Evolution: Natural Selection and Human Aelection Article | Biochemistry for MCAT PDF Download

Do you have the ability to metabolize cow's milk, or do you experience lactose intolerance? The capacity to digest milk into adulthood is believed to have arisen from the lifestyle of our ancestors thousands of years ago. Approximately 10,000 years ago, with the advent of dairy farming, individuals who possessed the ability to consume milk had a distinct advantage over those who could not, as they had an additional food source abundant in nutrients, fats, and proteins. This was particularly significant due to the scarcity of food, especially during winter months. Hence, if you can digest lactose, it is probable that one of your ancestors engaged in dairy farming.

What is evolution?

• Evolution refers to the process by which species undergo changes in their inherited traits over time. It is a gradual and cumulative process that occurs over many generations, often spanning thousands or millions of years. The ability of species to adapt and evolve is crucial for their survival and success in different environments.
• A notable example of evolution is the ability of humans to digest lactose as adults. In infancy, most humans have the ability to digest lactose, the sugar found in milk. However, as individuals grow into adulthood, some lose this ability, resulting in lactose intolerance, while others continue to digest lactose without any issues. This variation in lactose tolerance is due to a hereditary mutation that allows certain individuals to digest lactose throughout their lives.
• The lactose tolerance trait emerged from a mutation that occurred thousands of years ago. This mutation provided a selective advantage to individuals who could digest lactose, as it offered an additional food source high in nutrients. Over time, the mutation spread through the human population, becoming a common trait among many individuals today.
• Overall, evolution involves the accumulation of heritable changes in traits over long periods, and the example of lactose tolerance in humans highlights how beneficial mutations can shape the genetic characteristics of a population.

There are 4 mechanisms of evolution (how evolution happens):

1. natural selection
2. mutation
3. genetic drift, and
4. gene flow

Natural selection - one of the mechanisms of evolution

An example of natural selection at work - antibiotic resistance

• You may be familiar with the issue of antibiotic resistance, where certain bacteria have developed the ability to withstand the effects of antibiotics commonly used to treat bacterial infections in humans. This resistance has emerged due to the overuse and improper use of antibiotics, leading to a significant challenge in treating infections effectively.
• The decrease in the effectiveness of antibiotics can be attributed to the genetic diversity present within populations of pathogenic bacteria. While most bacteria within an infection are genetically similar, occasional genetic variations occur among them. This genetic diversity is crucial because if all bacteria were identical, they would respond in the same way to their environment and could be eradicated by a single harmful factor. However, when there are slight genetic differences, certain conditions such as the presence of an antibiotic may be lethal to most bacteria, but a few individuals with genetic variations could possess traits that enable them to survive and thrive.
• These advantageous traits allow the resistant bacteria to persist in the presence of antibiotics and continue causing illness. This situation arises due to natural selection, where different environments favor different traits. In the case of antibiotic exposure, the bacteria with genetic variations that confer resistance have a higher chance of survival and reproduction, leading to the propagation of antibiotic-resistant strains.

What is artificial selection or selective breeding?

• In addition to natural selection, another significant form of selection is artificial selection, also known as selective breeding. Unlike natural selection, which occurs without human intervention, artificial selection involves deliberate human interference in selecting and breeding individuals with desirable traits in plants or animals.
• Artificial selection is commonly employed in agriculture and animal breeding to produce desired characteristics in crops, livestock, or pets. Humans selectively breed individuals that exhibit desirable traits, such as higher crop yields, improved disease resistance, specific physical features, or desired behavioral traits. By choosing which individuals to breed and allowing them to reproduce, humans shape the genetic composition of subsequent generations.
• Through careful selection and breeding, humans can rapidly amplify specific traits within a population that would typically undergo gradual changes over time through natural selection. Artificial selection has been utilized for centuries to cultivate crops with higher nutritional value, develop dog breeds with specific physical or behavioral characteristics, and produce livestock breeds with desired meat or dairy production qualities.
• The key distinction between natural selection and artificial selection is that the latter involves intentional human decision-making to drive the evolution of specific traits, whereas natural selection operates through the survival and reproduction of individuals with traits that are advantageous for their environment.

An example of artificial selection - Dog breeding

• Approximately 30,000 to 40,000 years ago, humans initiated the process of domesticating wolves, which has led to the creation of what we now know as dogs. Domestication involves isolating a small group of organisms, in this case, wolves, from the larger population and selectively breeding them to enhance desired traits. Over the course of thousands of years, the domestication of wolves has resulted in the reduction of certain aggressive behaviors towards humans, such as barking, howling, teeth-baring, or displaying defensive postures like attacking or fleeing. Additionally, there have been changes in the size and shape of their teeth.
• While the precise reasons behind the initial domestication of wolves remain uncertain, evolutionary biologists speculate that wolves may have assisted humans in hunting during the ice age, which could have played a role in their domestication (Callaway, 2015). Some researchers even propose that this domestication may have contributed to the survival of early human ancestors during the ice age and potentially played a part in the extinction of Neanderthals (Callaway, 2015). Nowadays, humans selectively breed dogs based on personal preferences and for companionship rather than for survival purposes, resulting in a wide variety of traits present in different dog breeds.

Dog breeding serves as an excellent illustration of how humans selectively choose and prioritize desirable or trendy traits. Within the realm of dog breeds, there are three distinct categories:

1. Purebred dogs are the result of breeding within a lineage of the same breed without any crossbreeding. For instance, a purebred German Shepherd exclusively consists of German Shepherd ancestry.
2. Crossbreed dogs are the offspring of two different purebred breeds. For example, if a purebred German Shepherd mates with a purebred Husky, their offspring would be a crossbreed, inheriting traits from both parent breeds.
3. Mixed-breeds, on the other hand, encompass a blend of multiple breeds, where their parents are not purebreds. The potential combinations of mixed-breeds are virtually limitless.

One advantage of choosing crossbreeds or mixed-breeds over purebred dogs lies in their genetic diversity. Harmful genetic mutations that tend to occur frequently within specific lineages can be mitigated or diluted by the genetic background of the other breed(s) present in the individual. In purebred dogs, as there is only one lineage involved, these genetic errors are often more evident and can make purebreds more susceptible to certain diseases or conditions.

Common misconceptions about evolution

• Evolution should not be conflated with adaptation or natural selection. It's important to note that natural selection is a driving force behind evolution. The terms "progress" or "improvement" do not inherently define evolution since its outcomes depend on the specific context. Consider a scenario where a certain trait is highly advantageous in one environment but extremely disadvantageous in another. An illustrative example of this can be observed in the fur color of mice. Blending in with their surroundings enables mice to evade predators, making it advantageous for them. In a forest habitat, mice are more likely to have darker fur to match the earthy tones. 
• However, if a mouse with dark fur is placed in a contrasting environment like a white sandy beach, it becomes highly visible and susceptible to being captured by a bird due to the lack of camouflage.

Consider the following:

In the case of lactose tolerance, this is precisely what occurred. An European population and an African population both developed the ability to digest lactose as adults independently of each other. Both populations were engaged in dairy farming, and the lactose tolerance trait emerged around the same time. Interestingly, these populations were geographically distant enough to prevent interbreeding, making the development of the lactose tolerance trait a remarkable coincidence.