Ice Age Aridity Caused Pitcher Plant Speciation

North American pitcher plants belong to the sarracenia genus.  A study of sarracenia genetics determined the 11 species in this genus diverged about 4 million years ago.  This is when Ice Ages began occurring.  Pitcher plants grow in wetlands and moist meadows where the ground is saturated.  These acidic soils contain low levels of nutrients, so pitcher plants obtain additional nutrition by trapping insects in their modified leaves.  Other plants outcompete them on drier richer soils.  Glacial expansion caused environmental change in southeastern North America.  Dry oak scrub and prairie replaced wetlands and moist meadows and became the prevailing type of environment in this region.  Ice Age pitcher plants grew in relic marshes that managed to remain wet because of local conditions.  Pitcher plant populations became isolated from each other for long enough to evolve into different species.

Sarracenia flava var. ornata

 A yellow pitcher plant (Sarracenia flava variation ornate).  The hood keeps the pitcher from overfilling with rainwater.

The Okefenokee variety of the hooded pitcher plant.  There are 2 varieties of hooded pitcher plant.  The Okefenokee variety is tall and catches flying insects.  The other variety is short and more likely to trap ants.

I studied the range maps of all 11 species of pitcher plants. It appears as if there were 3 main refuges for pitcher plants during Ice Ages (assuming modern ranges reflect the locations of Ice Age refuges).  Hooded pitcher plants (Sarracenia minor) survived in some relic habitats on the Atlantic coastal plain.  Purple pitcher plants (S. purpurea) survived in relic Atlantic coastal plain and Gulf Coast habitats.  The pale pitcher plant (S. alata) and the white-topped pitcher plant (S. leucophylla) survived on relic habitat near the Gulf Coast.  The parrot pitcher plant (S. psitticina) and the yellow pitcher plant (S. flava) survived in the Atlantic coastal plain relic habitat and spread west following the end of the Ice Age or they survived in Gulf Coast relic habitat and spread east following the end of the Ice Age.  I can’t determine which scenario it was by just looking at range maps.  The green pitcher plant (S. oreophila) and the Alabama pitcher plant (S. alabamensis) survived in relic bogs located in the southern Appalachians.

Sarracenia minor range.png

Range map for hooded pitcher plant.  During the Ice Age they must have survived on relic habitat located on the Atlantic coastal plain.

Sarracenia alata range.png

Pale pitcher plant range map.  They became reproductively isolated from hooded pitcher plants when Ice Age aridity dried up wetlands between ancestral populations.

Sarracenia leucophylla range.png

Range map for white-topped pitcher plant–another species that survived near the Gulf Coast during Ice Ages.

Range map for Sarracenia purpurea, the only species of pitcher plant that colonized formerly glaciated regions.  There are several subspecies.

Pitcher plants originated in South America at least 50 million years ago and eventually split into 3 genera.  There are 18 species of pitcher plants in the South American heliophora genus.  Pitcher plants invaded North America from South America by island hopping across the Caribbean.  During the Eocene there was a chain of islands between the 2 continents.  Pitcher plant seeds float, allowing them to spread via water.  Eventually, these islands were inundated, and the foundling North American population became isolated.  Pitcher plants expanded their range across North America in the tropical wet climate of the Oligocene.  North American pitcher plants diverged into 2 genera by the Miocene, about 25 million years ago.  One species in the darlingtonia genus occurs in northern California and southern Oregon where it grows on serpentine seeps and in interdunal wetlands.  The other genera–sarracenia–thrives in the southeast with the exception of S. purpurea which ranges into Canada, the northeast, and the southeast.

Pitcher plants are an evolutionary marvel.  They’ve evolved a nectar that attracts insects, and a slippery leaf pit that traps the luckless arthropods.  Bacterial action in rainwater collected in the pit helps digest the protein.  All these plants needed to evolve these adaptations was time and chance.

Reference:

Ellison, A.M.; et. al.

“Phylogeny and Biogeography of the Carnivorous Plant Family Sarracenia”

Plos One 7 (1) 2012

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