Natural selection is a fundamental biological process that drives evolutionary change in living organisms. It describes how species adapt and change over time in response to their surroundings, shaping the characteristics of populations across generations. This process is one of the primary mechanisms explaining the vast diversity of life observed on Earth.
Genetic Variation Among Individuals
For natural selection to act, differences must exist among individuals within a population. This inherent variability, known as genetic variation, provides the raw material upon which selection operates. Without it, populations would lack the diverse traits necessary for some individuals to fare better in a given environment.
One primary source of genetic variation is mutations, which are random changes in the DNA sequence. These changes can occur spontaneously or due to environmental factors. While many mutations are neutral or harmful, some introduce new or altered traits into a population.
Genetic recombination, particularly during sexual reproduction, also contributes significantly to variation. This process shuffles existing genes from two parents, creating new combinations of traits in offspring. Gene flow, the movement of genetic material between different populations, introduces new genetic variants and increases diversity.
Heritable Traits
For any trait to be subject to natural selection, it must be heritable, meaning it can be passed down from parents to offspring. Traits acquired during an individual’s lifetime, such as a scar, are not typically passed on and do not contribute to evolutionary change.
Genes, which are segments of DNA, contain the information that determines an organism’s traits. These genes are inherited from parents, influencing the observable characteristics of an individual.
Heritability quantifies the extent to which variation in a trait is due to genetic differences among individuals. If a trait has high heritability, it is more likely to be transmitted across generations. This allows beneficial traits to become more common in a population over time, forming the basis for evolutionary adaptation.
Unequal Survival and Reproduction
Unequal survival and reproduction are central to natural selection, as they describe how certain individuals are favored. In any environment, some individuals possess heritable traits that make them better suited to their surroundings. This suitability gives them an advantage in surviving and producing offspring.
Individuals with advantageous traits are more likely to live long enough to reach reproductive age and pass on their beneficial genes. For example, a rabbit with a coat color that provides better camouflage might evade predators more effectively, increasing its chances of survival.
Reproductive success is the measure of an individual’s “fitness” in an evolutionary context. Fitness refers to an organism’s ability to produce offspring that survive and reproduce, passing on their genes. Traits that enhance an individual’s ability to find mates, reproduce, and ensure progeny survival become more prevalent in the population over time.
This differential reproductive success means individuals with less advantageous traits are less likely to survive or produce fewer viable offspring. As a result, their genes become less common in the population. Over many generations, the frequency of beneficial traits increases, leading to the gradual adaptation of the population to its environment.
Struggle for Existence
Populations typically produce more offspring than the environment can sustain. This leads to competition among individuals for limited resources, known as the “struggle for existence.” This competition creates the selective pressures that drive unequal survival and reproduction.
Organisms compete for essential resources such as food, water, shelter, and mates. For instance, plants in a dense forest compete for sunlight and nutrients, while animals might compete for feeding grounds or nesting sites.
This competition means not all individuals acquire enough resources to survive and reproduce. Those with traits allowing them to outcompete others for limited resources are more likely to thrive. For example, a faster predator might secure more food, or a plant with deeper roots might access more water during a drought.
The struggle for existence favors individuals whose inherited traits provide an edge in acquiring resources or avoiding dangers. This ongoing competition ensures that only a subset of the population survives to pass on their genetic material, shaping the evolutionary trajectory of the species.