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Evolution: Natural selection and human selection article

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Are you able to digest cow’s milk, or are you lactose intolerant? The ability to digest milk as an adult is thought to come from the lifestyle of our ancestors thousands of years ago. About 10,000 years ago, when dairy farming started, humans that could drink milk had an advantage over humans that couldn’t because they had an extra source of food that was high in nutrients, fats, and proteins. This was a really big deal because food wasn’t always easy to come by, especially in the winter months. So, if you are able to digest lactose, then one of your ancestors was likely a dairy farmer!
A cartoon of a cow with a spot that looks like DNA is saying "evo-MOO-tion"

What is evolution?

Evolution is how species experience heritable (passed from one generation to the next) changes in their traits over time. In order for evolutionary changes to occur, many generations over thousands to millions of years are often required - meaning, these adaptations don’t happen overnight! The ability of humans to digest lactose as adults is a perfect example of this. As babies, a majority of humans have the ability to digest lactose. As we develop into adults, some people lose the ability to digest lactose, and others continue to have no problem digesting milks, cheeses, and other food products containing lactose. A hereditary mutation responsible for the trait allowing humans to digest lactose well into adulthood is kept “on”, resulting in lactose tolerance. This trait is a result of a mutation from thousands of years ago. The mutation causing the trait was beneficial and heritable, so it spread throughout the human population and many of us today have this trait!
There are 4 mechanisms of evolution (how evolution happens):
  1. natural selection
  2. mutation
  3. genetic drift, and
  4. gene flow
Of these 4 mechanisms we will talk about natural selection.

Natural selection - one of the mechanisms of evolution

Natural selection is a process that favors heritable traits that increase the survival chances of an organism, allowing the organism to reproduce more!

An example of natural selection at work - antibiotic resistance

You may have heard about antibiotic resistance. Bacterial infections in humans are commonly treated with antibiotics. However, because of their overuse and misuse, some pathogenic bacteria have become resistant to some of these treatments. This is dangerous for humans, because treatments that used to cure potentially fatal diseases are now less effective in some cases, or not effective at all. How have antibiotics become less effective?
When you have an infection, the multiplying pathogenic bacteria inside of you are not all identical. Most of them are the same, but every now and again one of of them will be slightly different genetically. If they were all genetically identical, they would react to their environment the same way and all be harmed by the same things. One harmful thing could wipe out the entire species. But when there are slight genetic differences, harmful conditions, like the presence of an antibiotic, might kill most of the pathogenic bacteria but some may be better able to survive and then thrive. These characteristics are favored in this environment so these bacteria can continue to thrive, and make you ill. Different environments favor different traits and so natural selection has taken place!
A timeline showing a decrease in the number of bacteria over time when taking an antibiotic.

What is artificial selection or selective breeding?

There are other types of selection, in addition to natural selection, that are out there in the world. Think about some decisions you make about the types of pets you want or what kind of foods you prefer to eat. Artificial selection, also called "selective breeding”, is where humans select for desirable traits in agricultural products or animals, rather than leaving the species to evolve and change gradually without human interference, like in natural selection.

An example of artificial selection - Dog breeding

Around 30,000 to 40,000 years ago, humans began domesticating wolves. Nowadays, these domesticated animals are what we call dogs! Domestication is the act of separating a small group of organisms (wolves, in this case) from the main population, and select for their desired traits through breeding. Over thousands of years, the domestication of wolves resulted in the loss of some of the more aggressive traits, like the instinctual, defensive behavior in the presence of humans (barking or howling, bearing their teeth, poising to attack, or running away), and the size and shape of their teeth. The exact reasoning why humans began domesticating wolves isn’t known although evolutionary biologists suspect that wolves helped humans to hunt during the ice age (Callaway, 2015). In fact, they’ve also hypothesized that if this is true, their domestication could have been what helped human ancestors through the ice age and why Neanderthals went extinct (Callaway, 2015)! Now humans select for a variety of traits in dogs based on personal preference and companionship, instead of as a way to increase human survival.
A timeline showing how dogs became domesticated over a long period of time due to artificial selection.
Dog breeding is a perfect example of how humans select for desirable or fashionable traits. There are three different types of breeds that exist:
  1. Purebred is a type of dog that comes from a lineage of the same dog breed and that has never mated with another breed. For example, a purebred german shepherd is all german shepherd and nothing else.
  2. A cross-breed dog is a dog that was the offspring of two different types of purebreds. Let’s say your purebred german shepherd mated with a purebred husky. The resulting offspring would be a cross-breed of half german shepherd, half husky.
  3. Finally, mixed-breeds are a combination of multiple breeds, where their parents were not purebreds. There are too many possible combinations to count!
One advantage to choosing cross-breeds and mixed-breeds over purebred dogs is that harmful genetic mutations that tend to frequently occur in certain lineages can be covered up, or “bred out”, by the genetic background of the other dog breed(s) in the individual. In purebreds, since there is only one lineage, these mistakes are often more apparent and can make purebred dogs prone to certain diseases.

Common misconceptions about evolution

Evolution is not the same as adaptation or natural selection. Remember: natural selection is a cause of evolution. Terms like “progress” or “improvement” are not a part of the definition of evolution because evolution is context dependent. Imagine a scenario where one trait might be highly advantageous in one environment, but highly detrimental in another. A good example of this is the fur color of mice. It’s advantageous if mice blend in with their habitat so that they can avoid predators. In the forest, it will be more likely that mice take on a darker color to match the earth. If you were to take a mouse with dark fur out of their original habitat and place it on a white sandy beach, it will get snatched up by a bird very quickly because it’s so easy to see.

Consider the following:

Can beneficial traits arise in more than one area by accident? Yes! Let’s go back to our lactose tolerance example at the beginning of the article. When multiple environments favor the existence of a trait, these beneficial traits can pop up through mutation and spread throughout their individual populations completely independently. Evolutionary biologists call this convergent evolution. In the lactose tolerance example, this is exactly what happened. A population in Europe evolved the ability to digest lactose as an adult independently from an African population. Both populations had begun farming dairy, and both traits arose around the same time. What’s cool is that, when the lactose tolerance trait arose, these populations were far enough away that they were not able to reproduce with each other making the development of the trait a neat coincidence.
Cartoon showing a cow in Europe and a cow in Africa.

Want to join the conversation?

  • spunky sam blue style avatar for user Krish Patel
    Does artificial selection stop, trigger, or not affect evolution of a species.
    (3 votes)
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  • aqualine ultimate style avatar for user Txnnxr
    Where can you find a wild cow?
    (2 votes)
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  • blobby green style avatar for user skylarclay0
    How does artificial selection help to prove evolution taking place in bacteria
    (2 votes)
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    • male robot hal style avatar for user Charles LaCour
      One thing to keep in mind is that in science no theory is ever proven.

      The genetic code for a organism can be different from the genetic code that is passed to offspring because of genetic damage or errors in the process of replication of DNA. These changes are fairly random. This is an observed fact.

      This difference in genetic code between organism and offspring leads to random changes in the genetic code of the gene pool of the organism and its offspring.

      Genetic changes in a gene pool of an organism can cause changes in the organisms that affect how an individual organism functions. This is an observed fact.

      The natural or artificial selection based on these functional changes has been observed to cause specific genetic information to become more prevalent in a gene pool. This is also an observed fact.

      Based on these observation any many others is the basis of the modern theory of evolution.

      Artificial selection on bacteria is like a proof of concept of the theory of evolution. It is a study of these changes in a controlled and simplified environment to understand the process before looking for these types of processes and influences in nature.
      (3 votes)
  • blobby green style avatar for user brieanna.massung
    Why did people only drink milk if they were a dairy farmer? How did most people adapt to drink cow's milk?
    (2 votes)
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  • starky seedling style avatar for user Winona Nolan
    Do dogs get smaller I don't think so?
    (0 votes)
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  • stelly blue style avatar for user Azucena Jauregui
    Which type of artificial selection have humans been doing for thousands of year?
    (1 vote)
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  • hopper cool style avatar for user 秋のフォスター🏳‍🌈
    can artificial selection bring back extinct animals?
    (1 vote)
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    • male robot donald style avatar for user Tybalt
      It is definitely feasible; however, there are several things to consider:

      -Is the climate suitable for this organism anymore?
      -Why not spend the money protecting organisms that are going extinct right now?
      -Will there be a suitable food source for the organism?
      -Will the organism be healthy? Organisms that have been genetically altered in such a fashion are typically not in the best health. Clones like Dolly the sheep do not live for long, and hybrids like the mule are sterile.

      Did this help?
      (1 vote)
  • starky seedling style avatar for user mahi_fish
    Does artificial selection affect the species negatively?
    (1 vote)
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  • blobby green style avatar for user eleanor.bell
    How did people adapt to drinking milk.
    (0 votes)
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    • aqualine tree style avatar for user Fluffykitty :3
      Well, this is what I found off of google I hope this helps ^^

      "But then evolution kicked in: some people began to keep their lactase enzymes active into adulthood. This “lactase persistence” allowed them to drink milk without side effects. It is the result of mutations in a section of DNA that controls the activity of the lactase gene."
      (1 vote)
  • duskpin ultimate style avatar for user Ka Yu WONG
    Is artificial selection a type of natural selection? If not, where could it be found in the 4 mechanisms of evolution memtioned in the article(natural selection, mutation, genetic drift, and gene flow)?
    (0 votes)
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    • female robot grace style avatar for user tyersome
      Artificial selection and natural selection are thought of as different types of selection, but at a fundamental (genetic) level they are essentially the same process.

      Organisms with "better" traits have more offspring.

      The difference is in what "better" means.

      In the case of natural section "better" means more appropriate for surviving and reproducing in a "natural" environment.

      In the case of artificial section "better" means more likely to be allowed to survive and reproduce by the humans doing the selecting.

      Does that help?
      (2 votes)