Does Specialization Must Lead to an Evolutionary Dead - Term Paper Example

 Contents I.INTRODUCTION 2 II.EVIDENCES FROM FLORAL PLANTS 3 III.EVIDENCES FROM NON-FLORAL PLANTS 5 IV. EVIDENCES FROM PARASITES 5 V.EVIDENCES FROM INSECTS’ HABITAT 8 VI.EVIDENCE FROM MAMMALS (GIANT PANDA) 8 VII.POSSIBLE MECHANISM OF A SPECIALIST BECOMING A GENERALIST: ECOLOGICAL FITTING 9 VIII.CONCLUSION 11 REFERENCES 12 DOES SPECIALIZATION MUST LEAD TO AN EVOLUTIONARY DEAD END I. INTRODUCTION Academic community has always been intrigued by the highly specialized organisms, even though they have always been considered to be the final milestone on the road of evolution. Often termed as ‘inplastic’, ‘senescent’, ‘genetically fixed’ or ‘static’; they have been considered to have completed their course, to be at the brink of extinction, their survival threatened by environmental changes and geological upheavals (Amadon, 1943). The term ‘specialist’ refers to organisms that are adapted to a narrow range of environmental conditions and thus occupy one extreme of the adaptation band. On the other end are ‘generalists’, which are able to utilize a wider range of environmental conditions. Factors such as “jack of all, master of none” and freedom from natural enemies, have been used to justify the prevalence of specialists, however; it has also been postulated that in spite of these advantages, specialization would gradually lead to loss of genetic variations, thus disabling capacity of adaptation to altered set of environmental conditions. These factors would eventually lead to extinction of a specialized organism, a theory first put forward by Cope IN 1896 and termed “law of the unspecialized” (Kelly and Farrel, 1998). Another implication of this concept is that the specialists are derived from generalists and the change is irreversible. This theory has been challenged by larger number of recent studies, and the evidences contradicting the implications of this theory are mounting. These evidences have proved that either no directionality exists in the evolution of specialists and generalists or that the generalists have evolved from specialist ancestors (Stireman III, 2005). In either case specialization definitely does not lead to an evolutionary dead end. That this trend is not an exception or restricted to a few taxa, is evidenced by the fact that such reports are available for organisms belonging to varied phyla including angiosperms, insects, fishes, mammals and microorganisms. This paper aims to prove that specialization does not definitely lead to an evolutionary dead end using evidences from relevant researches on floral plants, parasites, and mammals. As further evidence of this concept a plausible mechanism through which transition of specialist to generalist has been reported to occur will also be discussed. II. EVIDENCES FROM FLORAL PLANTS Floral plants depend on various species acting as pollen vectors for the completion of their reproductive cycle. Thus pollen biology is an integral part of study of evolution of floral plants. Darwin had stressed the importance of the role of pollen vectors in the reproduction of floral plants and its ecological and evolutionary significance (Harder and Aizen, 2010). Specialization of plants in context to their pollination apparatus and pollinators has major impact on the diversification of the species and is of paramount significance in its sympatric reproductive isolation, even leading to speciation (Gegear and Burns, 2007). Cope’s law has been traditionally applied to pollination biology as well and the hypothesis that specialized pollination systems have evolved from ancestral generalized systems has been accepted unchallenged. This has neither been established nor studied. However, among the recent reports challenging the universality of Cope’s law, some belong to highly specific dependence of floral plants and their pollination vectors. The genera Ruellia with approximately 300 species is characterized by diverse floral morphology and varied pollinators namely bees, hummingbirds, hawk moths, bats, butterflies etc. Among these floral morphology of certain species renders the reproductive parts accessible exclusively to hummingbirds. Besides the hummingbird flowers, there are also hawk mouth flowers and bat flowers. The evolutionary trends in the pollination system of various Ruellia species were studied by Tripps and Manos (2008). Corolla color was found to be associated with specific pollinators in most cases; e.g. the pollination vector for all eight species with red corollas was hummingbird. Of the 11 purple flowered taxa, 10 had bees as pollinators and five had butterflies, homopterans and hummingbirds. So in order to study the evolution of pollination system of Ruellia, Tripps and Manos reconstructed the ancestral states of corolla colors. The results indicated that the pollination system of Ruellia has undergone several transitions which were multidirectional. From purple ancestral flowers there were at least six transitions to white flowers, two to yellow and only one to red flowers. On the other hand from red ancestral flowers, which were specialists with hummingbirds as the sole pollinators; there were 8-10 transitions to purple, 4-5 to white and at least one to yellow flowers. Thus specialist hummingbird pollinated species have not gone into an evolutionary dead end, but have undergone multiple transitions in reverse directions to give rise to bee or insect pollinated generalist species. Moreover data indicates that at least some of these morphological changes are adaptive, i.e. in response to selection pressure by pollinators. Thus in context to floral plants, the study by Trips and Manos has proved that not all specialization leads to an evolutionary dead end. While this might be true in some cases, e.g. species of Ruellia which have are pollinated by nocturnal vectors such as hawk mouths or bats, and have lost some of the corolla pigments e.g. anthocyanins. There are also examples of reverse transitions in hummingbird species which contradict the statement. Thus whether speciation leads to evolution or not; is a generalized question which should instead read as how much of specialization would lead to evolutionary dead end. III. EVIDENCES FROM NON-FLORAL PLANTS Springtail genus Willemia exhibits diverse habits with some species being generalized loam dwelling, while some being specialized psammophilous, living exclusively in sandy habitats. A cladistic analysis of the morphological characters of this genus revealed that psammophily, although a specialist trait is ancestral and generalists with loam habitat have evolved at least twice from these specialists. Thus, they provide evidence for ability of habitat specialist to revert to generalist and vice versa (D’Haese, 2000). IV. EVIDENCES FROM PARASITES Parasites exhibit host range specialization allocating them a position close to or exactly at the tip of the evolutionary tree and therefore predisposes them to high probability of extinction according to Cope’s law. However, many studies from broad range of families namely columbiformes (Johnson et al., 2009), tachinids (Stireman III, 2005), Dendroctonus spp. (Kelly and Farrel, 1998) have brought to light contradicting evidences disproving the definite evolutionary dead end theory of parasitic specialists. A study on ectoparasitic feather lice (Columbicola) of dove was conducted to test the hypothesis that evolution proceeds from generalists to specialists (Johnson et al., 2009). Species of the genus Columbicola exhibit a broad range of host specificity making them suitable for study of evolutionary patterns. The study made a comparative analysis of host specificity and competition using phylogenetic analyses of two mitochondrial (COI and 12S rDNA) and one nuclear gene (EF-1α) for Columbicola. A reconstruction of evolutionary tree clearly showed the direction of evolution to be from ancestral generalists to specialists, with at least six such incidences verified. The marginal probability of the specialists being ancestor to all dove wing lice was derived using maximum likelihood method to be 99%. Moreover presence of potentially competing species was found to be correlated to the evolution of host generalists (Johnson et al., 2009). Thus competition, in contradiction to conventionally accepted theory favored generalizations in this case. In 2005, Stireman III tested the same hypothesis of generalists being ancestral to specialists on the basis of a study on parasitoid flies of Tachinidae using parsimony and maximum likelihood methods. Tachinid flies include 8000 known species which are highly varied in the range of hosts and mechanisms of attack with both strictly monophagus and widely polyphagus species are recognized. On the basis of conventional theories it should be assumed that monophagus species would be derived from polyphagus ones and not vice versa. However, it was observed that it is the polyphagus species who occupy the phylogenetic trees reconstructed from the data. Moreover polyphagy has evolved repeatedly in this family (Stireman III, 2005). A study on the Chrysomaline leaf beetles analyzing the molecular aspects of their phylogeny and defense secretions provided evidence contradicting the generalist to specialist evolution theory. Ancestral beetle species were found to have autogenous metabolism involving synthesis of monoterpene iridoids, the insect being adapted to and dependent on chemistry of host plant. Two lineages independently developed from this, both involving host dependent metabolism (e.g. based on conversion of salicin to salicylaldehyde again dependent on single host plant of family Salicaceae). However, an interrupta group made a departure from this enclosure, and derived in to a generalist capable of forming multiple host plant associations, thus proving that generalists can be derived from specialists and specialists need not always be at the dead end of evolution (Termonia et al., 2001). In an older study done on bark beetle genus Dendroctonus, which is an ectoparasite of at least four different genera of family Pinaceae, the phylogeny of the genera was estimated to find whether specialization in this genera is derived (Kelly and Farrel, 1998). Using mitochondrial DNA sequencing and mapped transitions it was found that the evolution of Dendroctonus with its hosts Pinus and Picea was associated. However, there was lack of affiliations in the phylogeny of Dendroctonus and its various hosts from Pinus. On the basis of data gathered it was found that at least six times specialists were derived from the generalists, which changed feeding pattern from oligophagus to monophagus. Of these six, three have again departed from monophagy and reversed directions to polyphagy at least occasionally. There also is evidence to the effect that some generalists have added some host genera from Pinaceae to their diet (Kelly and Farrel, 1998). Thus this study once again successfully questions the universality of unidirectional derivation of generalists from specialists. While the above examples successfully present the argument specialization does not definitely lead to a dead end, they cannot fully counter the more frequent occurrences of transitions from generalists to specialists. A comparative study of rate of the two transitions in 15 groups of phytophagus insects showed that the rate of generalist to specialist forward transitions occur at a much higher rate than the reverse. The ratio of forward to .reverse transition arte was estimated to be 1.76 using Grafen branch lengths and 1.47 using uniform branch lengths (Nosil, 2002). The plausible explanation for this trend can be that the transitions from generalist to specialist results due to loss of certain abilities that help them interact favorably with a broader range of environmental conditions. For e.g. in Drosophila sechellia, a specialist; the ability to reproduce exclusively on ripe fruits of Morinda citrifolia (a species toxic to other Drosophila spp.) results due to loss of many of the putative bitter-taste genes. On the other hand a transition from specialist to generalist requires acquiring novel abilities, a process which may not be equally frequent. Hence, the higher rates of generalists to specialist transitions are observed in nature. Yet despite the high rate of null mutations in the Drosophila sechellia genome, maintenance of entire chemoreceptor gene family is indicative of the fact that an exposure to toxic plant can reverse the null mutations and lead to a reemergence of generalist population (Matsuo, 2008). V. EVIDENCES FROM INSECTS’ HABITAT An important set of evidence was obtained from the study of troglomorphic family of scorpions Typhlochactidae; six of nine species of which are troglobitic or hypogean, specialists adapted to the cave environment; while the rest are endogean and less specialized. It has long been reported that the evolution of troglobitics is irreversible and that they are at an evolutionary dead end. However a study by Prendini et al (2010) on the same revealed the above reports to be untrue, and proved that the troglobitics are capable of reverting to the endogean habitat and thus are not at an evolutionary dead end. VI. EVIDENCE FROM MAMMALS (GIANT PANDA) The giant panda (Ailuripoda melanoleuca) being one of the endangered mammals has been the subject of many ongoing researches that attempt to understand its evolutionary capability. A common factor observed in numerous studies employing varied techniques was the lack of genetic variability in its genome and this factor was held responsible for the declining population of the species. However, a study conducted by Zhang et al. (2007) analyzing 655 base pairs of mitochondrial (mt) control region (CR) DNA and 10 microsatellite loci for samples from 159 individuals of its five extant mountain populations gave surprising results. It was found that there is no lack of genetic variability in the giant panda population, and hence this definitely could not contribute to the decline of its population. The results were made possible from study of larger sample population using more sensitive technology compared to earlier studies. Moreover it was also reported that the population has not declined as significantly as has been reported, but the population decline had started thousands of years ago, which had been accelerated recently due to human infringement into their habitats. In fact as a consequence of conservation strategies the population of giant panda has been showing a rise in recent years. Besides the genetic diversification reported for one of the five populations, has resulted from habitat fragmentation and not due to specialization. Thus the conjectures that the species is on the verge of extinction due to it having arrived at an evolutionary dead end is untrue and the genetic structure still remains highly dynamic (Zhang et al. 2007). VII. POSSIBLE MECHANISM OF A SPECIALIST BECOMING A GENERALIST: ECOLOGICAL FITTING Possible mechanism of specialist becoming a generalist is explained by the mechanism underlying host switching by parasites or insect predators, which are adapted or specialist to one particular host. Host switching is made possible by two events; first host range expansion by colonization of new hosts and second host range narrowing y loss of ancestral host. Thus the process involves a specialist becoming a generalist and then reverting to specialist but with new host. This at least proves that specialist retain the ability to become a generalist (and vice versa) if needed for its survival and are not at an evolutionary dead end. It also is indicative of the fact that at least some of the characteristics imparting the ability to colonize new host had exapted or originated as a consequence of organisms past experiences, i.e. the organism is preadapted to its novel host (Agosta et al., 2010). Ecological fitting concept is based on the assumption that the chief concern for the parasite or insect is the resource which it requires for its nutritional needs or completion of life cycle. The packaging of the resource is irrelevant to this apparent specialist. Thus when singular host species with its resource requirement is available, it behaves as a specialist, vice versa when multiple host species capable of providing for its resource requirement are available, it behave as a generalist. This ability of the organisms to track a particular resource, irrespective of the source organism of the resource is referred to as ‘ecological fitting’ (Brooks et al., 2006). Ecological fitting has been defined as a process “where a geographical range expansion leads to a colonization or host shift with negligible initial evolutionary change” (Janz and Nylin, 2008). The mechanism has been well documented in context to various insect predators namely Greya polotellla (Janz and Nylin, 2008); Anurans and their platyhelminthes parasites (Brooks et al., 2006); frog lung flukes (Haematoloechus spp.) (Brooks et al., 2006) etc. Three underlying mechanisms of ecological fitting have been listed by Agosta and associates (2010). The first mechanism is phenotypic plasticity, as a consequence of which the organism is able to respond to the novel conditions. Second a set of correlated trait evolution which is able to produce phenotypes that are preconditioned to adapt to novel condition. Finally, retention of traits from experiences of resource constraints and phylogenetic conservation of traits that helped in the use of scarce resources such as design constraints enable to perform under a novel set of condition such as a host which is either similar or same as the ancestral host (Agosta et al., 2010). VIII. CONCLUSION Thus based on the evidences of host shifts available from host parasite (including phytophagus insects-plant host) studies, it is safe to claim that specialist do retain the ability to develop in to generalists. Considering the fact that parasites are the most stringent specialist known, the same theory can be extrapolated to include other living organisms as well. In fact evidences for specialists converting to generalists are now available from broader group of organisms including floral plants and mammals. The mechanism enabling this transition is ecological fitting, which not only provides a rational explanation for it, but also stipulates the preadaptation or preexistence of the ability to transform into a generalist in the organism as a consequence of its past experiences as well as phenotypic plasticity. Thus, the dogma claiming specialists to be at the tip of evolutionary tree is definitely challenged and that they are at an evolutionary dead end is proved to be untrue. REFERENCES Read More