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Think You Can Design a Better Fruit Fly?


What happens when you tinker with the design specs of a live flyer? Four European scientists with specialties in biology and aerodynamics got together and ran some clever experiments with fruit flies to find out. The biologists found ways to change the shape of the flies' wings, and the flight engineers measured what happened in performance tests. Their findings, published in Nature Communications, yield some lessons about the capabilities of mutation and natural selection to make modifications on the fly, so to speak.

Aircraft designers have the luxury of changing just one thing at a time and measuring the impact on flight performance. Living things, however, are bundles of requirements that must be optimized together as they develop from the embryo. The authors recognize this difference:

Wing shape is governed by the expression of a number of genes, the modulation of which leads to phenotypic variation. The results of changing the balance of expression bears little relation to the independent parameters engineers vary when designing aircraft wings, yet these are the gene expression-driven shape warps upon which natural selection acts. [Emphasis added.]

How far could natural selection vary a gene for wing shape without introducing unacceptable costs on other parts of the system? To find out, the scientists used RNA interference (RNAi) to alter expression patterns in a gene called narrow for only the wing tissues, without affecting other body parts. They bred four lineages with altered wing shapes that were progressively narrower than the wild type. Then they put them through a series of flight tests, including escape from a predator, a dragonfly.

All of the morphs could fly -- some with even better performance on tight turns. That "improvement" (from a human perspective) came at a cost (from the fly perspective).

Our aerodynamic model shows that all of the wing morphs we have tested show a decrease in aerodynamic efficiency in comparison with the control. Thus, measurable differences in flight performance are likely to be the result of a balance between the aerodynamic and mechanical modifications due to the shape change and its energetic cost. In the two milder morphs, we suggest that the energetic cost is not high enough to negate the performance benefits whereas, in the most extreme morph, an intersection is crossed beyond which the necessary power is more challenging to achieve. A key factor contributing to this shift from improved to inferior performance is likely to be the disruption we have introduced into the system by altering the wing planform without changing the complex musculoskeletal apparatus that drives it. In an engineering sense, the induced changes in wing shape force the flight motor to operate off-design.

Using intelligent design, aircraft engineers can correct for the cost of one alteration with a subsequent alteration. Lengthening a wing, for instance, might increase lift, but will require more engine power to compensate for the extra weight. Ann Gauger talked about this in a recent ID the Future podcast in response to charges that some designs in the human body are sub-optimal.

The next thing I would say is that good design doesn't necessarily mean optimal design for all things because design often means multiple constraints... you can't be perfectly designed for one feature because it often goes against what you need for a second feature. The example I give is from airplane technology. Airplanes have to have a lot of design features that match a lot of different constraints. They have to be light so they can fly but they have to be strong so that they can withstand the kinds of stresses they undergo. They have to be able to keep people warm and keep it from being too loud, but that all adds weight. They have to keep the atmosphere breathable, but that also adds weight. All of those things have conflicting needs....

In the case of the fruit fly, the interconnectedness of constraints is even more evident. The entire fly must develop from a zygote containing all the instructions for building the final product. That final product must not only be capable of powered flight, it has to get to the point of reproduction, or else any fitness gains will be lost. The authors realize this. As human engineers, we cannot presume to know what is most important to the fruit fly:

Insect flight performance is a direct consequence of the interaction of the wings with the air and is determined by a combination of kinematics and morphology. Routine behaviour can be dominant over escape responses as the predictor of survival in dragonfly-fruit fly interactions, with sharp turns highlighted as vital for evading capture. Selective pressures on fruit fly morphology may have been expected, therefore, to promote adaptations that enable a high degree of manoeuvrability. Our findings show that fruit flies do not develop wings that are best suited for agile flight even when driven by their existing flight motor. Moreover, flight performance envelopes could be widened by affecting the function of a single gene. That flies are suboptimal in this regard is not particularly surprising but symptomatic of at least one antagonistic developmental, physical or behavioural selection pressure: for example, sexual selection mediated by the effect of wing planform on auditory or visual cues.

The more agile flyer, in other words, might not be able to get a date with a female who thinks he buzzes too loud or looks weird. He would have to find a female who has mutated into a kind that likes those things about him. More seriously, the agile flyer might need more powerful flight muscles to handle the stresses of a tighter turning radius. If there is "at least one antagonistic developmental, physical or behavioural selection pressure" constraining maneuverability, there are more likely several -- perhaps many. Is it reasonable to expect blind mutations to occur simultaneously such that all the constraints are satisfied in a coordinated fashion in one individual?

Animal morphologies reflect the cumulative effect of non-adaptive variation and time-integrated selective pressures including -- but not limited to -- those optimizing form for function. Insect wings are under selective pressures driving towards local multi-objective optima, embodying a design compromise between features that are aerodynamically relevant (contributing to flight performance) and features that may contribute to fitness but are independent of aerobatic capability.

It's not surprising that genetic manipulation can improve one trait at the expense of others:

We did not encounter a physical limit along the principal component axis that we were able to influence; all our genotypes were able to fly, albeit with reduced performance maxima at the extremes of our morphological manipulations.

This is exactly what breeders do with artificial selection. Our prize cattle are optimized for humans' desires, but would likely not do well in the wild. By practicing "influence" and "manipulation" of existing champion flyers, the scientists did not provide insight into the "evolution of performance specialities in animals" as they had hoped. Rather, they showed the power of the engineering mind to optimize multiple competing constraints in a coordinated fashion.

In her podcast, Dr. Gauger recounted the epic flight of the Rutan Voyager, the first airplane to fly around the world without stopping or refueling. To achieve that feat, designer Burt Rutan had to optimize lift and discard every unnecessary weight. It's interesting that Rutgers scientists just identified a world-traveling insect of that caliber: a small dragonfly they determined is the "world's longest-distance flyer." Gene comparisons show the same species is found as far Texas, Korea, South America, and Canada. Pantala dragonflies can fly across oceans from continent to continent, some of them flying nonstop over 4,400 miles, besting the Monarch butterflies shown in Metamorphosis.

Pantala leaves many of its fellow dragonflies even farther behind. The mysteries of evolution are such that while Pantala and its cousin the Green Darner (Anax junius) have developed into world travelers, Ware says that by contrast, other members of the family "don't ever leave the pond on which they're born -- traveling barely 36 feet away their entire lives."

Could this be an example of "non-adaptive influences" that "may exhibit some features that are unrelated to fitness" the researchers spoke of? It doesn't appear necessary for any dragonfly to acquire international flight ranking when their pond-dwelling cousins are doing just fine. If world-traveling dragonflies are a mystery to evolution, then evolution is not doing a good explanatory job.

By contrast, we know the power of minds to create flying machines that range from puddle jumpers to global circumnavigators. And that's what we find in biological flyers (birds and insects), each species exhibiting not only perfect balance between multi-objective optima but the ability to faithfully transmit its design instructions across generations. That's positive evidence for designing intelligence.

Image credit: By André Karwath (Own work) [CC BY-SA 2.5], via Wikimedia Commons.