Stress.
Just that word brings on feelings of, well, stress. In our daily lives, stress sums up the physical and emotional factors that lead to mental tension and can cause bodily harm, even disease.
The word stress has many meanings, though. For a physicist, it is a constraining force or influence as one body pushes or presses on another.
For a chemist, stress can affect equilibria. Le Chatelier's principle says that a chemical equilibrium will respond to stress in a manner such that it will relieve that stress.
For biologists, stress can also affect equilibria. Indeed, it is stress in an ecosystem that can drive evolution.
Consider the introduction of fire ants to North America. There are some 285 different species of fire ants world-wide and many are relatively harmless. They are not invasive and do not harm people.
However, in the 1930s, Solenopsis invicta or the red imported fire ant was accidentally introduced into the southern United States. With no natural predators, the ants flourished.
If you have ever seen the ant attack in Indiana Jones and the Kingdom of the Crystal Skull, that is Hollywood hype but not by much. Solenopsis invicta have been known to attack and kill animals as large as a calf. They don't carry the killed animal back to the hive. Instead, they strip it down to the bone right on the spot.
In their native habitat, there are a number of predators and other natural controls on their population. With these constraints removed, Solenopsis invicta quickly set up shop throughout the south. In the last 70 years, the population has spread significantly and now occupies several states. It is estimated that between 30 per cent and 60 per cent of all humans in fire ant infested regions are bitten at least once each year.
On the other side of the equation, enter the indigenous fence lizard. Its favorite activity is sitting on fence posts enjoying the sun. They sit quite still for a very long time. And when an insect decides to crawl across their skin, it doesn't produce a response.
If anything, the lizard remains utterly motionless until the insect makes it way to within biting distance. In one swift move, it is gone. From a survival point of view, this is a strategy that has worked well for fence lizards for a very long time. After all, why hunt for food if it will walk right up and let you eat it?
Of course, the introduction of the red imported fire ant changed things. Suddenly, the insects walking along the lizard's back had switched roles. The fence lizard is now dinner. As few as 12 ants can inject enough venom to kill a juvenile fence lizard.
They do this by lifting the scales of the lizard and biting into the soft flesh underneath.
This could have meant the demise of the lizard population except that not all lizards enjoy having ants walk on them. When their scales are lifted, some lizards twitch vigorously throwing off the insects and flee the scene.
This evasive behavior is present in many juvenile fence lizards. Being smaller and with thinner scales than the adults, they tend to be more skittish. Most lizards stop dancing when attacked as they mature.
Most but not all. Some maintain the habit well into their adult life. In this case, the behavior provides a basis for evolution to work.
The fence lizards that keep throwing off insect invaders by twitching are the ones that survive attacks by fire ants. Instead of letting the insects attack and bite, they run away.
As a consequence, they are the lizards that survive long enough to reproduce and their genes get handed down to the next generation. This is an important part of the puzzle that is evolution. Natural selection operates on the phenotype expressed by the genotype.
In the case of fire lizards, the twitching behavior is now dominant in areas infested with fire ants. More than 70 per cent of adults will dance to dislodge the predator. In areas without fire ants, only 30 to 40 per cent of adults respond the same way.
Over the past 70 years, there has been an observable evolution of the fence lizard. It is not just in behavior either. Lizards in fire ant infested areas have longer hind limbs which help with the escape as they provide more leverage to shake off the ants and to run away.
While the evolutionary response is behavioral, it is driven by genetics. Unfortunately, the physiological adaptation to the venom in the fire ants bite has not been as rapid. The venom is still lethal. In this case, though, there is no variation upon which natural selection could act. No immune fence lizards that could breed an immune population.
Dancing lizards with longer legs is just one example of evolution under stress.
