John Collyer's fifteenth 'fact seldom taught to students' was the assertion that symbiotic relationships were too complex to evolve. His assertion "...the interdependence of two forms of life, such as the fig tree and fig-gall wasp, the yucca plant and Pronuba moth, pollen plants and the bee, each dependent for life upon the other, must have been formed complete at the same time" ignores the fact that such symbiotic relationships did not evolve in one step, but did so over time, evolving from a non-obligate relationship to one of obligate mutualism.
An excellent example are the codependent relationships that develop between time between two disabled people living together who can function as a unit, but are unable to look after each other when separated. As their disabilities developed over time, they became dependent on each other.
Special creationists such as Collyer tend not to mention that there there are many species of related insects which exploit the plants but do not give anything in return - these are referred to as cheater species. Special creation is completely unable to explain why God would create some plant-insect relationships that are mutualistic, while others are antagonistic. Evolutionary biology is however readily able to account for this.
Ultimately, Collyer's argument is yet again another example of special creationist incredulity leavened with ignorance of the facts.
One of the most amazing examples of symbiosis is that between the yucca plant and the yucca moth. The yucca plant is incapable of pollinating itself to grow more seeds and perpetuate. The yucca moth (Tegeticula, formerly Pronuba) pollinates the yucca plant while laying its eggs inside the plant.
After hatching, the moth larvae feed on the seeds of the yucca. Remarkably, the moth carefully calibrates the number of its larvae growing inside each flower so the larvae will not consume all the seeds of the yucca—because if they ate all the seeds the yucca plants would stop reproducing, thus eventually dooming the yucca moths as well!
By pollinating the plant, the moth develops food (yucca seeds) for its larvae while ensuring that the plant can perpetuate its own kind as well.
But that's not all. The life cycle of the yucca moth is timed so the adult moths emerge in early summer—exactly when the yucca plants are in flower.
How could this remarkable relationship have developed by random minor changes in both plant and insect over eons? It is obvious that it appeared abruptly or it never could have developed at all. [1]
Like many special creationist arguments, this is merely an argument from personal incredulity, as one can see from the final sentence, which is essentially stating "I can't imagine how evolution could have led to symbiosis, therefore it was created." Words such as "it is obvious" are not a substitute for careful research into a complex and well-studied problem. Of course, the whole argument as an attack on evolution per se falls apart because it implicitly conflates the fact of evolution (common descent) with the mechanism proposed (natural selection) and argues shortcomings in the latter invalidate the former. For example, one does not deny the reality of gravity because general relativity does not extend to the quantum level, yet special creationists argue that unsolved problems in ecology invalidate the considerable evidence [2] for common descent. They do not.
As mentioned earlier, two people who are mutually dependent on each other provide an example of social symbiosis. The sudden absence of either partner results in the other person being unable to function socially. No one would argue that such a co-dependent relationship is so complex it could not have happened gradually over time. Such relationships often begin with two healthy people living together and as both become frail, they gradually provide support for each other until they are incapable of living apart.
Herman Muller, one of the most influential geneticists of the early 20th century in a landmark paper noted that complex systems likely have evolved over time by the progressive interaction of multiple parts which originally were useful, but became interdependent and ultimately necessary:
Most present-day animals are the result of a long process of evolution, in which at least thousands of mutations must have taken place. Each new mutant in turn must have derived its survival value from the effect which it produced upon the “reaction system” that had been brought into being by the many previously formed factors in cooperation; thus a complicated machine was gradually built up whose effective working was dependent upon the interlocking action of very numerous different elementary parts or factors, and many of the characters and factors which, when new, were originally merely an asset finally became necessary because other necessary characters and factors had subsequently become changed so as to be dependent on the former. It must result, in consequence, that a dropping out of, or even a slight change in any one of these parts is very likely to disturb fatally the whole machinery; for this reason we should expect very many, if not most, mutations to result in lethal factors, and of the rest, the majority should be “semi-lethal” or at least disadvantageous in the struggle for life, and likely to set wrong any delicately balanced system, such as the reproductive system. [3]
Muller's seminal paper is nearly a century old, which cast into doubt the rigour of any creationist claim that "irreducible complexity" or symbiosis is a priori impossible. Where special creationists err is in assuming that complex organisms or elaborate symbiotic relationships could not have developed gradually, particularly when - contrary to their assertions - significant research has been done on these subjects.
Another point that Collyer neglected to mention (or did not know) was that there are many species of fig wasps and yucca moths, and not all of the relationships between these insects and the plants are mutualistic, with some of them being parasitic.
If we look within the Tegeticula moth lineage, we see species that do not have a mutualistic relationship with the plants, but rather are 'cheater species' that feed on the seeds without pollinating them:
In addition to interacting with the pollinator moths, yuccas are also utilized by other prodoxid moths that are not mutualists. Within the Tegeticula pollinator lineage, two cheater species have evolved that do not pollinate, yet feed on the developing seeds (Pellmyr and Leebens-Mack 1999, 2000; Pellmyr 1999; Marr et al. 2001). Another genus of yucca specialists, Prodoxus, also does not pollinate, but feeds on plant parts other than seeds (Davis 1967). This genus is widespread and species co-occur with the pollinator moths on almost every species of yucca with the possible exception of Yucca reverchoni (O. Pellmyr, personal communication). Adults rest in the flowers during the day and mate within yucca flowers at night, but they lack the specialized mouthparts used for pollination. Riley (1880) labeled Prodoxus decipiens as the “bogus yucca moth” because it was repeatedly misidentified as a pollinator. Since then, the name bogus yucca moth has been applied to the genus as a whole. There are currently 11 described bogus yucca moth species that differ in the yucca species utilized and the location of larval feeding. [4]
Furthermore, careful analysis of these relationships has shown evidence that mutualism evolved from parasitism, and that some of the anatomic adaptations that facilitate mutualism are found in related non-obligate mutualist species, providing more evidence that these allegedly 'irreducibly complex' mutualisms evolved over time incrementally, utilising existing anatomic structures and instincts.
One study on the moth Greya politella, which is related to the yucca moth family, and that feeds almost exclusively on members of the Lithophragma species found that unlike the yucca-yucca moth relationship:
…the interaction between G. politella and L. parviflorum is at least sometimes not mutualistic. Unlike the copollinators, Greya destroys some of the seeds as larvae, and this damage may outweigh its contribution to pollination when copollinators are abundant. The fluctuating availability of copollinators will mean that the interaction between G. politella and L. parviflorum varies between antagonism and mutualism. Consequently, selection for an obligate mutualism similar to that between yucca moths and yuccas may be unlikely.
Overall, the results indicate that several traits found in the yucca moths that are essential for the evolution of mutualism are present also in more primitive taxa, most of which are antagonists of their hosts. Hence, they are not necessarily special adaptations evolved for interactions with Yucca. (Emphasis mine) [5]
One can readily see that if the other co-pollinators became gradually extinct, leaving the only relationship being that between that of L. parviflorum and G politella, the relationship would likely evolve towards mutualism, if only because the cost of parasitism would far outweigh the benefit for the latter. The evolution of relationships from parasitism to mutualism is not speculation, examples of such evolutionary changes have been documented, such as that between a bacterium and a plasmid. [6]
The upshot of this is that in order to fully understand how mutualisms evolve, one will need to accurately characterise the evolutionary family trees of the moths. Pellmyr and Krenn [7] note that:
A long-recognized association of this kind, between yucca moths (Prodoxidae) and yucca plants (Agavaceae), has become an important model in understanding how obligate mutualisms coevolve. In this association, established at least 40 million years ago, yuccas are pollinated exclusively by yucca moths, whose larvae in turn consume some of the developing yucca seeds. This has been an evolutionarily and ecologically highly successful association, with some 30–45 yucca species being important vegetation components throughout much of the North American deserts and semiarid regions.
This is a well-studied problem, and far from posing an insoluble problem for science has shed much light on the mechanism of co-evolution. There are still areas of uncertainty, but this is hardly atypical for any area of science, so it betrays an agenda for special creationists to criticise evolutionary biology when other unsolved problems such as the absence of a quantum theory of gravity exist. As Yoder, Smith and Pellmyr point out:
The complexity and rarity of obligate pollination interactions lead to the question of how they arise in the first place: to what extent are less-specialized ancestors adapted for obligate pollination after a transition to their present host plant, and to what extent do they shift to the host with some necessary traits already available to be exapted for the interaction? One way to answer this question is to estimate ancestral states using well-resolved phylogenetic relationships between the partners in an obligate mutualism and proximal lineages. This approach has been applied extensively in studies of insect–host associations...However, because of a lack of phylogenetic resolution for relationships among lineages surrounding obligate mutualists, such an analysis has yet to be accomplished for any obligate pollination interaction. [8] (Emphasis mine)
One can hardly criticise ecologists for not having a final, complete answer to how and when each mutualise evolved when critical details such as well-resolved phylogenetic relationships for obligate mutualists remain to be done.
Yucca obligate mutualisms are one of the best-studied examples, with research extending over one hundred years. The female moth actively pollinates the plants using specialised mouth-parts, and lays her eggs in the plant ovary, where the hatching larvae eat exclusively yucca seeds or other fruit tissue. [9] What a century of research on yucca obligate mutualism has shown is that there is significant diversity [10] within the yucca moth family:
- At least 22 yucca moths in the genus Prodoxus lay their eggs in the plant ovary without pollinating the plant
- Two species in the genus Tegeticula are non-pollinating 'cheater moths'
Special creationists who advance the yucca mutualism as an insoluble problem for evolution need to explain why some yucca moths were created as cheaters that lay their eggs in the plants without pollinating them in turn. From an evolutionary point of view, this is valuable evidence which allows us to try to reconstruct a plausible evolutionary pathway for the origin of obligate mutualism. Pellmyr and Leebens-Mack [11] have postulated that reversal of mutualism in some yucca moths in fact lay behind an adaptive radiation. The Yucca moth / plant relationship is far more complex than the simplistic creationist story advanced in apologetic works.
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| Head of female Tegeticula carnerosanella with yucca pollen load. Black arrow = left tentacle; white arrow = proboscis. Source: Proc. Natl. Acad. Sci. USA (2002) 99(8): 5498-5502 |
Yoder, Smith and Pellmyr's paper contributes to the well-researched moth-plant mutualism by looking into the question of whether specialised traits evolved as part of establishing this relationship or existed in some form before the mutualism evolved, and were then pressed into use as the relationship developed and honed by evolution. They concluded:
To better understand the pattern of macroevolutionary change leading to this obligate pollination mutualism, we should then focus on factors leading to the colonization of arid habitats, which coincided with the stepwise colonization of two separate lineages of woody monocots. It is not until after these transitions that the morphologically and behaviourally minor, but ecologically major, changes appeared that led to the origin of the obligate mutualism. Thus it is reasonable to conclude that the obligate mutualism between yuccas and yucca moths arose at least in part because the early Prodoxidae were pre-adapted for the more specialized interaction. (Emphasis mine) [12]
Needless to say, the evolution of some of these traits required for this mutualism has been the subject of some research. Pellmyr and Krenn [13] have pointed out that the transition towards mutualism involved changes in existing traits, rather than evolution of novel traits, with one exception - the tentacle-like mouth parts that the moths use in precise handling of pollen. As it appeared quickly, and led to an adaptive radiation, understanding its evolution is, as the authors say, "central to understanding the coevolutionary history of diversification and changing interactions between yuccas and yucca moths." What they found was that the tentacles are similar to the galea (appendages that in moths zip together after pupation to form the distinctive moth drinking proboscis), and likely arose from a simple modification:
The tentacular appendages of the maxillary palp in the female yucca moths are a rare example of a complex key innovation that emerged in one clade without any homologous structure in related taxa. A considerable number of morphological synapomorphies are shared between the tentacle and galea, suggesting that the genetic template for the galea is fundamental in producing the tentacle as well.
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| A) Mouthparts of female T. yuccasella, lateral view. Tentacle (t) originates from first segment of maxillary palp (mp1); tentacle coiled laterally from the proboscis (p); distal segments of the maxillary palp (mp) covered with scales. (B) Male of T. yuccasella with maxillary palp (mp) showing a minor frontal elevation on first segment instead of a tentacle; proboscis (p) coiled under the head. (C) Tentacle surface ofT. yuccasella with microtrichiated surface and prominent trichoid sensilla (s) with hooked tips. (D) Proboscis surface of female T. yuccasella with microtrichiated surface and trichoid sensilla with straight tips. Source: Proc. Natl. Acad. Sci. USA (2002) 99(8): 5498-5502 |
Modifications in the tentacle from the galea are modest and mostly quantitative. The tentacle has a greater diameter than the galea, and the number of ventrolateral muscle fibers is much increased. The bristle-like sensilla are clustered in the ventral region, are larger, and have gained the distinctive terminal hook. They aid in handling the pollen and seem to evolve easily among pollen-collecting insects.
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| (A) Internal anatomy of the tentacle of female T. yuccasella in longitudinal section shows longitudinal tentacle musculature (tm), thickened dorsal epidermis and cuticle (arrowheads), a nerve (n), and a trachea (tr). (B) Cross section through tentacles (left and right sections) and proboscis, the latter of which is composed of the two galeae (Center), in female T. yuccasella. Shared internal features include proboscis muscles (pm) and tentacle muscles (tm) in ventral lumen, thickened dorsal epidermis and cuticle (arrowheads), nerve, and trachea (tr). Source: Proc. Natl. Acad. Sci. USA (2002) 99(8): 5498-5502 |
From a morphological perspective, it is striking that such a complex structure as the tentacle has evolved without any homologous features in the related prodoxid moths. The simplest explanation for the shared specializations between the tentacle and galea is that a shared developmental pathway is involved.
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(A) Tentacle rudiment (t) on first maxillary palp segment (mp1) of female T. intermedia; second maxillary palp segment (mp2) with scales; microtrichiated proboscis (p). (B) Tentacle rudiment of female T. intermedia in longitudinal section emerging from first maxillary palp segment (mp1) contains tentacle musculature (™).
Source: Proc. Natl. Acad. Sci. USA (2002) 99(8): 5498-5502
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In conclusion, the morphological data strongly indicate that the unique tentacles of female yucca moths originated through expression in a novel site of the genetic template for the elongated galeae of Lepidoptera. Prior work on the interaction has suggested that the tentacles are the only truly novel morphological trait in these moths, and the present findings show that acquisition of this complex trait and its mechanism of movement may have been evolutionarily simple. Meanwhile, the behavioral component of pollination is likely derived from a common probing behavior for nectar in floral tubes in more basal prodoxids and on the wet stigmas of yuccas, thus a passive, less efficient pollination mechanism may have preceded the origin of the tentacles and active pollination in the yucca moths. [14]
The evolution of mutualism is an active area of research, and many unresolved problems exist. However, significant work has been done, enough to show how this evolution has taken place. Furthermore, the existence of yucca moths that 'cheat' on the yucca plants is difficult to fit in with the creationist argument as one is forced to assume that the Creator arbitrarily chose to create some yucca moths as obligate mutualists that pollinated plants, and others as cheaters who do not pollinate the plants, but utilise the seeds as a food source for their young. Collyer’s argument reflects more his utter lack of familiarity with the primary research literature on the evolution of mutualism than any problem for evolutionary biology. It is, as one has seen repeatedly, a depressingly common feature of his anti-evolutionary arguments.
References
1. Competition or Cooperation: How Symbiosis Defies Darwin. http://www.ucg.org/b...fies-darwin.asp
2. Theobald, Douglas L. "29+ Evidences for Macroevolution: The Scientific Case for Common Descent." The Talk.Origins Archive. Vers. 2.89. 2012. Web. 23 July 2014
3. Muller HJ “Genetic Variability, twin hybrids and constant hybrids, in a case of balanced lethal factors.” Genetics 1918 3(5): 422-99. Emphasis in the original
4. David M. Althoff . Kari A. Segraves, Jed P. Sparks "Characterizing the interaction between the bogus yucca moth and yuccas: do bogus yucca moths impact yucca reproductive success?" Oecologia (2004) 140: 321–327
5. Pellmyr O., Thompson J.N., "Multiple occurrences of mutualism in the yucca moth lineage" Proc. Natl. Acad. Sci. USA (1992) 89:2927-2929
6. Bouma J.E., Lenski R.E., "Evolution of a bacteria / plasmid relationship" Nature (1988) 335:351-352
7. Pellmyr O, Krenn HA “Origin of a complex key innovation in an obligate insect-plant mutualism" Proc. Natl. Acad. Sci. USA (2002) 99(8): 5498-5502
8. Yoder JB, Smith CI, Pellmyr O “How to become a yucca moth: minimal trait evolution needed to establish the obligate pollination mutualism.” Biological Journal of the Linnean Society (2010) 100:847–855.
9. ibid p 848
10. loc cit
11. Pellmyr O, Leebens-Mack “Reversal of Mutualism as a Mechanism for Adaptive Radiation in Yucca Moths” The American Naturalist Supplement: Species Interaction and Adaptive Radiation (2000) 156(4):S62-S72
12. Yoder JB, Smith CI, Pellmyr O op cit p 853
92.
13. See ref. 7
14. Pellmyr O, Krenn HA, op cit p 5500-2
