There Are Two Main Types of Variation
- Variation is one of the defining features of life.
- No two individuals are exactly alike, even within the same species.
- These differences are shaped by genetics, environment, or the interaction of both, and can be separated into two types:
- Continuous variation and discrete variation.
What Is Continuous Variation?
Continuous variation
Continuous variation describes traits that exhibit a range of phenotypes without distinct categories.
- This type of variation is often represented as a bell-shaped curve, with most individuals displaying intermediate traits and fewer at the extremes.
- Continuous traits are typically influenced by multiple genes (polygenic inheritance) and environmental factors.
Skin color in humans, which varies from very light to very dark, with countless shades in between.
What Is Discrete Variation?
Discrete variation
Discrete variation involves traits that fall into distinct, separate categories.
ABO blood groups, where individuals are type A, B, AB, or O, with no intermediates.
Continuous Traits and the Bell Curve
- Continuous variation results in a range of phenotypes, often forming a normal distribution curve.
- Traits like height, body mass, and wrist circumference are examples.
Applying Measures of Central Tendency
- Scientists use measures like mean, median, and mode to analyze continuous traits:
- Mean: The average of all data points.
- Median: The middle value when data is ordered.
- Mode: The most frequently occurring value.
If the skin tone measurements are 3, 4, 4, 5, 6, the mode is 4 because it appears most often.
ExampleIn the same dataset (3, 4, 5, 6, 7), the median is 5. If there is an even number of data points, the median is the average of the two middle values.
ExampleIn studying corn cob length (a continuous trait), researchers calculate these measures to summarize population trends.
Why Skin Color is A Continuous Trait
- Early humans in Africa evolved dark skin to protect against intense sunlight and UV rays.
- As humans migrated to regions with less sunlight, lighter skin evolved to synthesize vitamin D more effectively.
- Despite this genetic influence, skin colour is still considered a continuous variable because it shows a range of values influenced by multiple genes and the environment.
Polygenic traits like skin color demonstrate how evolution balances genetic and environmental pressures to ensure survival.
Key Differences Between Continuous and Discrete Variation
| Continuous Variation | Discrete Variation |
|---|---|
| A continuous range of phenotypes, no distinct categories. | Distinct categories, no intermediates. |
| Influenced by multiple genes (polygenic inheritance). | Typically influenced by one or a few genes. |
| Environmental factors can significantly influence traits. | Environmental factors usually have no effect. |
| Examples: Skin color, height, body mass. | Examples: ABO blood groups, number of eggs laid by birds. |
Reflection
- Continuous variation arises from the interaction of polygenic inheritance and environmental factors.
- Understanding this helps us appreciate the complexity of traits like skin colour and the importance of both genetics and the environment in shaping phenotypes.
- How does the interplay between genetics and the environment challenge the idea of nature versus nurture?
- Can you think of other traits influenced by both factors?
