Unraveling Reality {25}

Convergent Evolution

Evolution is not mystical. Instead, its tendency toward certain traits makes its outcomes frequently foreseeable.

“Is evolution predictable? To a surprising extent the answer is yes.” ~ Canadian evolutionary biologist Peter Andolfatto

The predictability of evolution is amply illustrated by convergent evolution (aka parallel evolution), whereupon vastly different organisms come to the same solution. Flight, vision, singing, parasitism, toxins, and camouflage are but a few of countless examples of convergent evolution.

Plants present a litany of parallel evolution. To begin, there were at least 5 independent evolutions of single-celled photosynthetic organisms into multicellular plant forms by 600 million years ago (MYA). Green algae alone gave birth to the land-based life commonly called flora, which became a prodigious potpourri.

Plants independently developed precise nanostructures to harvest light and channel it to the molecules which perform photosynthesis. Engineering at the nanometer scale enabled plants to translate light energy into chemical energy with an incredible 90% efficiency.

Despite striking differences in climate, soils, and evolutionary lineages, plants share common needs and means of achieving them. If one were to design an organism to optimize light collection, a branched structure with flat-bladed leaves might seem obvious. Nature created a wild variety that all aim at the same end, constrained in assortment only by biomechanics. An astonishing diversity of appearance belies functional convergence.

Phloem is the conductive tissue in green land plants which transports sugars from sunlit tissue to those tissues that live in perpetual shade; an essential distribution of nutrients. Kelp is a large seaweed that forms marine forests. It has cells like phloem.

As all plants are photosynthesizers, they have the same need to capture light and internally exchange water and nutrients. Relationships in body-plan geometry are crucial, and thereby constrained in terms of practical possibilities.

Over 350 MYA, competition for access to sunlight drove plants in divergent families to reach for the sky. Vast forests of towering trees emerged during the Carboniferous period (359–299 MYA). Independent innovations for height continued since then. Many tall plants employ wood, but others, such as palms and bamboo (an ambitious grass), do not.

Acquiring carbon dioxide for photosynthesis through controllable pores (stomata) causes water loss by transpiration. Although plants evolved various mechanisms to alleviate this problem, one well-established solution is C4: a technique that independently evolved at least 45 times, starting 32 MYA, and most recently 4 MYA.