Convergent Evolution

Nicholas Simpson

July 2017

 Convergent evolution is a phenomenon where species without a common ancestor evolve to have similar traits. Convergent evolution demonstrates that an undirected process of adaptation, like evolution, will often present the same solutions to the same problems, even in vastly different organisms.

Convergent evolution can be seen in many places in nature. Take for example the evolution of flight. Powered flight has evolved at least four separate times in Earth's history. Birds, bats, insects, and the now extinct pterosaurs all individually evolved the ability to take advantage of the atmosphere as a method of movement. Excluding the insects, the wings of the other three flight-capable groups are remarkably similar. Each has elongated arm and finger bones that support the structures necessary for providing lift, with stretched skin in the case of the bats and pterosaurs and feathers for the birds. And though these are the only groups known to have evolved fully powered flight, other species have also adapted to the air, using their skin, scales, and fins to glide and maneuver through the atmosphere. Snakes that flatten their bodies to glide from tree to tree, frogs that use their large webbed feet, and the only slightly misnamed flying fish and flying squirrels have all evolved the adaptation of flattening part of their bodies to control their movement through the air.

Evolution is a fascinating and sometimes misunderstood process. It is important to remember that there is no blueprint for the evolution of a species. Often it is said that a certain feature of an animal or a person is 'designed' to do something, which creates the impression of there being some kind of designer or a plan. There is no plan to how a species evolves, evolution cannot see into the future and realize that a certain adaptation would be useful a million years down the line. The only thing that matters in evolution is if the organisms with a certain heritable trait have more prolific offspring than organisms without that trait. This means that often species will evolve adaptations that are not the best possible solutions but simply the ones that happened to randomly occur first.

This evolution of imperfect solutions can be seen in the convergent evolution of the eyes of vertebrate animals and of cephalopods like squids or octopus. Both eyes are a solution to the problem of needing to visually perceive the world and in many ways they are remarkably similar. Both eyes are roughly spherical and have lenses for concentrating light onto groups of photosensitive cells which can then relay visual information to the brain. The major difference is where the nerves relaying this information are located. In vertebrates the nerves run along the inside of the eye and must pass through the wall of photosensitive cells to get to the brain. This is why humans and all other vertebrates have a `blind-spot', an area in our field of vision that we cannot see. In cephalopods the nerves run out the back of the eye, behind the wall of photosensitive cells. This means that squids and octopus have no blind spots when they view their underwater worlds. Why are vertebrate eyes built the way they are, when there is clearly a better solution? The answer is that evolution does not look for all the solutions to a problem and then choose the best one. Sometime in the distant past an early vertebrate ancestor mutated to have a light sensitive organ with nerves that ran, for whatever reason, in front of the light sensitive cells. This gave the organism a survival advantage, despite the blind-spot drawback, and so the trait was passed down and eventually a fully-fledged eye evolved, with the nerves still running along the inside.

This process of evolution, an almost blind stumble through possible solutions to a universe of ever shifting problems of survival and reproduction, is what makes convergent evolution so interesting and impressive. That such an inexact method of problem solving, applied consistently for millions of years, will produce the same solutions over and over is nothing short of amazing. Take echolocation, for example. Echolocation is most notably employed by bats and whales, two organisms in hugely different environments, flying through the air and swimming through the ocean. Each, entirely separately, adapted to the need to navigate in the dark by evolving the ability to sense their surroundings by making rapid clicking noises and then listening to the way that they bounced off the environment.

There are many examples of ways in which different species of animals have evolved similar solutions to the same problems. The streamlined shapes of fish, dolphins, and birds are all solutions to the problem of moving rapidly through a fluid medium. Both primates and pandas evolved opposable thumbs to grasp branches and bamboo, respectively. But sometimes the most revealing aspect of convergent evolution is not how organisms have become similar, but how they remain different. These differences show that there are often multiple ways to adapt to the same environment. Whales and dolphins have tails that move up and down, while fish swim by moving side to side. Birds rely on a single line of bones at the front and strongly connected feathers to form their wings while bats use elongated finger bones and stretched skin to move through the air. Evolution may be slow and inefficient but it is also a constant and aggressive optimization process that has been running since life began. There are few niches that can't be filled and usually they will be filled more than once. These variations show that while different solutions may often look similar there is usually more than one way to solve the same problem.