Heterochrony and Evolutionary Processes

K- and r- selection





Historical Antecedants

Recognising Heterochrony

Modern examples: Sexual Dimorphism

Cambrian trilobites

Cope's Rule

K- and r- selection: Tertiary echinoids

Consequences for debates on adaptation, constraints and evolutionary dynamics


K- and r- selection are terms that describe the effect of selection on so-called "life-history". A K- selective regime is an environment where selection pressures favour long generation times, small numbers of off-spring and greater specialisation. It is associated with stable environments. r- selective regimes, on the other hand, are associated with unstable environments, creating a general selection pressure that favours a fast generational turnover, large numbers of off-spring and a low degree of niche specialisation. Gould (1977) proposed that one might expect K-selective regimes to be associated with peramorphic evolution, as delaying the onset of maturity and increasing developmental time has the effect of lowering generational turnover. Similarly, in a r- selective environment, one would expect to find paedomorphosis as precocious maturation directly affects generation times. Since peramorphs tend to be larger than their ancestors, the incidence of Cope's rule size increases in K-selective regimes may be neatly explained by heterochrony.

McKinney's (1986) examination of a Tertiary echinoid suite provides some compelling evidence in this regard.

From McKinney (1986): Relationships between water depth, stability, selection regime, organism size and heterochrony in a Tertiary echinoid suite.

In this environment, stability is a function of water depth, with shallow depths indicating unstable environments (high sedimentation rates, more temperature and current perturbations, et. al.). Heterochronic relations were examined along a lineage of echinoids affected by a number of transgressive and regressive phases. Heterochronic response to K and r selection had the consequence of increasing and decreasing, respectively, echinoid's overall size as well as other morphological differences. If selection was indeed targeted towards generational turnover, these "by-products" do not represent adaptations.