INTEGRATIVE BIOLOGY: THE NEXUS OF DEVELOPMENT,
ECOLOGY, AND EVOLUTION
By
Professor Marvalee H. Wake, President, IUBS
Department
of Integrative Biology and
A plenary
lecture given at the IUBS General Assembly on 18 January 2004, in
It is becoming increasingly
apparent that ‘integrative biology’ provides both a philosophy and a mechanism
for the amalgamation of expertise from different but relevant fields of science
to be brought to bear on complex questions.
Often, when the term is mentioned, people ask “What is integrative biology?” It is both an attitude about science and an approach to the
practice of science. It seeks both
diversity and inclusiveness. It deals
with questions across all levels of biological organization, and includes
physical and social dimensions as appropriate to the questions. The conceptual
basis of ‘integrative biology’ requires a hierarchical approach to the
exploration of complex questions/problems, the use of multiple techniques, and
novel but relevant analyses that lead to syntheses of appropriate arenas of the
sub-disciplines of science, biological and otherwise, to give broader and more
innovative insights into the consideration and even resolution of major
questions. It has applications to issues
in biodiversity science (Wake, 1995, 1998) and many other areas (see Wake,
2001, 2003). At the same time, it cannot be successful until it becomes a
framework for education and training, so that the practice of science is truly
changed (Wake, 2000). Similarly, if the
promise of integrative biology is to be
realized, it must be explicitly developed as a framework for research on the
complexity of life. An integrative
approach may then become the paradigm of 21st century biology, if
not all of science.
Integrative biology promotes several principles and goals:
•the delineation of
complex questions,
•the organization of
expertise to tease apart the questions hierarchically,
•the extension of
expertise into non-traditional arenas, and
•the development of new educational/
training modes.
It is these principles and
goals that IUBS seeks to promote with its adoption of its program called
“Towards an Integrative Biology”.
An emerging arena
for the integrative approach to major questions in biology is the developing
interest in exploring the interaction of ecology, development, and evolution in
organismal biology, including such questions as the basis of developmental
change, the origin of new body forms, the nature of evolutionary processes, and
the effects of environmental factors on development and evolution. It is an area of science ripe for synthesis
because new and ‘old’ questions can now be dealt with differently, as a consequence
of the employment of new tools and
approaches. ‘Eco-evo-devo’ is rapidly
adopting integrative principles, as I will illustrate.
The realization of such a
paradigm shift has a history. In 1973 (Science,
180:488), Leigh Van Valen, with his usual prescience, stated that “A plausible
argument could be made that evolution is the control of development by
ecology.” It has taken many years for
that insight to be appreciated, but it is now being exemplified in the current
wave of research that is exploring the interface and interactions of ecology,
development, and evolution. I will
consider the integrative and hierarchical properties of that research.
More than two
decades ago, a synthesis of research in development and evolution began to
develop, with the tools of molecular genetics facilitating the investigation of
mechanisms of development, and of the mechanistic bases of changes in
developmental programs. “Evo-devo” has
become a successful research paradigm, represented by at least two international
journals, a division of a major biological society devoted to it, and many
researchers espousing it. The molecular
biology of the genetics of development, the organization of phenotypes, and
insights into mechanisms of evolutionary change are all being explored. Now, that synthesis is beginning to integrate
an additional, significant, component---the environmental factors that both
regulate and disrupt differential gene expression, affect rates of development,
determine reproductive and developmental patterns, and provide selection
mechanisms that drive evolutionary change, as Van Valen advocated. “Ecological developmental biology is the
meeting of developmental biology with the real world. It involves studying development in its
natural context rather than only in the laboratory” (Gilbert, 2001). ‘Eco-evo-devo’ includes a host of areas of
investigation, including density-dependent morphological polyphenisms,
predator-induced alteration in developmental rates, context-dependent
life-cycle progressions (e.g., temperature/photoperiod dependent
metamorphosis), egg protection against radiation, hormone mimics, and many
others. Work on these diverse problems
is highly integrative of research at all levels of the hierarchy of biological
organization, and the genetics of development can complement the ecology and
physiology of development, so that a new understanding of those elements that
both stabilize populations and provide for evolutionary change is emerging.
In fact, the broadening of
developmental biology into evolutionary developmental biology, ecological
developmental biology, and medical developmental biology represents in part a
return to questions that embryology had abandoned as it became developmental
biology. Historically, ecological concerns had played a
major role in the development of experimental embryology in the late 19th
century, but the ecological paradigm was overtaken by the physiological one, as
experimental embryology became an investigation of the causal physiology of
development in the early 20th century. Integrations are
now occurring among many ‘disciplines’—molecular genetics, molecular
development, organismal development, morphology, biochemistry, physiology,
endocrinology, ecology, behavior, systematics, population biology, and
evolution. This integrated network is
changing our ideas about evolution, variation, and morphogenesis, and it is
highlighting questions that have long been at the margins of developmental
biology. As Scott Gilbert declared at a recent symposium, an expanded evolutionary developmental biology
should become the central processing center to integrate the cellular and
molecular levels of biology with the organismal and populational areas of the
biological sciences.
An ‘emerging value’ of the
new synthesis is that, with
new approaches to complex interactions, biologists are again focusing attention
on the organisms that are involved,
as well as their interactions and effects.
Organismal biology is returning to the forefront of investigation
because of better understanding of the history of lineages and the responses of
species, populations, and individuals to biological processes. I write as an evolutionary biologist who
focuses on the biology of organisms, and who employs the techniques and
approaches from many areas of biology in order to be hierarchical in my
approach—that is, an integrative biologist.
Many examples of
the integrative biology of ‘eco-evo-devo’ involve research on amphibians. Members of that class illustrate many of the
issues, questions, and problems involved, ranging from the evolution of
development to the ecology of extinction.
Also, not only are several amphibian taxa ‘model animals’ for
developmental, evolutionary, and ecological studies, but recent research on the biology of amphibians is
re-exploring the interface of development, ecology, and evolution. Tools and techniques from molecular and cell
biology, genetics, development, physiology, morphology, ecology, behavior,
systematics, and evolutionary biology are being brought to bear on complex
interactions. Research on such interactions is conducted hierarchically, often
from molecular to ecosystem and evolutionary levels.
I will illustrate the integrative and hierarchical approach to the
effect of ecology on development, and of development on evolution and vice
versa, with four examples, all from recent research by several scientists on
the biology of frogs, mostly, and also salamanders. All of these examples involve research that is
truly integrative, in that questions from the molecular to the ecological,
behavioral, and evolutionary are explored; multiple techniques and analyses are
employed; major implications for science and in some cases for public policy
are involved. The examples are:
•Limb development in direct-developing frogs,
•The effect of parasites on limb development,
•The effect of predation on hatching time and
success, and
•The effect of UVB on
hatching success.
Eleutherydactylus coqui is a species of small leptodactylid frogs that
inhabits Puerto Rican forests. There it
is ubiquitous and has large populations.
It has been introduced in
The question arose in the fertile mind of James Hanken, an
evolutionary developmental biologist who works on amphibians, whether direct
development imparted any specific modifications to developmental pathways that
might be concomitant to bypassing the larval period. He and his colleagues have found that there
are several novel features of E. coqui embryonic development – in particular, many
larval features are lost (mouthparts, cartilages, muscles), and nearly all of
development occurs before hatching, as indicated above. In fact, bone, rather than
cartilage, is the dominant tissue type, the paired limbs develop nearly
simultaneously, an egg tooth develops apparently to facilitate hatching from
the egg membrane. In addition, the tail
of the unhatched tadpole is expanded and appressed to the egg membrane wall,
apparently to serve as the principal embryonic respiratory organ, apparently
effecting gaseous exchange with the environment.
Limb development in E. coqui
illustrates a number of modifications, and at the same time, the analysis of
the phenomenon is an illustration of integrative biology, as the genetics of
limb development are employed to assess changes correlated with life history
and ecology. Limb development in E. coqui is a mosaic of conserved and novel
features. Those conserved are the
patterns of Hox and Shh
expression, and distal-less (dll) in the zone of polarizing activity, as well
as many aspects of the general gross
pattern of development at the cellular level.
Novel are the fact that limb outgrowth is slow, so that the limbs
develop nearly simultaneously; limb development is completed before hatching,
rather than being a post-hatching event; and at the cellular level, the apical
ectodermal ridge, typical of vertebrate limb development, is not present; the
zone of proliferating growth (ZPA) also typical of limb development, is
reduced, and its induction properties cease earlier than in other vertebrates
examined, and the limb can continue to grow and differentiate after removal of
the distal ectoderm (Elinson, 1994; Richardson et al., 1998; summarized in Hanken et al., 2001). These are
significant timing differences in aspects of limb development relative to the
several frog species with the ancestral life history pattern that have been
studied to date.
The main point of
this examination of limb development in a species that has evolved direct
development, a derived mode of reproduction, is that cellular and molecular
aspects of development are both conserved and novel. One emphasis of this research for
developmental and evolutionary biology is that it is important to consider that
the concept of the limb as a uniform developmental module must be modified to
allow for the effects of evolutionary change in other factors of development of
the animal (Hanken et al.,
2001). The influence of ecology on
development and life history strategy remains to be evaluated fully for this
system.
In the last decade or so, many multi-legged frogs have been found
to occur in populations in which they had not been observed previously. The phenomenon has been reported in many
parts of the world, particularly in disjunct areas of the
The developmental biologist of the team devised experiments to see
whether the multiple limb anomaly could be simulated by inserting small glass
beads the size of the cercaria in developing joint regions in tadpoles near
metamorphosis, especially the pelvis. He
found that the beads did induce multiple limbs.
The experimenter concluded that the beads, and the parasites, exert a
mechanical effect at the site of the outgrowth of the cellular condensation
that will form the bones and muscles, causing a replication (Sessions and Ruth,
1990). The action of the cercaria themselves now has been further delineated
(Stopper et al., 2002).
The interaction of ecology with development in this case seems to
indicate that new phenomena are occurring.
The presence of the trematodes, and their use of frogs as hosts, in the
ponds is an old one. Why, then, are
anomalies in the frogs suddenly being found in large numbers in ponds in which
they had not occurred previously? Are the trematodes more numerous? If so, why?
Are the frogs more sensitive to the cercaria? If so, why?? A new
parasitic disease in amphibians may be emerging (Johnson et al., 2003). At the same
time, increased numbers of the same kind of anomaly are being reported in other
species of frogs whose tadpoles inhabit ponds that do not have trematode parasites.
Acid rain and increased retinoic acid have been implicated in inducing
the anomaly, as well as the parasites (summarized by Sessions et al., 1999). Again, the question arises why there are ecological
and physiological changes that result in developmental anomalies. The evolutionary implications in terms of
reduced reproductive capacity of multi-legged adults and the population
involved, and consequently potential extinction, are profound.
The Red-eyed Tree Frog, Agalychnis
callidryas, a colorful hylid frog featured on many calendars, postcards,
and notecards, inhabits the forests of lower
When snakes prey on clutches of eggs/embryos before they are
competent to hatch, nearly all of the eggs are consumed. Data for many clutches show high, nearly
complete decimation of the clutch (Warkentin, 1995, 1999a). However, when the snakes attack clutches with
embryos capable of hatching, hatching ensues, and most of the clutch escapes
into the water. This response occurs at
any time during the plastic period, and survivorship is enhanced. Warkentin and her students are now
determining the factors that induce the hatching response. She and her students record the vibration
signals (frequency and amplitude) of rain (no hatching) and snake movement
(especially the main predator). Playback
experiments show that the embryos sense and distinguish the vibrations of snake
movement, and commence hatching. This
work is in progress, and it includes a diversity of environmental variables
(Warkentin, pers. comm.).
Similarly, wasp predation is most massive when the eggs are
attacked before the embryos are competent to hatch. However, they are not as efficient predators
as are the snakes---they remove the eggs or embryos one at a time, so that
often more of the clutch is left uneaten.
If wasps pierce an egg with an embryo not competent to hatch, they
consume some yolk and often extract all or part of the embryo. However, wasp predation induces hatching
during the plastic period. If the embryo
is able to hatch, it usually escapes into the water below its place on its
leaf, but occasionally the wasp is swift enough to catch the embryo. Only if the embryo escapes does the wasp then
pierce a nearby egg in the clutch. In
contrast to the snake predation, however, the embryos that are near the egg
being attacked are the only ones that will hatch, leaving most of the clutch to
continue development. Few eggs are
consumed by wasps, several are induced to hatch, persistence by the wasp at a
clutch increases the hatch rate, but not at the rate that snake predators effect. This suggests that the embryos at 5-8 days
post-fertilization are capable of sensing the nature of the predator, and
responding accordingly (Warkentin, 2000a).
Warkentin is now investigating embryonic development, especially that of
the neurosensory system, in order to assess how young embryos sense predator
type and make responses that appear to be ‘decisions’ whether or not to hatch
(Warkentin, pers. comm.). The situation
for a fungus attack is similar; the fungus destroys the clutch if it attacks
the eggs before the embryos are competent to hatch, but during the embryos’
plastic stage, a fungus infection induces hatching, and the differences are
highly significant (Warkentin et al.,
2001). Warkentin (2000b, 2002; Warkentin
and Wassersug, 2001) has recently begun to explore the relationships of oxygen
availability, external gill loss, and hatching time, thereby adding an
additional physiological-developmental-ecological dimension to her research
program.
Warkentin and her group are assessing the kinds of
signals---vibrations, water-borne chemicals, oxygen levels, etc.--- that might
induce the hatching response, and she is also examining the development of the
embryos and their sensory reception and integration systems to see how early
the response can be effected, and what its pathways are. This is truly integrative research, examining
development, morphology, physiology, ecology, life history strategies, and
evolution, with a strong theoretical base to guide the analysis. It will provide empirical and theoretical
information about the development of behaviors, predator-prey system function,
a new way of considering phenotypic plasticity and reaction norms, and on the
interaction of multiple ecological components on development, physiology, behavior,
and the evolution of complex biological interactions.
The example of
the integrative approach that I will develop most fully herein is that of
research on global climatic variables as they affect multiple species in
diverse habitats. It considers the
integrative biology of molecular biology (DNA repair), development
(developmental rate, presence or absence of anomalies, hatching success),
ecology (oviposition sites), climatic variables (decreased ozone and increased
UV-B irradiation), behavior (e. g., activity level of tadpoles, egg-wrapping
behavior of salamander larvae) and the implications of the decline of
populations of amphibians (which has been reported around the world) (Blaustein
and Wake, 1995) for evolutionary and conservation biology. Andrew Blaustein and his colleagues have
studied these interactions in the field and in the lab for more than a decade,
and for all of the species of frogs and salamanders that are endemic to the
Cascade Mountains of Oregon in the western
Blaustein and his
group first concentrated on the effects of ultraviolet-beta (UV-B) on egg
hatching and survival rates among the species of frogs and salamanders of the
Cascades. They did both field and lab
experiments (see Blaustein et al., 1998 for a summary). Their field experiments took place at natural
oviposition sites, and used an experimental design in which fixed numbers of
eggs/embryos from naturally laid clutches were placed in same-sized containers. One-third of the containers were shielded
with UV-B blocking filters, one-third with an acetate filter that transmitted
UV-B, and the remainder lacked filters.
The containers were placed in a randomized block design, and the
experiments were ended when the embryos either hatched or died, as they were
monitored daily. The experiments were
repeated at a number of sites at different elevations, and site differences
were controlled for (Blaustein et al., 1997). They found that there were no differences in
Pacific Tree Frog (Hyla regilla) or
Red-legged Frog (Rana aurora) hatching
success for the three treatments.
However, hatching success of the Western Toad (Bufo boreas), the Cascades Frog (Rama cascadae), and the Northwestern Salamander (Ambystoma gracile) all had significantly
greater hatching success under treatments that blocked UV-B radiation compared
to those that had the eggs exposed (Blaustein et al., 1998). Further, UV-B can cause deformities--more
than 90% of the Long-toed Salamander (A.
macrodactylum) embryos had edema, lateral flexure of the tail, etc., under
the UV-B transmitting situations (Blaustein et al., 1997). Similar experiments document developmental
responses to UV-B in several amphibian species (Hays et
al.,
1996). In a recent experiment, Belden
and Blaustein (2002a, b) tested the predictability of effects of increases in
UV-B. They submitted Rana aurora tadpoles, currently immune
to UV-B, to increased dosages of UV-B, and found that a slight increase caused
significantly reduced hatching success.
They conjecture that climatic changes that continue to produce
increasing amounts of UV-B will soon cause the declines of species now
resistant to UV-B.
Blaustein and his
group have found that the endemic frog and salamander species vary in their
levels of photolyase, an enzyme that is responsible for DNA repair. The species that are resistant to UV-B in
general have much higher levels of photolyase (e. g., Hyla regilla), and those that are vulnerable (e. g., Bufo boreas) have low levels of the enzyme. Levels of the DNA repair enzyme in the
various species are significantly correlated with their resistance to UV-B
exposure, and with certain behaviors (Blaustein et al., 1996, 1999,
2001b).
Over evolutionary
time, species in the Cascades have evolved a number of defenses that apparently
deal with UV-B radiation, among other factors (summarized in Blaustein and
Belden, 2003). They are molecular (e.
g., photolyase level), physiological, and behavioral. For example, the species of salamanders and
frogs with low levels of photolyase preferentially lay their eggs in shade,
while Hyla regilla, which has high
levels of the enzyme, often lays its eggs in water under bright, unfiltered,
sunlight. In a correlated study, Marco et al. (2001) found that European newts
(Triturus cristatus), that wrap their
eggs in leaves and grass fronds, effect greater hatching success under UV-B
exposure. In order to test ways that
behaviors might have evolved, Belden et
al. (2003) examined survival, the hormonal stress response, and UV-B
avoidance in Cascades Frog (Rana
cascadae) tadpoles. They found that the tadpoles apparently do
not perceive UV-B, or react with a stress response to its presence. However, UV-B exposure significantly reduces
survival. Therefore the lack of
perception and the lack of hormonal response indicate that the species’
preference for laying its eggs in shade may be light-mediated, but likely not
UV-B mediated. The authors suggest that,
consequently, as UV-B increases, the frogs would appear to lack mechanisms for
compensating directly for it, and their capacity for modifying their molecular
biology, physiology, and behavior as has occurred over evolutionary time is
likely to be outstripped by environmental change, thus causing significant
population declines and ultimately the extinction of the species.
Blaustein and his
group have also considered whether or not there might be synergistic effects
among climatic and other variables in causing anomalies, illness, and decreased
survival in the species that they have studied.
They have found that UV-B radiation interacts with a pathogenic fungus (Saprolegnia), often present in the
field, so that the mortality of the tadpoles is greater when exposed to both
variables than it is with exposure to either UV-B or the fungus alone. The data were obtained from experiments on
three species of frogs subjected to the UV-B experimental design in ponds that
included the fungus (Kiesecker and Blaustein, 1995). They also found that a combined effect of
UV-B, nitrates from fertilizer runoff, and low pH (acidification) significantly
reduced Cascades frog survival and activity levels, but the tadpoles were not
affected by any of the three factors alone (Blaustein and Hatch, 2000). These results indicate that multiple
stressors have a synergistic effect.
Blaustein et al. (2003) reviewed the effects of UV, the synergistic
effects of other components (e. g., such contaminants as pesticides, heavy
metals, acidification, and fertilizers) with UV, and such effects as endocrine
disruption that the contaminants mediate.
They note that the effects are not only at the population and species
levels, but also at community levels.
There is a generally increased susceptibility to disease that is
emerging, and environmental stressors are implicated. In fact, Kiesecker et
al.
(2004) recently presented evidence for a correlation of amphibian population
declines with emerging diseases, and strongly recommended that global climate
change and environment stressors be investigated as part of concern about new
diseases.
Blaustein and his
colleagues, and others, have examined their data regarding the issue of
declining amphibian populations (Blaustein et al., 1994), global
biodiversity loss, and the effects of climate change in order to assess whether
there might be patterns that suggest causality.
Blaustein et al. (2001a) document that climate changes may be
influencing the breeding patterns of a number of amphibians, especially in
terms of earlier breeding with increased temperature. Pounds et al. (1999) and
Pounds (2001) illustrate that global warming is causing a rise in the level at
which cloud forest begins in Costa Rica, so that cloud forest amphibians are
losing their breeding sites and the timing of breeding is being affected---and
some species appear to have been lost from the fauna. It is clear that causes of amphibian
population declines are complex, and the interactors that appear to contribute
to declines vary in different locations in the world, that different species
respond differently to the same potential causative agents, and that even
populations of the same species can respond differently. In addition, effects of causal agents may
occur either at fixed stages during the life cycle, or at all stages (Blaustein
et al., 2003; Kiesecker et al., 2001). Blaustein and Kiesecker (2002) make the point
that though some generalizations can be made about amphibian population
declines, the generalizations must take into account the context-dependent
(abiotic and biotic factors that act together) dynamics of the ecological
systems in which the species studied occur (as Wake [1991] had earlier
suggested). Only then can lessons about
biological complexity result in important pragmatic ventures such as
conservation measures designed to protect species and habitats.
Obviously, many factors recently have been demonstrated to be
implicated in the ecology and physiology of population declines, particularly
of amphibians, but also of many other organisms. These factors are largely anthropogenic, as
illustrated by the following list:
•Habitat modification by humans (houses, farms,
roads, dams, etc.),
•Pollution of water, land, and air,
•Secondary effects, such as increased UVB, acid rain,
•Global climate change,
•Introduced predators, competitors,
•Exploitation for food or other resources,
•New, or newly virulent, pathogens (chytrid fungi, algae,
arboroviruses, etc.),
•Many other causes, and the effects of causes are often
synergistic (list from D. B. Wake, pers. comm.).
Such anthropogenic effects can affect any and all phases of the
life cycle of the organisms that respond to them, including the molecular
biology and biochemistry of their physiological processes, their development
from molecular and cell-level responses to whole-animal and whole-population
effects, their reproductive capacity, and many other aspects of the components
of life. It appears that many
anthropogenic effects outstrip an organism’s or a population of organisms’
capacity to adapt, physiologically and over evolutionary time, to the changes
in its environmental regime, so that decline and ultimately extinction are the
consequence, rather than the evolution of the population in response to the
selection pressure. We must learn our
lessons immediately, and take steps to respond responsibly to global change and
the issues of environmental degradation, population declines, emerging diseases
and developmental anomalies, etc. An integrative approach is necessary at all
steps in this consideration, from observation and recognition, to
experimentation, to analysis, to appropriate responses.
Conclusions
The several cases
presented as examples of research on questions of the complexity of the
relationship of ecology to developmental biology and developmental biology to
evolutionary biology, and their interactions, allow several conclusions to be
drawn about the effectiveness of an integrative approach to such issues. They include:
1. An integrative and hierarchical approach to
questions of interactions yields new and different insights than those obtained
by focusing on single levels of problems.
2. The tools and techniques of many ‘branches’
of biology can be successfully integrated.
3. The impact of humans on the ecology of the
planet has more dimensions than we normally consider.
4. We have a responsibility to conduct our
research with both scientific and societal information and responses in mind.
5. We must train
our students to be integrative in approach and attitude, so that they can
contribute effectively to ‘the new biology’ and to society.
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