Within-niche pace of life acceleration as a fundamental evolutionary principle: a mammal pilot test case

Abstract

Background: In the competition for resources, an evident mechanism by which a taxon or clade can prevail is by out-reproducing competitors, or, in other words, by a faster life history. Hence, for organisms that share a certain niche space, there should be strong directional evolution of life history characteristics towards faster reproduction. One reason why this mechanism is rarely considered may be the conceptualization of life history strategies as a set of trade-offs subjected to fixed overall physical laws, rather than as a set of morphophysiological adaptations that may evolve towards a higher efficiency.
Hypothesis: Among extant taxa, directional evolution towards a faster reproduction should be reflected in a higher diversity in those clades of a niche that have a faster pace of life. Assuming phylogenetic inertia in the pace of life, fossil representatives of clades whose extant representatives are characterized by a slower pace of life should have been replaced in their niche by representatives of clades whose extant representatives have a faster pace of life.
Data description: We use life history data from extant eutherian mammals from the PanTheria database, and examples from the mammalian fossil record, focusing particularly on large herbivores.
Pilot results: We showcase examples that indicate differences in offspring production per unit time in eutherian mammals of similar niches; e.g., the sequence of gestation period length in which cattle, horses, dromedaries and okapis produce offspring of similar number, size and maturity (280, 340, 390 and 440 days, respectively) reflects the current species diversity and past displacement sequences of bovids, equids, camelids and giraffids.
Conclusion: The demographic mechanism of the ‘survival of the fittest’ can be expected to have consequences on the evolution of properties determining demographic life history. Considering life history as clade-specific, and life history characteristics of extant species as a snapshot in evolutionary time, can prominently enhance interpretations of clade turnovers and species diversity.