Evolutionary response to fluctuating climate in the Quarternary
Sheldon's "Plus Ça Change" hypothesis predicts that there is a tendency towards evolutionary stasis in response to long-term climatic fluctuations (i.e. during the Quaternary) and gradualism during phases of relative climatic stability. Published Plio-Pleistocene climatic proxy data are examined and used to subdivide previously published and newly collected evolutionary series into "unstable" and "stable" climate subsets. Tests on these data are conducted using two similar fractal-based methods that determine evolutionary modes as deviations from a null hypothesis of random walk. Both methods are critically examined and are found to produce robust results in most cases. The results reveal a wide range of evolutionary modes during both the Pliocene and Quaternary with no overall tendency towards increasing stasis during the Quaternary. However, a slight tendency towards less stasis is also exhibited in temperate/polar forms during the Quaternary. Examination of three newly collected evolutionary series is conducted: (l) Morphometric trends in the ostracod Limnocythere inopinata from a high-resolution African Holocene sequence (25 - 37 years/sample) reveal short-term (10[sup]1 years) correlation between morphology and Sr/Ca-inferred salinity, interpreted to represent non-genetic physiological response. Evolutionary mode analysis also reveals relaxation of stasis where Sr/Ca ratios are more widely fluctuating, although the overall trend is one of stasis. (2) Attempts at confirming stasis in a Pliocene to recent bivalve, Glycymeris glycymeris are unsuccessful due to the low-resolution nature of the data. It is proposed that such studies should not be over-emphasised in analyses concerned with summarising evolutionary patterns in the fossil record unless sampling resolution is greatly improved. (3) Morphometric trends in the Miocene to recent foraminifera Globigerina bulloides exhibit stasis throughout with no significant change in evolutionary mode during the Quaternary. Previous work on latitudinal diversity gradients has revealed that there are no significant differences between speciation rates in tropical and temperate clades. Also, evidence from studies on recent floral and faunal changes has shown that both tropical and temperate communities respond in similar ways to a globally averaged change in temperature. It is proposed that these latitudinal similarities can be transposed on a temporal scale, representative of the changes between Pliocene to Pleistocene conditions: therefore no significant change would be expected at the onset of Northern Hemisphere glaciation. The higher degree of disturbance encountered in colder regions may, however, have resulted in conditions that led to more evolutionary change during the Quaternary. The mechanisms underlying the latter response can be invoked by using genetic algorithm modelling: where simulations are run with a large-scale shift in the environmental target, population means are seen to shift further from the optimum. It is expected that in "real" populations, this would lead to evolutionary change (or migration) in order to avoid extinction in unfavourable conditions. Further studies, utilising improved genetic algorithm modelling, tied to further compilation and testing of evolutionary series is expected to provide further insights into the mechanisms underlying evolutionary processes.