Fig. 3

A summary of correct (left) and incorrect (right) conceptions of heredity as it pertains to adaptive evolutionary change. The panels on the left display the operation of “hard inheritance”, whereas those on the right illustrate naïve mechanisms of “soft inheritance”. In all diagrams, a set of nine squares represents an individual multicellular organism and each square represents a type of cell of which the organisms are constructed. In the left panels, the organisms include two kinds of cells: those that produce gametes (the germline, black) and those that make up the rest of the body (the somatic line, white). In the top left panel, all cells in a parent organism initially contain a gene that specifies white coloration marked W (A). A random mutation occurs in the germline, changing the gene from one that specifies white to one that specifies gray marked G (B). This mutant gene is passed to the egg (C), which then develops into an offspring exhibiting gray coloration (D). The mutation in this case occurred in the parent (specifically, in the germline) but its effects did not become apparent until the next generation. In the bottom left panel, a parent once again begins with white coloration and the white gene in all of its cells (H). During its lifetime, the parent comes to acquire a gray coloration due to exposure to particular environmental conditions (I). However, because this does not involve any change to the genes in the germline, the original white gene is passed into the egg (J), and the offspring exhibits none of the gray coloration that was acquired by its parent (K). In the top right panel, the distinction between germline and somatic line is not understood. In this case, a parent that initially exhibits white coloration (P) changes during its lifetime to become gray (Q). Under incorrect views of soft inheritance, this altered coloration is passed on to the egg (R), and the offspring is born with the gray color acquired by its parent (S). In the bottom right panel, a more sophisticated but still incorrect view of inheritance is shown. Here, traits are understood to be specified by genes, but no distinction is recognized between the germline and somatic line. In this situation, a parent begins with white coloration and white-specifying genes in all its cells (W). A mutation occurs in one type of body cells to change those cells to gray (X). A mixture of white and gray genes is passed on to the egg (Y), and the offspring develops white coloration in most cells but gray coloration in the cells where gray-inducing mutations arose in the parent (Z). Intuitive ideas regarding soft inheritance underlie many misconceptions of how adaptive evolution takes place (see Fig. 2)