Friday, October 23, 2015

What is an adaptation?

What do we mean by adaptation?  We can use that word  both as a process and as the observable result of that process.   Adaptation is the process of evolutionary change under the guidance of natural selection.  It is the process in which populations become genetically modified to function more efficiently in their specific environment, to respond to changes in the environment, or to move into new environments.  The result of that process is new or altered characteristics that we refer to as adaptations.

An important working assumption, or hypothesis, in biology is that every observable characteristic or trait of an organism has some adaptive significance, or at least had adaptive significance sometime during the ancestry of the organism.  A related assumption is that the total set of adaptations (and hence the total set of observable characteristics) is unique for each species, and defines a unique ecological niche.  That in turn means that each species "fits" into the biosphere in a different way from every other species. Discovering the adaptive meaning of everything from leaf shape to flower color is to me the most exciting part of botany, or biology in general.

Let's just take one example: the shape of cactus stems.  First, of course, cactus stems are succulent, i.e. filled with water-storage tissue.  They gather water during the brief and infrequent rain storms, store it, and utilize it sparingly during the long dry spells.  It allows cacti to continue to function, even to bloom at predictable times, rather than become dormant during those dry periods. That is the signature adaptation made by early members of the cactus family.

Cactus stems are also, in the absence of leaves, photosynthetic.  The two major functions of cactus stems requires some interesting compromises. They need to gather light, but exposure to the intense sunlight and heat of the desert environment can potentially result in overheating and tissue damage.   Imagine leaving a plastic jug of water out in the full sun, with surrounding air temperatures over 100 degrees F.

The approximately 1500 species of the cactus family have evolved a variety of mechanisms to cope with this heating problem.  The evolution of many species from a single common ancestor is called adaptive radiation.

Many cacti are round but narrow,optimizing water storage while reducing
exposure along the sides at mid-day sun and optimizing exposure 
the early morning or late afternoon, Photo by RC Designer t-w-m-c
Most cacti have adaptations that minimize exposure during the hottest hours of the mid-day.  One strategy is to take on an erect and narrow shape.  This allows full exposure to early morning and late afternoon sun, when temperatures are somewhat cooler.  In the middle of the day, however, only the small tip of the stem faces directly into the sun, and the sides receive light obliquely.

Beavertail cacti (genus Opuntia) take that strategy a step further. Their stems develop as flattened segments, which expose even less surface to the noon-time sun, and even more direct exposure  early and late in the day.

The flattened segments of a beavertail cactus (Opuntia) gather
light optimally when the sun is low in the sky, and provide
minimal exposure in mid-day.  Photo by Stan Sherm, Wikipedia.
A spherical or barrel-shaped stem would seem to be all wrong - exposed maximally at high noon.   It is however the most efficient way to store water.  The round shape provides the minimum ratio of evaporative surface area to water storage volume, but it does potentially provide the greatest proportion of its surface facing directly to the noonday sun.  To compensate, barrel cacti often have large curved spines, or numerous long hair-like spines that provide protection from the intense sun.
Cactus spines, which are modified leaves, are particularly well-developed in
broadly rounded cacti, and serve both for protection against herbivores and for
shading from mid-day sun.  Photo by t-w-m-c

Most barrel cacti are ribbed, allowing expansion of the water-storage tissues, and
 also decreasing exposure of the surface tissues to direct sunlight.
Photo by F. B. Essig
Barrel cacti are often fluted, or corrugated, as well - their surfaces consist of accordion-like ridges and valleys.  This further reduces the amount of surface exposed to intense sunlight.   This fluting has a second function as well, allowing the stems to shrink or expand neatly as their internal water stores fluctuate.  The vascular tissues in these stems are concentrated into a series of parallel ribs, so as to allow the expansion of tissues between them.

Another aspect of adaptation is how they are chained together over time, one leading to another to arrive at the characteristic features of a current organism.  We can say that adaptive change is canalized, (see G. L.Stebbins and the process of adaptive modification) and develops momentum in a particular direction.  Certain kinds of change come naturally based on what has come before; others are extremely unlikely.  I have referred to this as "adaptive parsimony" in some of my other essays (see Were the first monocots syncarpous?A flying elephant is unlikely, but the evolution of flight is quite possible in lightweight animals that already leap around in trees (e.g. the ancestors of bats and flying squirrels). 

Another thing lightweight arboreal mammals can become is human.  When I taught introductory biology, I had the students do a thought experiment dealing with human evolution: could humans (or equally sentient beings) have evolved from some other starting point than primates adapted to life in the trees?  Could they have evolved from grazing ungulates or dog-like carnivores?  Could they have evolved from octopi or cuttlefish? Or from insects? The evolution of stereoscopic color vision, grasping hands with opposable thumbs, and rotatable arms in arboreal primates pre-adapted some of their descendants to walk upright and use their hands to craft and utilize tools and weapons - an essential ability for developing technology.  What other path to humanity could have occurred?  We also applied this logic to fictional aliens: how might Wookies or Huts have evolved (especially the huts!)?  Well, that's another story altogether.

Returning to plants, a number of my previous postings (including the ones mentioned above) have centered around logical chains of adaptations.  In the case of cacti, the original adaptations for storing water within the stems led to modifications of the stem to avoid overheating.  In other succulent plants, leaves were modified for water storage instead of the stems (aloes, sedums, etc.).  An aloe is not likely to abandon its water-filled leaves and transfer that function to its stem, just as a cactus is highly unlikely to sprout leaves and transfer water storage to them.  The modification of stems or leaves for water storage is an either/or situation, constrained by their separate canalized adaptive trends.

Stem segments of some epiphytic cacti, such as this Schlumbergera, have
become thin and leaf-like. Photo by Peter Coxhead, Wikipedia.
When some species of cactus adapted to life as epiphytes in tropical rain forests, overheating was not as big a problem, but they needed to absorb more of the light that came to them, They did not sprout leaves again, but instead developed flattened, leaf-like stem segments.  It was a simpler adaptive path.  They also ditched the stem fluting and heavy spines so as to expose more of their light-gathering surface.

Focusing on adaptation can be a highly useful way to teach botany.  It allows one to tell engaging stories that combine systematics (the differences among plants), ecology, anatomy, and physiology.