Macroevolutionary Processes—
Radiations
Major Speciation Models
Ancestor
B
A
A
Allopatric
C
B (island)
A’
A
A’ A” B
(ancestor dies out)
Founder
Phyletic gradualism
Concepts Involving Radiations
• Definition of “radiation”—relatively rapid
diversification of an initial ancestral
population into several derivatives (species)
• Often associated with opening of a new
geographic area or set of new niches (e.g.,
ecological, behavioral, nutritional)
• Often accompanied or provoked by one or
more novelties/innovations
Concepts Involving Radiations
• Character displacement—one species
affects direction of evolution or at least
local behavior, in one or more competitors,
not often explicitly demonstrated but often
implicitly invoked in studies of radiations
• Parallelisms—multiple independent origins
of similar traits within lineages or among
closely related lineages is often compelling
evidence of a radiation
Adaptive Radiation
• “The rise of a diversity of ecological roles
and attendant adaptations in different
species within a lineage" (Givnish and
Sytsma)
• Term “adaptive radiation” has been recently
loosely applied to all bursts of
diversification; attempts being made to
restrict definition
• Does not always result in large species
numbers or depend on a key innovation
Adaptive Radiation
• Correctly defined examples require empirical
evidence:
– Adaptive value of phenotypic traits
• Comparative methods—distantly related, ecologically similar
species show convergent form, physiology or behavior
• Functional analyses—functional significance of traits (e.g.,
stomata)
• Populational studies—phenotypic traits linked to survivorship
and reproduction
– Environmental sorting of different phenotypic forms,
tracking of multiple new niches/adaptive zones by sister
taxa; similar phenotypic traits and occupation of
“equivalent” habitats by non-sister taxa
Adaptive Radiation
• Examined (or at least postulated) most
intensively in oceanic islands
• Could further subdivide examples
– Diversification within one habitat—e.g.,
pollinator exploitation
– Diversification across habitats—e.g., classic
AR
• Examples later on
Non-adaptive Radiation
• Also termed “diffusive evolution”
• "Evolution abhors a vacuum—whenever possible,
organisms will evolve by chance into and come to
occupy all regions in the domain of theoretically
possible phenotypes that permit survival and
reproductive success" (Niklaus)  speciation without
appreciable ecological divergence and evolution of
corresponding adaptations
• Usually results in single origin of key trait
• Usual pattern of geographic or other speciation
• Better explains certain traits in putatively adaptive
radiations, e.g., arborescence
Canary Island Land Snails (Theba)
Africa
Photos: GoogleEarth; Greve et al. 2012. PLOS One 7(4): e34339
Canary Island Land Snails (Theba)
T. impugnata
T. grassetti
Photos: Greve et al. 2012. PLOS One 7(4): e34339
Canary Island Land Snails (Theba)
• Many phenotypically and genetically
differentiated land snail species
• Phenetic/morphometric analysis revealed
only two unusually divergent species
• The anomalous species showed no clear
association with extreme habitats  no
obviously “adaptive” traits corresponding
with ecological specialization
 NOT an adaptive radiation
Developmental Radiation
• e.g., new body-plan rearrangements in
Precambrian, with possible advent of newly
arising, divergent sets of homeotic genes
controlling different organs/systems
• Could be involved at lower level in each
adaptive radiation
Sexual Radiation
• Result of sexual selection toward premating
isolation mechanisms
• Differentiation is primarily in courtship
behaviors, genitalic features, other traits
associated with reproduction
• Direction of selection does not track
environmental factors
• e.g., fruit fly (Drosophila) diversification
Coevolutionary Radiation
• Intimate association with and parallel speciation in
different organismal lineages
• Must demonstrate closely correspondent
diversification patterns between organism groups;
often revealed by congruent molecular
phylogenies and tight host-user relationships
• Generally demands sole utilization of one host by
an organism (no generalist behavior)
• e.g., figs and fig wasps
• e.g., yuccas and yucca moths
Interesting Questions
• Do phenotypic or ecological similarities
reliably reflect phylogenetic relationships in
a group produced in a radiation?
• What evidence exists to indicate certain
species differences are adaptive, fitting
them to their divergent evolutionary roles?
• What environmental factors may have
"driven" diversification of particular
phenotypic traits, or syndromes
Interesting Questions
• Does selection for ecological divergence
result in speciation?
• What is the level of genetic divergence
among species and their populations in a
radiation?
• How labile are the phenotypic traits and
physiological tolerances in species of a
radiation?
Interesting Questions
• What is the relation between local geography,
ecology and speciation in a radiation?
• Have similar-looking radiations occurred in very
distantly related groups, leaving behind some
fundamental patterns of diversification and
convergence?
• Are radiations more likely to proceed in certain
places or under certain circumstances?
Evolutionary Radiations
• Virtually all studies explicitly concerned
with evolutionary radiation identified with
"adaptive radiation", many attempt to
identify key innovations
• Many strictly phylogenetic/molecular
systematic studies could be used for
inference of diffusive evolution with small
amount of additional information
Evolutionary Radiations
• Adaptive radiation still commonly assumed
prior to investigation; results then used to
characterize “an example of adaptive
radiation”—circular reasoning!!
• Few studies have adequately demonstrated
divergence in both phenotypic (e.g.,
morphological, anatomical) traits and
ecological differentiation among sister taxa
Evolutionary Radiations
• Few studies have adequately investigated the
evolution of derivative taxa relative to the sister
group (nearest relative[s])
• Extraordinarily few groups have been investigated
intensively for comprehensive information on
evolutionary processes, relevant speciation models,
isolation mechanisms, microevolutionary (genetic)
processes, etc.
• Most studies have focused on island groups—easier
to work with and get funded, sexier; but many of the
same processes should hold for continental groups
Molecular Data in Radiations
• Phenotypic features in any type of radiation may
be prone to extensive parallelism where more
distantly related (=non-sister) taxa grow in same
habitat and have evolved similar morphologies
• Conversely, closely related taxa may have
diverged dramatically in morphology and ecology
and do not resemble each other
• "Weird" or extreme phenotypic traits in certain
organisms sometimes confound interpretation of
relationships; e.g., bizarre families like
carnivorous plant groups
Molecular Data in Radiations
• Use of phenotypic traits to reconstruct phylogeny of a
group and to interpret phenotypic changes is
controversial, considered by many to be circular
reasoning
• Molecular markers provide a more "neutral" data set
from which to generate a phylogeny
• Molecular phylogeny can be used to infer relationship
of morphological traits, ecological diversification,
divergence in feeding behavior, etc., and can be used
as starting point for investigating
molecular/developmental basis of traits
Case Studies of Radiation
• e.g., evolution in African cichlid fishes
– Several distinctive groups, many very different looking
species in each, with divergent feeding strategies within
lakes
– Several hundred cichlid species in each lake, most
endemic to one lake
– Extreme phenotypic features among species within groups
make interpretation of relationships difficult
– Similar forms with similar mouth structures, feeding
behavior and ecological niche grow in different lakes; are
they related? Or parallel products of adaptive radiation?
Case Studies of Radiation
Case Studies of Radiation
• Evolution in African
cichlid fishes (cont.)
– mtDNA phylogeny reveals
that cichlid species in
different African lakes with
equivalent body form and
mouth-feeding structures are
NOT sister species 
rampant parallelism
– phenotypically and
ecologically divergent
species typically are sisters
extensive divergence in
relatives
Reinthal & Meyer (1997)
Case Studies of Radiation
• Evolution in African cichlid fishes
(cont.)
– Ecologically equivalent species in different
lakes occupy similar microhabitats, eat same
food items  strong selection for similar
phenotypes
– Suggestion of sympatric speciation within
individual lakes, accompanied by adaptive
radiation based on mouthparts for feeding
 reinforcement by competitive exclusion?
Case Studies of Radiation
• e.g., “pitcher plants”
(Brocchinia) on
Venezuelan tepuis
– About 20 species on tall,
nutrient-poor (often boggy)
sandstone mesas (tepuis)
jutting up out of the
Venezuelan lowland
rainforest
– Several growth habits and
feeding strategies--"tank"
habit and carnivory,
epiphytes, tree forms, antplants
Givnish et al. (1997)
Case Studies of Radiation
Case Studies of Radiation
• “pitcher plants” (Brocchinia) on Venezuelan
tepuis (cont.)
– Morphological and anatomical traits related
intimately to growth form and nutrition; tank habit
found only at higher elevations
– Divergent growth forms and feeding strategies
obscure the relationships  chloroplast DNA
phylogeny used to interpret morphological and
ecological evolution
– Two sister lineages occur primarily on tepuis in
different geographic areas
Case Studies of Radiation
•“pitcher plants” (Brocchinia) on Venezuelan
tepuis (cont.)—parallelism of carnivorous traits
Givnish et al. (199
Case Studies of Radiation
•“pitcher plants” (Brocchinia) on Venezuelan
tepuis (cont.)—stepwise evolution of
carnivorous habits
Givnish et al. (199
Case Studies of Radiation
• e.g., Hawaiian violets (Viola)
– Nine taxa, seven species distributed over most islands
– Species occupy several different habitats across five islands
•
•
•
•
•
dry forest
dry cliff
mesic streambank
swamp (cloud) forest
open bog
– Species growing in same habitat on different islands are
almost identical morphologically, anatomically
Case Studies of Radiation
Case Studies of Radiation
• Hawaiian violets (cont.)
– Nuclear gene (ITS)
phylogeny used to interpret
relationships
– Morphologically and
ecologically equivalent
species on different islands
are not sisters
– Closest relatives on the same
island (= derivatives of local
radiation) are phenotypically
and ecologically divergent
Review
• A “radiation” is a relatively rapid burst of
speciation, producing multiple species from
a recent common ancestor
• Not all lineage radiations are adaptive, must
demonstrate a causal link between
environmental selection and phenotypes
• Molecular data are valuable to provide a
basis for inferring morphological evolution
Review
• Adaptive radiations common on oceanic
islands but probably overlooked on
continents
• Two frequent situations in adaptive
radiations
– Non-sister species inhabiting similar ecological
zones are morphologically convergent
– Sister species in different habitats are
morphologically very different
Bibliography
• Givnish, T. J. and K. J. Sytsma (eds.). 1997. Molecular
evolution and adaptive radiation. Cambridge University
Press, Cambridge, United Kingdom. 621 pp.
• Givnish, T. J., K. J. Sytsma, J. F. Smith, W. J. Hahn, D. H.
Benzing, and E. M. Burkhardt. 1997. Molecular evolution
and adaptive radiation in Brocchinia (Bromeliaceae:
Pitcairnioideae) atop tepuis of the Guayana shield. In:
Givnish, T. J. and K. J. Sytsma (eds.), Molecular evolution
and adaptive radiation. Cambridge University Press,
Cambridge, United Kingdom. pp. 259-311.
• Niklas, K. J. 1997. The evolutionary biology of plants.
University of Chicago Press, Chicago, Illinois. 449 pp.
Bibliography
• Nitecki, M. H. (ed.). 1990. Evolutionary
innovations. University of Chicago Press,
Chicago, Illinois. 304 pp.
• Reinthal, P. N. and A. Meyer. 1997. Molecular
phylogenetic tests of speciation models in Lake
Malawi cichlid fishes. In: Givnish, T. J. and K. J.
Sytsma (eds.), Molecular evolution and adaptive
radiation. Cambridge University Press,
Cambridge, United Kingdom. pp. 376-390.
• Schluter, D. and J. D. McPhail. 1993. Character
displacement and replicate adaptive radiation.
Trends in Ecology and Evolution 8:197-200.