14.9 Ontogeny (Development)

211

simultaneously, and if those that do so first can then prevent their neighbours from

doing so, segregation is assured. A great variety of specific molecular mechanisms

is available for the realization of such processes.

14.9.3

The Epigenetic Landscape

Waddington introduced the term “epigenetics” as the name for the study of “the

causal interactions between genes and their products, which bring the phenotype

into being”, ⁶⁴ and it is particularly associated with the ontogenic level of pheno-

type; that is, possibly stable and preferably heritable changes in gene expression and

phenotype not requiring changes in the sequence of the four fundamental bases of

DNA (Fig. 15.3) in the genome. Waddington is also credited with introducing the

vivid imagery of the “epigenetic landscape” (Fig. 14.5); this represents the process of

successive decision-making during cellular development, most decisions implying

different ultimate outcomes of cell differentiation. ⁶⁵ More recently, attempts have

been made to describe the epigenetic landscape in more mathematical terms. ⁶⁶

Development of course continues throughout the lifetime of the organism, influ-

enced by its environment and its response to that environment. It is enabled through

the mechanisms of DNA methylation (Sect. 14.8.4) and chromatin structural modu-

lation; to some degree at least the former can be inherited, ⁶⁷ engendering the notion

of “soft inheritance”. This has in turn led to a revival of Lamarck’s ideas about the

inheritance of acquired characters. ⁶⁸

14.9.4

rr and upper KK Selection

In an ecological void (i.e., a new environment empty of life), at least of the types we

are considering, or a highly fluctuating environment, growth is limited only by the

coefficientrr in Eq. (12.5) (rr-selection). This circumstance favours progenesis—rapid

proliferation at the cost of sophistication; and slight acceleration of development (cf.

Table 14.6) leads to a disproportionately greater increase in fecundity.

In an older, more complex ecosystem (with a high density of organisms and

intense competition for resources), or a very stable environment, growth is limited

by its carrying capacity—the coefficientupper KK in Eq. (12.5) (upper KK-selection). This circum-

stance favours neoteny. Development is stretched out to enable the development of

more sophisticated forms. There is no pressure to be fecund; the young offspring have

64 Goldberg et al. (2007).

65 See Gilbert (1991) for a critique and Buss and Blackstone (1991) for an experimental exploration.

66 Bhattacharya et al. (2011).

67 Richards (2006).

68 Burckhardt (2013), Skinner (2015), Penny (2015).