FITNESS AND ADAPTATION II
Adaptation is a central issue or concept in evolution, but one must
be very specific when defining or deciding that one is actually "looking
at" an adaptation or that something is adapted. The issue revolves
around the general belief that the environment presents problems
for the organism and that adaptations provide solutions to these
First: clarify some terminology. Adaptation come from ad (to,
towards) and aptus (a fit). But it is important to distinguish different
uses of the word "adaptation" in the biological sciences. An
adaptation in physiology is a change in response to a certain problem:
you heat up and respond by taking off your jacket (a behavioral
"adaptation" to an environmental problem); you continue to heat
up and respond by sweating (a physiological response to an environmental
In an evolutionary context: also a change in response to a certain problem.
This time the change is genetic, is achieved by the process of natural
selection and takes place over a period of time considerably longer
than the physiological time scale. But note: the physiological response
itself could be an "adaptation" in the evolutionary sense: it
can be (is) adaptive (genetically) to adapt physiologically
The word Adaptation is both a state of being (phenotypic
trait or character) and it is a process by which such traits
come to be called "adaptations".
Implicit in the term adaptation is the belief that an adaptation serves
some function or purpose. Dispersal and reproduction are the function
or purpose of an apple, and apples are an adaptation apple trees
use to achieve reproduction. This assumes natural selection lead to the
apple as the agent of dispersal and reproduction. Avoiding predation is
the function or purpose of leaf-like coloration in katydids
and preying mantids and their coloration is an adaptation these insects
use to avoid predation.
As argued by G. C. Williams it is important to distinguish adaptations
from "effects": an effect of being an apple is to provide
food for insect larvae or humans; food is an effect of an apple's phenotype
(good resources); apple farming is and effect of apples' good taste
and nutritional value (apples did not evolve to solve the problem of providing
work for apple farmers); the cryptic coloration of a katydid is not for
the purpose of demonstrating adaptation in evolution lectures; demonstration
is an effect of the striking morphology.
To reiterate: we identify traits as adaptations only when they evolved
for the solutions of a specific problem (function/purpose).
Selection is myopic: evolutionary trends are clear and presumably
adaptive; some of these lead to intensification other trends lead
to diminution of characters. Examples: Elaborate secondary sexual
characteristics: increased horns, weapons in males displaying for females;
winglessness in insects: fleas adapted to reaching scalp/skin in
hairy animals; eye loss and reduced pigmentation in cave organisms:
increase fitness by not shunting energy into useless organs/tissues. All
of these trends are adaptive but adaptation is not seeing
the "goal" of larger antlers or no eyes or smaller wings.
Now we have a problem of identifying adaptations in this context. Many
traits evolved under one selective regime and are now being used under
a very different selective regime. The current function may not reflect
the context in which a trait evolved. We have to be able to distinguish
current utility from historical origin. Some traits may have evolved
in one context but later such a trait may be co-opted for use in
a different role. One term used to refer to such traits is Preadaptation.
Some evolutionary biologists dislike this term (some get nauseous when
they hear it!) because the term implies that an adaptive trend was anticipating
some future need. We know that evolution is "blind", "shortsighted"
and can't foresee or anticipate new selective regimes. Key point is that
a trait's function can change faster than its form.
S.J. Gould and E. Vrba (1982, Paleobiology vol 8 pg 4-15) have
suggested a different term: exaptation to stress the cooptedness
Three examples: the evolution of bone tissue is believed to have
proceeded under selection for a tissue that stores inorganic ions (e.g.
phosphate ions). The ions need to be stored and released depending on the
physiological demands of the body. The tissue best at doing this became
rigid and could be coopted as a structural member. Thus organisms
with "bone" as a structural tissue entered a new "adaptive
zone" and adapted for various functions. Skull sutures in mammals
appear as an adaptation for birth since they allow the skull to deform
when passing through the birth canal (a tight squeeze). But reptiles and
birds have them and they hatch out of eggs. Sutures evolved in one context
(allow for growth of brain, head) but are an exaptation for birth
in mammals (they do allow for the head to change shape during birth which
is adaptive). Isolating mechanisms that prevent gene flow between
incipient species. These evolved in allopatry prior to any exposure to
the sister taxon; isolating mechanisms may undergo subsequent adaptive
changes after being challenged by the related species. In all these cases
the historical origin is quite distinct from the current utility.
Exaptation also allows for the evolution of traits that originally had
no "adaptive" function, but later get coopted for a function.
The Adaptationist Program as it has been called by Gould and Lewontin
(Section reading) seeks to find adaptive explanations for every characteristic
of the organism. Some things are not "for" the "purpose"
or "role" they seem to be filling (e.g. "Spandrels"
[you must understand the analogy of spandrels!]; read the paper!)
some things are just nonadaptive and can be distinguished from maladaptive
(former = neutral; latter = bad). Think of your chin; it's not "for"
something, it is there due to differential growth rates of two growth fields
(dentary and maxillary) of your skull. Its just there. The striking pattern
of white triangles on the Conus shell: looks like it is "for"
something but they live under the sand and mud and are not visible. Could
be due to chemistry of shell deposition or might have been "useful"
in the shell's ancestor.
Notion of optimality needs to be considered in a historical context:
again, consider current utility vs. historical origin (see figure 13.5,
Another important point contra the Adaptationist Program is that
some traits may not be capable of achieving "maximal adaptedness"
selection acts on the entire phenotype (debatable sensu units of
selection; later) and phenotypes are compromises. Orians' central
place foraging model: bird sits in middle of territory, may fly a certain
route depending on availability and size of food items. Could
design and optimal foraging strategy BUT, when the birds leaves
nest, young are available for predation. Foraging strategy may not be the
best foraging strategy, but the best compromise given predation
risk. Green sea turtle: excellent swimmer; terrible digger (not
designed for it) but must use flippers to dig hole for laying eggs. Flippers
are not "optimal" for digging, but they work.
Phenotypes as compromises underscores the importance of constraints.
Evolution of one trait can be constrained due to correlation
among traits: selection for body weight in broiler chickens: get more
fat with it; selection for increase milk yield in cows: more milk (with
higher water content); selection for yield in soybeans: get less protein
per bean; selection for nicotine content in tobacco: tar content increases.
These are artificial selection examples but can apply to natural selection
Constraints can be phylogenetic or developmental. (see figures
13.7, 13.8, pgs, 356-7; figure 13.11, pg. 361). It might be adaptive for
certain mammals to be able to breath under water, but their phylogenetic
history and developmental program constrains them from evolving gills.
They "solve" this "problem" by tolerating high levels
of lactic acid in their blood (and other physiological adaptations
of the diving response).
An informative means of analyzing adaptations is through the comparative
approach. Wise to put adaptations in a phylogenetic context
example: rhinoceros horns. Experiments need to be done to determine whether
each unique pattern is uniquely adaptive or simply neutral variations about
a general adaptive theme (difficult experiments! can you design some?)
In seeking adaptive explanations for phenomena we should seek parsimonious adaptive explanations: Flying fish goes out of water; how does it get back down. Physics offers a parsimonious explanation; we could be adaptationist about it and say the fish evolved the means of returning to water. Problem is not how it comes down, but why it takes so long to do so. Gliding must be a result of natural selection. Point is: use the most efficient means to explain the existence of a trait. G. C. Williams: Parsimony demands that we recognize adaptation at the level necessitated by the facts and no higher.