On the Tendency of Species to Form Varieties…

On the Tendency of Species to Form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection. By Charles Darwin, Esq., F.R.S., F.L.S., & F.G.S., and Alfred Wallace, Esq. Communicated by Sir Charles Lyell, F.R.S., F.L.S., and J.D. Hooker, Esq., M.D., V.P.R.S., F.L.S., &c. (London, June 30th, 1858). Darwin 1858

Charles Darwin, in conjunction with Alfred Wallace, published this work in 1858 through the Linnean Society, the world’s first and longest standing group dedicated to the study and dissemination of matters concerning natural history, taxonomy, and evolution. Born in 1809, Charles Darwin had an all-consuming desire to collect biological specimens. His father wanted to make a doctor of him, but he dropped out from the University of Edinburgh whereby his dad ‘re-assigned’ him to a career in the church. Shortly thereafter, an old professor informed him that the HMS Beagle would embark soon for South America then around the world (Mastery, Robert Greene). Darwin joined as the crew’s naturalist and for much of the subsequent five years he filled countless notebooks with plant, animal, and geological observations. With this and the thousands of specimens he collected and crated back to England, the theory of evolution began to take shape (American Museum of Natural History). His partner in the matter, Alfred Wallace, is undoubtedly the unsung hero of this story.


Darwin Wallace
Alfred Wallace (Left) & Charles Darwin (Right), World

Nearly everyone you ask will attribute the Theory of Evolution to Charles Darwin, but Alfred Wallace was an equal contributor to the matter. It was on an eight year expedition to the Dutch East Indies (Indonesia) when Wallace concluded that living things evolve. However, it wasn’t until three years later that he pinpointed how: they adapt to their environment. He conveyed his findings to Darwin, an old mentor, which eventually led to the co-authorship of this Earth shattering paper (NPR). Lastly, one notable and relevant figure is Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (Understanding Evolution). Yes, that is his comically long name. Although he presented a full-blown theory of evolution in 1801, before Darwin was even born, he differed in one major way. Lamarck proposed the Theory of Inheritance of Acquired Characteristics, whereby any changes an organism acquires over the course of their life will be passed to their offspring (What Lamarck Believed). Darwin debunks this absurd idea in the paper.


In a nutshell, the sole driving force for any animal to do anything in nature is to have as much sex a possible to produce as many children as possible. The central dogma of evolution, which follows hereafter, explains how this optimizes itself.

Genetic mutations produce new varieties or forms within a species, which, if chance has it, can confer a survival advantage for that individual over its peers. If this change is heritable and allows that organism to increase its fitness level – their reproductive success – then it will eventually dominate the gene pool and the variant will replace the species it spawned from.

“De Candolle, in an eloquent passage, has declared that all nature is at war, one organism with another, or with external nature” (46).

Along the Pacific coast of California boaters and beachgoers can see ‘forests’ of kelp. These aptly named kelp forest habitats provide refuge for juvenile fish by offering protection from predators and fast moving water. The California Current continually transports water from B.C. to Baja and without this sanctuary, juvenile fishes would lack an area to grow and develop in safety. Giant kelp (Macrocystis pyrifora) maintains its connection to the seafloor via an iron clad hold fast of root-like structures. It’s gas-filled bladders buoy its photosynthetic blades to the ocean’s surface to reach the sun. Meanwhile, droves of purple urchins (Strongylocentrotus purpuratus) threaten to detach its grasp by grazing at their biological anchors. Thankfully, though, the sea otter (Enhydra lutris) and the California sheephead (Semicossyphus pulcher) both feed on these pin-cushion predators.

“Lighten any checks in the least degree, and the geometrical powers of increase in every organism will almost instantly increase the average number of the favoured species” (48).

Unfortunately for the sea otter, their hair density can reach upwards of a million per square inch, making them the warmest in the world and highly sought after. The Legacy of the Fur Trade explains how Russian explorer Vitus Bering stumbled upon a raft of otters on the Commander Islands in the Russian Far East, triggering the ensuing  fur trade. “By the time conservationists got the International Fur Seal Treaty signed in 1911, which protected otters from hunting, scientists estimate that only 13 small groups of sea otters still survived on the wide arc stretching from Mexico to the Kamchatka Peninsula on Asia’s east coast.” With sea otters out of the way, purple urchin density grew unchecked and kelp forests paid the price. Still today, large swaths of Northern California coastline are ‘urchin barrens’, areas devoid of kelp and their associated fishy friends. With no Macrocystis to house sea otters, sheephead, or other predators of the purple urchin, the population now saturates the seafloor, smothering any new recruits before they can take hold, literally.

“But the external conditions of a country alter” (48).

Prior to Vitus Bering’s arrival, the complex and dynamic kelp forest ecosystem had checks and balances that prevented species from out competing each other. This system evolved over time by means of natural selection to suit the existing state of the environment. However, by anthropologically eliminating the sea otter, he altered the conditions of the food web leading to the downfall of the kelp forest.

To introduce evolution and natural selection, please watch this video by Minute Physics. Simply put, evolution means change over time and natural selection is the mechanism by which organisms better suited for their environment tend to survive and produce offspring. Anything can evolve; Pokémon, the weather, our dreams and desires, a popsicle in the hot, summer sun; the list goes on.

Natural Selection

Take even a small cross-section of humanity and a wide variety can be seen in a multitude of external traits, not to mention internal biology. It is impossible to argue against the existence of a range in hair, eye, and skin colors, that height varies greatly, and our aptitude to discern certain sights and smells are different. These are all variants in the human species. This is not dissimilar to any other species on planet Earth. Darwin and Wallace are saying that these ever so slight differences in our “structure, habits, or instincts” could confer a better chance of survival. If these particular traits are heritable, that is, coded for in our DNA, our children will acquire and be advantaged by them just the same. In reality, these differences have an equal likelihood of negatively or neutrally effecting us.

Natura non facit saltum” (52), to mean, “Nature does not make a jump.”

Whales did not evolve from ocean dwelling creatures into different ocean dwelling creatures. Scientists have established that the lineage of whales traces back to a terrestrial vertebrate (Smithsonian). However, they did not jump from one to the other. This process took millions of years and likely hundreds or thousands of generations of unknown failed varieties and unknowable small successes in the advancement of their form and function to live in the ocean. In Matthew Syed’s book, Black Box Thinking: Why Most People Never Learn From Their Mistakes – But Some Do, he explains the ‘Nozzle Paradox’.

In the 1970s, a Liverpool soap factory called Unilever was manufacturing detergent in their usual manner. The process involved forcing high temperature chemicals through a nozzle then collecting the ensuing powder. However, the nozzles were inefficient and prone to blockage. When their team of experts couldn’t generate a superior design based in “phase transition”, chemical analysis, and fluid dynamics, they asked the biologists for help.

“They took ten copies of the nozzle and applied small changes to each one, and then subjected them to failure by testing them. ‘Some nozzles were longer, some shorter, some had a bigger or smaller hole, maybe a few grooves on the inside,’ Jones says. ‘But one of them improved a very small amount on the original, perhaps by just one or two percent.’ They then took the ‘winning’ nozzle and created ten slightly different copies, and repeated the process. They then repeated it again, and again. After 45 generations and 449 ‘failures’, they had a nozzle that was outstanding. ‘It worked many times better than the original’. Progress had been delivered not through a beautifully constructed master plan (there was no plan), but by rapid interaction with the world. A single, outstanding nozzle was discovered as a consequence of testing, and discarding, 449 failures” [my bold] (126).

Leave it to biologists to relate manufacturing optimization to natural selection. The boldface section is so vital because it states two key points about natural selection. There is no master plan! Every minute change in a species is the direct result of its attempt to optimize to the environment as it is now. I repeat, as it is now. Organisms don’t ‘hold onto’ a trait because it might be beneficial in the future. Optimization occurs through rapid interaction with the world. If a variant arrives that is less favorable, it is quickly squelched from existence; these variants are those 449 failures.

“Most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individual” (57). “It is also evident that most changes would affect, either favorably or adversely, the powers of prolonging existence” (58).

This is the kicker if I haven’t already made it clear. By procuring more food or securing greater protection for oneself, this new, genetically coded trait, allows that individual to live longer, bear more offspring than its peers, and have its successful trait appear in a greater proportion in the next generation. The cycle repeats.

We will end this section by addressing my favorite example of evolution and natural selection, and frankly an underused one: the Peppered moth (Boston betularia) during the industrial revolution. The air in England was relatively clean before the onset of the Industrial Revolution, but the transition to new manufacturing processes and a precipitous increase in production output led to a fouling of many air and water sources. Soot, ash, and other pollutants began to accumulate on tree bark and change their color from a lighter tone to black.

Imagine if you will, how well the darker variant would camouflage itself against a dirty, black tree bark background. Hungry, on looking birds would have trouble spotting and eating these. On the other hand, the lighter, marbled variant would stand out unfortunately well and be easy pickings for their predators. This would lead to an uneven ratio of black and white variants such that more black variants would live to reproduce and pass on their successful genes. Eventually this imbalance would lead to a shift in the overall population of Peppered moths whereby the white variant would either be reduced to extremely low numbers or completely eliminated altogether. So, in this instance, evolution describes the change from a population primarily dominated by the white moth to a population primarily dominated by the black moth. Natural selection describes the survival advantages conferred by being a dark variant (National Geographic).

Artificial Selection:

Strawberries the size of apples, apples the size of melons, and melons the size of, well, really large melons. Just look at the infographic below. This might seem like an exaggeration, but it’s not far from the reality of our produce aisle in today’s marketplace. Genetically modified organisms (GMOs) and other modern technology aside, we have been subconsciously altering the agricultural landscape for thousands of years and consciously doing so for hundreds.

“The “roguing”, as nurseryman call the destroying of varieties which depart from their type, is a kind of selection. I am convinced that intentional and occasional selection has been the main agent in the production of our domestic races; but however this may be, its great power of modification has been indisputably shown in later time.”

If environmental pressures are the dictator of natural selection then humans are the dictator of artificial selection. We can call the early period of humanity’s sedentary life a period of domestication. During this time we settled down and began growing crops to support ourselves instead of constantly chasing ephemeral supplies. We would sow a field of some wild plant species, just as it can be found in nature, and wait for it to produce the desired yield. Let’s take wheat for example. According to Nancy A. Eckardt’s paper Evolution of Domesticated Bread Wheat, “Two of the most important traits in the evolution of bread wheat and other cultivated grasses were an increase in grain size and the development of nonshattering seed. The former has been associated with successful germination and growth of seedlings in cultivated fields, whereas the latter trait (a hallmark of domestication) prevents natural seed dispersal and allows humans to harvest and collect the seed with optimal timing.” If left to itself in nature, natural selection would likely select for, as in ‘promote’ the survival of, wheat plants endowed with large seeds and effective dispersal methods. However, as humans we have desires that depart from the natural trajectory. As a result, we actively seek out the large seeded, non-shattering variant to replant and reap. From here the evolutionary process is the same.

Wheat Domestication.png

Please enjoy this lovely infographic I made using the Venggage website.

“Our quickly fattening pigs, short-legged sheep, pouter pigeons, and poodle dogs could never have come into existence in a state of nature, because the first step to such inferior forms would have led to the rapid extinction of the race; still less could they now exist in competition with their wild allies.” “Domestic varieties, when turned wild, must return to something near the type of the original wild stock, or become altogether extinct” (60).

Human intervention into the state of affairs of nature immediately jeopardizes the survival of the variant. By selecting for traits we find desirable, we are inherently selecting against traits that confer survival in the wild. It might be hard to see how strong ox bred for plowing fields couldn’t go toe-to-toe with their wild cousins. Traits like raw strength and added muscle mass, which we might see as universally advantageous, could be kryptonite in nature. If a population of farm ox was re-introduced to Portugal’s Côa Valley to roam free with the aurochs – the common ancestor to all domestic cattle breeds – they might not be so lucky (Return of the Aurochs). The irony here is that the aurochs went extinct in Poland in 1627, yet the modern ox lives on. This is contradictory to the outcome I just predicted because we assume they’re governed by the same laws.

“The life of wild animals is a struggle for existence. The full exertion of all their faculties and all their energies is required to preserve their own existence and provide for that of their infant offspring” (54). “The domestic animal, on the other hand, has food provided for it, is sheltered, and often confined, to guard it against the vicissitudes of the seasons, is carefully secured from the attacks of its natural enemies, and seldom even rears its young without human assistance” (60).

Is there much more to say than what this quote provides? If our plowing bovine left the care and comfort of our civilization, they would go extinct in the blink of an eye.

NvA Watermelon
James Kennedy, VCE Chemistry Teacher; Artificial vs. Natural Watermelon & Sweetcorn

Sexual Selection:

A lavish home, fancy sports cars, and a chiseled body, these are modern examples of secondary sexual characteristics used to attract a mate. Although these traits are indicative of higher earning individuals – whereby the money they earn can be used to secure food or purchase high-end security systems for protection – it’s these flashy, appealing qualities we associate with having an expanded dating pool. Plenty more women would rather sleep with a billionaire than a homeless man.

“…there is a second agency at work in most unisexual animals, tending to produce the same effect, namely, the struggle of the males for the females. These struggles are generally decided by the law of battle, but in the case of birds, apparently, by the charms of their song, by their beauty or their power of courtship, as in the dancing rock-thrush of Guiana” (50). “…the effect chiefly produced would be the modification of the secondary sexual characters, which are not related to the power of obtaining food, or to defence from enemies, but to fighting with or rivaling other males” (50).

By falling into the demographic mentioned above, men aren’t physically eliminating the competition. Rather they’re making themselves more marketable to women by making themselves look better in comparison to other suitors.

The Field Museum’s Division of Birds provide examples of how plumage, song, and behavior can all be used to win over females. The Indian Peafowl (Pavo cristatus), known by most as the peacock, is a classic example of how plumage is used to attract mates. It has been shown that females prefer males with a greater number of eyespots, which the male displays by raising and fanning its tail feathers. Thus, males have evolved to elongate their trains and produce more eyespots. Remember, in order for this trait to evolve it must be heritable and it is.

Peafowl Male
Male Courting; Bird Eden
Peafowl M:F
Male (Left), Female (Right); JFF

It should be noted that with sexual selection there is generally sexual dimorphism – a condition where sexes within a species exhibit different characteristics beyond their reproductive organs. Here you can see that the female is much more drab than her male counterpart.

The Superb Lyrebird (Menura novaehollandiae) is unrivaled in it’s ability to mimic other sounds in its courting songs. Their vocal repertoire can consist of up to 20-25 other song birds’ calls and even manmade sources like chainsaws. Wild Ambience has a beautiful example of a Superb Lyrebird’s song.

Lastly, the bowerbirds of Australia and New Guinea are often cited for their exquisite behavioral displays while mating. A bower is a structure built to woo females. “The males go to great lengths to gather objects it deems to be beautiful and to then artistically arrange them around the entrance to the bower.” These items often include colorful objects like flowers, bugs, and trash. The females subsequently chooses a mate based on their architecture, not their survival skills.


So, what did we learn today? Evolution is the change of something, anything, over time and ‘evolution by natural selection’ describes how species change to suit the demands of their environment. This process is slow, too slow to observe in a single lifetime, and progresses in small steps, not leaps and bounds. Artificial selection is identical to natural selection, except humans decide which traits are desirable rather than nature. We breed animals that exhibit traits suited for our needs and in doing so we render them unfit to return to the wild. This is how we got dogs. Lastly, sexual selection involves attracting mates using characteristics unrelated to survival. These are often ‘showy’ and not indicative of a male’s ability to procure food or ward off enemies.

Everyone should read the actual paper by Darwin and Wallace. As far as scientific literature goes, it’s not terribly long and they present the material much more concisely than I have.

Additional Resources:

Boundless Biology


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