A new method for quantifying heterochrony in evolutionary lineages

The occupation of new environments by evolutionary lineages is frequently
associated with morphological changes. This co-variation of ecotype and
phenotype is expected due to the process of natural selection, whereby
environmental pressures lead to the proliferation of morphological
variants that are a better fit for the prevailing abiotic conditions. One
primary mechanism by which phenotypic variants are known to arise is
through changes in the timing or duration of organismal development
resulting in alterations to adult morphology, a process known as
heterochrony. While numerous studies have demonstrated heterochronic
trends in association with environmental gradients, few have done so
within a phylogenetic context. Understanding species interrelationships is
necessary to determine whether morphological change is due to
heterochronic processes; however, research is hampered by the lack of a
quantitative metric with which to assess the degree of heterochronic
traits expressed within and among species. Here I present a new metric for
quantifying heterochronic change, expressed as a heterochronic weighting,
and apply it to xiphosuran chelicerates within a phylogenetic context to
reveal concerted independent heterochronic trends. These trends correlate
with shifts in environmental occupation from marine to non-marine
habitats, resulting in a macroevolutionary ratchet. Critically, the
distribution of heterochronic weightings among species shows evidence of
being influenced by both historical, phylogenetic processes and external
ecological pressures. Heterochronic weighting proves to be an effective
method to quantify heterochronic trends within a phylogenetic framework
and is readily applicable to any group of organisms that have well-defined
morphological characteristics, ontogenetic information, and resolved
internal relationships.