be the same, but they are different concepts. Consider pantherine cats: In the wild, lions, leopards, jaguars, etc. are morphologically and behaviorally distinct, but in captivity they produce fertile hybrids frequently.

Paleontological applications: There have been many minor variations on these themes. For paleontologists, however, the Phylogenetic Species Concept is most frequently employed, simply because we just can't test reproductive isolation in fossil taxa. We can, however, test hypotheses of phylogeny - the branching pattern of evolution, in such a way as to determine whether individuals might have belonged to single lineages.

Caveats:

Speciation: The Origin of Species

So far, we have only considered the present time slice. When we look at the past, other issues rise up. Remember that despite the foregoing, "paleospecies" are necessarily morphospecies. Whatever species concept we favor, we can't absolutely assess their mating habits.

Two major models of species origins in geologic time:

Additional reading:

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