HOW CAN A CHARACTER BE DEVELOPMENTALLY CONSTRAINED DESPITE VARIATION IN DEVELOPMENTAL PATHWAYS

A fundamental riddle of evolutionary developmental biology is the cons ervation of adult morphological patterns (Hall, 1992). Conservative pa tterns are either called body plans if they concern overall body desig n, or homologues if they concern parts of the body (Riedl, 1978; Roth, 1982; Sattler, 1984; Van Valen, 1982; Wagner, 1989a, 1989b). An adult pattern is considered conservative if it remains unchanged in spite o f changes in function, as indicated by the original definition of homo logy by Owen, as a similarity of organs regardless of form and functio n (Owen, 1848). Conservation of anatomical features despite different adaptive pressures is naturally explained by developmental constraints (Wagner, 1986). However, this approach to explain the biological basi s of homology is plagued by the fact that developmental pathways are o ften more variable than the characters that they produce (see Tab. 1) (Hall, 1992; Roth, 1988, 1991; Spemann, 1915; Wagner, 1989b). This is also true for any other application of the concept of developmental co nstraints. The widely held opinion that early stages of development ar e conservative because any early perturbation is likely to interfere w ith later development, is far from absolute, since a vast amount of da ta in comparative developmental biology speaks to developmental variat ion (see e.g. the examples in Tab. 1). The question then is, how can d evelopmental constraints on adult variation be reconciled with the fac t of developmental variation?