An Introduction to the Invertebrates, Second Edition - tiera.ru

280 INVERTEBRATE EVOLUTIONARY HISTORY20.9 Can we now define homology?Homology, provisionally defined as similarity in morphology reflectinga common evolutionary origin, is said to be the central conceptof comparative biology. Can we give it more than a superficialdefinition? The difficulty indicated at the beginning of Chapter 2can now be examined.Historically, homology is a pre-Darwinian concept. The processof classification (ordering) of organisms acquired new significanceafter Darwin wrote, ‘On my theory, unity of type is explained byunity of descent’. The resemblances on which classification is basedwere seen to be due to descent with modification from a commonancestor. The importance and difficulty of distinguishing betweenhomology and convergence soon became recognised as paramount.Homology is phylogenetic continuity, recognised where particularstructures have been established and maintained by evolution.We must ask, continuity of what? Of structure alone? Developmentalprocess? Gene action?Structural homology, the sole basis of the original definition,already demands discrimination between different levels of organisation.For example, the wings of birds and bats can be calledconvergent because they are very differently constructed and, consideredat the level of wings, have no common ancestor. Equallyreasonably they can be called homologous when considered at thelevel of the whole animal, because they have evolved from thesame pattern of vertebrate forelimb, and that can be traced to acommon ancestor. A major problem with structural homology is thatif it is used to construct a phylogeny and that phylogeny is thenused to deduce homology, the circularity does need to be noticed!A phylogeny needs to be constructed first, on widely based evidence,before homology is postulated.Development and evolution are now recognised as closely linked:identity of developmental pathways might therefore appear to bean important criterion of homology. However, different developmentalprocesses can produce the same adult form by differentroutes: examples include methods of gastrulation, and the differentdevelopmental pathways of regeneration and embryonic developmentin the hydroid Tubularia (see Chapter 19). There are manysuch examples, and conversely the same developmental pathwayscan produce different adult forms. Nor can embryonic developmentalorigin provide a conserved adult morphology, since that must dependin part on later stabilising interactions between cells. There seemsto be no basis in developmental mechanisms for a general guide tohomology.The molecular level is unhelpful because genetic identity doesnot coincide with morphological identity; gene behaviour is morecomplicated than that. A striking example of how genes work isgiven by the pax-6 gene, which is expressed at the earliest stage of