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II.13 Coevolution John N. Thompson OUTLINE 1. All complex organisms depend on coevolved mutualistic interactions 2. Coevolution shapes defenses and counterdefenses 3. Coevolution of competitors further structures the web of life 4. Species coevolve as a geographic mosaic 5. Coevolution may sometimes foster speciation 6. Coevolution may result in predictable webs of interaction 7. The coevolutionary process is pervasive in human endeavors Coevolution is reciprocal evolutionary change among interacting species driven by natural selection. It is the evolutionary process by which many predators and prey, parasites and hosts, competitors, and mutualists adapt to each other in the constant struggle for life. It is also a process that can sometimes lead to new species, as different populations of interacting species coevolve in different ways in different geographic regions. Through its effects on adaptation and speciation, coevolution continually reshapes the web of life. Moreover, human society is increasingly altering the coevolutionary process through manipulation of ecological relationships among species within and among ecosystems, alteration of the genetic structure of crop plants, and development of novel strategies for mitigation of human diseases. GLOSSARY coevolution. Reciprocal evolutionary change in interacting species driven by natural selection coevolutionary cold spot. Geographic regions in which one of a set of interacting species does not occur or in which the interaction, although occurring, does not result in reciprocal evolutionary change coevolutionary hot spot. Geographic regions in which interactions between two or more interacting species result in reciprocal evolutionary change local adaptation. Adaptation of populations to the local physical environment or to the local populations of other species with which they interact 1. ALL COMPLEX ORGANISMS DEPEND ON COEVOLVED MUTUALISTIC INTERACTIONS Coevolution has been a major part of the process of evolution at least since the beginnings of complex life on Earth. In fact, many of the major events in the history of life are a direct result of the coevolutionary process that has created mutualistic symbioses among species. All complex organisms rely on mitochondria for cellular respiration. Those mitochondria are ancient bacteria that coevolved with their hosts and eventually became obligate organelles within the cells of all eukaryotic life. Every multicellular organism therefore has two genomes, a nuclear genome and a mitochondrial genome, as a direct result of this ancient coevolutionary process. Most animal species harbor one or more other coevolved symbionts that are necessary for their survival and reproduction. Among the most common are gut symbionts that aid digestion and nutrition. In many insect species, for example, coevolved symbionts provide one or more essential amino acids that are missing in the diet. Plants harbor yet other ancient coevolved partners: obligate symbionts called chloroplasts. These organelles drive photosynthesis, and few plant species can survive without them. Many plants also rely on mycorrhizal fungi that attach to the roots of plants and aid in nutrition. Legumes and a few other plant taxa have coevolved relationships with rhizobial bacteria that convert atmospheric nitrogen into a form usable by the plant. In addition, the leaves of some herbaceous plants and some trees are laced with endophytic fungi, whose coevolved relationships with plants are only now being explored in depth. A majority of plants also rely on animal pollination for reproduction. Hence, survival and reproduction in most plant species require inter- actions with multiple other species, and many of these interactions are highly coevolved. In general, all major biological communities are based on coevolved mutualistic relationships that form the underpinnings for community structure and succession . Most terrestrial communities rely on lichens (which are mutualistic symbioses between fungi and algae), mycorrhizal fungi, rhizobial bacteria, and chloroplasts to create the organic base on which other microbial life and animal species rely. Take away those coevolved mutualisms, and terrestrial life as we know it would disappear. The same central ecological role for coevolved interactions holds for marine communities. Coral reefs, which harbor so much of the diversity of marine life, are a result of coevolved mutualistic symbioses between corals and the dinoflagellates (sometimes called zooanthellae) they harbor. The phenomenon called ‘‘coral bleaching,’’ which has increasingly devastated coral reefs worldwide, results from the loss of the dino flagellate symbionts brought about by multiple forms of environmental change. Much deeper in the ocean, deep-sea vents harbor species that rely on complex symbioses that oxidize sulfide to produce energy, much as plants use the sun’s energy for photosynthesis . As new molecular tools continue to be developed to study microbial species, it is becoming increasingly evident that coevolved symbioses with microbial taxa permeate oceanic...


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