Gerald Edelman:
TOPOBIOLOGY (Basic, 1988)

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The title refers to location-dependent development of body cells: how can a cell know where in the body it is supposed to grow in order to generate the shape and function of the animal? Edelman's molecular embryology claims that development is based on topobiological events (division, movement, death and so forth of cells, which are regulated by cell-adhesion and substrate-adhesion molecules on the surface of the cell). A cell's competence is due essentially to its location.
Animate systems exhibit three properties that allow them to exist: heredity, variation in their hereditary material, competition as the environment changes. Animate systems are self-replicating systems, whose genetic code undergoes mutation and whose variant individuals undergo natural selection.
Characteristic of animate systems is development, in particular morphogenesis, the emergence of form during embryonic development. Roughly the same cell types appear in different parts of the body. The difference in position and shape results from the interaction of a number of driving forces (namely cell division, cell motion and cell death), which determine the number of cells in a particular region, and regulatory processes (namely cell adhesion and cell differentiation), which determine the interaction among cells.
Evolution can be viewed as a process of phenotypic transformation resulting largely from genetically mediated change in developmental dynamics that is itself altered throughout phylogeny.
Edelman than analyzes in detail what he considers the molecular mechanisms of epigenesis.
Development is under genetic control, but developmental events are nonetheless epigenetic and topobiologically controlled. Pattern, and not mere cell differentiation, is the evolutionary basis of morphogenesis. The cell surface, not its core, plays the fundamental role in this process, because it mediates signals from other cells and links with other surfaces to form tissues. A sequence of interactions between certain special types of genes via epigenetic signal paths provides the basis of pattern by controlling temporal sequences of mitosis, movement, death and further signaling.
In order to explain how this process can be reconciled with extensive changes in animal form in relatively short evolutionary time periods, Edelman points to the nonlinear relation between genetics, development and evolution.

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