Wednesday, July 16, 2014

G. L. Stebbins and the process of adaptive modification

The convergence of modern phylogenetics, genetics and evolutionary development (EVO-DEVO) has allowed us to make better and better hypotheses about the evolutionary history of plants, for which the fossil record still leaves many gaps.  There is another dimension to this fascinating pursuit, an older one that isn't talked about as much: the analysis of adaptive or evolutionary trends.  This was the core logic of evolutionary biology through much of the twentieth century, before cladistic analysis came to dominate.  The publication of "Flowering Plants: Evolution above the Species Level" by G. L. Stebbins in 1974 represented the finest of this approach.  Far from being obsolete, the thought processes of  Stebbins and other 20th century evolutionary biologists are still applicable to the various, sometimes controversial theories that are appearing currently.  In my opinion, Stebbins' book should be required reading for all graduate students of evolution.

Adaptive modification along the lines of least resistance

  One of the general principles of evolutionary biology is that evolutionary change, under the force of natural selection, will tend to proceed along the lines of least resistance - i.e. the simplest route. For example, a leafless cactus adapts to epiphytic life in the rain forest, not by reconstructing the leaves abandoned by its ancestors, but by flattening its stem segments into leaf-like units (e.g. genus Schlumbergia - the Christmas cactus).  This is one aspect of the bigger picture of evolutionary canalization, which essentially states that the possible adaptations of a plant species or individual organs are limited by what they already are.  A coconut has little prospect of evolving into an orchid-like capsule with millions of tiny, wind-pollinated seeds, just as an elephant has little prospect of evolving wings (or flying with its ears!).

Let me expand upon one of Stebbins' most lucid examples.  Suppose there is selective pressure for an increase in seed production.  This could occur for a variety of reasons: improved growing conditions, adaptation to a sunnier environment where smaller seeds can be created in greater numbers, and/or an increase in the numbers of seed-eating animals present.  All could all favor a species that increases its output.  How a species would respond to such a challenge would depend on its starting equipment.

A legume is a modified follicle containing a variable
number of seeds. Ovules are created in a
meristematic zone in the young carpel
 that can be increased or decreased. Photo by
by Renee Comet - NCI, posted on Wikipedia.
A species that has a fixed number of carpels in its flowers, but a variable number of ovules in each carpel, might respond by producing more ovules within each carpel.  The capsules of lilies, irises, and orchids, along with simpler follicles of columbines, delphiniums, etc. produce ovules in sequence from meristematic (embryonic) tissues within the carpel.  Simply continuing that activity for a longer period of time would result in more ovules.  To some extent, this flexibility is important on a day-to-day basis.  When conditions are poor, fewer ovules may be produced, and when better, more may be produced.  Extended onto an evolutionary scale, adaptations for more and more seed production can result.  Orchids represent one culmination of this trend, producing millions of tiny airborne seeds in each capsule. By the same token, reduction to a single seed is also possible from an ancestor in which the number is variable.

In the flower of a strawberry, the numerous carpels are
produced by the apical meristem in the embryonic bud.
Each will become a seed-like achene on the outside of
the swollen, Fruit-like receptacle (central stalk). Photo by
Papas2010, posted on Wikipedia.

In species that have a fixed number of ovules in each carpel, but a variable number of carpels in each flower, the number of carpels can be increased.  This is what happens in something like a strawberry (see "Why are the seeds of a strawberry on the outside?").   The tiny fruits of the strawberry (the seed-like structures on the outside of the swollen receptacle) are adapted as achenes. The number of seeds in each tiny carpel is rigidly fixed at one.  It would take an extraordinary amount of genetic and developmental reorganization to increase the number of seeds within each achene, which in the process would have to adapt to a different dispersal strategy.  The meristem in the center of the flower, which produces the carpels sequentially, however, can continue to operate a little longer and easily produce many more single-seeded carpels.
the number of carpels might be increased.

In a third "starting point," we have sunflowers and their relatives (family Asteraceae), in which flowers are highly canalized.  The actual flowers are tiny and crowded onto a dense head.  Each contains a single seed in a highly specialized ovary. It is inconceivable that a pressure for higher seed output would result in more seeds or more carpels being produced in each flower.  It is vastly simpler to increase the number of flowers within the composite head.  This is what we see in the massive cultivated sunflowers, which have evolved under human selection, compared with their wild relatives.
The massive sunflower is actually a composite head of several hundred tiny flowers that each bear a single seed.  The
size of the head depends on how long the central apical meristem actively produces new flowers (oldest flowers
around the outside are open, newer ones toward the inside are still in bud).  Photo by Amada44, posted in Wikipedia.

The imperative of evolutionary canalization, and modification along the lines of least resistance can be stronger than the imperative of parsimony in phylogenetic analysis.  It is the basis for my conclusion that the single-seeded drupes of Amborella represented a specialization from a more flexible common ancestor (see "What's so primitive about Amborella.")  I will also invoke these principles in some upcoming posts.

References cited

Stebbins, G. L. 1974.  Flowering Plants: Evolution above the Species Level.  Belknap Press of Harvard University.

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