The Need for Speed Constrains Evolution in Mammals

Despite the seemingly limitless potential of mutation and selection, evolution is often startlingly conservative. In many cases, populations can end up trapped with certain traits, constrained by the fact that any changes may come at too high a fitness cost. While the idea of evolutionary constraints is hardly new, it's always important to understand what factors are constraining the evolution of a given trait. In a paper appearing in PNAS, a team of scientists from the Naturalis Biodiversity Center in the Netherlands and the University of Utah have shown how a need for speed and agility have limited changes to the vertebral columns of a variety of mammals, curbing their evolutionary options.

Although mammal species can have very different vertebral columns, in accordance with the variety of their lifestyles, the number of vertebrae in the main part of the column seems to be highly conserved in many groups. Some researchers have suggested that this apparent evolutionary stasis may reflect underlying developmental constraints, and the team decided to test this hypothesis. The reasoning is based on the fact that altering the number of vertebrae in this region necessarily involves a homeotic transformation — for example, a sacral vertebra has to change into a lumbar vertebra — and many genes have to mutate simultaneously for that identity change to happen smoothly. Since it's unlikely for all the right genes to mutate together at once, the outcome is usually an incomplete transformation resulting in the formation of an intermediate, 'transitional' vertebra which doesn't integrate properly with the rest of the column, reducing its flexibility and durability at that point. The lumbosacral joint, the connection between the vertebral column and the hips, has to be very flexible in fast running mammals with an agile lifestyle, and any mutations which reduce this flexibility would be strongly selected against. The team reasoned that the irregular lumbosacral joints resulting from single mutations would constrain the evolution of changes in the number of vertebrae in fast, agile mammals; slow-moving mammals, by contrast, wouldn't face the same biomechanical constraints, and would have therefore have more variation in the number of vertebrae.

To test this idea, the team analysed 774 skeletons from 90 mammal species. They found very few individuals with extra vertebrae in fast-running carnivores and many of their prey species, while variation was much more common among slow-moving mammals. Long-limbed, fast-paced species need to be able to flex their spine as they swerve and leap, so they can't afford to develop an abnormal joint. "The locomotion of slow mammals with a stiff back is only marginally affected by irregular lumbosacral joints, but for fast running mammals such joints are fatal," said Clara ten Broek, one of the authors of the study.

It's a wonderful insight an excellent example of how different factors interact in the course of evolution. The concerted influence of developmental processes and biomechanical requirements based by life-history effectively constrain the evolution of fast-running mammals, limiting the options available to them. The team also found that this pattern doesn't hold amongst domesticated mammals, which I found particularly interesting. Domesticated animals — even those derived from fast-running stock, like cats, dogs, and horses — tend to have transitional vertebrae more frequently than their wild counterparts. It seems that human selection and husbandry is enough to overcome the fitness cost of an odd vertebral column, freeing these species from the shackles of their evolutionary constraints.

Ref
Galis, F. et al. Fast running restricts evolutionary change of the vertebral column in mammals. PNAS 111(31):11401-11406. (2014) doi:10.1073/pnas.1401392111

Image credits
The photo of a Thomson gazelle skeleton is by Joris van Alphen and is used with permission.