Increased diversification rates are coupled with higher rates of climate space exploration in Australian Acacia (Caesalpinioideae)

Abstract

Summary Australia is an excellent setting to explore relationships between climate change and diversification dynamics. Aridification since the Eocene has resulted in spectacular radiations within one or more Australian biomes. Acacia is the largest plant genus on the Australian continent, with around 1000 species, and is present in all biomes. We investigated the macroevolutionary dynamics of Acacia within climate space. We analysed phylogenetic and climatic data for 503 Acacia species to estimate a time‐calibrated phylogeny and central climatic tendencies for BioClim layers from 132 000 herbarium specimens. Diversification rate heterogeneity and rates of climate space exploration were tested. We inferred two diversification rate increases, both associated with significantly higher rates of climate space exploration. Observed spikes in climate disparity within the Pleistocene correspond with onset of Pleistocene glacial–interglacial cycling. Positive time dependency in environmental disparity applies in the basal grade of Acacia, though climate space exploration rates were lower. Incongruence between rates of climate space exploration and disparity suggests different Acacia lineages have experienced different macroevolutionary processes. The second diversification rate increase is associated with a south‐east Australian mesic lineage, suggesting adaptations to progressively aridifying environments and ability to transition into mesic environments contributed to Acacia's dominance across Australia.

Adapting to extremes: reconstructing evolution in response to changing climate over time and space in the diverse Australian plant genus Acacia

“…This interpretation indicates evolutionary flexibility in Acacia, which is supported by the high rate of niche evolution along both aridity and salinity axes in the three recent radiations. This enhanced rate of niche evolution was also found by a recent study of diversification in Acacia using traditional PCMs, which hypothesized that Pleistocene glacial-interglacial climate cycling (< 2.5Ma) drove rapid climatic adaptation, which promoted diversification and led to large radiations (Renner et al 2020).…”

“…Phylogenetic analyses such as these are useful, but have a number of limitations. To permit the application of standard stochastic models of trait evolution, such as Brownian Motion (BM) or Ornstein-Uhlenbeck (OU), in most macroevolutionary studies, the environmental niche is simplified to a point-estimate for a species, for example by taking the mean value across a species distribution (Evans et al 2009;Kozak & Wiens 2010;Münkemüller et al 2015;Renner et al 2020). But this is problematic for two main reasons.…”

Phyllodes and bipinnate leaves of

“…Our methods followed those of Renner et al (2020). We used the dataset published by Mishler et al (2014), comprising two nuclear (nrITS and ETS) and four chloroplast markers (psbA-trnH, trnL-trnF, rpl32-trnL, matK) from 510 species, for phylogenetic reconstruction (see supplementary table 1 in Mishler et al (2014) at https://doi.org/10.1038/ncomms5473, for voucher specimen details and GenBank accession numbers).…”

“…Owing to the geographic location of the research groups working on this phylogenetic dataset for Acacia, the sampling of species for this dataset was skewed towards the eastern coast of Australia. This means that some groups of Acacia are over-represented in the data, for example, section Botrycephalae for which 82% of its constituent species were sampled, in contrast to an overall sampling rate of 47% for all species of Acacia (Renner et al 2020). However, this is not as straightforward to assess as these numbers indicate, because section Botrycephalae is not monophyletic, similar to other sections of Acacia, so it is difficult to translate a geographic bias into an estimation of phylogenetic bias, especially as the relationships of unsampled species can be difficult to estimate on the basis of morphological data.…”

“…Acacia can be subdivided into two or three macroevolutionary cohorts with different diversification rates (Nge et al 2020;Renner et al 2020), one of which comprises a lineage occurring in mesic south-eastern Australia (Renner et al 2020). This south-eastern mesic lineage has a higher mean diversification rate than the other extant Acacia lineages, contains both phyllodinous and bipinnate species, and may provide additional insight into the contribution that foliage type has made to the radiation of Acacia.…”