Evolution without standing genetic variation: change in transgenerational plastic response under persistent predation pressure.

Transgenerational phenotypic plasticity is a fast non-genetic response to environmental modifications that can buffer the effects of environmental stresses on populations. However, little is known about the evolution of plasticity in the absence of standing genetic variation although several non-genetic inheritance mechanisms have now been identified. Here we monitored the pea aphid transgenerational phenotypic response to ladybird predators (production of winged offspring) during 27 generations of experimental evolution in the absence of initial genetic variation (clonal multiplication starting from a single individual). We found that the frequency of winged aphids first increased rapidly in response to predators and then remained stable over 25 generations, implying a stable phenotypic reconstruction at each generation. We also found that the high frequency of winged aphids persisted for one generation after removing predators. Winged aphid frequency then entered a refractory phase during which it dropped below the level of control lines for at least two generations before returning to it. Interestingly, the persistence of the winged phenotype decreased and the refractory phase lasted longer with the increasing number of generations of exposure to predators. Finally, we found that aphids continuously exposed to predators for 22 generations evolved a significantly weaker plastic response than aphids never exposed to predators, which, in turn, increased their fitness in presence of predators. Our findings therefore showcased an example of experimental evolution of plasticity in the absence of initial genetic variation and highlight the importance of integrating several components of non-genetic inheritance to detect evolutionary responses to environmental changes.

Branching patterns in phylogenies cannot distinguish diversity-dependent diversification from time-dependent diversification.

One of the primary goals of macroevolutionary biology has been to explain general trends in long-term diversity patterns, including whether such patterns correspond to an upscaling of processes occurring at lower scales. Reconstructed phylogenies often show decelerated lineage accumulation over time. This pattern has often been interpreted as the result of diversity-dependent (DD) diversification, where the accumulation of species causes diversification to decrease through niche filling. However, other processes can also produce such a slowdown, including time dependence without diversity dependence. To test whether phylogenetic branching patterns can be used to distinguish these two mechanisms, we formulated a time-dependent, but diversity-independent model that matches the expected diversity through time of a DD model. We simulated phylogenies under each model and studied how well likelihood methods could recover the true diversification mode. Standard model selection criteria always recovered diversity dependence, even when it was not present. We correct for this bias by using a bootstrap method and find that neither model is decisively supported. This implies that the branching pattern of reconstructed trees contains insufficient information to detect the presence or absence of diversity dependence. We advocate that tests encompassing additional data, for example, traits or range distributions, are needed to evaluate how diversity drives macroevolutionary trends.