The morphogenetic cross road of development and evolution: remembering Pere Alberch (1954–1998)

Ever since the modern synthesis, evolution is more often than not seen through the lens of genes: allelic frequencies, changes in gene sequence and also relationships between specific genes and function.From the beginning, this view excluded what one could call 'structuralist' arguments, that were very common during the nineteenth century.From this perspective, form and embryonic development fuelled the arguments in search of principles that could relate organisms to each other.And, these arguments went a long way, creating evolutionary relationships that genes have finessed but not overturned.A relationship between development and evolution has never been disputed but the modern synthesis relegated it to background discussions.This is probably because, when you come to think of it, shape and form, patterns, are not linearly related to genes.The notion that embryos tell us something about evolution was first hinted at by Karl Ernst von Baer in his comparative studies.His laws about the order of appearance of characters (from more general to more specific) and the scaffolding nature of structures in the evolution of body plans had an impact in biological thought.Darwin, in his great book, used similarities and divergences of embryos to support his theory of descent with modification.These ideas were later turned into a famous mantra by Ernst Haeckel when he stated, for the most part wrongly, that during development animals revisited their evolutionary history: ontogeny recapitulates phylogeny.Much later in his monumental 'Ontogeny and phylogeny', Gould arbitrated many disputes on the topic in favour of von Baer and interpreted Darwin in this light, leaving ideas about recapitulation by the side.What all these views have in common is the observation that animals appear to be variations on specific structural themes.This is clearly seen in the relationship between fins, wings and limbs but it also applies to different organs that appear to progress from simple to complex, e.g. the heart, from the dual-chamber organ in fish to the complex machine with four chambers that is in mammals.The word 'constraint' hovers over these discussions.Is what we see that exists and has existed the only variation possible?Is the natural world we live in inevitable?The question is important to understand both animals and plants, but is more often than not discussed in animals.It has been said that from the perspective of animals, the great crucible was the Burgess shale and that the world as we know it came out from those ponds under the close supervision of selection.It is to that place where, beholding the panoply of forms around us, we can trace back their overall segmental morphology, with their openly bilaterally polarized body plans, as well as probably their different organs, tissues and appendices.However, the mechanisms that generate novelties and evolve those novelties remain an open question.Is there a slot machine of shapes operating in the background from which the environment selects what works?Or does the environment only see a small subset of forms because not all forms are possible?We have learnt that, from the perspective of genetics, there are no constraints, that given enough time myriads of possibilities can be, and are probably being exploited.However, is this possible from the point of view of multicellular organization?Or, are there large structural constraints that limit that