Yoel E Stuart and Jonathan B Losos’ article “Ecological character displacement: glass half full or half empty?” looks into the past notions on Ecological Character Displacement (ECD), and how the notions changed over the years. In addition, they also did a literature review on several past and recent studies. ECD is based on the notion that sympatric species compete for scarce resources; hence, the species that natural selection should favor the species to diverge in the usage of resources and phenotype (Stuart & Losos 402). Over the years, ECD has received both acceptance and rejection from the science community; hence, Stuart and Losos took the initiative to investigate the various studies that support ECD, to find out whether they answer the knowledge gaps that were identified by opposing scientists.
Tamar Dayan and Daniel Simberloff’s article “Ecological and community-wide character displacement: the next generation” argues that an increasing number of studies on ECD are based on functional morphology and other studies employ resource partitioning that is morphological (875). In addition, phylogenetic models and experimental work have made significant additions to the depth and scope to earlier theoretical studies (875). However, there are ecological and evolutionary challenges in relation to selective forces at intra-specific and inter-specific stages as well as resultant patterns that need to be attended (875). Tamar Dayan and Daniel Simberloff go through the evolutionary significance of ECD and cite Schluter and McPhail’s criteria as a credible measure for determining the credibility of ECD (875).
Stuart and Losos point out that ECD rose in the 1950’s and some of the supporting claims came from MacArthur who argued that factors such as differences in feeding times, perches used, and canopy heights came between the competitive exclusion of the Warbler species (402). Connells also showed that the barnacle communities where structured by inter-specific competition, while Hutchinson argued that beak size differences in sympatry observed in three species of the Galapagos tree finch were a necessity to permit at least two species to co-occur at different niches but within the same food web level (Stuart & Losos 402). These studies and many others are based on mathematical models of competitive exclusion, verbal models, limiting similarities, and minimum size ratios (Stuart & Losos 402).
Some of the counter-arguments against ECD include the assumption that resource competition is obvious in nature and has nothing to do with ECD observations (Stuart & Losos 402). In addition, Weins pointed out that resources competition is illogical because ECD empirical studies always identify changes in resource use, the prediction of difference in resource use and that the assumption that an observed difference is the result of competition, without adequate experimental proof (Stuart & Losos 403).
In the 1980’s various scholars came up with stronger evidence that supports ECD and inter-specific resource competition via experimental studies and realistic theoretical models (Stuart & Losos 404). Moreover, Schluter and McPhail proposed six testable attributes that a strong case of ECD should fulfill. These attributes address the queries the ECD-opposing scientists of the 1970s and early 1980s (Stuart & Losos 404). The Schluter and McPhail’s criteria (ECD criteria) are the same ones that Stuart and Losos used to review the studies published from 2005. In addition, they re-examined the reviews by Dayan and Simberloff, as well as Schluter (Stuart & Losos 404).
Stuart and Losos put to consideration any study that met at least one of the ECD criteria within the period of 1964 and 2000. The study revealed that only 20 of Schluter’s 64 studies fulfill at least four attributes of the ECD criteria, and only 5 studies met all the attributes (Stuart & Losos 405). In the case of Dayan and Simberloff’s studies, only 40 studies fulfilled at least one attribute, 10 studies met four attributes while one case met all six attributes. A major point to note is that approximately 30% of the studies assessed by Stuart and Losos meet at least 4 ECD criteria attributes while approximately 5% of the studies assessed meet all 6 attributes. As a result of this observation, Stuart and Losos note that accepting studies that do not meet all six attributes is comparable to accepting the half-empty-glass view. Hence, future studies ECD should strive to meet all six attributed to eliminate the chances of doubt.
Benjamin M Fitzpatrick and Michael Turelli’s article “The Geography of Mammalian Speciation: Mixed Signals from Phylogenies and Range Maps,” points out that the Age Range Correlation (ARC) has been used to show on that allopatric speciation is more common than sympatric speciation. They add that very few studies prove this claim and the methods used are inadequate (601). Hence, they recommend that changes can be included into ARC such as changes in the clades overlap quantification and the inclusion of the Monte Carlo approach in testing a null hypothesis if there is no relationship between phylogenetic relations and the geographic range relations (601). 14 clades of mammals were analyzed because of availability of data and the general agreement by mammalogists that mammal’s speciation is mainly allopatric. This was the case although some data showed that some clades were not allopatric (601).
The lack of relation between phylogenetic distance and range overlap is as a result of the scarcity of range overlap, changes in post speciation change, or a variety of geographic forms of speciation (602). For there to be a relationship between divergence and the range of overlap, three conditions must be met; first, at various time there must be comparable pairs, including the periods that geographical ranges are enlightening on the geography of speciation (Fitzpatrick & Turelli 602). Secondly, the dataset should have a predominant form of cladogenesis, and finally, there should be sufficient variation in pairs of range overlap, for the detectability of the pattern (Fitzpatrick & Turelli 602). All these conditions can be realized by the critical execution of ARC as well as the unambiguous consideration of alternative and null hypotheses.
Benjamin M Fitzpatrick and Michael Turelli used data from the “Mammalian Species” accounts published by the American Society for mammologists and UICN status surveys (603). The range maps were digitized, and then they were superimposed over the world map in the ArcView GIS 3.2 application (603). Thereafter a script was formulated to compute overlaps and range areas (603). The results of the study did not show consistency in the geography speciation (610). This finding disputes earlier notions that sympatric speciation is a norm among mammals. In addition, this finding also means that ARC is ineffective for several mammals (610). Inconclusive results should be expected if diversification involves an assortment of non-sympatric and sympatric speciation or when the ranges have blurred the geography of speciation (610). Therefore, ARC gives conclusive results when speciation is exclusively sympatric or allopatric.
Daniel I Bolnick and Benjamin M Fitzpatrick’s article “Sympatric Speciation: Models and Empirical Evidence” primarily addresses the nature of sympatric speciation, and how it can be distinguished from a non-sympatric form of speciation. Daniel I Bolnick and Benjamin M Fitzpatrick argue that there are few instances that prove the occurrence of Sympatric speciation, because of biogeographic changes that blur geographic patterns that host speciation occurrences from the past (459). In addition, there is positive progress in coming up with measures to ascertain the validity of the theoretical circumstances for sympatric speciation. Moreover, there is biological proof that a disruptive selection and direct selection on mating characteristics assist sympatric speciation (459).
Genuine sympatric speciation occurrences should satisfy four of the standards set by Coyne and Orr (Bolnick and Fitzpatrick 461). The standards include; the species should have a widely overlapping geographic range, speciation should be comprehensive, clades that arise from speciation should related species, and lastly, the evolutionary and biogeographic history of the groups must eliminate the chances of an allopatric stage (Bolnick and Fitzpatrick 461-62). Very few studies met all four standards. Bolnick and Fitzpatrick also present more evidence to prove the occurrence of sympatric speciation, the frequency of occurrence, the stages, the models, as well as criticism of the models.
In conclusion, all the authors present vital points that are important in filling knowledge gaps left by previous research works, and they can set precedence for future studies. It is healthy to question the validity and credibility of research work as a way of eliminating doubt and promoting healthy debate.
Works Cited
Bolnick, Daniel I and Benjamin M. Fitzpatrick. “Sympatric Speciation: Models and Empirical Evidence”. Annual Review of Ecology, Evolution, and Systematics. 38, (2007); 459–487. Print
Dayan, Tamar and Daniel Simberloff. “Ecological and community-wide character displacement: the next generation”. Ecology Letters. 8 (2005); 875–894. Print
Fitzpatrick, Benjamin M and Michael Turelli. “The Geography of Mammalian Speciation: Mixed Signals from Phylogenies and Range Maps”. The Society for the Study of Evolution. 60.3, (2006); 601–615. Print
Stuart, Yoel E and Jonathan B Losos. “Ecological character displacement: glass half full or half empty?”. Trends in Ecology & Evolution. 28.7, (2013); 402-408. Print
