Phylogenetic Systematics

Since the advent of the theory of evolutio n, o ne of the tasks of biology has bee n to i nvestigate the phyloge netic relatio nship betwee n species. This task is especially impor ta nt because all of the differe nces which exist betwee n species, whether i n morphology, physiology , or ecology, i n ways of behavior, or even i n geographical distributio n, have evolved , like the species themselves, i n the course of phyloge nesis. The prese nt-day multiplicity of species a nd the structure of the differe nces betwee n them, first becomes i ntelligible whe n i t i s recognized that the differences have evolved i n the course of phyloge ne­ sis; i n other words, whe n the phyloge netic rela tio nship of the species i s u nderstood. I nvestigatio n of the phyloge netic relatio nship be twee n all existi ng species a nd the expressio n of the results of this research , i n a form which ca nnot be misu nderstood, i s the task of phyloge netic systematics. The problems a nd methods of this importa nt province of biology can be u nderstood o nly if three fu ndame ntal questio ns are posed a nd a nswered: what i s phyloge netic relatio nship, how is it established, a nd how is knowledge of i t expressed so that misu ndersta ndi ngs are excluded? The defi nitio n of the co ncept "phyloge netic relatio nship" i s based o n the fact that reproductio n is bisexual i n the majority of orga nisms, a nd that i t usually takes place o nly within the framewor k of co n fi ned reproduc tive communities which are genetically isola ted from each o ther. This i s especially true for the i nsects, with which this paper i s mainly concerned. The repro­ ductive communities which occur i n nature we call species. New species origi nate exclusively because parts of existi ng reproductive commu nities have first become exter nally isolated from o ne a no ther for such exte nded periods that ge netic isolatio n mecha nism s have developed which make reproductive relatio nships betwee n these parts impossible whe n the exter nal barriers which have led to their i solatio n are removed. Thus, all species ( = reproduc­ tive communities) which exist together at a given time, e.g., the present, have originated by the splitting of older homoge neous reproductive communities. O n this fact i s based the defi nitio n of the co ncept, "phyloge netic relatio n­ ship": u nder such co ncept, species, B, is more nearly related to species, e, than to a nother species, A, whe n B has at least o ne a ncestral species source i n common with species e which i s not the a ncestral source of species A [Hennig (8)) . "Phylogenetic relatio nship" i s thus a relative co ncept. I t i s poi ntless (si nce i t is self -evide nt) to say , as is ofte n said, that a species or species­ group is "phylogenetically related" to a no ther. The questio n is rather o ne of