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Our work aims to answer the question: "Is ecological specialization always an evolutionary dead-end"?


Evolutionary specialization occurs when an organism is capable of optimally using a narrow fraction of the resources available. It is usually a result of competition or coevolution. Specialized organisms frequently have phenotypic traits (e.g. morphological, physiological, behavioural, molecular or a combination of thereof) which permit an optimal exploration of the resources on which they are specialized. A controversial topic is whether the evolution of such traits makes it difficult for specialist clades to give rise to generalist ones. This review aims to abridge the studies that have addressed to explain the evolutionary patterns of generalization and specialization.


Specialist organisms utilize only a subset of suitable and available resources, while generalist species use a wide array of resources (Futuyma & Moreno 1988) depending on the environmental resource availability. The evolution of specialization is often coupled with the appearance of phenotypic traits that optimize the use of a specific kind of resource. On the other hand, once an organism becomes specialist, it gets trapped in the ecological niche and loses the ability to explore other types of resources as efficiently as before. Thus, there is a trade-off between the ability to explore a wide variety of resources and the efficiency in using each one of them (Futuyma & Moreno, 1988) imposed by the phenotype. Specialization may arise in different morphs of the same species (e.g. Meyer, 1989) or among different species (e.g. flower-visiting insects that explore either pollen or nectar, Krenn et al. 2005; spiders that feed on woodlice employ different grasping tactics, Rezác et al. 2008). Species exploring similar resources often have similar phenotypic modifications even if they are not phylogenetically related.

Many factors have been proposed to explain the evolution of specialization as a habitat-specific adaptation (e.g. Schluter 1993, 1995; Joshi & Thompson 1995), competition for resources (MacArthur & Levins 1964), predator evasion (e.g. Bernays & Graham 1988; Bernays 1989) and mate-finding. Many of these factors have involved the modification of morphological structures, changes in feeding habits or in the case of symbiotic species several types of host selection. These changes enhance the competitive ability so that specialists are predicted to have higher fitness in their native environment than do plastic generalists (Donohue et al. 2000).

One of the most controversial and debatable topics in evolutionary biology is regarding to the evolution of specialists and generalists. Ecological specialization is often described as a derived state where further diversification is hampered, representing thus an “evolutionary dead-end” (Futuyma & Moreno 1988; Moran 1988; Kelley & Farrel 1998). This theory is based on the assumption that highly specialized structures or organisms will rarely evolve into a new structure or give rise to another species (Holmes 1977). Generalist species on the other hand could be more likely to diversify into specialists ones.

Here, we present scenarios and mechanisms of specialization that do not lead to an evolutionary dead-end. We employ a phylogenetic approach to understand if generalists always give rise to specialists or whether specialists have also resulted in generalist clades, indicating a shift of specialization to generalization.

Is there phylogenetic evidence for ecological specialization being an evolutionary dead-end?

Futuyma & Moreno (1988) suggested definition of specialization and generalization in reference to particular axes of ‘n-dimensional’ hypervolume; where environmental variables or resources form the axes. Ecological specialization is hard to define quantitatively because the specialist-generalist spectrum is a continuum trait. Hence, we define specialized species by comparing their range of resource use with that of closely related species. This highlights the importance of studying specialization in a phylogenetic framework.

We assessed a number of phylogenies (Table 1) of different groups of organisms containing varying degrees and types of specialization (such as host plants, feeding behaviour, habitat type, etc.) highlighting scenarios that do not lead to an evolutionary dead-end. Three of the 23 phylogenies we analysed were consistent with the idea that specialization leads to an evolutionary dead-end (e.g. Fig. 1A), while most other scenarios (15) indicated the diversification of specialists into the generalist clades (e.g. Fig. 1B). However, in five of the phylogenies we analysed the specialists occupied the tips of the tree because of the recent divergence and hence were not ideal for such comparisons. In some of the aforementioned cases, the authors suggest that some ways of specialization may lead to an evolutionary dead-end (moth- and bat-pollination), while others do not (humming bird-pollination; Tripp & Manos 2008). In some of these cases pertaining to host range evolution, the generalists were repeatedly derived from specialist lineages and occupied terminal branches of the phylogeny. Some authors suggested that the parasite and parasitoid generalist lineages may be less likely to diversify due to few opportunities of genetic isolation (Stireman 2005).

It is difficult to discern why a few specialist clades in a phylogeny diversify into generalists while others don’t. The degree of specialization has a greater impact on the ability of the specialist to reverse to the ancestral state. Here we discuss several mechanisms that could explain why and how ecological specialization can be reversed.

Mechanisms of reversal to generalization

1. Character loss, gene expression and re-evolution of ancestral characters.
Specialization often leads to the reduction or complete loss of morphological features. According to the Dollos’ law the reappearance of complex features could be nearly impossible. The theory is based in the assumption that after a structure is lost the genes related to it would degenerate (Gould 1970; Kohlsdorf and Wagner 2006). Therefore, it is important to understand whether a gene is lost completely or if it is present but remains unexpressed.

The reduction and loss of limbs in some reptiles has evolved several times and is often associated to specialization towards a burrowing behaviour (Kohlsdorf & Wagner 2006, but see Wiens & Slingluff 2001). The expression of Hox genes is responsible for patterning and limb positioning during tetrapod embryonic development (Burke et al. 1995; Shubin et al. 1997). Limb development and hox gene expression patterns therefore constitute a good model for testing the hypothesis that specialization is an evolutionary dead-end.

The reversibility of digit loss was documented for newts (Wagner et al. 1999). This study shows that the expression of the Hoxa-11 gene in Xenopus is similar to those described in chick and mice while in the newt Notophthalmus has an autopodial expression of the Hoxa-11 gene. Their results suggest that the postaxial digits in urodeles may be evolutionary innovations probably arising from an ancestor with reduced digits (Wagner et al. 1999). The reversibility of digit loss was also reported for Gymnophthalmidae (Kohlsdorf and Wagner 2006). A study regarding the morphological transition from lizard to snakelike body forms in reptiles however indicates that the digit loss was never recovered in the studied group (Wiens & Slingluff 2001). The reduction and loss of the digits might have occurred multiple times in evolution in a gradual way. The results suggest for a gradual evolution towards limb reduction which is neither necessarily associated to burrowing behaviour nor with the expression of the hox gene (Wiens & Slingluff 2001). The implications of these unexpected results for specialization and gene expression should be further explored. In summary, there is clear evidence for evolutionary reversals at the gene-expression level, indicating that specialization is not an evolutionary dead-end. Genes related to development seem to be highly conserved and do not disappear with a morphological innovation (Carroll et al. 2001). Hence, the factors regulating their expression and the time necessary for a character re-evolution still needs to be clarified.

2. Obligatory versus Facultative Specialization
It is widely believed that specialist species decline and experience higher extinction rate relative to the generalist counterparts (Clavel, Julliard & Devictor). It is the very “jack of all trades is a master of none” nature of generalists that seems to be conferring them the ability to adapt when the resource they depend on depletes (McKinney 1997; Devictor et al. 2008; Colles, Liow & Prinzing 2009). Contrastingly, the specialist species are considered to be so adapted to exploit the narrow range of resource they depend on that they cannot switch to a different type of resource when confronted by the aforementioned scenario of resource depletion.

For a long time ecological specialization was most often considered to be accompanied with the limitation of niche breadth, resulting from evolutionary trade-offs between the ability of species to exploit a range of resources and their capacity to use each one (the ‘jackof-all-trades is master of none’ hypothesis; MacArthur 1972). Though this seems like a more plausible scenario, there are specialist organisms that do not follow this rule and restrict their specialization to a narrow range resource; for instance, although phytophagous insects are extremely adapted to feed on plants, some species could feed on an array of plants.

Aipysurus eydouxii (marbled sea snake) is yet another interesting example of a facultative specialist. Although most sea-kraits are equipped with a deadly arsenal of venom, A. eydouxii has lost its ability to produce venom because of its switch of diet from fish to fish-eggs (Glodek & Voris 1982; H. K. Voris & H. H. Voris 1983). It is so highly evolved for eating fish-eggs that it has even lost the fangs and its venom glands have greatly atrophied (McCarthy 1987; Gopalakrishnakone & Kochva 1990). Despite the greater degree of specialization, A. eydouxii feeds on the eggs of a broad range of fish and hence is not dependent on a single species for survival. Facultative specialization might decrease the risk of extinction relative to obligatory specialization as the organism will not be dependent on narrow range of resources or a smaller niche breadth.

3. Role of phenotypic plasticity, sloppy fitness space and correlation trait evolution
One of the most obvious and intriguing features of parasitism is the pronounced conservatism in the range of host usage (high host-specificity), both on ecological (Thomson 1994, 2005) and evolutionary time-scales (Ehrlich & Raven 1964, Futuyma & Mitter 1996, Winkler & Mitter 2008). Many authors suggest that the host specificity holds the key to understand the evolution of host-parasite associations.
Agosta & Klemens (2008) proposed a general framework for ecological fitting as a mechanism behind the assembly of ecological communities and the formation of novel interactions between species within communities. They propose three factors leading to ecological fitting and the ability of the organisms to achieve realized fitness under novel conditions. First, phenotypic plasticity can allow organisms to mount a response to novel conditions (West-Eberhard 2003). Second, correlated trait evolution (Lande & Arnold 1983) can produce phenotypes that are ‘pre-adapted’ to some future novel conditions. Third, phylogenetic conservatism in traits related to resource use including design constraints (e.g. Gould & Lewontin 1979) and retention of traits from past selection pressures (e.g. Jansen & Martin 1982) that provide the latent ability to perform under apparently novel conditions: for instance, a parasite encountering a new host that is actually the same or sufficiently similar to some ancestral host (Brooks & Mc Lennan 2002). This leads to organisms possessing potential fitness outside the range of conditions in which the species evolved. Agosta & Klemens (2008) termed this region of fitness space “Sloppy fitness space”, a by-product of direct selection under some other set of conditions, the ancestral operative environment. It comprises all components refining a host as resource and therefore the range of host-related variables affecting parasite evolution (Fig. 3).

Host shift not only requires the acquisition of new traits to exploit the new host species but also often results in the loss of traits that were required to infect the ancestral host. The extent to which such modification will happen will depend on both local circumstances and evolutionary history. The butterfly Pieris napi regularly oviposit on the introduced plant Thalaspi arvense, despite the latter being lethal to the larvae (Chew 1977) and strangely, this process has not changed (Agosta et al 2010). Fox et al (1997) reported that initial colonization of Chloroleucon ebano by the seed beetle Stantor sp. depended on pre-existing variance in the capability to utilize this novel host. These aforementioned examples signify that successful host shifts depend on the history of the association as well as on the life history, abundance and distribution of the species.

4. Horizontal transfer
Illegitimate recombination or horizontal gene transfer (HGT) leading to the insertion of new DNA sequences could enhance the ecological niche of a bacterial species through the acquisition of new metabolic capabilities (Jain, R. et al. 2003). The addition of a new DNA fragment through HGT could promote speciation among bacterial lineages, as this would lower the rate of homologous recombination among initially promiscuous lineages (Vetsigian, K et al. 2005). Nitrogen-fixing bacteria that interact with leguminous plants belonging to the genus Medicago appear to be an ideal biological model for such studies (Bailly, X. et al. 2007). Phylogenetic analyses reveal that several interspecific horizontal gene transfers occurred during the diversification of Medicago symbionts. According to Bailly et al. 2007, incongruence between symbiotic and housekeeping gene phylogenies suggests that HGT of nod genes occurred between these two genera. In this context, the analyses performed up to now suggest that Sinorhizobium species acquired the ability to nodulate Medicago species through HGT of nod genes from the ancestor of a Rhizobium species, most probably R. mongolense (Bailly et. al. 2007). Bailly et al. 2007 say that horizontal transfer of nod genes might have occurred between S. meliloti and S. medicae after their speciation. Another example of HGT as source of capabilities for an organism is shown by Futterer et al. 2004, where they say that many genes that enhanced the abilities of Picrophilus torridus (an organism able to grow around pH 0 at up to 65°C ) to cope with its extremely acidic environment have been obtained by horizontal gene transfer. This includes some of the organic acid degradation pathways, the main components of the electron transport chain, and mechanisms to deal with oxygen stress. These examples show us that horizontal transfer is one of the mechanisms that could drive specialization. Gene acquirements can be reversed by high rates of non-neutral mutations, stochastic loss, deletions, post-transcriptional gene silencing, etc. Hence, ecological specialization can reverse to the ancestral generalized state.

Concluding Remarks

In this review we summarize several sources of evidence indicating that specialization does not always leads to an evolutionary dead-end. The re-evolution of morphological features related to generalization may happen in the reviewed circumstances: i) if the genes related to the generalist condition are not degenerated, ii) if the species is phenotypically plastic and iii) if the specialization degree is not high. Despite the wide diversity in the degree and type of specialization, it was possible to find a general pattern. We conclude that specialists are continually evolving and are not trapped in evolutionary dead-ends.

Table 1. List of phylogenies used to evaluate the hypothesis of evolutionary dead-end. Abbreviations as follow: (N) number of species used in the phylogeny; (N-spe) number of specialist; (Types of spec.) type of specialization: (H-phy: Host-phytophagous); (H-p: Host-parasitoids); (H-pa: Host-parasites); (H-m: Host-mutualist); (F: feeding); (H: habitat); (C: chemical); (P: pollinator); (G→S): transition from generalist to specialist; (S→G): transition from specialist to generalist; (S→S): specialist that explore different resources; Root: most plausible ancestor (G: generalist; S: specialist; U: unknown); (dead-end): Evidence of dead-end.

~Taxa ~N ~N-spe ~Type of spe. ~GS SG SS Root Dead-end References
Chrysomelinae (Coleoptera) 35 10 H-phy 2 1 0 G No Termonia et al. (2001)
Dendroctonus (Coleoptera) 19 6 H-phy 6 0 0 G Equivocal Kelley & Farrel (1998)
Timema (Phasmida) 17 17 H-phy 5 1 2 U No Crespi & Sandoval (2000)
Tomoplagia (Diptera) 19 19 H-phy 6 2 ? U No Yotoko et al. (2005)
Nymphalini (Lepidoptera) 26 13 H-phy 5 2 0 S No Janz et al. (2001)
Troidini (Lepidoptera) 33 5 H-phy 5 0 0 G Equivocal Silva-Bandrão et al. (2005)
Nymphalini (Lepidoptera) 26 15 H-phy 8 2 0 S No Janz et al (2001)
Parides (Lepidoptera) 33 4 H-phy 4 0 0 G Equivocal Silva-Bandrão et al. (2005)
Acrididae (Orthoptera) 13 8 H-phy 0 3 1 S No Rowell & Flook (2004)
Cotesia (Braconidae) 45 45 H-p 0 0 15 S No O'Connor (2011)
Hymenoptera ? ? H-p 1 ? ? U No Whitfield (1998)
Schistosoma (Trematoda) 23 23 H-pa 0 0 9 S No Brant & Loker (2005)
Dactylogerus (Trematoda) 51 51 H-pa 0 6 30 S No Simková et al. (2004)
Monogenea (Trematoda) 74 51 H-pa 0 10+ 0 U No Sasal et al (1999)
Tachinidae (Diptera) 49 39 H-pa 0 5+ 0 S No Stireman (2005)
Amphiprion and //Premnas (Perciformes) 8 2 H-m 1 1 1 S No Elliot et.al (1999)
Tylomelania (Gastropoda) 92 39 F 32 0 0 G Equivocal von Ritelen (2004)
Thamnophilidae (Passeriformes) 70 14 F 2 0 0 G Yes Brumfield et al. (2007)
Diadasia (Hymenoptera) 28 28 F 0 2 4 S No Sipes & Wolf (2000); Sipes & Tepedino (2005)
Araneae 224 50 F 31 0 ? G Yes Pekár et al. (2011)
Emydidae (Testudines) 64 23 F 7-8 2-3 0 G No Stephens & Wiens (2003)
Emydidae (Testudines) 64 58 H 2 1-2 1 S No Stephens & Wiens (2003)
Dendrobatidae 61 ? F / C 4 0 0 U Equivocal Darst et.al (2005); Santos et al. (2003)
Ruellia (Acanthaceae) 115 ? P ? ? ? S Yes/No Tripp & Manos (2008)

Fig. 1. Topology of a group of spiders (Araneidae), showing the transition from generalist ancestor (grey circles) to specialist species (filled circles) that are mostly specialized to feed on Lepidoptera.


Fig. 2. Ancestral character reconstruction for diet (a) and habitat use (b) of an ecological diverse group of turtles (Emydidae), based on molecular phylogeny. Diet specialist states (herbivorous and carnivorous) / generalist state (omnivorous). The analysis suggests that the ancestor of the group was a dietary generalist. Figure extracted from Stephens & Wiens (2003).


Fig. 3. A schematic illustration show 2 dimension of the operative environment associated with fictional host resources (black circles). Sloppy fitness space can allow realized fitness in an area outside from the operative environment to which the parasite is adapted (light green circles). Panels a, d and g illustrate a specialist adapted to a single host resources, panels b, e and h illustrate a polyspecialist that has adapted independently to 3 different resources and panels c, f and i illustrates an generalist with a more general host recognition and tolerance system that allows it to utilize any resource that falls within the dark green area. The open circles in panels d-f represent 3 novel resources. The specialist in panel e can colonize resource 1 (which is more or less identical to de ancestral resource) but no resource 2 and 3. The polyspecialist in panel e can colonize resource 1, but also resource 2 that falls within its sloppy fitness space, and the generalist in panel f can colonize resource 1, but also resource 2 that falls within its sloppy fitness space, and the generalist in panel f can colonize all three resources. In panels g-i only host is available, and all three parasite species will appear to be specialists, but their ecological and evolutionary potential will be different.


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Asorey, Cynthia M. - PhD candidate, Universidad Catolica del Norte, Chile
Canales-Aguirre, Cristian B. - PhD candidate, Universidad de Concepcion, Chile
Kalsoom, Saima - PhD student, Quaid-i-Azam University, Islamabad, Pakistan
Kaminski, Valéria L. - Undergraduate student, Universidade Federal de Santa Maria, Brazil
Macias-Hernandez, Nuria- Post-doc, Aarhus University, Denmark
Magalhães, Ivan L. F. - Masters student, Universidade Federal de Minas Gerais, Brazil
Marcelino, Vanessa R. - Masters student, Ghent University, Belgium
Sunagar, Kartik B. - Phd candidate, CIIMAR/University of Porto, Portugal

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