Shown separately are the phenotypes resulting when the presence of b and g are necessary or sufficient conditions for switching genes E and F. The results show that a variety of different phenotypes can be obtained by phenotypic changes affecting a single cycle and that sufficient causes tend to protect the network system against phenotypic changes associated with cycle breakage, while necessary causes have the reverse effect. A mechanism for the addition of a new cycle to a pre-existing network is shown in Figure 1 (d). Gene products a, b and c switch on genes D, E and F which are not part of a cycle. These switches could have arisen by genetic assimilation24 of adaptive responses associated with the production of d, e and f. In this situation selection might then favour genetic changes which ensure switching of genes D, E and F by interaction between d and E, e and F and f and D as indicated by dotted arrows in Figure 1(d). This would add a new cycle which could be broken independently of the cycle A-B-C-A if d, e and f become necessary for switching genes E, F and D. In this way a network of cycles as in Figure 1(c) could be built up by selection.
In multicellular animals greater possibilities exist for cycles than in single celled organisms, particularly in the female line. Figure 2 symbolises the association between maternal and offspring phenotype and germline. Three types of cycle are represented as dotted lines marked with arrows; letters are omitted. Cycle I is confined to the germ cell line representing the phenomena discussed in relation to Figure 1.