The concept of fitness in evolutionary biology is intimately related to the different forces of selection. Fitness refers to an organism's ability to survive and reproduce in its environment. In the context of stabilising selection, individuals with average or intermediate traits have higher fitness as these traits are optimally suited to stable environments. In disruptive selection, individuals with extreme traits have higher fitness in heterogeneous environments. Finally, in directional selection, individuals with a specific advantageous trait have increased fitness due to changing environmental conditions or pressures. Thus, each type of selection favours traits that maximise an organism's fitness in its specific environmental context.

Human activities can significantly influence directional selection in wild populations. One of the most prominent ways is through environmental changes, such as habitat destruction, pollution, and climate change. For example, pollution can alter the environment in a way that favours certain traits over others, as seen in the case of industrial melanism in peppered moths. Additionally, activities like overfishing or hunting can lead to directional selection, favouring traits that help species evade human capture. Agricultural practices, such as the use of pesticides, can also drive directional selection by favouring pest species that develop resistance. These human-induced selective pressures often lead to rapid evolutionary changes in wild populations.

Genetic drift, a random change in allele frequencies in a population, plays a significant complementary role with selection forces in shaping a population's genetic makeup. While selection forces act on traits based on their adaptive value, genetic drift can cause certain alleles to become more or less common independently of their impact on fitness. This is especially significant in small populations, where random events can have a larger impact on the gene pool. When combined with selection forces, genetic drift can either reinforce or counteract the effects of selection. For instance, beneficial traits favoured by selection can be lost due to drift, or harmful traits can become more common. The interplay of genetic drift and selection forces adds a layer of complexity to the evolution of populations, making it a less predictable and more dynamic process.

Disruptive selection can indeed lead to increased genetic diversity within a population. By favouring individuals at both extremes of a trait distribution, this type of selection encourages a wider range of genetic variations to persist and proliferate. In a population where two or more distinct phenotypes are advantageous, individuals with these traits have higher survival and reproductive success. Over time, this leads to an increase in the frequency of genes responsible for these traits, enhancing the overall genetic diversity. Such diversification is particularly pronounced in heterogeneous environments, where different traits can be advantageous in different microhabitats or under varying conditions, leading to a broader spectrum of successful genetic combinations.

Stabilising selection plays a critical role in maintaining a species' genetic health by promoting genetic stability and consistency. By favouring average or intermediate phenotypes, this type of selection reduces the frequency of extreme traits that might be detrimental to a species' survival and reproductive success. For example, in a stable environment, extreme phenotypic variations might result in lower fitness, as these extremes could be less adapted to prevailing conditions. By preserving the genetic stability of key traits, stabilising selection helps ensure that a species remains well-adapted to its environment. This is crucial for long-term survival, as it helps maintain the optimal genetic makeup that has proven successful over generations.