Genetic assimilation in the fossil record: phenotypic plasticity and later accommodation in Cambrian arthropods

• Genetic assimilation is a hypothesised process in which an initially plastic developmental phenotypic response of an organism to the environment is fixed genetically, i.e. assimilated into its genome 1,2. One central prediction of genetic assimilation is that “phenotypic accommodation can precede, rather than follow, genetic change, in adaptive evolution”3. Here we test this prediction in the fossil record. Agnostus pisiformis, a Cambrian Series 3 trilobite-like arthropod, has been shown to exhibit subtly different patterns of pygidial morphological variation across coeval assemblages4, varying chiefly in the degree to which the axial lobe dominates the pygidium. We demonstrate that this morphological variation as well as that of the slightly younger closely related Homagnostus obesus is significantly correlated with geochemical indicators of dysoxia/euxinia and thus stressed environments5-7. In addition, the variances of high and low-stress assemblages also differ significantly, suggesting that the morphological variability of the different assemblages is induced by environmental stress and can be understood as a reaction norm8. We include in our analysis the younger relative Trilobagnostus holmi and interpret its morphology, which has a strongly reduced variance, as representing a more canalized9 or stabilized1 stage of the assimilation process. Thus our data contain all stages of adaptation via phenotypic plasticity and genetic assimilation10 and support the main predictions of the ‘Extended Evolutionary Synthesis’3.

Ämnesord

NATURVETENSKAP  -- Biologi -- Evolutionsbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Evolutionary Biology (hsv//eng)

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