Adaptation is the process whereby a population
becomes better suited to its
habitat. This
process takes place over many generations, and is one of the basic
phenomena of biology.
Also, the term
adaptation may refer to a feature which is
especially important for an organism's survival. For example, the
adaptation of horses' teeth to the grinding of grass, or their
ability to run fast and escape predators. Such adaptations are
produced in a variable population by the better suited forms
reproducing more successfully, that is, by
natural selection.
General principles
The significance of an adaptation can only be
understood in relation to the total biology of the
species.
Julian Huxley
Adaptation is, first of all, a
process, rather than a
physical part of a body. The distinction may be seen in an internal
parasite (such as a
fluke), where the bodily structure is greatly
simplified, but nevertheless the organism is highly adapted to its
unusual environment. From this we see that adaptation is not just a
matter of visible
traits: in such
parasites critical adaptations take place in the
life-cycle, which is often quite
complex. However, as a practical term, adaptation is often used for
the
product: those features of a species which result from
the process. Many aspects of an animal or plant can be correctly
called adaptations, though there are always some features whose
function is in doubt. By using the term
adaptation for the
evolutionary
process, and
adaptive trait for the
bodily part or function (the product), the two senses of the word
may be distinguished.
Adaptation may be seen as one aspect of a two-stage process. First,
there is
speciation (species-splitting or
cladogenesis), caused by
geographical isolation or some other
mechanism. Second, there follows adaptation, driven by natural
selection. Something like this must have happened with
Darwin's finches, and there are many other
examples. The present favourite is the evolution of
cichlid fish in African lakes, where the question of
reproductive isolation is much more complex.
Another great principle is that an organism must be viable at all
stages of its development and at all stages of its evolution. This
is obviously true, and it follows that there are
constraints on the evolution of development, behaviour and
structure of organisms. The main constraint, over which there has
been much debate, is the requirement that changes in the system
during evolution should be relatively small changes, because the
body systems are so complex and interlinked. This is a sound
principle, though there may be rare exceptions:
polyploidy in plants is common, and the
symbiosis of micro-organisms that formed the
eukaryota is a more exotic example.
All adaptations help organisms survive in their
ecological niches. These adaptative traits
may be structural, behavioral or physiological. Structural
adaptations are physical features of an organism (shape, body
covering, defensive or offensive armament); and also the
internal organization).
Behavioural adaptations are composed of inherited
behaviour chains and/or the ability to learn: behaviours may be
inherited in detail (
instincts), or a
tendency for
learning may be inherited (see
neuropsychology). Examples:
searching for food,
mating,
vocalizations.
Physiological adaptations permit the organism
to perform special functions (for instance, making
venom, secreting slime,
phototropism); but also more general functions
such as
growth and
development,
temperature regulation,
ionic balance and other aspects of
homeostasis. Adaptation, then, affects all
aspects of the life of an organism.
Definitions
The following definitions are mainly due to
Theodosius Dobzhansky.
- 1. Adaptation is the evolutionary process whereby an
organism becomes better able to live in its habitat or habitats.
- 2. Adaptedness is the state of being adapted: the
degree to which an organism is able to live and reproduce in a
given set of habitats.
- 3. An adaptive trait is an aspect of the developmental
pattern of the organism which enables or enhances the probability
of that organism surviving and reproducing.
Adaptedness and fitness
From the above definitions, it is clear that there is a
relationship between adaptedness and
fitness (a key
population genetics concept). Fitness is
an estimate and a predictor of the rate of natural selection. What
natural selection does is change the relative frequencies of
alternative phenotypes, insofar as they are
heritable. Although the two are connected, the one
does not imply the other: a phenotype with high adaptedness may not
have high fitness. Dobzhansky mentioned the example of the
Californian redwood, which is highly
adapted, but a relic species in danger of extinction.
Elliott Sober commented that adaptation was a
retrospective concept since it implied something about the history
of a trait, whereas fitness predicts a trait's future.
- 1. Fitness. The degree of demographic difference among phenotypes. Usually a relative measure: the
average contribution to a breeding population by a phenotype or a
class of phenotypes. This is also known as Darwinian
fitness, relative fitness, selective
coefficient, and other terms.
- 2. Adaptedness. Usually an absolute measure: the average
absolute contribution to the breeding population by a carrier of a
phenotype or a class of phenotypes. Also known as absolute
fitness, and as the Malthusian
parameter when applied to species as a whole.
Brief history
Adaptation as a fact of life has been accepted by all the great
thinkers who have tackled the world of living organisms. It is
their explanations of how adaptation arises that separates these
thinkers. A few of the most significant ideas:
- Empedocles did not believe that
adaptation required a final cause (~ purpose), but "came about
naturally, since such things survived". Aristotle, however, did believe in final
causes.
- In natural theology, adaptation
was interpreted as the work of a deity, even as evidence for the
existence of God. William Paley
believed that organisms were perfectly adapted to the lives they
lead, an argument that shadowed Leibniz, who
had argued that God had brought about the best of all possible worlds.
Voltaire's Dr Pangloss is a parody of this
optimistic idea, and Hume also argued against
design. The Bridgewater Treatises are a product of natural
theology, though some of the authors managed to present their work
in a fairly neutral manner. The series was lampooned by Robert Knox, who held quasi-evolutionary views,
as the Bilgewater Treatises. Darwin broke with the
tradition by emphasising the flaws and limitations which occurred
in the animal and plant worlds.
Lamarck
- Lamarck. His is a proto-evolutionary
theory of the inheritance
of acquired traits, whose main purpose is to explain
adaptations by natural means. He proposed a tendency for organisms
to become more complex, moving up a ladder of progress, plus "the
influence of circumstances", usually expressed as use and
disuse. His evolutionary ideas, and those of Geoffroy, fail because they cannot be reconciled
with heredity. This was known even before Mendel by medical men interested in human
races (Wells, Lawrence), and especially
by Weismann.
Many other students of natural history, such as
Buffon, accepted adaptation, and some also accepted
evolution, without voicing their opinions as to the mechanism. This
illustrates the real merit of
Darwin
and
Wallace, and secondary
figures such as
Bates, for
pushing forward a mechanism whose significance had only been
glimpsed previously. A century later, experimental field studies
and breeding experiments by such as
Ford
and
Dobzhansky produced
evidence that natural selection was not only the 'engine' behind
adaptation, but was a much stronger force than had previously been
thought.
Types of adaptation
Adaptation is the heart and soul of
evolution.
Niles Eldredge
Changes in habitat
Before
Darwin, adaptation was seen as
a fixed relationship between an organism and its habitat. It was
not appreciated that as the
climate changed,
so did the habitat; and as the habitat changed, so did the biota.
Also, habitats are subject to changes in their
biota: for example,
invasions of species from other areas. The
relative numbers of species in a given habitat are always changing.
Change is the rule, though much depends on the speed and degree of
the change.
When the habitat changes, three main things may happen to a
resident population: habitat tracking, genetic change or
extinction. In fact, all three things may occur
in sequence.
Of these three effects, only genetic change brings
about adaptation.
Habitat tracking
When a habitat changes, the most common thing to happen is that the
resident population moves to another locale which suits it; this is
the typical response of flying insects or oceanic organisms, who
have wide (though not unlimited) opportunity for movement. This
common response is called
habitat tracking. It is one
explanation put forward for the periods of apparent stasis in the
fossil record (the
punctuated
equilibrium thesis).
Genetic change
Genetic change is what occurs in a population when natural
selection acts on the
genetic
variability of the population. By this means, the population
adapts genetically to its circumstances. Genetic changes may result
in visible structures, or may adjust
physiological activity in a way that suits the
changed habitat.
It is now clear that habitats and biota do frequently change.
Therefore, it follows that the process of adaptation is never
finally complete. Over time, it may happen that the environment
changes little, and the species comes to fit its surroundings
better and better. On the other hand, it may happen that changes in
the environment occur relatively rapidly, and then the species
becomes less and less well adapted. Seen like this, adaptation is a
genetic
tracking process, which goes on all the time to
some extent, but especially when the population cannot or does not
move to another, less hostile area. Also, to a greater or lesser
extent, the process affects every species in a particular
ecosystem.
Van Valen thought that even in a
stable environment, competing species had to constantly adapt to
maintain their relative standing. This became known as the
Red Queen hypothesis.
Intimate relationships: co-adaptations
In
co-evolution, where the existence of
one species is tightly bound up with the life of another species,
new or 'improved' adaptations which occur in one species are often
followed by the appearance and spread of corresponding features in
the other species. There are many examples of this; the idea
emphasises that the life and death of living things is intimately
connected, not just with the physical environment, but with the
life of other species. These relationships are intrinsically
dynamic, and may continue on a trajectory for millions of years, as
has the relationship between flowering plants and insects (
pollination).
Pollinator constancy: these honeybees selectively visit flowers
from only one species, as can be seen by the colour of the pollen
in their baskets:File:Plumpollen0060.jpgFile:Bee PD foto
explained1.jpgFile:Carnica bee on sedum telephium.jpg
The gut contents, wing structures, and mouthpart morphologies of
fossilized
beetles and
flies suggest that they acted as early pollinators.
The association between
beetles and
angiosperms during the early
Cretaceous period led to parallel radiations of
angiosperms and insects into the late Cretaceous. The evolution of
nectaries in late Cretaceous flowers
signals the beginning of the
mutualism
between
hymenopterans and
angiosperms.
Mimicry
Henry Walter Bates' work on
Amazonian
butterflies led him to develop
the first scientific account of
mimicry,
especially the kind of mimicry which bears his name:
Batesian mimicry. This is the mimicry by a
palatable species of an unpalatable or noxious species. A common
example seen in temperate gardens is the
hover-fly, many of which – though bearing no sting
– mimic the warning colouration of
hymenoptera (
wasps and
bees). Such mimicry does not need to be perfect
to improve the survival of the palatable species.
Bates,
Wallace and
Müller believed that Batesian and
Müllerian mimicry provided evidence
for the action of
natural
selection, a view which is now standard amongst biologists. All
aspects of this situation can be, and have been, the subject of
research. Field and experimental work on these ideas continues to
this day; the topic connects strongly to
speciation,
genetics and
development.
The basic machinery: internal adaptations
There are some important adaptations to do with the overall
coordination of the systems in the body. Such adaptations may have
significant consequences. Examples, in
vertebrates, would be
temperature regulation, or
improvements in
brain function, or an
effective
immune system. An example in
plants would be the development of the reproductive system in
flowering plants. Such adaptations
may make the
clade (
monophyletic group) more viable in a wide range
of habitats. The acquisition of such major adaptations has often
served as the spark for
adaptive
radiation, and huge success for long periods of time for a
whole group of animals or plants.
Compromise and conflict between adaptations
It is a profound truth that Nature does not know best;
that genetical evolution... is a story of waste, makeshift,
compromise and blunder.
Peter Medawar
All adaptations have a downside: horse legs are great for running
on grass, but they can't scratch their backs; mammals' hair helps
temperature, but offers a niche for
ectoparasites; the only flying penguins do is
under water. Adaptations serving different functions may be
mutually destructive. Compromise and make-shift occur widely, not
perfection. Selection pressures pull in different directions, and
the adaptation that results is some kind of compromise.
- Since the phenotype as a whole is the target of selection, it
is impossible to improve simultaneously all aspects of the
phenotype to the same degree. Ernst Mayr
Consider the antlers of the
Irish elk,
(often supposed to be far too large; in
deer
antler size has an
allometric
relationship to body size). Obviously antlers serve positively for
defence against predators, and to score victories in the annual
rut. But they are costly in terms of resource.
Their size during the
last glacial
period presumably depended on the relative gain and loss of
reproductive capacity in the population of elks during that time.It
is, of course, not possible to test selective pressures on extinct
populations in any direct way. (1974): Origin and function of
'bizarre' structures - antler size and skull size in 'Irish Elk',
Megaloceros giganteus.
Evolution 28(2):
191-220. (First page text) Another example:
camouflage to avoid detection is destroyed when
vivid colors are displayed at mating time. Here the risk to life is
counterbalanced by the necessity for reproduction.
The
peacock's ornamental train (grown anew
in time for each mating season) is a famous adaptation. It must
reduce his maneuverability and flight, and is hugely conspicuous;
also, its growth costs food resources. Darwin's explanation of its
advantage was in terms of
sexual
selection: "it depends on the advantage which certain
individuals have over other individuals of the same sex and
species, in exclusive relation to reproduction." The kind of sexual
selection represented by the peacock is called 'mate choice', with
an implication that the process selects the more fit over the less
fit, and so has survival value. The recognition of sexual selection
was for a long time in abeyance, but has been rehabilitated. In
practice, the blue peafowl
Pavo
cristatus is a pretty successful species, with a big
natural range in India, so the overall outcome of their mating
system is quite viable.
The conflict between the size of the human
foetal brain at birth, (which cannot be larger than
about 400ccs, else it will not get through the mother's
pelvis) and the size needed for an adult brain (about
1400ccs), means the brain of a newborn child is quite immature. The
most vital things in human life (locomotion, speech) just have to
wait while the brain grows and matures. That is the result of the
birth compromise. Much of the problem comes from our upright
bipedal stance, without which our pelvis
could be shaped more suitably for birth.
Neanderthals had a similar problem.
Shifts in function
Adaptation and function are two aspects of one
problem.
Julian Huxley
Pre-adaptations
This occurs when a species or population has characteristics which
(by chance) are suited for conditions which have not yet arisen.
For example, the
polyploid rice-grass
Spartina townsendii is better adapted than either of its
parent species to their own habitat of saline marsh and mud-flats.
White Leghorn fowl
are markedly more resistant to
vitamin B
deficiency than other breeds.Lamoreux W.F and Hutt F.B. 1939. Breed
differences in resistance to a deficiency in vitamin B
1
in the fowl.
J. Agric. Res. Washington
58, 307–315. On a plentiful diet there is no
difference, but on a restricted diet this preadaptation could be
decisive.
Pre-adaptation may occur because a natural population carries a
huge quantity of
genetic
variability. In
diploid eukaryotes, this is a consequence of the system
of
sexual reproduction, where
mutant alleles get partially shielded, for example, by the
selective advantage of
heterozygotes.
Micro-organisms, with their huge populations, also carry a great
deal of genetic variability.
The first experimental evidence of the pre-adaptive nature of
genetic variants in micro-organisms was provided by
Salvador Luria and
Max Delbrück who developed
fluctuation analysis, a method to
show the random fluctuation of pre-existing genetic changes that
conferred resistance to phage in the bacterium
Escherichia coli.
Co-option of existing traits: exaptation
The classic example is the
ear ossicles of
mammals, which we know from palaeontological and embrological
studies originated in the upper and lower jaws and the hyoid of
their
Synapsid ancestors, and further back
still were part of the gill arches of early fish. We owe this
esoteric knowledge to the comparative anatomists, who, a century
ago, were at the cutting edge of evolutionary studies. The word
exaptation was coined to cover
these shifts in function, which are surprisingly common in
evolutionary history. The origin of wings from feathers that were
originally used fortemperature regulation is a more recent
discovery (see
feathered dinosaurs).
Related issues
Non-adaptive traits
Some traits appear to be not adaptive, that is, selectively
neutral. There may be various causes: the utility of a trait is
lost and does not now appear adaptive; the utility of a trait is
unknown; the trait is a consequence of another trait that is
adaptive (i.e.
spandrels).
Because genes have
pleiotropic effects,
not all traits may be functional. Of course, a trait may have been
adaptive at some point in an organism's evolutionary history, but
habitats change, leading to adaptations becoming redundant or even
a hindrance (
maladaptations). Such
adaptations are termed
vestigial. So, the
utility of adaptations may ebb and flow.
Fitness landscapes; drift
Sewall Wright's explanation for
evolutionary stasis was that
organisms come to occupy
adaptive peaks. In order to
evolve to another, higher peak, the species would first have to
pass through a valley of maladaptive intermediate stages. This
could happen by
genetic drift if the
population were small enough. This was Wright's
shifting
balance theory of evolution. There has been much skepticism
among evolutionary biologists as to whether these rather delicate
conditions hold often in natural populations.
Ronald Fisher felt that most populations in
nature were too large for these effects of genetic drift to be
important.
Vestigial organs
Many organisms have vestigial organs, which are the remnants of
fully functional structures in their ancestors. As a result of
changes in lifestyle the organs became redundant, and are either
not functional or reduced in functionality. With the loss of
function goes the loss of positive selection, and the subsequent
accumulation of deleterious
mutations.
Since any structure represents some kind of cost to the general
economy of the body, an advantage may accrue from their elimination
once they are not functional. Examples:
wisdom teeth in humans; the loss of pigment and
functional eyes in cave fauna; the loss of structure in
endoparasites.
Extinction
If a population cannot move or change sufficiently to preserve its
long-term viability, then obviously, it will become extinct, at
least in that locale. The species may or may not survive in other
locales. Species
extinction occurs when
the death rate over the entire species (population, gene pool ...)
exceeds the birth rate for a long enough period for the species to
disappear. It was an observation of
Van
Valen that groups of species tend to have a characteristic and
fairly regular rate of extinction.
Co-extinction
Just as we have co-adaptation, there is also co-extinction.
Co-extinction refers to the loss of a species due to the extinction
of another; for example, the extinction of
parasitic insects following the loss of their
hosts. Co-extinction can also occur when a flowering plant loses
its
pollinator, or through the disruption
of a
food chain. "Species co-extinction
is a manifestation of the interconnectedness of organisms in
complex ecosystems ... While co-extinction may not be the most
important cause of species extinctions, it is certainly an
insidious one".
Flexibility, acclimatization, learning
Flexibility deals with the relative capacity of an
organism to maintain themselves in different habitats: their degree
of
specialization.
Acclimatization is a term used for
automatic
physiological adjustments
during life;
learning is the term
used for improvement in behavioral performance during life. In
biology these terms are preferred, not adaptation, for changes
during life which improve the performance of individuals. These
adjustments are not inherited genetically by the next
generation.
Adaptation, on the other hand, occurs over many generations; it is
a
gradual process caused by natural
selection which changes the genetic make-up of a population so the
collective performs better in its niche.
Flexibility
Populations differ in their
phenotypic plasticity, which is the
ability of an organism with a given
genotype to change its
phenotype in response to changes in its habitat,
or to its move to a different habitat.
To a greater or lesser extent, all living things can adjust to
circumstances. The degree of flexibility is inherited, and varies
to some extent between individuals. A highly specialized animal or
plant lives only in a well-defined habitat, eats a specific type of
food, and cannot survive if its needs are not met. Many herbivores
are like this; extreme examples are
koalas
which depend on
eucalyptus, and
pandas which require
bamboo. A generalist, on the other hand, eats a range
of food, and can survive in many different conditions. Examples are
humans,
rats,
crabs and
many carnivores. The
tendency to behave in a specialized
or exploratory manner is inherited – it is an adaptation.
Rather different is
developmental flexibility: "An animal
or plant is developmentally flexible if when it is raised or
transferred to new conditions it develops so that it is better
fitted to survive in the new circumstances". Once again, there are
huge differences between species, and the
capacities to be
flexible are inherited.
Acclimatization
If humans move to a higher altitude, respiration and physical
exertion become a problem, but after spending time in high altitude
conditions they
acclimatize to the pressure by increasing
production of
red blood corpuscles.
The
ability to acclimatize is an adaptation, but not the
acclimatization itself.
Fecundity goes
down, but deaths from some tropical diseases also goes down.
Over a longer period of time, some people will reproduce better at
these high altitudes than others. They will contribute more heavily
to later generations. Gradually the whole population becomes
adapted to the new conditions. This we know takes place, because
the performance of long-term communities at higher altitude is
significantly better than the performance of new arrivals, even
when the new arrivals have had time to make physiological
adjustments.
Some kinds of acclimatization happen so rapidly that they are
better called reflexes. The rapid colour changes in some
flatfish,
cephalopods,
chameleons are examples.
Learning
Social learning is supreme for humans, and is possible for quite a
few mammals and birds: of course, that does not involve genetic
transmission except to the extent that the capacities are
inherited. Similarly, the
capacity to learn is an
inherited adaptation, but not what is learnt; the capacity for
human speech is inherited, but not the details of language.
Function and teleonomy
Adaptation raises some issues concerning how biologists use key
terms such as
function.
Function
To say something has a
function
is to say something about what it does for the organism, obviously.
It also says something about its history: how it has come about. A
heart pumps blood: that is its function. It also emits sound, which
is just an ancillary side-effect. That is not its function. The
heart has a history (which may be well or poorly understood), and
that history is about how natural selection formed and maintained
the heart as a pump. Every aspect of an organism that has a
function has a history. Now, an adaptation must have a functional
history: therefore we expect it must have undergone selection
caused by relative survival in its habitat. It would be quite wrong
to use the word adaptation about a trait which arose as a
by-product.
It is widely regarded as unprofessional for a biologist to say
something like "A wing is for flying", although that is their
normal function. A biologist would be conscious that sometime in
the remote past feathers on a small dinosaur had the function of
retaining heat, and that later many wings were not used for flying
(e.g.
penguins,
ostriches). So, the biologist would rather say that
the wings on a bird or an insect usually had the
function
of aiding flight. That would carry the connotation of being an
adaptation with a history of evolution by natural selection.
Teleonomy
Teleonomy is a term invented to describe
the study of goal-directed functions which are not guided by the
conscious forethought of man or any supernatural entity. It is
contrasted with
Aristotle's
teleology, which has connotations of intention,
purpose and foresight. Evolution is teleonomic;
adaptation
hoards hindsight rather than foresight. The following is a
definition for its use in biology:
- Teleonomy: The hypothesis that adaptations arise without the
existence of a prior purpose, but by the action of natural
selection on genetic variability.
The term may have been suggested by
Colin Pittendrigh in 1958; it grew out of
cybernetics and
self-organising systems.
Ernst Mayr,
George
C. Williams and
Jacques Monod picked up the term and used it
in evolutionary biology.
Philosophers of science have also commented on the term.
Ernest Nagel analysed the concept of
goal-directedness in biology; and
David
Hull commented on the use of teleology and teleonomy by
biologists:
- Haldane can be found remarking,
"Teleology is like a mistress to a biologist: he cannot live
without her but he’s unwilling to be seen with her in public."
Today the mistress has become a lawfully wedded wife. Biologists no
longer feel obligated to apologize for their use of teleological
language; they flaunt it. The only concession which they make to
its disreputable past is to rename it ‘teleonomy’.
References
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adaptation as "Any change in the structure or functioning
of an organism that makes it better suited to its
environment".
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- Patterson C. 1999. Evolution. Natural History Museum,
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biology." p5
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Chapter 2
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was his mechanism of adaptation that caught the attention of later
naturalists". (p90)
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