If climate change during the Quaternary shaped the macroevolutionary dynamics of a taxon, we expect to see three features in its history: elevated speciation or extinction rates should date to this time, more northerly distributed clades should show greater discontinuities in these rates, and similar signatures of those effects should be evident in the phylogenetic and phylodemographic histories of multiple clades. In accordance with the role of glacial cycles, speciation rates increased in the Holarctic Enallagma damselflies during the Quaternary, with a 4.25x greater increase in a more northerly distributed clade as compared with a more southern clade. Finer-scale phylogenetic analyses of three radiating clades within the northern clade show similar, complex recent histories over the past 250,000 years to produce 17 Nearctic and four Palearctic extant species. All three are marked by nearly synchronous deep splits that date to approximately 250,000 years ago, resulting in speciation in two. This was soon followed by significant demographic expansions in at least two of the three clades. In two, these expansions seem to have preceded the radiations that have given rise to most of the current biodiversity. Each also produced species at the periphery of the clade's range. In spite of clear genetic support for reproductive isolation among almost all species, mtDNA signals of past asymmetric hybridization between species in different clades also suggest a role for the evolution of mate choice in generating reproductive isolation as species recolonized the landscape following deglaciation. These analyses suggest that recent climate fluctuations resulted in radiations driven by similar combinations of speciation processes acting in different lineages.

We used mitochondrial [cytochrome c oxidase subunit I (CO I), cytochrome b, and 16S] and nuclear [internal transcribed spacer (ITS) phylogenies of Skistodiaptomus copepods to test hypotheses of Pleistocene divergence and speciation within the genus. Mitochondrial (mt)DNA sequence divergences do not support hypotheses for Pleistocene speciation and instead suggest much more ancient speciation events in the genus. Skistodiaptomus oregonensis and Skistodiaptomus pygmaeus (i.e. two morphologically similar and parapatric species) exhibited uncorrected mtDNA sequence divergences exceeding 20%. Similarly, we identified three divergent clades of Skistodiaptomus pallidus that exhibited mtDNA sequence divergences exceeding 15%, suggesting that even intraspecific divergence within this morphospecies predates the Pleistocene. We found clear evidence of CO I pseudogenes in S. pygmaeus, but their presence did not lead to significant overestimates of sequence divergences for this gene. Substitution saturation and strong purifying selection have most likely led to underestimates of sequence divergences and divergence times among Skistodiaptomus. The widespread phenomenon of morphological stasis among genetically divergent copepod groups indicates that speciation often occurs with little or no morphological change. Instead, morphological evolution may occur idiosyncratically after speciation and create discordant patterns of morphological similarity, shared ancestry and divergence time. Cryptic species complexes are therefore common in copepods, and morphological species concepts underestimate their true species diversity.

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Hybridization may stimulate the evolution of invasiveness in human-impacted habitats if unique hybrid genotypes have higher fitness than parental genotypes. Human efforts to control invasive taxa frequently involve the intentional alteration of habitats, but few studies have considered whether hybridization can result in decreased sensitivity to control measures. Here, we investigate whether interspecific hybrids between introduced Eurasian watermilfoil (Myriophyllum spicatum) and native northern watermilfoil (M. sibiricum) are more invasive than parental Eurasian watermilfoil, especially in regard to their relative responses to an herbicide commonly applied for their control (2,4-dichlorophenoxyacetic acid; 2,4-D). In two separate laboratory experiments, hybrids on average grew faster and were less sensitive to 2,4-D compared with parental Eurasian watermilfoil. These two invasive traits appear to be common in hybrid watermilfoils, as opposed to being restricted to a few unique lineages, because they were found in a diversity of hybrid genotypes from several independent hybridization events. In addition, we found that hybrids occurred more frequently than parental species in natural lakes previously treated with 2,4-D. Our results provide compelling empirical evidence that hybridization is associated with the evolution of increased invasiveness in watermilfoils, and have important implications for their management.