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More Findings Shed Light on the Complexity of Butterflies

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One expectation of intelligent design applied to biology is that biological systems will look more complex and specified the closer you look. Let’s see if that holds true in butterflies.

Host Specificity

The Illustra documentary Metamorphosis: The Beauty and Design of Butterfliesintroduced the case for intelligent design in the butterfly world. One of the first things shown was how accurately the female finds the correct host plant on which to lay her eggs. “Butterflies just don’t make mistakes,” remarked Ronald Boender of Butterfly World. Now, news from UC Irvine explores the details of the sensors they use.

Biologist Adriana Briscoe and colleagues found that females of the Heliconius species express gustatory, or taste, receptor genes when choosing a host on which to lay their eggs. Many plants defend themselves by producing toxic chemicals, so it’s vital to their larvae’s survival that the butterflies pick the right kind. Heliconius females have 80 taste organs called sensilla on their forelegs that they use to sample potential host plants, while male butterflies have none. (Emphasis added.)

The resulting paper in PLoS Genetics provides more detail. The number of systems involved in host plant detection is truly remarkable:

Genes for vision, taste and smell are likely to be crucial genomic loci underlying the spectacular diversity of butterfly-plant interactions. The availability of genomes for two butterfly species, the postman Heliconius melpomene (Nymphalidae) and the monarch (Danaus plexippus), as well as the silkmoth (Bombyx mori), enables us to examine the evolutionary diversification of gustatory (Gr) and olfactory (Or) receptor genes that mediate insect-plant interactions. Each of these species feeds on hosts from different plant families. Silkmoth larvae feed on mulberry (Morus spp., Moraceae) and monarch larvae feed on milkweed (Asclepias spp., Apocynaceae). The larvae of Heliconius feed exclusively on passion flower vines, primarily in the genus Passiflora (Passifloraceae). In addition, adult Heliconius are notable for several derived traits such as augmented UV color vision, pollen feeding ... and the ability to sequester substances from their host plants that are toxic to vertebrate predators such as birds.

The authors believe that host plant sensing evolves according to an “evolutionary arms race” between plants and butterflies, and that most of the taste sensors evolved during gene duplications. The variability of existing senses is an interesting topic for debate, but such speculation adds nothing to the major question of how these organs and functions came into being in the first place. Scientists know of sixty genes for taste sensing in fruit flies. The chance origin of one functional gene is outlandishly improbable; how much more so for sixty!

The problem for Darwinian evolution is aggravated when you consider that each gene is needed to be able to identify the taste or smell of a plant and pass the signal to the brain through neurons. The brain then has to make sense of the signal, and adjust the insect’s behavior accordingly. The female identifies the plant by tasting with her feet, smelling with her antennae, and imaging with her ultraviolet-sensing eyes. Only when the identity of the host plant is assured will she lay her eggs -- utilizing another complex system of organs and behaviors, without which the taste sensing is useless for survival of the species.

Some butterflies are specialists (very picky about their host plants) and others are generalists. Saying that the butterflies diversified their senses through gene duplications or gene losses, though, adds little in the way of understanding. How did all these multiple systems come together for this function originally? Duplicating a gene for bitter taste and having it “evolve” into a gene responsive to sweet taste, even if that step is plausible, would be useless unless the entire architecture is already in place. And needless to say, pointing to gene loss does nothing to advance the Darwinian hypothesis.

The authors only give the flimsiest support for evolution’s explanatory role:

Taken together, the whole-genome and whole-transcriptome data suggest that Gr genes in particular are highly evolutionarily labile both on short and long evolutionary timescales, and begin to offer an insight into the likely molecular basis for the rapid coevolution observed between these butterflies and their host plants. Understanding the remarkable diversity underlying this ecological interaction at a molecular level has remained a challenge.... Thanks to technological innovations in sequencing, the genetic basis of taste and olfaction involved in host-plant adaptation in Heliconius is beginning to be uncovered.

Molt Timing

Another system described in the Illustra film is molting. Three or four times during its crawling phase, the caterpillar sheds its skin as it outgrows it. One final molt is done clinging to the cremaster, right before the caterpillar forms the chrysalis. A new paper in PNAS describes a hormonal “molt timer” that acts as a quality control checkpoint to ensure the caterpillar is ready to undergo its dramatic metamorphosis. Working on tobacco hornworm moths (Manduca sexta), the authors showed that the larva has to pass a “critical weight” test to get through the checkpoint:

Manduca sexta larvae are a model for growth control in insects, particularly for the demonstration of critical weight, a threshold weight that the larva must surpass before it can enter metamorphosis on a normal schedule, and the inhibitory action of juvenile hormone on this checkpoint. We examined the effects of nutrition on allatectomized (CAX) larvae that lack juvenile hormone to impose the critical weight checkpoint. Normal larvae respond to prolonged starvation at the start of the last larval stage, by extending their subsequent feeding period to ensure that they begin metamorphosis above critical weight. CAX larvae, by contrast, show no homeostatic adjustment to starvation but start metamorphosis 4 d after feeding onset, regardless of larval size or the state of development of their imaginal discs....

The authors further found that the hormone-deprived larva below the critical weight, after entering metamorphosis, eventually died.

This constant period between the start of feeding and the onset of metamorphosis suggests that larvae possess a molt timer that establishes a minimal time to metamorphosis. Ligation experiments indicate that a portion of the timing may occur in the prothoracic glands....

These observations argue that the critical weight checkpoint, mediated through the CA [corpora allata] and JH [juvenile hormone], ensures that the larva has enough nutrient reserves to deal with the demands of metamorphosis.

The authors are not sure where the molt timer is located, but believe it is started by feeding on protein. From then on, it runs regardless of nutrient input. The amount of juvenile hormone, though, is sensitive to weight. It is able to delay the onset of metamorphosis (i.e., hit the snooze button) until a critical weight is reached. It’s an amazing system that is usually fail-safe unless human researchers (or mutations) interfere. Many insects and worms may have similar mechanisms:

The presence of such intrinsic timers ensures that each individual reaches sexual maturity even in the face of adverse environmental conditions.

The authors toss in some evolutionary speculation as a mere afterthought, saying, “This positive system that promotes molting and the negative control via the critical weight checkpoint provide antagonistic pathways that evolution can modify to adapt growth to the ecological needs of different insects.”

Well, OK. Perhaps evolution can modify the system. It’s the origin of the system, however, that is the interesting question. How did the checkpoint come into being, with the genes, prothoracic glands, weight sensors, and timers all working together to ensure a successful metamorphosis? Remember that in the Illustra film, Paul Nelson said that the chrysalis (for moths, the cocoon) is like a “casket” unless all the systems to rebuild the larva into a flying adult are in place.

A scan of the paper shows that Darwinian theory had practically nothing to do with the science. It only provided a narrative gloss, stating that “The existence of both positive and negative mechanisms that impinge on metamorphosis provide pathways that evolution can modify to adapt growth to the ecological needs of different insects.”

Conclusions

In both the taste-sensing findings and the molt-timer story, scientists have described multiple independent complex systems working together for function. Darwinian evolution might be able to vary certain pieces of the systems (if one is predisposed to accept that possibility), but Darwinism proves useless for explaining the origin of the systems, as demonstrated de facto by the utter absence of discussion about what mutations produced the innovations through an aimless, unguided process.

Those innovations include sensors, timers, and integrated behaviors. From our uniform experience, the only cause necessary and sufficient to produce such purposeful innovations is intelligence. We see, once again, that design forms the structure of the science, while evolution supplies merely the fa├žade.

Image credit: Illustra Media.