Biological Concepts: Clades

After Physical Anthropology in 1950, I had a few requests that I clarify evolutionary terms I had used and distinctions I had drawn, plus a request for an explanation of one of the illustrations for the article.  Hopefully people will continue to read that article, and hopefully I will write more on biology in the future, and if so it would be useful to answer these kinds of questions in a central location.

Be forewarned!  This article is less ambitious than what I aim for in the similarly-named series on political concepts [to date: Fascism • Servitude ].  I claim no authority on biology, and I welcome any corrections or guidance (but sourced corrections if I don’t know you, thanks).  Hopefully I can add a certain clarity and organization for someone diving into the subject for the first time.

Genetic — This is a fun little word that can confuse people without their having realized they’ve been confused.

(I)  The modern science of genetics owes its name to Charles Darwin, who called his (now-obsolete) theory of inheritance pangenesis.  Darwin called the units of inheritance in this theory gemmules, but later disciples called them pangenes, shortened to genes.  Gregor Mendel called the hypothetical unit of inheritance identified by his pea-plant experiments (published in 1865) factors, but his work remained unknown until after 1900.

(II)  Thus up through at least the 1940s the primary meaning of genetic was in its root sense of γένεσις (origin, source, generation; cf. γένος, family, stock, race).  The genetic fallacy is to conflate criticism of the original source of an argument (its genesis)  with a logical demonstration of its invalidity.  A genetic relationship, in the original and most general sense, is any relationship which implies a common origin: descent, so to speak, from a common ancestor.

(III)  Ever since the theoretical Mendelian gene was convincingly identified with long strings of deoxyribonucleic acid, there has been a movement to use genetic to label (a) the deoxyribonucleic acid molecules themselves; the experiments performed on them; observed sequences of nucleotides (the genetic code); hypothesized sequences; the sum of all such sequences carried by one organism (the genome); traits linked to these sequences (genetic disease, genetic syndrome); and (b) biological theories and classification systems which emphasize descent from a common ancestor; and in particular those drawing out the implications of the specific mechanics in which genes dictate why and how descendants from a common ancestor resemble both each other and the ancestor.  As the latter use grows, midwits increasingly assume that any reference to a genetic relationship either asserts shared DNA, or rests on some kind of dubious analogy to common biological descent.  This trend is particularly irritating to linguists; linguistics not only systematized the investigation of genetic relationships between languages long before Mendel, but actually invented many concepts and tools which were later borrowed by the DNA-geneticists.

Ancestral, derived — A trait that an organism shares with its ancestors because the genetic code which causes the trait to develop has been passed down in an unbroken line from the ancestral population to the present day is ancestral.  All traits wherein an organism differs from its ancestors due to mutations and subsequent natural selection are derived (i.e., derived from the ancestral trait).  — I do not know whether there is any specific term to describe situations where an organism regains an ancestral trait which his immediate ancestors had lost.

Clade, phylum, taxon  On a family tree picturing an ancestral population (the trunk) and the populations descended from the common ancestor all the way to the modern day (the twigs and leaves), each subfamily branching off the rest of the tree is a clade: which is just Greek for “branch, shoot”.  — Phylum (“race, tribe”) can also describe one of these subfamilies in a generic way (and is the traditional way to describe this kind of genetic relationship outside of biology, e.g. in linguistics), but it has a technical use in biology as referring to the level of classification between kingdom and family. The phyla roughly correspond to “types of body plan” inside the animal kingdom.  Phylogeny/phylogenetics is an approach to classification which assumes the items within a classification should have a closer genetic relationship to one another than to any item excluded from it.  (While phylogeny now refers to the DNA-conception of genetics specifically, the concept was introduced by Haeckel, who proposed a phylogeny based on similarity in embryogenesis before either Darwin or Mendel had published their theories.) — A taxon is a neologism (and a back-formation, no less!) for sets of objects to be treated as a unit for classification (i.e. taxonomy).  In the phylogenetic approach to taxonomy, every taxon should be a clade… and even, according to phylogenetic purists, a monophyletic clade.

139417Monophyletic, paraphyletic, polyphyletic  The word clade shares an ambiguity with its etymon “branch”: for any given (tree-)branch, there is no precise answer to whether a bit of wood growing off a point on the branch is a continuation of the same branch, one of the branch’s twigs, or a new branch in its own right.

(I)  A monophyletic clade is one which includes all the descendants of some common ancestor (with no exceptions); purists who use “clade” exclusively for monophyletic clades might say “monophyletic taxon” to avoid tautology.

(II)  A paraphyletic grouping is one which includes the original ancestor of an entire clade, plus some (but not all) of its descendants.  Paraphyly comes up whenever we observe many taxa, all distinctively similar due their shared ancestral traits, which would be a monophyletic clade if not for a small number of subfamilies which have lost the traits in question.  Thus reptiles are paraphyletic with respect to mammals and birds; together reptiles, mammals, and birds are the clade of amniotes, and amphibians are paraphyletic with respect to them; wasps are paraphyletic with respect to ants and bees.  Because paraphyly divides conservative branches of a clade from those which have evolved away from the ancestral form, these groupings were previously called evolutionary grades; I am not sure whether this term has fallen out of use for technical reasons, or is merely unfashionable.

(III) A polyphyletic grouping, on the other hand, is neither a proper clade nor almost-a-clade; it is just a set of several taxa which share a derived trait with each other, but not with their most recent common ancestor. (I’ll put additional illustrations of these relationships at the end of the post.)

Analogy, homology — A trait or feature in two species is said to be an analogy between them if it merely “looks the same”, has the same function, or generally can be described in the same way.  A homology is a relationship between traits/features which evolved out of the same ancestral trait/feature, even if these features do not currently resemble one another or serve common functions.  Thus human arms, elephant fore-legs, dolphin flippers and bat wings are all homologous (they all develop out of a single feature of the body plan of the common mammalian ancestor) whereas butterfly wings and bird wings are analogous (both are wings, both allow an organism to fly, but there is no similarity in the ancestral traits from which the wings are derived).  — Now, are bat wings and bird wings homologous?  As limbs, bat wings and bird wings developed out of the same pair of limbs on the common ancestor of birds and bats, but these limbs were not wings in the common ancestor, and bats and birds evolved to fly completely independently, with the result that the functions were entirely different.  This is easiest to see if you look into the structure of the bones in a bat wing and a bird wing.  When you divide the whole into parts, you can see that bones which are homologues play different roles in the two wing-plans, and structurally similar parts of the wing are formed in completely different ways.

517px-apomorphy_-_homoplasy-svgApomorphy, homoplasy — A derived trait, when we are looking at the exact place where it emerges in the family tree, is an apomorphy.  If this trait only emerges once, and taxon in which the trait emerges never gives rise to any daughter taxa, the trait is autapomorphic.  If the derived trait can be found in an entire clade descended from the original mutants, it is synapomorphic.  Homoplasy, by contrast, is a situation where the derived trait in question evolves independently in several populations (these traits are homoplasious/homoplastic).  An ancestral trait, shared by all the lines where it did not disappear, is symplesiomorphic. — As best I can tell, the two series synapomorphic : homoplasious : symplesiomorphic and monophyletic : polyphyletic : paraphyletic divide out the same three possible topological relationships between a set of nodes in a tree; they differ only in that the first series describes relationships between nodes when they are grouped with respect to a single trait.  So for example the shared traits which justify a useful paraphyletic grouping must be symplesiomorphic, but the reverse is not true.

§§§

A full toolbox of technical ways of thinking and talking about evolutionary relationships is valuable for all sorts of different purposes.  If you care about the irresistible scuttling of Gnon, then you want to be sophisticated about what kinds of analogies you could draw when (inevitably) you draw analogies between Darwinian-selection and the other chambers in Gnon’s mansion.  This is especially true if are interested in history and anthropology; whether you are looking at cultures, languages, hominids, or haplogroups, you are looking at descent with modification and you want to be as thoughtful as possible about the relationships you’re observing.

But the most politically urgent reason to stay clear about different kinds of evolutionary terms and relationships is the one we encountered in Physical Anthropology in 1950.  Leftist dogma teaches that in the bad old days of biology and anthropology, scientists believed races were real because they were confused, biased, and fundamentally ignorant.  Leftists, of course, have trouble with certain kinds of complex thought.  They will often argue that a certain biological difference can’t be real because it doesn’t define a monophyletic clade.  At least, that is the gist of their confused bluster; if they actually knew what “monophyletic” meant, they would probably be familiar with “paraphyletic” too, in which case they would know many important biological relationships aren’t monophyletic!

Let me finish by repeating what I said at the beginning: I am not a biologist and I welcome any correction, instruction, or suggestions for revision.

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