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Adaptive Radiation Rosemary Gillespie I.19 Adaptive Radiation Rosemary Gillespie OUTLINE GLOSSARY 1. Conditions promoting adaptive radiation adaptive radiation. Rapid diversification of an ancestral 2. Are certain taxa more likely to undergo adaptive species into several ecologically different species, radiation? associated with adaptive morphological, physiolog- 3. Examples of current adaptive radiations ical, and/or behavioral divergence 4. Initiation of adaptive radiation attenuation. Decline in number of species represented 5. Speciation in adaptive radiation on islands with distance from a source of colonists 6. Community assembly divergent natural selection. Selection arising from en- 7. Molecular basis for adaptive change vironmental forces acting differentially on pheno- 8. Testing adaptive radiation typic traits (morphology, physiology, or behavior) resulting in divergent phenotypes; reproductive iso- Adaptive radiation is generally triggered by the appearance lation may occur as a side effect, either in sympatry of available niche space, which could result from (1) in- or allopatry trinsic factors or key innovations that allow an organism to ecological character displacement. Divergence in eco- exploit a novel resource, and/or (2) extrinsic factors,in logical traits (which may lead to reproductive iso- which physical ecological space is created as a result of lation as a by-product) caused by competition for climatic changes or the appearance de novo of islands. shared resources There are no general rules as to what taxa are more likely ecological release. Expansion of habitat or use of re- to undergo adaptive radiation, although some lineages sources by populations into areas of lower species may have certain attributes that facilitate adaptive radia- diversity with reduced interspecific competition tion in the appropriate setting. The process of adaptive ecological speciation. Process by which barriers to gene radiation is described below, as well as some prime ex- flow evolve between populations as a result of eco- amples of the phenomenon. Adaptive radiation is generally logically based divergent natural selection initiated by expansion of ecological amplitude of a taxon ecomorph. A group of populations, species, etc., whose into newly available ecological space, followed by special- appearance is determined by the environment ization, the process possibly facilitated through adaptive escalation/diversification. Diversification of a herbivore/ plasticity. Speciation associated with adaptive radiation parasite in concert with its host in which the adapta- may involve one or more of the following: founder events, tions of the host to counter exploitation by the her- divergent natural selection, sexual selection, and hybrid- bivore or parasite build one on each other, and vice ization. Competition is generally implicated in divergent versa natural selection and in dictating the communities of spe- escape and radiation. Diversification of a herbivore/ cies formed during the course of adaptive radiation. Current parasite in concert with its host in which the host is research is focused on (1) examining the molecular un- generally considered to radiate before exploitation derpinnings of apparently complex morphological and be- and subsequent radiation by the herbivore or para- havioral changes that occur during the course of adaptive site, and vice versa radiation, and (2) experimental manipulation of bacteria founder event. Establishment of a new population with to assess the conditions under which adaptive radiation few individuals that contain a small, and hence occurs. unrepresentative, portion of the genetic diversity 144 Autecology relative to the original population, potentially it will be colonized repeatedly by taxa from those leading to speciation habitats, and patterns of species diversity will be gov- key innovation. Any newly acquired structure or prop- erned by ecological processes of immigration and ex- erty that permits the occupation of a new environ- tinction, rather than by evolutionary processes. ment, or performance of a new function, which, in turn, opens a new adaptive zone Intrinsic Factors: Key Innovations nonadaptive radiation. Elevated rate of speciation in the absence of noticeable ecological shifts A key innovation is a trait, or a suite of traits, that sexual selection. Form of natural selection based on an allows an organism to exploit a novel resource or in- organism’s ability to mate such that individuals with crease the efficiency with which a resource is used, attributes that allow them greater access to the op- thereby allowing a species to enter a ‘‘new’’ adaptive posite sex, either through (1) combat with the same zone; the ecological opportunity thus provided may sex or (2) attributes that render them more attrac- permit diversification. For example, the evolution of tive to the opposite sex, mate at higher rates than C4 photosynthesis, which enhanced rates of carbohy- those that lack these attributes drate synthesis in open environments, likely served as taxon cycle. Temporal sequence of geographic distri- a key innovation preceding radiation of most of the bution of species from (1) colonizing to (2) differ- major lineages of grasses. Among recent adaptive ra- entiating to (3) fragmenting and to (4) specializing diations, one of the best-known key innovations is that of the pharyngeal jaw apparatus of cichlid fish (figure Adaptive radiation is the rapid diversification of a 1). Although most bony fishes have pharyngeal gill lineage into multiple ecologically different species, arches modified to process prey, the cichlid pharyngeal generally associated with morphological or physiolog- jaw is unique in having upper pharyngeal jaw joints, a ical divergence. The phenomenon can be characterized ‘‘muscular sling,’’ and suturing that functionally fuses by four criteria: common ancestry, a phenotype– the lower pharyngeal jaw. The features of the jaw ap- environment correlation, trait utility, and rapid specia- pear to have allowed cichlids to exploit a diversity of tion. The concept of adaptive radiation, and particularly prey types, including large fish and hard-shelled prey diversification of ecological roles by means of natu- that are unavailable to most other aquatic vertebrates. ral selection, has had a long history, beginning with In flowering plants, floral spurs have evolved at least the observations of Charles Darwin on the Gala´pagos seven times, each time resulting in higher rates of di- Islands, and has played a pivotal role in the development versification, perhaps the best known radiation being of the Modern Synthesis (see Givnish and Sytsma, 1997, in the columbines (Aquilegia, Ranunculaceae). for a detailed history of the concept). Interacting species, such as herbivores or parasites and their hosts, may themselves create ecological op- portunity and provide some notable examples of key 1. CONDITIONS PROMOTING ADAPTIVE RADIATION innovations. Here, the host may develop a defense to the The most familiar present-day adaptive radiations are herbivore or parasite (e.g., a plant may develop toxic- known from isolated archipelagoes or similar island- ity), but in due course the herbivore or parasite may like settings (e.g., lakes). However, it is quite likely that develop resistance to the defense of the host. Paul Ehr- much of the diversity of life originated through epi- lich and Peter Raven (1964) examined such coevolu- sodes of adaptive radiation during periods when eco- tionary responses and hypothesized that when plant logical space became available for diversification. lineages are temporarily freed from herbivore pressure Within this context there are two primary mechanisms via the origin of novel defenses, they enter a new through which ecological space can become available: adaptive zone in which they can undergo evolutionary (1) intrinsic changes in the organism often associated radiation. However, if a mutation then arises in a group with key innovations, and (2) extrinsic effects, includ- of insects that allows them to overcome these defenses ing environmental change and colonization of isolated and feed on the plants, the insect would then be free to landmasses. The two situations may be linked; for ex- diversify on the plants in the absence of competition. ample, an intrinsic change may allow an organism to The major radiations of herbivorous insects and plants exploit a new environment. In either case, individuals may have arisen through repeated steplike opening of exploiting the newly available niche space must be novel adaptive zones that each has presented to the isolated to some extent from the remainder of the pop- other over evolutionary time. Often referred to as escape ulation to allow for genetic divergence associated with and radiation, the host is generally considered to radiate adaptive radiation. If a new habitat becomes available before exploitation and subsequent radiation by the in close proximity to other such habitats (not isolated), herbivore or parasite. This idea has been supported by Adaptive Radiation 145 more recent studies of insect diversification in the con- A. text of their host plants, with repeated evolution of an- a giosperm feeding in phytophagous beetles associated with an increased rate of diversification. There is a con- sistently greater diversity of beetles among plants in which latex or resin canals have evolved as protection b against insect attack. In the same way, adaptive radia- tion of parasites may occur as a consequence of host switching to a new lineage of hosts.
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