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The world around us is full of astounding variety. Every moment you spend outside is a moment spent surrounded by dozens of species and subspecies, each with their own unique characteristics. Though the rigors of the wilderness have been a driving force in creating diversity among living things for millenia, we humans have also begun to play an increasingly large role in modifying the plants and animals all around us. Many of the changes we encourage are unintentional–like the coal boom in the early 1900s causing many animal species to develop darker coats. However, some staggering changes have also been brought about by our own intentional direction, through the process of artificial selection.
In nature, forces such as predation drive change within a species by allowing some individuals to survive better than others. The lucky survivors then get a chance to pass down their beneficial genetics, and, over time, the entire species may completely change. This is known as natural selection. In domestic species, however, many natural pressures are minimized or completely absent, leaving room for us humans to step in and create the changes we want to see. We often have control over which organisms produce offspring, and that control allows us to pair up specimens with useful traits to quickly change the standard within a species. This is called artificial selection, and it is a tool that we can use for good–and ill.
Take broccoli, brussels sprouts, and kale, for example. These three plants are all vastly different in terms of taste and appearance, but they actually come from the exact same plant: the wild mustard plant. In fact, other popular veggies like cauliflower and cabbage also come from this very same species. Over many generations of wild mustard, various individuals were singled out for their interesting traits. The more these interesting traits were concentrated, the more unique each “subspecies” became, until we had a whole range of fascinating veggies derived from a single plant.
The many faces of Brassica oleracea. Image from the BBC.
This is the kind of thing that artificial selection is good at: taking small variations and turning them into massive differences. If there’s something we want to achieve, like breeding a mustard plant to have larger leaves, we can pick up on the tiniest variations and use them to build toward that end goal. In nature, individuals with less ideal traits can still survive and produce offspring; in human hands, it is often only the best specimens that are given the opportunity. What would take thousands of years in the wild can take only a few decades under human guidance.
However, the traits we try to optimize aren’t always wholly beneficial–for the species in question or ourselves. Take dogs, for example. Though many dog breeds are generally sound and healthy, many others find themselves stricken by the consequences of artificial selection.
Image from Dr. Jeffery Shoenebeck.
Above are the skulls of two dog species. The one on the right comes from a German Shepherd, and it resembles the skulls of the wolves from which dogs originated. The one on the left? Well, it comes from a Pug.
Though it’s easy to take Pugs and other flat-faced dog breeds “at face value,” their cuteness does come with consequences. Because of how intensely we selected for flat faces, the Pug breed didn’t have time to adjust to the changes. The snubbed nose seen in the skull above isn’t just a short snout–it is a scrunched version of the longer snout the breed began with. Instead of gradually shortening, the Pug’s nasal passages were rapidly “squished” by a few generations of artificial selection, and the breed now struggles with significant respiratory issues because of it.
Where companion animals are often artificially selected for their looks, other domestic species find themselves selected for utility. The domestic turkey offers a staggering example of how much artificial selection (and other forms of human intervention) can change a species. In 1960, the average domestic turkey grew to about 15.1 pounds; three years ago, in 2019, the average turkey weighed a whopping 30.5 pounds! In just fifty-nine years, our dogged artificial selection has more than doubled the size of the average turkey, allowing us to get more meat from fewer birds. However, this tremendous gain in musculature has resulted in a surprising issue for this familiar fowl.
The historical journey of turkeys. Image from BusinessInsider.
As it stands today, the domestic turkey is a species that cannot sustain itself without human intervention. Because of how quickly modern turkeys pack on muscle mass, most male turkeys are no longer capable of physically mating with their female counterparts. Turkey farming is currently sustained almost entirely by a technology known as artificial insemination, where semen collected from male turkeys is administered to the females by human technicians using a special device called an artificial insemination straw. By emphasizing one trait to an extreme, other aspects of the species' functionality took a pretty significant hit.
Artificial selection is a double-edged sword: we have the ability to improve particular traits, but, the farther we push them, the greater the strain on the species in question. Without a holistic approach that considers each species as a functioning whole, it is very easy to go from creating something good and useful–like cabbage, broccoli, and brussels sprouts–to something dubious, like Pugs with their compromised respiratory systems. Many species under the care of humanity are already on the tipping point of artificial selection, with many generations of pressure behind them. With modern advances like genetic analysis and artificial insemination now on our side to make selecting for certain traits easier, it is especially important to examine our efforts critically to prevent our improvements in certain areas from becoming detriments in others. When it comes to genetics, there is certainly such a thing as “too far”!
References & Recommended Readings
This brief article from the BBC sums up artificial selection in one bite-sized installment. If you want a good summary to keep on tab, this is the resource for you!
Don't let the simple title of this article from the Exploratorium fool you; though the titular broccoli is indeed the article's focus, it also contains a great discussion of how artificial selection works. If your interest was piqued by learning of broccoli's shared mustard plant heritage, definitely give this short article a peep! Not only will you learn more about artificial selection in regard to broccoli, but, if you're a bit of a green thumb, you might learn a few tips for saving seeds from broccoli, too.
This article from PBS dives deeper into some of the issues that have come with artificial selection in dogs. Though we focused on pugs for this introduction to artificial selection, many other breeds suffer from conditions linked to their selected characteristics. In addition to conditions with a direct link to artificial selection, this article also covers an unintentional side effect of concentrating on specific traits: damaging recessive mutations. When it comes to desirable traits, it isn't uncommon for breeders to pair related animals with one another. This makes it much easier for dangerous recessive traits to become the norm in certain breeds.
This article by the Atlantic goes further into detail on some of the fascinating science behind the modern turkey's many differences from its kin from the 60s. It's more than just size; artificial selection has produced turkeys that can grow much faster than their ancestors, among other things!
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