Think about the fast sprint of a cheetah or the rapid undulation of a swimming fish.

All biological motion is dependent on the rules of mechanics, which is a branch of physics that deals with the motion of material bodies and the forces exerted upon them.

But, how do the static laws of physics impact the dynamic process of evolution? Do stronger relationships between a morphological trait and swimming speed, for example, facilitate or hinder evolution? Virginia Tech and Duke University researchers answer this question with their most recent research. 

Using various biomechanical systems in animals, the researchers have demonstrated that mechanical relationships in the structural traits of animals impart distinct, predictable footprints on biological diversity. Specifically, morphological traits that more strongly impact the way an animal moves also evolve faster.

“Our study demonstrates that evolution is shaped by the general laws of physics. We have long known that there are fundamental laws of motion that shape performance space for organisms. But, the laws of mechanics don't just define the parameter space that organisms can occupy. Mechanical laws also shape the process of evolution itself by guiding the rate of morphological evolution as well as shaping its pattern throughout evolutionary history,” said Muñoz, an assistant professor of biological sciences in the College of Science at Virginia Tech.

Muñoz did much of this research as a post-doctoral fellow in the lab of Sheila Patek, a professor of biology at Duke University. Their findings were recently published in the journal eLife.

“Our findings provide a compelling case for a strong influence of biomechanics on the pace of evolutionary change. We know that physics and biomechanics are central to evolutionary diversification, and our finding of a consistent increase in the rate of evolutionary change in the most tightly correlated parts of the system is surprising and exciting,” said Patek.

Their evolutionary finding opens up numerous possibilities across different organisms and different mechanical systems. The evolutionary footprints that Muñoz and Patek have discovered may be widespread in biological motion.

With the help of researchers from the University of Rhode Island and University of Illinois, Urbana-Champaign, Muñoz chose to focus on four-bar linkages, a simple movable closed chain linkage common in nature that is comprised of four levers connected in a loop by four joints. Examples of four-bar linkages in human-engineered systems include the pedaling of a bicycle or the movement of a pair of locking pliers.  

bicycle and locking pliers
Examples of four-bar linkages in the pedaling of a bicycle and locking pliers

Muñoz’s research on these linkages focuses on four biological systems: wrasses, cichlids, sunfish, and mantis shrimp.

“In order to conduct evolutionary analyses, I needed biomechanical and morphological data from numerous species and a good working phylogeny, or evolutionary history, to be available. With these requirements in mind, I was able to study three independent evolutions of four-bar linkage systems: the oral four-bar (wrasses and cichlids), the opercular four-bar (sunfish), and the raptorial four-bar (mantis shrimp),” said Muñoz, an affiliated faculty member of the Global Change Center, an arm of the Fralin Life Science Institute.

 

Four-bar linkages
A. Four-bar linkage systems have evolved independently multiple times across animals and consist of four rotatable links that transit motion and force. B. The raptorial appendage of a mantis shrimp and four-bar linkage system. Figure courtesy of Martha Muñoz.

Each of these four-bar systems represents an independent evolutionary experiment in a common mechanical system — the same laws of mechanics apply to all of these four-bars, but each one is used in a different ecological context. Mantis shrimp use their raptorial four-bar to rapidly strike at prey, whereas fish use their four-bar linkages to suction food into their mouths. Thus, Muñoz was able to examine whether similar laws of mechanics result in similar evolutionary patterns in various independently evolved mechanical systems.

In multiple groups of fishes and mantis shrimp, the researchers discovered that four-bar linkages evolve in predictable ways: links that impact mechanical output of the system the most evolve the fastest.

mantis shrimp striking a snail
A mantis shrimp striking a snail. Image courtesy of Roy Caldwell.

This recent study establishes the connection between mechanical sensitivity and evolutionary rate. Muñoz’s next question is how natural selection factors into the equation.

“Are links of high mechanical effect experiencing strong directional selection, or are links of weak mechanical effect experiencing strong stabilizing selection? In other words, I've documented an evolutionary pattern, and I'd like to next examine the underlying evolutionary process.” said Muñoz.