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I.19 Adaptive Radiation Rosemary Gillespie OUTLINE 1. Conditions promoting adaptive radiation 2. Are certain taxa more likely to undergo adaptive radiation? 3. Examples of current adaptive radiations 4. Initiation of adaptive radiation 5. Speciation in adaptive radiation 6. Community assembly 7. Molecular basis for adaptive change 8. Testing adaptive radiation Adaptive radiation is generally triggered by the appearance of available niche space, which could result from (1) intrinsic factors or key innovations that allow an organism to exploit a novel resource, and/or (2) extrinsic factors, in which physical ecological space is created as a result of climatic changes or the appearance de novo of islands. There are no general rules as to what taxa are more likely to undergo adaptive radiation, although some lineages may have certain attributes that facilitate adaptive radiation in the appropriate setting. The process of adaptive radiation is described below, as well as some prime examples of the phenomenon. Adaptive radiation is generally initiated by expansion of ecological amplitude of a taxon into newly available ecological space, followed by specialization , the process possibly facilitated through adaptive plasticity. Speciation associated with adaptive radiation may involve one or more of the following: founder events, divergent natural selection, sexual selection, and hybridization . Competition is generally implicated in divergent natural selection and in dictating the communities of species formed during the course of adaptive radiation. Current research is focused on (1) examining the molecular underpinnings of apparently complex morphological and behavioral changes that occur during the course of adaptive radiation, and (2) experimental manipulation of bacteria to assess the conditions under which adaptive radiation occurs. GLOSSARY adaptive radiation. Rapid diversification of an ancestral species into several ecologically different species, associated with adaptive morphological, physiological , and/or behavioral divergence attenuation. Decline in number of species represented on islands with distance from a source of colonists divergent natural selection. Selection arising from environmental forces acting differentially on phenotypic traits (morphology, physiology, or behavior) resulting in divergent phenotypes; reproductive isolation may occur as a side effect, either in sympatry or allopatry ecological character displacement. Divergence in ecological traits (which may lead to reproductive isolation as a by-product) caused by competition for shared resources ecological release. Expansion of habitat or use of resources by populations into areas of lower species diversity with reduced interspecific competition ecological speciation. Process by which barriers to gene flow evolve between populations as a result of ecologically based divergent natural selection ecomorph. A group of populations, species, etc., whose appearance is determined by the environment escalation/diversification. Diversification of a herbivore/ parasite in concert with its host in which the adaptations of the host to counter exploitation by the herbivore or parasite build one on each other, and vice versa escape and radiation. Diversification of a herbivore/ parasite in concert with its host in which the host is generally considered to radiate before exploitation and subsequent radiation by the herbivore or parasite , and vice versa founder event. Establishment of a new population with few individuals that contain a small, and hence unrepresentative, portion of the genetic diversity relative to the original population, potentially leading to speciation key innovation. Any newly acquired structure or property that permits the occupation of a new environment , or performance of a new function, which, in turn, opens a new adaptive zone nonadaptive radiation. Elevated rate of speciation in the absence of noticeable ecological shifts sexual selection. Form of natural selection based on an organism’s ability to mate such that individuals with attributes that allow them greater access to the opposite sex, either through (1) combat with the same sex or (2) attributes that render them more attractive to the opposite sex, mate at higher rates than those that lack these attributes taxon cycle. Temporal sequence of geographic distribution of species from (1) colonizing to (2) differentiating to (3) fragmenting and to (4) specializing Adaptive radiation is the rapid diversification of a lineage into multiple ecologically different species, generally associated with morphological or physiological divergence. The phenomenon can be characterized by four criteria: common ancestry, a phenotype– environment correlation, trait utility, and rapid speciation . The concept of adaptive radiation, and particularly diversification of ecological roles by means of natural selection, has had a long history, beginning with the observations of Charles Darwin on the Galápagos Islands, and has played a pivotal role in the development of the Modern Synthesis (see Givnish and Sytsma, 1997, for a detailed history of the concept). 1. CONDITIONS...


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