The Elements of Evolution (44-1) Plants – Convergent Evolution


Despite striking differences in climate, soils, and evolutionary history among diverse biomes ranging from tropical and temperate forests to alpine tundra and desert, there are similar interspecific relationships among leaf structure and function and plant growth in all biomes. This demonstrates convergent evolution and global generality in plant functioning, despite the enormous diversity of plant species and biomes. ~ American plant ecologist Peter Reich et al

Plants present a litany of convergent evolution. To begin, there were at least 5 independent evolutions of single-celled photosynthetic organisms into multicellular plant forms by 600 MYA. The best known are brown, red, and green algae. Green algae alone gave birth to the land-based life commonly called flora.

Despite substantial variations in the morphology of the lineages of plant forms, they share common functionalities, and means of achieving them. Kelp, which are members of the brown algae group, possess trumpet cells which are like phloem, the conductive tissue found in green plants that transports sugars from sunlit tissue to those below which live in perpetual shade.

Selfsame problems gave rise to similar solutions between evolutionarily remote plant groups. The non-vascular structure of some brown algae is strikingly like the arrangement of leaf-stem-root in vascular plants.

Algae live liquidly, where the needs of life are readily available in the immediate surroundings. The move to terra firma required extensive modification in body plan, and strategies to cope in a completely different habitat. Needed were novel methods of CO2 and nutrient acquisition, dealing with desiccation, body support, and reproduction. These were first met with economy, and later succeeded with embellishment.

As all plants are photosynthesizers, they share the same requirements to capture light and internally exchange water and nutrients. Relationships in body plan geometry, such as surface areas and volumes, are crucial, and thereby constrained in terms of practical possibilities.

A critical feature in plant physiology is the presence of a cell wall. Beyond securing internal contents from environmental exposure, cell walls provide rigidity and strength to hold shape and protect against mechanical stress. Plant cell walls, along with characteristics dependent upon cell cycle and growth conditions, including rigidity-control mechanics, evolved independently many times.

If one were to design an organism to optimize light collection, a branched structure with flat-bladed leaves might seem obvious. There are constraints in the anatomical and biomechanical features of leaves and petioles (leaf-supporting stalks). These again exhibit convergent evolution in various plant lineages.

Another driving factor arose from competition for access to sunlight. The ultimate achievement would be a perennial lifestyle: literally being able to perpetually stand one’s ground, and growth potentiality to lift one’s leaves above others. Thus, in various forms and means, evolved trees. Many employ woods, 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. This technique separately evolved at least 45 times, starting 32 MYA, and most recently 4 MYA.

The familiar fixation of nitrogen via symbiotic microbes independently evolved in over a dozen families of flowers. Their accommodation by plants varies, indicating loose constraints to a decided functional need.

Convergence is also shown with parasitic and carnivorous plants. There are at least 4,000 species of parasitic plants distributed among 19 different flowering families, with 6 different lifestyles which establish the type of parasitism. All parasites use a haustorium to penetrate host tissue and connect their vascular system to that of the host.

Some 600 species of carnivorous plants are known, in 6 angiosperm families, including both monocots and dicots. Typically living in bright, low-nutrient, water-logged environs, many produce adhesive areas on stems and leaves to lure, capture and digest insects. Sundews are exemplary. With at least 194 species, they are the most specious genus of parasitic plant. Their flypaper lifestyle independently arose 5 times. Similarly, the pitfall traps of pitcher plants convergently evolved in 4 families (3 dicot and 1 monocot).

Over 1,064 species in 7 genera of euphorbias are succulents that store large volumes of water in their short, broad stems, which are green and photosynthesize. Desert conditions led to the same adaptations employed by the unrelated cactus family, with over 2,000 species in ~175 genera. Similarly, conifers in the forests independently converged on the same genetic changes to better withstand cold weather.

Further demonstration of plant convergence is that vegetation in selfsame biomes tends to be similar. The Mediterranean climate – mild, wet winters and hot, dry summers – occurs in California where chaparral plants live, South Africa, central Chile, and southern Australia, as well as the Mediterranean. In all these areas, native vegetation is a dense scrub, dominated by woody evergreen sclerophyllous shrubs. Sclerophyll is a type of vegetation that has hard leaves oriented parallel or oblique to direct sunlight, with short internodes (the distance between leaves along the stem).

Likewise, Alpine plants typically have small, hairy leaves and prostate stems; a response to often perpetual brisk winds and soil mineral deficiency.

Genera with very different flower shapes are often very closely related, and genera with highly similar flowers share such similarity via convergent evolution. Floral traits are more prone to rapid evolutionary changes in response to local ecological conditions, whereas vegetative and fruiting traits are more conserved and not readily shaped by local conditions. ~ Brazilian botanist Domingos Cardoso