I woke up thinking about taxonomy. Hence, today’s topic. Go
figure.
(Picture from here.)
First, taxonomy is about successful lineages. Evolution
doesn’t select for failed systems. Relative reproductive success is it’s only
criteria. Therefore, even of species or families or classes go extinct, they existed because they were successful.
This doesn’t mean a
thing about successful individuals—an individual that doesn’t reproduce is an
evolutionary failure in that its genes were not contributed to the gene pool. Individuals
compete solely on that basis. If individual A outcompetes individual B by
killing individual B’s offspring, that’s a win for individual A. If individual
A outcompetes individual B by being so social cooperative that A is sought
after as a mate, that’s a win for individual A, too. The means of competition
is immaterial. What’s important is individual A’s genetic contribution outweighs
other individuals. The behaviors, mechanisms, physiology, and characteristics
of A are then passed down.
The common
characteristics that represent taxonomic groupings reflect those evolutionary
successes. Species that are members of Class Mammalia are
characterized by milk-producing glands, a neocortex in the brain, middle ear
bones. But no species became mammals on purpose. A successful root species
happened to have those characteristics—evolved from variation and selection—and
passed those characteristics on to its descendants.
Imagine a collection
of pictures of a particular individual—call her Claire. Each year a picture is
taken in the same pose from birth to death. Now, imagine all of those pictures superimposed
on one another such that common features remain in the aggregate and individual
characteristics—hair cut, size, clothing—are discarded. The remainder should
show all of the common features of that individual’s physical appearance over
their lifetime.
A taxonomic tree is
a similar artificial construction. An individual species partakes of the
general qualities of the group containing it. That group partakes of the
qualities of its containing group like a set of nesting Russian dolls.
Carl Linnaeus gave us
the first scientifically based taxonomy called binomial
nomenclature. This is the
genus/species combination we’re so familiar. For animals, modern taxonomy
starts at the top in a set of hierarchies. Here’s the hierarchy for Homo sapiens.
-
Domain: Eukaryota:
eukaryotic cells. I.e., cells with nuclei.
-
Kingdom: Animalia:
The animal kingdom. Everything from jellyfish to jaguars.
-
Phylum: Chordata:
Animals with a notochord— in mammals this is protected in the spine
-
Subphylum:
Vertebrata: any animal with a backbone containing a notochord.
-
Class: Mammalia:
mammals. See above.
-
Order: Primates: Old
and New World monkeys, great apes, lemurs, tarsiers, lorises etc.
-
Suborder:
Haplorhini: Old World monkeys and apes and New World monkeys.
-
Infraorder:
Simiiformes: (Simians) Old World monkeys and apes.
-
Family: Hominidae:
The Great Apes: chimpanzees, orangutans, gorillas, bonobos, humans
-
Subfamily:
Homininae: humans, chimpanzees and bonobos, and gorillas
-
Tribe: Hominini:
all members of genus Homo and genus Pan (chimpanzees and bonobos)
-
Genus: Homo: includes
extant and extinct members that emerged from genus Australopithecus. Includes H. erectus, H. habilis, H.
neanderthalis, H rudolfensis, H. sapiens, etc.
-
Species: H. sapiens:
us
Therefore, H. sapiens shares the common characteristics of genus Homo. All members of genus
Homo share common characteristics of Tribe Hominini, etc.
This is the record
of our success.
Different Kingdoms
and Domains have different taxonomies. Linnaeus was a mammal and brought a
mammal’s narrow perspective to the problem. Bacteria share DNA all the time. Plants
hybridize. Animals don’t hybridize near as much and vertebrate hybridization even
less, and mammals hardly at all. Thus, these fairly rigid categories hold
mostly well—but even here, the rules can be bent or broken to the point that
there have been arguments that the only real grouping is a species and all
these other groups are human simplifications. There are even those that say
there’s not really such a thing as a species.
An example of the
problem can be shown in one of my favorite animals, the common box turtle, Terrapene carolina.
Box turtles are
clever, colorful, and highly individualized animals. Each has their own
personality within the confines of their species. There are several “subspecies”:
T. carolina carolina, T. carolina bauri, T. carolina major, T. carolina mexicana, T. carolina yucatana, and T. carolina putnami. These
subspecies represent changes that correlate with their range. T. carolina carolina is the Eastern Box Turtle. It’s a bright,
colorful animal with a high dome. They can range from southern Maine down to Florida.
T. carolina major, the Gulf Coast Box turtle is larger and
less colorful and found along the Gulf of Mexico.
The Eastern Box tends towards
deciduous fortests and the Gulf Coast Box along swamp and estuaries. Both are omnivorous.
I have worked with the Eastern Box and Gulf Coast Box extensively. The
reproductive behavior between the two subspecies is somewhat different but not
extremely so and there is intergrading where the range is shared.
However, the other
subspecies are more contained and do not intergrade as much. At what point does
a species become sufficiently isolated to become a new species?
The Eastern Box
occasionally has three toes on the hind foot. T. triunguis, the Three
Toed Box Turtle, is a small box turtle that has three toes on the back feet but
some four toed examples have been noted. Some speculate that T. triunguis is actually an Eastern Box in disguise. Both have high domed shell.
Thought the coloration is markedly different. The Three Toe is not brightly colored. The male display colors are different between the
two species.
So, what is a
species?
In our own lineage,
there is strong evidence that humans and Neanderthals interbred. Is
it, then, H. neanderthalensis
or H. sapiens neanderthalensis?
But I think dwelling on minutia is missing the point. Taxonomy
is a tool, not sacred writ. It marks historical qualities that reach down to
the current species. For a long time, dinosaurs were in reptiles. Now, dinosaurs
reside in Class Dinosauria. Birds were Class Aves and they are still generally
so considered. However, it’s become ever more clear that birds descended directly from
dinosaurs and still so resemble them that some have suggested Aves should,
instead, be a subclass of Dinosauria. Has the evolution of birds since their
origin a sufficient difference as to justify Class Aves? Arguments abound. Thus,
the taxonomy continues to reflect both our current understanding and confusion.
Taxonomic groups (at least in animals) reflect a heritage
from a common ancestral population. Genus Homo came from an ancestral group
representing genus Australopithecus. At some point, the changes in that population
were such they could no longer easily breed back to the ancestral genus. As
time went on, further populations subdivided, sharing common characteristics but
developing their own distinctions.
Always successful. Always changing over time.
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