week 1. Evolutionary Convergence: strong evidence for selection, but a problem for taxonomy...........
This demonstration illustrates a central conundrum of evolutionary biology: how much of the nature of organisms is due to selection, and how much to phyletic history? and why does it matter? Below is the text of the preamble to this demonstration, so that you may look at it again later, at your leisure. For its continuation, see the sheets on display in the demonstration itself.
A recent paper of mine for the Argentine general-interest science magazine "Ciencia Hoy" (Science Today) discusses material centering around these same matters - it may be useful in your attempts to come to grips with things........
In a pre-evolutionary world, this question could only be answered vaguely, in terms such as divine providence, the unknowable mysteries of creation, and the like.
But after long years of more and more closely specifying the detailed structure of organisms, and seeing thereby the many ways in which they resembled or differed from one another, it became obvious to students of living creatures that there was a more or less clear structure to the degrees of similarity and difference, which suggested a way of classifying life. There was a system of levels of resemblance evidently inherent in the structure of organisms: a Natural System which was, moreover, hierarchical. There were nested degrees of resemblance - groups containing groups containing groups, and the higher the hierarchical level, the more and more dissimilar were the creatures included. So a more refined question was:
As such work progressed, some similarities among organisms, though very impressive, could be understood as only "at the surface," reflecting adaptation rather than genetic affinity. That is, these similar characters could be recognised as analogous (superficial), rather than homologous (fundamental.) In so doing, the phenomenon of Evolutionary Convergence became recognised: convergent forms, despite their often great similarity, could be seen as only ecologically equivalent rather than closely related by descent - they had independently acquired similar characters through adaptation.
The existence of equivalent forms, superficially alike yet only distantly related, provided some of the more powerful evidence for the efficacy of natural selection in moulding the form of organisms so as to fit their environmental contexts. Selection had been able, so to speak, to provide false clues to the decipherment of the phylogenetic (historical) relationships of the organisms: the clues appeared to imply close relationship, because the characters were so similar, but close scrutiny permitted their recognition as in fact superficial, and the eventual uncovering of more fundamental characters, truly indicative of distinct histories.
But notice that the recognition of such convergence requires that distinct ancestry indeed can be clearly established: that homologous characters can be readily distinguished from analogous ones. But, of course, characters don't come with labels on them; they aren't colour-coded according to homologue or analogue status, fundamental or superficial form, phylogenetic or adaptational meaning. This distinction is one which must be inferred, and, therefore, it is a distinction prone to error. Thus the systematists' perpetual search for the "non-adaptive character" immune from the taint of the selective process which can muddy the trail of ancestry. So:
So this demonstration is by way of raising and illustrating issues closely associated with this matter of resemblance - why it is there and what it means; it concerns the issue of:
To the extent that the differences among species substantially represent the operation of Natural Selection, we will expect that phylogenetic radiations will comprise arrays of species (or higher-level taxa) which each possess sets of characteristics functionally appropriate to the conditions encountered: they will be Adaptive Radiations. This should be more or less true, regardless of the ancestral taxa concerned; as a consequence we would expect similar selective régimes to evoke similar kinds of species - what we call ecological equivalents.
As an example, take a look at the accompanying figure showing the evolutionary/ecological radiation of the Hawaiian Drepanididae, a family of passerine birds closely related to the same stock that gave rise to the Cardueline finches, such as the Goldfinch. On their arrival in the Hawaiian archipelago, they encountered a multitude of little-exploited ecological opportunities, and rapidly, and within their single family, generated an array of morphological forms comparable to many found elsewhere in the world, under similar ecological circumstances, in several other bird families.
To the extent that the ancestral taxa are phyletically (genealogically; historically) distinct, these functional attributes of equivalent species will be analogous rather than homologous; that is, they will be evolutionarily convergent structures rather than having been inherited, as is, from ancestral forms.
Often, it is straightforward to distinguish analogue characters from homologues, and the taxonomy of the species is therefore clear. But, if selection is very powerful, leading to very close convergence, analogues become more and more difficult to discriminate from homologues and, accordingly, taxonomic errors become more probable.
In this lab. demonstration we see a variety of examples illustrative of this issue. As is so often the case, a historical perspective is useful:
Time and again, accepted taxonomies have been modified or even overturned when evidence comes to light that presumed homologies are, in fact, analogies - and in each case, we are offered evidence of the power of selection to erase history's footprints ................
Perhaps the most dramatic recent taxonomic overturning followed from the abandoning of the old Plant Kingdom /Animal Kingdom dichotomy, when it was realized that, say, the similarities between so-called Blue-green Algae and Green Plants was not due to any close phyletic relationship.............. Blue-green Algae aren't plants, or even algae, at all just because they possess chlorophyll, and use it to photosynthesize oxygenically. The similarity is convergent - a matter of life-style; Blue-green algae are actually Cyanobacteria.
You will, no doubt, recall some classic examples of evolutionary convergence from earlier courses, such as Bio 284a, or from your own reading, such as:
various kinds of "anteaters" - true S. American anteaters, Old World pangolins, Australian anteaters (marsupial numbats), Australasian spiny anteaters (monotremes).
various placental/marsupial parallels - thylacine & wolf; marmot & wombat; placental and marsupial 'moles;' placental and marsupial 'cats.'
several large, high-speed, ocean carnivores - sharks, tuna, dolphins, ichthyosaurs.
It is very clear to us now, in these cases, that the similarities these taxa show are only superficial - it is clear that those similarities are due to adaptive convergence and not to common ancestry. But this wisdom is relatively recently won...................
It really wasn't so long ago that loons and grebes were placed in the same taxonomic group, for example; or swifts and swallows.......... but all that silliness is behind us now, isn't it? - and we may now confidently display the following examples of convergent forms - ecological equivalents...........
(From ths point, you are dependent upon the notes you made in the demonstration...........)
The 2005 list of papers to be discussed in the weeks after this demonstration follows below:
Week 2. the analysis of adaptation
Cain, A.J. & P.M. Sheppard 1954 Natural selection in Cepaea. Genetics 39: 89-116.
Pemberton, S.D. etal 1991 Countervailing selection in different fitness components in female Red Deer. Evolution 45: 93-104.
Week 3. the units of selection
Emlen, S.T. 1988 The role of kinship in helping decisions among White-fronted Bee-eaters. Behavioral Ecology & Sociobiology 23; 305-315.
Packer, C. et al. 1991 A molecular genetic analysis of kinship and cooperation in African lions. Nature 351: 562-565.
Week 4. adaptive explanation
Reimchen, T. 1979 Substratum heterogeneity, crypsis and colour polymorphism in an intertidal snail. Can J. Zool. 57: 1070-1085.
Johnston, R.F. 1992 Evolution in the Rock Dove: Skeletal Morphology. Auk 109: 530-542.
Week 5. classification & evolution
Avise, J.C. et al. 1990 Mitochondrial gene trees and the evolutionary relationship of Mallard and Black ducks. Evolution 44: 1109-1119.
Lanyon, S. & J. Hall 1994 Re-examination of barbet monophyly using mitochondrial-DNA sequence data. Auk 111: 389-397.
Week 6. the idea of a species
Patton, J.L. & M.F. Smith. 1994 Paraphyly, polyphyly and the nature of species boundaries in Pocket Gophers (Thomomys). Systematic Biology 43: 11-26.
Ellsworth, D.A. et al. 1995 Phylogenetic relationships among N. American grouse inferred from Restriction Endonuclease analysis of Mitochondrial DNA. Condor 97: 492-502.
Week 7. speciation
Thompson, C.E., E.B. Taylor and J.D. McPhail. 1997 Parallel evolution of lake-straem pairs of Threespine sticklebacks (Gasterosteus) inferred from mitochondrial DNA variation. Evolution 51: 1955-1965.
Carroll, S.P. & C. Boyd 1992 Host race radiation in the Soapberry bug: natural history with the history. Evolution 46: 1052-1069.
Week 8. the reconstruction of phylogeny
Lanyon, S.M. 1994 Polyphyly of the blackbird genus Agelaius and the importance of assumptions of monophyly in comparative studies. Evolution 48: 679-693.
O'Brien, S.J. et al. 1985 A molecular solution to the riddle of the Giant Panda's phylogeny. Nature 317: 140-144.
Week 9. evolutionary biogeography
Ellsworth, D.L. et al. 1994 Historical biogeography and contemporary patterns of mitochondrial DNA variation in White-tailed deer from the southeastern United States. Evolution 48: 122-136.
Wenink, P.W. et al. 1996 Global mitochondrial phylogeography of holarctic breeding Dunlins (Calidris alpina) Evolution 50: 318-330.
Week 10. rates of evolution
Boag, P. and P. Grant 1981 Intense natural selection in a population of Darwin's finches (Geospizinae) in the Galapagos. Science 214: 82-85.
Gingerich, P. 1983 Rates of evolution: effects of time and temporal scaling. Science 222: 159-161. Followed by a comment by Gould, S. (1984) Science 226: 994-995, and a response by Gingerich (1984) Science 226: 995-996.
Week 11. macroevolutionary change
Williamson, P. 1981 Paleontological documentation of speciation in Cenozoic molluscs from the Turkana basin. Nature 293: 437-443.
Bell, M.A. et al. 1985 Patterns of temporal change in single morphological characters of a Miocene stickleback fish. Paleobiology 11: 258-271.
Week 12. macroevolutionary trends
Jablonski, D. 1986 Background and mass extinctions: the alternation of macroevolutionary regimes. Science 231: 129-133.
McKinney, M. 1987 Taxonomic selectivity and continuous variation in mass and background extinctions of marine taxa. Nature 325: 143-145.
Week 13. extinction & mass extinction
Raup, D.M. 1991 A kill curve for Phanerozoic marine species. Paleobiology 17: 37-48.
Raup, D. & J. Sepkoski 1986 Periodic extinction of families and genera. Science 231: 833-836.
Patterson, C. & A. Smith 1987 Is the periodicity of
extinctions a taxonomic artefact? Nature 330: 248-251.
Return to Main 441a document.