Natural selection represents a fundamental process within biology, driving changes across life on Earth. It describes how certain organisms are better suited to their environment and therefore more likely to survive and pass on their traits. A common question arises regarding this process: does natural selection primarily impact individual organisms during their lifetime, or does its influence manifest over generations within entire populations?
The Mechanism of Natural Selection
Natural selection operates through a series of interconnected conditions. Within any given population, individuals display a range of variations in their traits. These differences can involve physical characteristics, metabolic processes, or behavioral patterns.
Many of these variations are heritable, meaning they can be passed down from parents to their offspring. This transmission of traits forms the basis for how characteristics persist or change across generations.
The environment plays a significant role in determining which of these heritable traits are beneficial. Individuals possessing traits that make them better adapted to their specific surroundings are more likely to survive, acquire resources, and ultimately reproduce. Consequently, they pass on their advantageous traits with greater frequency, thereby influencing the makeup of the next generation.
Why Individuals Don’t Change
Natural selection acts upon the existing traits of individual organisms. However, it does not alter an individual’s genetic makeup during its lifetime. An organism’s genes, which determine its characteristics, are essentially fixed at the moment of conception or fertilization.
Environmental pressures or the process of natural selection itself cannot directly modify an individual’s DNA to make it better suited to its surroundings. For example, if a rabbit lives in a cold environment, its fur will not spontaneously grow thicker because of the cold. The rabbit either has thick fur from birth, or it does not.
Therefore, an individual organism’s ability to survive and reproduce is based on the traits it already possesses. Those with suitable traits thrive, while those with less suitable traits may struggle or perish. This means natural selection is not a mechanism for personal adaptation within a single lifespan.
How Populations Evolve
The true impact of natural selection becomes evident when observed across generations within a population. While individuals do not change, the proportion of specific traits within a group of organisms can shift over time. This happens because individuals with advantageous heritable traits are more successful at surviving and reproducing.
As these successful individuals produce more offspring, their beneficial traits become more common in the subsequent generations. This gradual increase in the frequency of advantageous genes, or alleles, within the population’s gene pool is the essence of evolution. The population, as a whole, becomes better adapted to its environment.
Consider a conceptual example of a population of insects that feed on a particular plant. If a new, tougher variety of this plant emerges, only insects with stronger mandibles might be able to effectively consume it. Over many generations, the insects with stronger mandibles would survive and reproduce more successfully, leading to a population where strong mandibles are more prevalent.
Observable Shifts in Populations
Real-world observations provide compelling evidence of how natural selection drives changes in populations. A well-documented example is the development of antibiotic resistance in bacteria. When bacteria are exposed to antibiotics, most are killed, but a few individuals may possess a natural genetic mutation that allows them to survive. These resistant bacteria then reproduce, leading to a new population dominated by the resistant strain.
Similarly, agricultural practices have illustrated pesticide resistance in insect populations. When pesticides are applied, some insects naturally have genetic variations that confer resistance. These resistant individuals survive the chemical treatment and pass on their resistance genes to their offspring. Over time, the entire insect population becomes increasingly resistant to the pesticide, rendering it ineffective.
Another instance involves finches on the Galápagos Islands, where beak size can change in response to environmental shifts. During droughts, only large, hard seeds may be available, favoring finches with larger, stronger beaks capable of cracking them. Finches with smaller beaks may struggle to find food and reproduce less successfully. Consequently, the average beak size within the finch population increases in subsequent generations.