Literature Review: Hickory Evolution

by Carl Strang

The scientific papers that attract my attention are often ones that fill in pieces of life’s long history on Earth. What is happening today is plenty interesting, of course, but the present moment occupies only the tiniest fraction of billions of years of interesting stuff. One aspect of that story is the similarity of plants between Asia and North America. A study from earlier this year focused on hickories (trees in genus Carya), which occur in both places. The authors state their results well in their abstract, from which I’ll quote.

A shagbark hickory bud opening in spring

A shagbark hickory bud opening in spring

Zhang J-B, Li R-Q, Xiang X-G, Manchester SR, Lin L, et al. (2013) Integrated Fossil and Molecular Data Reveal the Biogeographic Diversification of the Eastern Asian-Eastern North American Disjunct Hickory Genus (Carya Nutt.). PLoS ONE 8(7): e70449. doi:10.1371/journal.pone.0070449

Abstract: “The hickory genus (Carya) contains ca. 17 species distributed in subtropical and tropical regions of eastern Asia and subtropical to temperate regions of eastern North America. Previously, the phylogenetic relationships between eastern Asian and eastern North American species of Carya were not fully confirmed even with an extensive sampling, biogeographic and diversification patterns had thus never been investigated in a phylogenetic context. We sampled 17 species of Carya and 15 species representing all other genera of the Juglandaceae as outgroups, with eight nuclear and plastid loci to reconstruct the phylogeny of Carya. … Our results support two major clades within Carya, corresponding to the lineages of eastern Asia and eastern North America. The split between the two disjunct clades is estimated to be 21.58 (95% HPD 11.07-35.51) Ma [millions of years ago, early Miocene]… [A]nalyses incorporating both extant and extinct genera of the Juglandaceae suggested that Carya originated in North America, and migrated to Eurasia during the early Tertiary via the North Atlantic land bridge. Fragmentation of the distribution caused by global cooling in the late Tertiary resulted in the current disjunction. The diversification rate of hickories in eastern North America appeared to be higher than that in eastern Asia, which is ascribed to greater ecological opportunities, key morphological innovations, and polyploidy.”

Juglandaceae is the plant family to which the hickories belong. It also includes the walnuts. The “eight nuclear and plastid loci” are genes in cell nuclei and chlorophyll-bearing organelles called plastids that the researchers analyzed. Mutations over the years accumulate at the same time species are splitting apart, allowing relationships to be discovered and time estimates for the process to be made.

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