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Darwin did so, shocked that he had been "forestalled".
When comparing DNNSresid of potential invaders in invaded and non-invaded communities we found support for Darwin’s naturalization hypothesis (phylogenetic overdispersion) at the fine sampling resolution at which competitive interactions take place on Mediterranean coastal dunes. In other words, invaders were more likely to be present in plots when they were more phylogenetically distant from their native relatives occurring in that plot (e.g. in the case of C. acinaciformis, one of the most invasive species in these environments. See for a discussion on patterns of single invaders and their likely interactions with native species). This finding was reported at relatively small spatial scales in other contexts (, ). As we found that several traits tended to show a phylogenetic signal (), it is likely that relatedness of the invader indeed reflected high functional similarity and thus niche overlap with the native species. Consequently, our results are in line with Darwin’s and Elton’s theoretical expectations on the biotic resistance of the native community to invasion. At coarser resolutions we instead found an opposite pattern suggesting a more dominant effect of habitat filtering (phylogenetic clustering). At the coarsest resolution (ca 35 km2) a high proportion of invaders was more related to the invaded communities than expected by chance. This trend mirrors patterns previously observed at regional and continental scales, for example in the floras of New Zealand and Australia (, ). In fact, when considering a coarser resolution, species can co-occur while avoiding direct biotic interactions. The main reason is presumably that greater environmental variation is encompassed within larger sites, providing opportunities for species to sort across environmental gradients (). It is therefore possible to reconcile apparently contrasting hypotheses and results for the patterns of relatedness of invading plants through the explicit consideration of the scale or resolution at which communities are sampled and defined. This is true in our system even though we restricted our analyses to coastal dunes, so that our coarse scale species pool is already quite filtered compared to earlier studies (, , ). However, given the strong sea-inland environmental gradient and strong zonation of the vegetation the species pool is still broad enough to detect the underdispersion in a high proportion of invaders.
When considering only the most related native species (DNNSresid – ), results from randomization tests corroborated Darwin’s naturalization hypothesis (phylogenetic overdispersion) at the finest sampling resolution (2 × 2 m). For this resolution, a high proportion of invaders (ca 25%; Fisher’s p-value = 0.002) were more distantly related to the closest relative in the invaded local community than under random expectations. However, none of the invaders showed a DNNSresid greater than expected by chance at the intermediate and coarse sampling resolutions. The trend was even inverted at the coarsest resolutions, with ca 10–15% of the invaders having smaller DNNSresid values than expected by chance (Fisher’s p-value
ad hoc hypothesis - The Skeptic's Dictionary - …
Quite surprisingly, in a recent study addressing ‘Darwin’s naturalization hypothesis’ at different scales, found that introduced plant species were more likely to become invasive in the absence of close relatives in the overall native flora of the Azores, but could not confirm this trend with a fine sampling resolution. The authors argue that on these islands the exclusion of similar invaders seemed to be mostly driven by the clustering of common enemies, such as herbivores and pathogens rather than by competition. They argue that enemy release should act at all spatial scales in a system like the Azores and thus produce a signal of over-dispersion not only at small but also at large scales, explaining why their outcome partially contradicts theoretical expectations. A similar result is reported by for reptiles in North America.
A standard methodological framework to address Darwin’s naturalization hypothesis was outlined in a recent review paper (). The authors suggest that the niche overlap between the invader and the members of the recipient community can be explored through a series of metrics based on functional or phylogenetic distances among species (when traits show a phylogenetic signal) and then tested with an appropriate null hypothesis and associated algorithm (, ). As different metrics and methods have specific assumptions and may lead to different conclusions, adopting a combination of approaches both for the quantification of niche overlap and for the statistical test is a good method of corroborating results.
What is Darwin's most famous ad hoc hypothesis?
There are two opposing hypotheses originally proposed by Darwin to link the phylogenetic relatedness between potential invaders and native communities with probabilities of successful invasion (). On the one hand, close relatedness is predicted to hamper local naturalization due to niche overlap and competition with native species (i.e. ecologically similar species compete more than dissimilar species; ‘Darwin’s naturalization hypothesis’) (). The resulting pattern is commonly referred to as phylogenetic overdispersion in studies of community assembly rules. On the other hand, appropriate niche adaptation may instead favor the naturalization of closely-related introduced species due to habitat filtering, which leads to a spatial pattern of phylogenetic clustering or underdispersion of niches (). Previous studies have found support for both of these hypotheses, leading to a fierce controversy in recent literature (, , , , ). Two aspects are likely to play a major role in explaining the discrepancy between theoretical predictions and among empirical studies: methodological differences and most importantly differences in the scale considered ().
In conclusion, we found that the relationship between phylogenetic distance and probability of occurrence of an invader changes with spatial resolution and that we can confirm Darwin’s naturalization hypothesis for fine resolutions where biotic interactions are also expected to be most important. Our results appear robust as both statistical tests applied supported the same conclusions. This paper therefore offers a new methodological framework for using the composition of local native species assemblages as a predictive tool for the new establishment of invaders. The specific resolution at which a community is no longer driven by biotic interactions, but rather by habitat filtering depends on habitat heterogeneity, and should therefore vary depending on the system being studied. In general, our results are promising for the perspective of incorporating information on the phylogenetic identity of resident native species into fine-grained predictive models for species invasions.
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ad hoc hypothesis - Unnatural Acts that can improve …
Extensionist theories are clearly less ontologically parsimonious thanDispersal Theories, since the former are committed to extra entitiessuch as land bridges or movable tectonic plates. Moreover,Extensionist theories were (given the evidence then available) notmanifestly superior in other respects. Darwin was an early critic ofExtensionist theories, arguing that they went beyond the“legitimate deductions of science.” Another critic ofExtensionist theories pointed to their “dependence on ad hochypotheses, such as land bridges and continental extensions of vastextent, to meet each new distributional anomaly” (Fichman 1977,p. 62) The debate over the more parsimonious Dispersal theoriescentered on whether the mechanism of dispersal is sufficient on itsown to explain the known facts about species distribution, withoutpostulating any extra geographical or tectonic entities.
Is the hypothesis of a universal common ancestor an ad hoc hypothesis
Darwin proposed two seemingly contradictory hypotheses for a better understanding of biological invasions. Strong relatedness of invaders to native communities as an indication of niche overlap could promote naturalization because of appropriate niche adaptation, but could also hamper naturalization because of negative interactions with native species (‘Darwin’s naturalization hypothesis’). Although these hypotheses provide clear and opposing predictions for expected patterns of species relatedness in invaded communities, so far no study has been able to clearly disentangle the underlying mechanisms. We hypothesize that conflicting past results are mainly due to the neglected role of spatial resolution of the community sampling. In this study, we corroborate both of Darwin’s expectations by using phylogenetic relatedness as a measure of niche overlap and by testing the effects of sampling resolution in highly invaded coastal plant communities. At spatial resolutions fine enough to detect signatures of biotic interactions, we find that most invaders are less related to their nearest relative in invaded plant communities than expected by chance (phylogenetic overdispersion). Yet at coarser spatial resolutions, native assemblages become more invasible for closely-related species as a consequence of habitat filtering (phylogenetic clustering). Recognition of the importance of the spatial resolution at which communities are studied allows apparently contrasting theoretical and empirical results to be reconciled. Our study opens new perspectives on how to better detect, differentiate and understand the impact of negative biotic interactions and habitat filtering on the ability of invaders to establish in native communities.
on ad hoc hypotheses, such as ..
The Festival of Bad Ad Hoc Hypotheses (or BAH!Fest for short) that was held Sunday night in Kresge auditorium was essentially one enormous anti-joke. The premise? Have seven educated individuals present seven well-argued, but astronomically absurd evolutionary theories in front of a panel of distinguished judges. To the victor go the spoils. And by spoils, they mean a 3-D printed rendering of Charles Darwin looking doubtful and saying, “I guess so?”….
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