Natural selection is a fundamental process that drives evolutionary change. It explains how populations adapt and diversify over vast spans of time. This mechanism relies on the presence of variation among individuals within a population, but not all variations are equally relevant. Natural selection can only act upon variations that are heritable.
Understanding Heritable Variation
Heritable variation refers to differences in characteristics among individuals that are encoded in their genetic material, DNA, and can be passed from one generation to the next. These variations arise primarily from random mutations in DNA sequences, or through genetic recombination during sexual reproduction. For example, a person’s natural eye color or their maximum potential height are largely determined by heritable genetic factors. In contrast, acquired traits are characteristics that an individual develops during its lifetime due to environmental influences or personal experiences, and these are not encoded in their DNA. A scar from an injury, muscles gained through exercise, or the specific language a person speaks are examples of acquired traits, which cannot be passed down to their offspring because they do not alter the genetic information in reproductive cells.
The Mechanism of Natural Selection
Natural selection operates on several basic principles. First, there must be variation among individuals within a population. This variation can manifest in diverse traits, such as size, coloration, or disease resistance. Second, there is differential survival and reproduction, meaning some individuals with certain traits are better suited to their environment, making them more likely to survive and produce offspring. Finally, for natural selection to lead to evolution, these advantageous traits must be inherited by the next generation. Over many generations, individuals possessing beneficial heritable traits tend to produce more surviving offspring, causing those traits to become more common in the population. This gradual increase in the frequency of advantageous heritable traits ultimately leads to the adaptation and evolution of the population.
Why Heritability is Essential for Natural Selection
Heritability is foundational for natural selection because evolution is defined as a change in the genetic makeup of a population over time. If a trait is not heritable, it cannot contribute to this long-term genetic shift. Even if a non-heritable trait offers a significant survival or reproductive advantage to an individual, that advantage effectively “dies” with the individual, as the trait cannot be propagated through the population because it lacks a genetic basis that can be passed on to offspring. Natural selection works by increasing the proportion of beneficial genes within a population across successive generations. Only variations encoded in DNA that can be reliably transmitted from parents to offspring provide the raw material upon which natural selection can act. Without heritability, there would be no mechanism for advantageous traits to accumulate and spread, preventing any lasting evolutionary change.
What Happens When Variation is Not Heritable
When variations are not heritable, natural selection cannot drive long-term evolutionary change, even if these traits impact an individual’s success. Acquired traits are prime examples of non-heritable variations that selection cannot act upon. For instance, a bodybuilder’s developed muscles are the result of environmental factors like diet and exercise, not genetic predispositions that can be passed to their children. Similarly, a bird that learns a complex song does not pass this learned behavior genetically to its own chicks.
Another example involves phenotypic plasticity, where an organism’s traits change in response to environmental conditions without any alteration to its underlying genes. A plant growing taller in abundant sunlight compared to one grown in shade might appear different, but their genetic potential for height remains the same. While these variations can influence an individual’s survival or reproduction, they do not lead to a change in the genetic composition of future generations. Ultimately, only variations encoded in genes and passed across generations can facilitate lasting evolutionary change through natural selection.