Evolution by Natural and Sexual Selection

Downtown Menomonee Falls in fall; taken in October 2020.

Look outside: what life do you see? You may see trees, grass, perhaps some insects, squirrels, or rabbits, or birds; perhaps you see other people or dogs. How would you feel, now, if I told you that every living thing you see out there – grass, birds, dogs, and trees alike – is your distant cousin? You may shake your head in disbelief, as did I when I first learned of it.  Yet, I promise you, it is true: the grass you see, the weeds you see, the trees you see, the birds you see, the squirrels you see, the people you see, the dogs you see – all of them share a significant proportion of their genome with you. Whether you like it or not, every species on the planet is the distant cousin of every other species. Welcome to the insanity of evolutionary biology.

Evolution is an odd concept – it seems inconceivable that we share over half our genes with a banana, or even 98.8% of our genes with a chimpanzee; yet it all, somehow, is true. Knowing now, somehow, that every organism on earth is part of our extended family, our immediate questions are these: how did this happen, and why is this happening? The goal in this article will be to convey the biological law of evolution and discuss the two most widely accepted theories explaining its process: the theories of evolution by natural selection and by sexual selection. 

If you read the last entry (regarding the beginning of the solar system and life on earth), you would know that earth’s life has existed for 3.7 billion to 4.1 billion years (general scientific consensus lingers around 3.9 billion years). Life at this time was extremely simple, prokaryotic (pro, as in no envelope enclosed nucleus, as we learned in 7th grade biology), and generated energy through processing surrounding naturally-occurring macromolecules. Ancient microscopic, unicellular organisms were far less sophisticated and complex than the somatic cells in our bodies, even. This fundamental evolving complexity we have seen led us to the great theory of evolution: Darwinian evolution

Evolution by natural selection is the process by which most organisms are likely to have evolved since, well, the beginning. In natural selection, an individual organism that has genes more suitable for the environment they reside in would be more likely to reproduce and continue their genes than an individual organism of the same species that has traits less suitable for their environment. It is not the survival of the fittest; rather, it is the survival of those most adaptable to change.

A flagship example of natural selection is the story of the post-Industrial Revolution peppered moth. The peppered moth is a white organism with black spots that easily camouflages itself against lichen-covered trees (which are of a peppered white color) in Great Britain. There is a genetic mutation in the population of the peppered moth that resulted in some of the individual moths having almost pure black wings. These wings do not blend in well with the lichen and, therefore, make these moths far more susceptible to predation. Normally, there would be far more peppered moths than monochromatic moths, but in 19th century England, such was not the case. The Industrial Revolution began widespread electricity usage in England, and the main producer of energy at the time was coal. The pollutants produced by coal killed off the lichens that the peppered moths used to camouflage, exposing and darkening the tree bark below. As a result, the non-mutated moths (peppered) were preyed on far more, and the melanic (mutated peppered moths) could utilize the dark, polluted bark to camouflage themselves. Such resulted in a vast decrease in population for the peppered moths and a vast increase in population for the melanic moths. A campaign in the mid-20th century in the efforts to reduce air pollution allowed for peppered moths to again be more populous than the melanic moths. The example provides evidence for the fact that more environmentally-suitable traits will survive and reproduce for the longest.

An example of natural selection relating to human evolution is the genetic discrepancy between ethnic peoples of the Himalayas versus those of the lowlands. Himalayan natives live their entire lives in high altitudes (around 3,200-4,100 meters); therefore, they live in low-oxygen environments. Because of the environment they live in, a mutation that significantly lowers the amount of hemoglobin in their blood is more dominant in their population. The lower amounts of hemoglobin results in a lesser presence of Monge’s Disease, otherwise known as chronic mountain sickness. The mutation allowed for carriers to live longer and thus reproduce more. The proportion of Himalayan natives with this mutation, therefore, gradually increases by generation. This is natural selection at work.

Evolution by sexual selection seeks to explain the peculiar mating strategies of animals on earth (including humans). While natural selection deals with an individual’s suitability with their environment, sexual selection deals with specifically a male’s ability to mate with females, and therefore reproduce and transfer their genes to another generation. Sexual selection occurs more often in males because of a principle called “Batesman’s Principle,” in which female reproduction is limited by their ability to nourish and produce large and strong gametes (sex cells), whereas male reproduction is limited only by their access to mating with females. Mating competition between males may result in genetic changes aimed specifically at sexual reproduction. 

There are many interesting and freaky traits of some male organisms that are strong examples of sexual selection. The male peacock, for example, has incredibly beautiful, luminant, and flamboyant feathers that dramatically hamper the male peacocks’ chances of survival. Mating is the only purpose of this trait. When male peacocks compete for a female peacock, they expand their feathers into a fan shape and shake the feathers, producing a rattling noise intended to grab the female’s attention. The female peacock examines the feathers of the competing male peacocks very closely, choosing one to mate with. The better the male’s feathers, the better his chances of reproducing more than his male competitors – and, therefore, the better the chances that the more appealing sexual trait grows more dominant.

Sexual selection, while less evident, is also applicable to many human traits, including male AND female characteristics. For males, sex-specific traits that are important to female mates would include broad shoulders, beards, and ‘masculine’ facial features. For women, sex-specific traits that are important to male mates would be broad hips, breasts, and ‘feminine’ facial features. Another intrinsic characteristic of human sexual selection has less to do with physical features and more to do with emotional connection. Male peacocks, unlike humans, mate many times each mating season, and peacocks, also unlike humans, do not feel complex emotional bonds with other peacocks. Human mating rituals have much more to do with the person rather than the procreation. Even in modern human societies, mating means connection and bonding rather than examination merely of physical traits. This love-based mating ritual of humans is what makes our species, and in fact all apes, so unique in the animal kingdom. We tend to value bonding over sex.

Evolution is a hotly-debated scientific subject that is seemingly inexplicable to many, yet by no means is it hard to grasp. We must admit that the law of evolution – and its associated theories – are far more controversial than they should be, especially once we learn the processes themselves.

If you have any questions, comments, or corrections, please comment on this post or email learningbywilliam@gmail.com with your concerns. Thank you.


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