Notes on Novelty series:
2. Historical considerations – before and after evolution
3: The meaning of evolutionary novelty
4: Examples – the beetle’s horns and the turtle’s shell
5: Evolutionary radiations and individuation
6: Levels of description
8: Conclusion – Post evo-devo
Given that novelty plays such a big role in the thought of so many people, it might be useful to review the senses in which they mean this. Fortunately, Pigliucci (2008) has done this already. He lists several examples: turtle shells, flight, flowers, the ability of great tits to open milk bottles in the 1950s, the transition of jaw bones to the
inner middle ear bones in mammals, eyes, hearts, and the origins of Hox genes. The definitions are equally heterogeneous. Mayr (1963: 602) defined them as
any newly acquired structure or property that permits the performance of a new function, which, in turn, will open a new adaptive zone
Mayr’s definition, which can stand in for many similar definitions in the post Modern Synthesis era, we might call the evolutionary definition, which relies upon novel functions and adaptive niche occupancy (and all that relies upon – adaptive landscape theories, the existence of objectively and non-arbitrarily delimitable niches, and uncontroversial atomisation of the parts of organisms).
Then there’s Wallace Arthur’s (2000: 811) more recent phylogenetic definition:
Novelties and apomorphies are essentially the same.
Apomorphies are characters (or character states in fact) that are unique to a clade. This definition relies upon there being only some shared set of traits unique to the clade (indeed, apomorphies define clades). As Pigliucci notes, this makes every change an evolutionary novelty. As soon as some trait evolves (or more exactly, as soon as we can identify an evolved trait, since we do not have direct non inferential access to the past), and a clade can be delineated, there is an evolutionary novelty. While exact (and objective just to the extent we have empirical data), this fails to capture the importance that evolutionary novelists ascribe to evolutionary adaptive advances.
In addition to the evolutionary and phylogenetic definitions, Pigliucci gives some recent definitions based upon developmental criteria, developmental definitions. Muller and Wagner (1991: 243) define a novelty as
A morphological novelty is a structure that is neither homologous to any structure in the ancestral species nor homonymous to any other structure of the same organism.
Some definitions are in order here
, which we shall give later; for now note that homonymy means here that a structure is repeated; in other contexts it would be called iterative homology. Also, note that Wagner and Muller refer to morphological novelty. That is, they refer to properties of the organism that are outcomes of developmental processes (i.e., not genes).
The second definition Pigliucci cites is West-Eberhard’s; according to her (2003: 140), the origin of a trait is:
the initial appearance of as a qualitatively distinct developmental variant.
and a phenotypic novelty (2003: 198):
a trait that is new in composition or context of expression relative to ancestral traits
Here the definition, if indeed it is intended to be one (much weight is put on scientists’ passing comments), has to do with quality, development, and ancestral traits. In other words, both are still evolutionary accounts, but as Pigliucci notes, neither involves adaptation or ecological niches.
Now the point of the developmental accounts is to put the novelty bearers in a developmental context: some trait is novel if it differs from parts of previous developmental systems in some significant manner. Where the evolutionary definition of Mayr is defined in terms of novelty of function, the developmental view is defined in terms of novelty of development of parts, and the phylogenetic definition involves only novelty of homologies. These three approaches are not independent of each other, but they have clear differences of emphasis.
In order to evaluate these different approaches – let’s not call them definitions in a conscious manner – we need to deal with some real cases. Fortunately, an excellent paper provides them ready made (Shubin et al. 2009), and in particular offers an account of the horns on beetles, an example offered to me by Brett Calcott and which was classically discussed by Darwin himself in The Descent of Man. Another is the evolution of the turtle’s shell, which another recent (as yet online only) paper summarises beautifully (Nagashima et al. 2011). That is the subject of the next post.
Arthur, Wallace. 2000. Intraspecific Variation in Developmental Characters: The Origin of Evolutionary Novelties. American Zoologist 40 (5):811-818.
Mayr, Ernst. 1963. Animal species and evolution. Cambridge MA: The Belknap Press of Harvard University Press.
Muller, Gerd B., and Gunter P. Wagner. 1991. Novelty in evolution: restructuring the concept. Annual Review of Ecology and Systematics 22:229-256.
Nagashima, Hiroshi, Shigehiro Kuraku, Katsuhisa Uchida, Yoshie Kawashima-Ohya, Yuichi Narita, and Shigeru Kuratani. 2011. Body plan of turtles: an anatomical, developmental and evolutionary perspective. Anatomical Science International:1-13.
Pigliucci, Massimo. 2008. What, if Anything, Is an Evolutionary Novelty? Philosophy of Science 75 (5):887-898.
Shubin, N., C. Tabin, and S. Carroll. 2009. Deep homology and the origins of evolutionary novelty. Nature 457 (7231):818-823.
West-Eberhard, Mary Jane. 2003. Developmental plasticity and evolution. New York; Oxford: Oxford University Press.