Biology and Philosophy 13: 119–124, 1998. c 1998 Kluwer Academic Publishers. Printed in the Netherlands. Bridges between Development and Evolution EVA JABLONKA and MARION J. LAMB The Cohn Institute for the History and Philosophy of Science and Ideas Tel-Aviv University Tel-Aviv 69978 Israel Abstract. Adaptive evolution is usually assumed to be directed by selective processes, devel- opment by instructive processes; evolution involves random genetic changes, development involves induced epigenetic changes. However, these distinctions are no longer unequivocal. Selection of genetic changes is a normal part of development in some organisms, and through the epigenetic system external factors can induce selectable heritable variations. Incorporating the effects of instructive processes into evolutionary thinking alters ideas about the way envi- ronmental changes lead to evolutionary change, and about the interplay between genetic and epigenetic systems. Key words: acquired characters, epigenetic inheritance, induced variation, Lamarckism Evelyn Fox Keller and James Griesemer have done an admirable job in summarizing the principal ideas, arguments and goals of our book, Epigenetic Inheritance and Evolution, so we need not reiterate its main themes. However, their reviews give us a welcome opportunity to clarify some points, and to take up some of the issues they have raised. The common theme that unites many of their comments is the role of selective and instructive processes in evolution, and the part epigenetic inheritance plays in them. Evolution is usually presented as being directed by selective processes, whereas development is best described as being dependent on instructive processes. There is a fundamental difference in the logic underlying the two types of process. Instructive processes are directed responses to factors outside the responding unit. 1 Embryonic induction is a classical example of an instructive process, as is transcriptional regulation. With instructive processes, an external factor “instructs” a system that is already constructed to be able to respond to this type of factor in a precise, and often (but not necessarily) adaptive manner. When the responding system is very plastic, the external factor seems “to mould it to shape”; but even with extremely plastic systems, it is clear that the very pliability of the system is to some extent a pre-established evolved property.