Question

Describe two scenarios in human populations, one of which accounts for a gradual cline, and one for an abrupt cline.

Short answer

Gradual cline in human populations can be observed in the distribution of skin color, where the skin tone gradually changes from dark near the equator to light near the poles. An example of an abrupt cline is the distribution of lactose tolerance, which changes abruptly between regions where dairy farming is common versus where it is not.

Step-by-step solution

Example of a Gradual Cline

Gradual clines can be seen in the distribution of skin color in human populations around the globe. There is a steady gradation from the equator towards the poles. At the equator where solar radiation is highest, people have evolved darker skin to protect against harmful UV rays. As we move towards the poles, where solar radiation is lower, the skin tone becomes progressively lighter.

Example of an Abrupt Cline

Abrupt clines can be observed in the distribution of lactose tolerance in human populations. This is largely due to cultural factors linked to dairy farming. For instance, in regions where dairy farming is historically prevalent such as in Northern Europe, tolerance to lactose is high in adults. However, in regions close by where dairy farming is not practiced widely such as in Southern Europe, lactose intolerance increases abruptly.

Key concepts

Gradual Cline

Gradual clines are fascinating examples of how human populations adapt slowly over geographical distances. As you traverse wider regions, especially in areas with varying environmental factors, you might notice changes in certain traits. A prominent example is the skin color distribution across the globe.

Imagine the Earth as a large sphere with the equator receiving the most sun, while the poles get the least. Over generations, human populations have gradually adapted to the sun exposure levels in their respective regions.

• Near the equator, where ultraviolet (UV) radiation from the sun is intense, you'll find populations with darker skin. The darker pigmentation serves as a protective shield, minimizing the damage from UV rays.
• As you move away from the equator, towards the poles, the amount of solar radiation decreases. Therefore, populations in these regions have evolved lighter skin that facilitates efficient vitamin D synthesis, given the lower sunlight exposure.

This creates a smooth and continuous transition in skin color from darker near the equator to lighter towards the poles. Hence, the gradual cline in skin color is a profound testament to human adaptation over time.

Abrupt Cline

Abrupt clines represent sharp, sudden changes in human traits or characteristics across populations. These changes often occur due to cultural or lifestyle differences rather than environmental gradients. A classic example of an abrupt cline is the distribution of lactose tolerance among human populations.

Lactose tolerance in adulthood is primarily common in populations with a history of dairy farming. In certain regions such as Northern Europe, dairy farming has been a significant cultural practice. So, people in these areas have maintained the genetic ability to digest lactose—the sugar in milk—into adulthood.

• In Northern Europe, lactose tolerance is prevalent due to a long history of consuming milk and dairy products.
• However, if you move towards Southern Europe, you will notice a sudden increase in lactose intolerance. People in these regions historically have less emphasis on dairy products, resulting in different genetic adaptations over time.

This stark contrast between neighboring regions is an illustrative example of an abrupt cline, where the change happens over a relatively short geographical distance, driven largely by lifestyle and dietary practices.

Skin Color Distribution

Skin color among human populations is a classic example of a trait that displays a cline. It's influenced by sunlight exposure and ties deeply to our history and evolutionary adaptations.

The connection between skin color and geographic distribution is significant:

• At the equator, there is high exposure to the sun, which necessitates natural protection against harmful UV radiation. Darker skin, rich in melanin, offers this protection by absorbing and diffusing UV rays, thus reducing the risk of damage such as skin cancers.
• In contrast, regions farther from the equator receive significantly less UV exposure. Here, a lighter skin tone is advantageous for maximizing vitamin D production, which is crucial for bone health and immune function.

This gradual change in skin tone serves as evidence of adaptation, highlighting the beautiful complexity of human evolution in response to our environment.

Lactose Tolerance

Lactose tolerance is another intriguing example of how human populations adapt genetically in response to cultural practices. In most mammals, the ability to digest lactose diminishes after weaning, but in some human populations, this isn't the case.

This extended lactose tolerance aligns closely with the history of dairy consumption:

• Regions such as Northern Europe have a long tradition of dairy farming. With milk being a staple of the diet, natural selection favored individuals who could digest lactose into adulthood, leading to higher rates of lactose tolerance in these populations.
• Conversely, in areas where dairy is not a significant part of the diet, such as in East Asia or parts of Southern Europe, lactose intolerance remains common. Here, there was less evolutionary pressure to maintain lactase activity into adulthood.

This trait illustrates how cultural habits, such as diet, can direct genetic evolution, resulting in stark differences between populations that are relatively close in geographical terms.