  Non-uniform rates of morphological evolution and evolutionary increases in
organismal complexity, captured in metaphors like "adaptive zones", "punctuated
equilibrium" and "blunderbuss patterns", require more elaborate explanations
than a simple gradual accumulation of mutations. Here we argue that non-uniform
evolutionary increases in phenotypic complexity can be caused by a
threshold-like response to growing ecological pressures resulting from
evolutionary diversification at a given level of complexity. Acquisition of a
new phenotypic feature allows an evolving species to escape this pressure but
can typically be expected to carry significant physiological costs. Therefore,
the ecological pressure should exceed a certain level to make such an
acquisition evolutionarily successful. We present a detailed quantitative
description of this process using a microevolutionary competition model as an
example. The model exhibits sequential increases in phenotypic complexity
driven by diversification at existing levels of complexity and the resulting
increase in competitive pressure, which can push an evolving species over the
barrier of physiological costs of new phenotypic features.
