© Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. VI.10 Adaptive Radiation Peter R. Grant OUTLINE GLOSSARY 1. Biodiversity Allopatry. The occurrence of species that occupy sepa- 2. Origin and development of the concept rate environments, such as islands. 3. The ecological theory Competition. The struggle between two or more individ- 4. Speciation uals or species for a resource in limited supply that 5. Ecological opportunity they jointly consume (e.g., nutrients for a plant, prey 6. Species interactions for a predator, or places for prey to avoid being 7. Intrinsic factors: Key innovations eaten). Competition may take the form of an aggres- 8. Hybridization sive interaction such as fighting, or differential de- 9. Testing the ideas pletion of a resource by the competitors. 10. Future prospects Disparity. The degree of phenotypic difference among individuals or species in one or more traits. The world has millions of species, and they display an Ecomorph. A recognizable association between morphol- astonishing variety of size, color, and behavior. Adap- ogy of individuals or species and use of the environ- tive radiations comprise groups of distinctive yet closely ment. related species that have evolved from a common an- Evolvability. The genetic and developmental properties cestor in a relatively short time. Studies of these radia- of members of a species that determine the likeli- tions help reveal the causes of their evolution. As a result hood that it will undergo evolutionary change. of natural selection during and after speciation, descen- Introgressive Hybridization. The interbreeding of two dant species differ morphologically or physiologically species or genetically divergent populations and sub- in the way they exploit different environments. Adap- sequent breeding of the offspring with members of tive differentiation also depends on the absence of con- one of the parental populations, resulting in the trans- straints from competitor species. The guiding force of fer of genes. natural environments is revealed in the observation Parapatry. The occurrence of species in adjacent or that the same evolutionary pathway is often taken by contiguous distributional ranges. different organisms in the same environment. Taxonom- Sympatry. The occurrence of species that occupy the ic groups vary in their intrinsic potential to diversify same area. because they possess traits that are key evolutionary inno- vations or because they readily exchange genes through hybridization. Invasion of an underexploited environ- ment allows species to initially multiply at a high rate, 1. BIODIVERSITY and diversify morphologically and ecologically. The fossil record and reconstructions from molecular phylo- For many evolutionary biologists the most important genies show that both speciation and diversification rates pair of questions that need to be answered are: Why do later decline. Experiments in the laboratory with bacteria so many species exist on this planet? And why do they replicate the pattern of diversification through observ- differ so greatly from one another? Species number in the able time. Bacteria respond to ecological opportunity by millions, varying in size from viruses to whales and from diversifying into a maximum number of ecologically algae to trees; varying in color from bright butterflies to differentiated types. dull and cryptic moths; varying in behavior from solitary © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. 560 Speciation and Macroevolution polar bears to highly social honey bees; and varying in adjective is applied because the products of a radiation numerous other ways in exploiting the environment for are conjectured or known to be adapted to exploiting the food, avoiding their enemies, and reproducing. How is environment in different ways. all this variation to be explained? In the last 20 or 30 years the range of extant organ- Adaptive radiations provide rich material for seeking isms that have been studied in detail has increased dra- answers to these questions because they comprise groups matically, owing largely to the availability of molecular of distinctive yet closely related species. An adaptive ra- phylogenies for inferring relatedness among species and diation is the product of differentiation of an ancestral the pattern and rates of diversification. With these stud- species into an array of descendant species that differ in ies has come increasing scrutiny of the term itself, and the way they exploit the environment. When the differ- debate on definitions. Should an unusually high rate of entiation has proceeded rapidly, the evolutionary tran- diversification be an essential ingredient of the defini- sitions from one state to another can readily be char- tion? How is an adaptive radiation to be distinguished acterized and strongly interpreted. from a nonadaptive radiation? These questions become Angiosperm plants, dinosaurs, and marsupial mam- important in comparative studies when generalizations mals are typical examples at high taxonomic levels. are sought across a broad taxonomic range of organ- Typical examples at lower levels are Darwin’s finches on isms. There are no simple answers because there is no the Gala´pagos Islands, honeycreeper finches, Drosoph- clear line of demarcation or break point between adap- ila, spiders, the silversword alliance of plants in the tive radiations—defined by numbers of species, variety Hawaiian archipelago, cichlid fish in the Great Lakes of or rates of diversification—and all others. As used in this Africa, and Anolis lizards in the Caribbean. These ex- chapter the term adaptive radiation is most usefully amples have the following in common: (1) they comprise applied to those groups that have diversified rapidly and several to many species, (2) the species vary morphol- interpretably, such as the ones cited earlier. ogically in conspicuous ways, and relatedly, (3) they oc- cupy a diversity of ecological niches. Most of the species 3. THE ECOLOGICAL THEORY were (4) derived from a single ancestor in their current environment, and (5) most diverged relatively rapidly. By placing the occurrence of fossils within a time frame, Cichlid fish in Lake Malawi are an outstanding example. paleontologists like Simpson were able to detect a pat- Hundreds of species—the exact number is unknown— tern in the history of a radiation. A radiation was seen to were derived from one or a few common ancestors in the begin with rapid multiplication of species as well as di- last 2 million years, and they have diversified into many versification of morphological types. As the radiation trophic forms, including algae-, insect-, snail-, and fish- proceeded, both species proliferation and morpholog- eating specialists. Their mouth and teeth morphologies ical evolution slowed down. The observed pattern gave reflect their diets, and for this reason the variation is in- rise to an inferred process, as follows. Invasion of an ferred to be adaptive, that is to say, the product of diverse underexploited environment allowed species to multiply natural selection. One group alone, the rock-dwelling at a high rate. At the same time they diversified pheno- “Mbuna” of the genus Tropheops, comprises 230 spe- typically and ecologically. Eventually, both speciation cies. Ole Seehausen has calculated that one new species and diversification rates declined as competition in- arose every 46 years! creased for a diminishing variety of unexploited or un- derexploited resources. This is an explicitly ecological interpretation. On one occasion Simpson used T. H. 2. ORIGIN AND DEVELOPMENT OF THE CONCEPT Huxley’s phrase “filling the ecological barrel” to capture The term adaptive radiation was coined in 1902 by the essence of a limited environment. The greatest op- a paleontologist, H. F. Osborn, and the phenomenon it portunity for occupancy occurs at the beginning, when refers to was popularized by another, G. G. Simpson, the barrel is empty, and by implication there is an in- about 50 years later. Simpson viewed the evolutionary creasing difficulty for newcomers to fit as the barrel fills. radiation of a major group of animals, such as marsupial Dolph Schluter converted a coherent explanation mammals, as various lines of descent from a common into a theory by identifying all the elements and framing ancestor arising more or less simultaneously and diverg- them as hypotheses to be tested by their predictions. The ing in different morphological and ecological directions, key elements of the theory are (1) phenotypic differ- rather like spokes radiating from the hub of a wheel. This entiation caused by natural selection arising from dif- image is powerful yet fails to represent the correct evo- ferences among environments, (2) competition for re- lutionary pathway of bifurcating branches in a treelike sources, and (3) speciation governed by both processes. structure. Nonetheless, the term has stuck. The adaptive The theory is not one to be rejected by the first contrary © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. Adaptive Radiation 561 observationsomuchasanorganizationalframeworkfor environments,