Forbidden Evolution
We are already well-versed with the indelible greatness of evolution and how it helps competent species survive, and creates astounding traits in species. This article will discuss intriguing questions like why can’t humans evolve wings or why can’t even bigger animals exist, and some questions which you might never have pondered upon like- why can’t animals evolve wheels. We will discuss the limitations of evolution and why certain traits are impossible to evolve in a particular species. However, one must also keep in mind that phenotypic constraint is not absolute and could also depend on the environment surrounding the species.
By learning more about traits that are impossible to evolve, we can learn more about how evolution actually works. Phenotypes arising through these traits are aptly named ‘forbidden phenotypes.’
A phenotype is a set of observable characteristics of an individual resulting from its interaction with the environment. First, when we think about why animals cannot evolve wheel-like structures, it seems quite obvious and funny. But let’s delve deeper and understand what are some of the crucial reasons behind it.
First of all, a structure capable of rolling around an axle is physically separable from the body and is difficult to build in a developing animal. First of all, in my opinion, having wheels as a body part is biologically impossible as there seems to be no plausible way by which the body can provide nutrition, adequate blood supply and nerve impulse to these structures. But that’s just me speaking.
A less apparent reason is that biological adaptations of the organism and its evolution depend upon the environment. Although wheeled vehicles are dominant in transportation on flat surfaces and wheels provide a significant advantage in locomotion on flat surfaces, they are at a severe disadvantage in rough terrains where most animals live and therefore, animals cannot evolve such a disadvantageous trait.
FITNESS LANDSCAPE
Fitness in terms of evolution doesn’t necessarily mean ‘strength’ but the ability to survive and reproduce. Convergent evolution is the process wherein organisms that are not closely related independently evolve similar traits or phenotypes. An example of this includes the wings of birds and bats, which have evolved to serve a similar purpose due to similar environmental conditions.
As the structure of bat and bird wings is also highly similar to our hands, it seems viable that our hands can evolve into structures that could resemble wings. But it’s just not possible.
This brings us to a more technical argument. For evolution to occur successfully from point A to point B (i.e. Feature A to Feature B), every stage of its evolution should provide an advantage or at least not be harmful to the organism. Even if the final stage is beneficial, every step in the journey must be advantageous to the organism. This can be better understood through a Fitness Landscape.
An example of a simple fitness landscape is given in the above picture. The x and y axes represent different phenotypes of a given trait that occur in the environment of interest while the z-axis represents the fitness conferred in the presence of a given phenotype. Each point on this “wayward plane” is thus a variant; with the crests representing the fittest variations and the troughs representing the less fit and the distance between these points shows us how close these variations are to each other.
One of the main features of a fitness landscape is that you can only move uphill, i.e. towards higher fitness. Evolution in terms of fitness landscape can be considered as walking around this landscape from the ground level and evolving different variations of the trait as it reaches different peaks.
As a result, we get different variations of traits sitting on the crests (also called local optima). Therefore even if there exists a higher peak with perhaps the best fitness in the landscape, it can happen that the variant of the trait can’t evolve further because it has to then travel from the smaller peak to the highest peak through a valley which is forbidden. After all, it has to go through troughs, which represent more unfit traits in the given environment.
The best example of this is the human eye. As the optic nerve and blood vessels run through the eyes and not from behind them, we have a blind spot in our vision. In comparison, octopus eyes don’t have a blind spot and are much better than human eyes. But here, the problem is that we can’t go to the higher fitness peak without going through a valley, making our eyes worse in the meantime.
Coming back to the original argument, if humans wanted to evolve wings from their hands, each stage of evolution should be beneficial, which is similar to climbing uphill in the fitness landscape. For birds, the early wings were used to glide and conserve energy, which gave them a fitness advantage. In the case of humans, variations that will help us to glide will be disadvantageous to our daily activities. Therefore even if the ability to fly is a fitness advantage overall, the intermediate steps are harmful, and it is similar to moving down the fitness hill and into a valley.
In the same way, another question would be — Can Humans can grow wings as an extra pair of limbs?
Mutations in a class of body-patterning genes called Hox genes can lead to an organism having extra hands, feet etc. So even if the rare mutation occurs and we get an extra limb, it will be a huge burden on the body. The new limb would need additional bones, joints, muscle and nerves which would need countless mutations in other genes. Also, as much nutrition will then go towards evolving wings it will prove to be disadvantageous for us.
SELECTION PRESSURE AND THE NON EXISTENCE OF BIGGER LAND ANIMALS
One of the more easily perceptible ways in which evolution works is that the traits need a certain type of pressure to evolve. This pressure, which arises due to challenging living conditions for a population of organisms is called ‘selection pressure.’ Thus, unless there is a selection pressure for gliding, we won’t start to evolve adaptations for flight.
The reason why bigger sized land animals aren’t present is because of gravity and the square-cube law. The Square-cube law states that as an organism becomes bigger, its volume increases much faster than its surface area. As a result, all processes that depend on surface area start lagging, which is disadvantageous for the organism. But in the case of whales, as they live in water, they can grow bigger in size due to the buoyancy force from the water, which counters their gravitational force or weight. This indicates that phenotypic restrictions and forbidden phenotypes for an organism depend upon the environment around them.
So, organisms don’t evolve just for the sake of it. Evolution occurs in concordance with what our environment demands from us, and phenotypes show up accordingly. But having said that, although evolution may have its rules and limitations, we should not forget the things that evolution can mould and create, are pretty impressive, and we should not stop appreciating the power of evolution.
PS:- Your dream of flying like an Angel (or Lucifer) is far from becoming true.
