10 



ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



Table 1 . Comparison of Three Methods Used in Biological Classification. 



Evolutionary' 



Character weighting 

 Convergence 

 Homology 

 Fossil History 

 Eco-evolutionary Data 

 Rale of Evolution 

 No. of Characters 

 No. of Specimens 

 Branches from a Node 

 End Product 



Test of Goodness 



No 



Not Considered 



Not Considered 



Not Considered 



Not Considered 



Not Considered 



Many 



Few 



Two to Many 



Perhaps a Phylogeny 



Replicability 



Yes 



Important 

 Important 

 Not Important 

 Not Important 

 Not Important 

 One to Medium 

 Few to Many 

 Two when Possible 

 Phyiogenetic Classification 

 Based on Genealogy 



Parsimony 



Yes 



Important 



Important 



Important 



Important 



Important 



One to Medium 



Few to Many 



Two to Many 



Phyiogenetic Classification Based 



on Genealogy and Degree of 



Difference 

 Predictability 



is the same pail or organ in different animals under every variety 

 of form and function." He goes on to note, however, that some 

 earlier workers defined the concept as we now define analogy. 

 But our problem remains identical with that of Owen— how to 

 define same. In a recent discussion of homology, Patterson (1982) 

 proposed similarity in ontogeny as part of a test of homology. 

 But the use of similarity in development to help define Owen's 

 "same" is tautological. 



Palaeontologists proceed in a basically circular fashion in their 

 use of homology. They depend upon a time series to trace the 

 history of transformed states of a presumably homologous char- 

 acter along a sequence that is interpreted as a genealogy. But of 

 course the characters are considered homologous because they 

 are part of a genealogy. Whether they admit to it or not, most 

 systematists use pure phenetics in the search for homology, and 

 although this common sense, intuitive, non-scientific approach 

 works much of the time, still, many systematists have misin- 

 terpreted as homologues characters that are actually analogous 

 and have filled the literature with many misdiagnosed conver- 

 gences. In comparative vertebrate anatomy and systematics, the 

 convention has grown up that certain organ systems are more 

 conservative than others and therefore provide a better method 

 for detecting homologies. The nervous system is generally con- 

 sidered the best, the skeleton the next best, followed by viscera 

 and muscles, with the integument the least good. In fishes, for 

 example, Freihofer (1963, 1970) has used the patterns of the 

 ramus lateralis accessorius and ramus canalis lateralis nerve 

 systems relative to elements of the skeleton to propose groupings 

 of fishes. But even here the possibility of convergence cannot 

 be ignored (Gosline, 1968), and again the problem of circularity 

 arises because many ichthyologists define osteological features 

 on the basis of their topographic relation to elements of the 

 nervous system. Another example relates to homologies of pho- 

 tophore series in lantemfishes as determined by studies of their 

 innervation (Ray, 1950). Here also, the conclusions based on 

 this method appear to be equivocal (Moser and Ahlstrom, 1 972). 



A direct method for demonstrating the homology of structures 

 would be to trace them back during development to their an- 

 lagen. De Beer (1951) has commented on the apparent failure 

 of experimental embryology to validate this approach. Even so, 

 a survey of the development of bony structure during fish on- 

 togeny presented by Dunn ( 1 983b) lists some observed instances 

 of losses, gains, and modifications, chiefly in the caudal fin skel- 

 eton, which interpret homologies in adult structure; unfortu- 



nately, these instances are too few. Ahlie had a long interest in 

 the caudal fin skeleton, particularly of flatfishes, and the com- 

 pletion of his work by colleagues hopefully will constitute an 

 additional contribution to the use offish ontogeny in identifying 

 homologous structures. 



The concepts of ontogeny and homology are intimately as- 

 sociated in the idea that the study of early life history stages of 

 an organism will reveal its adult ancestral stages— ontogeny re- 

 capitulates phylogeny— as proposed by Ernst Haeckel in the 

 latter half of the 19th century. Taken at its most extreme, the 

 biogenetic law has been interpreted as meaning that an entire 

 genealogy is encapsulated in an ontogenetic series. If adults of 

 extant species of a group were to be matched up with their closest 

 approximations in an ontogenetic series, homology would un- 

 fold before our eyes. Of course its value to us in unraveling 

 phylogeny would be redundant, because phylogeny would be 

 there as well. It was soon evident however that the biogenetic 

 model is far too crude to approximate nature. The embryologist 

 von Baer had previously formulated four "laws" or general 

 propositions about embryology that have been restated in var- 

 ious forms by many authors and applied to the interpretation 

 of phylogeny. The following are taken from De Beer ( 1 95 1 ): ( I ) 

 In development from the egg the general characters appear be- 

 fore the special characters. (2) From the more general characters 

 the less general and finally the special characters are developed. 

 (3) During its development, an animal departs more and more 

 from the form of other animals. (4) The young stages in the 

 development of an animal are not like the adult stages of other 

 animals lower down on the scale, but are like the young stages 

 of those animals. These propositions are useful generalizations 

 and we can all think of obvious instances of fish , ontogeny that 

 can be interpreted by one or more of them . Consider for example 

 the bilaterally symmetrical larvae of flatfishes, the early presence 

 and subsequent loss of a swimbladder in stromateoids (Horn, 

 1970a), the sequence of fusions during ontogeny in the caudal 

 fin skeleton of myctophids (Ahlstrom and Moser, 1976), the 

 ontogeny of the upper jaw bones and dentition in notosudids 

 (Berry, 1964a), and the presence of a pectoral fin in larval Tac- 

 tosloma and its loss in adults (Ahlstrom, lecture notes). On the 

 other hand, a plethora of early life history stages of fishes man- 

 ifests character states that represent morphological specializa- 

 tions occurring early in development. Consider the egg stages 

 of macrourids with their hexagonal patterns, atherinomorphs 

 with their filaments, and argentinoids with their pustules. Other 



