Artificial selection tends to reduce genetic variation within a species by focusing on specific traits deemed desirable by humans. This reduction occurs because only a subset of the population, those individuals possessing these traits, are chosen for breeding. Over time, the gene pool becomes narrower, leading to a decrease in genetic diversity. The long-term consequences can be significant. Reduced genetic diversity may result in a population that is less adaptable to environmental changes or new diseases. This is because a genetically diverse population is more likely to contain individuals with variations that could be beneficial under changing conditions. Moreover, limited genetic diversity can increase the incidence of certain genetic diseases or defects, particularly if harmful recessive traits become more common in the gene pool. This phenomenon is observed in many purebred dog breeds, where years of selective breeding have increased the prevalence of specific health issues. Hence, while artificial selection can produce immediate benefits in terms of desired traits, it also poses risks to the long-term health and adaptability of species.

Artificial selection can indeed lead to the development of new species, a process known as speciation. This occurs when the artificially selected traits become so pronounced that the selectively bred individuals can no longer interbreed with the original population. One classic example is the domestic dog (Canis lupus familiaris), which has been bred from the gray wolf (Canis lupus) through extensive artificial selection. Over thousands of years, various breeds of dogs have been developed, each with distinct physical and behavioral traits. While dogs are still considered the same species as their wild ancestors due to their ability to interbreed, the vast differences in certain breeds suggest the early stages of speciation. In agricultural contexts, different crops like cabbage, broccoli, kale, and Brussels sprouts have been artificially selected from wild mustard plants. Although these vegetables are not distinct species, they exhibit significant differences in morphology and usage, demonstrating how artificial selection can drastically alter organisms. However, it's important to note that true speciation, where interbreeding becomes impossible, is a long process and may not always be the direct result of artificial selection.

Artificial selection, particularly in agricultural contexts, has inadvertently contributed to the development of resistance in pests and pathogens. This occurs primarily through two mechanisms. First, when crops are bred for traits such as high yield or specific physical characteristics, they may lose resistance to pests and diseases. This can happen because the genetic focus on certain traits may neglect or even eliminate genes that confer natural resistance. Second, in an effort to combat pests and diseases, plants are often bred for resistance to specific threats. However, this can lead to a selective pressure on the pests or pathogens, encouraging the evolution of resistant strains. For example, the repeated use of a single type of pest-resistant genetically modified crop can lead to the emergence of pest populations that are no longer affected by the resistance gene. This is analogous to the way overuse of antibiotics has led to antibiotic-resistant bacteria. Therefore, while artificial selection has provided numerous benefits in terms of crop yield and quality, it also has the unintended consequence of driving the evolution of more resistant pests and pathogens.

Artificial selection can play a nuanced role in conservation efforts, especially for endangered species. On one hand, it can be used to enhance the genetic health of small, endangered populations. Through careful selective breeding, conservationists can increase genetic diversity and reduce the incidence of inbreeding, which is a common problem in small populations. For instance, in captive breeding programs, individuals may be selectively bred to ensure that genetic diversity is maximized, thereby improving the overall health and viability of the population. On the other hand, there are concerns that artificial selection in conservation could lead to the loss of natural behaviors and adaptations, as the selection criteria are set by humans and may not reflect the survival needs of the species in the wild. Furthermore, there is a risk that artificially selected traits may not be beneficial if the animals are reintroduced into their natural habitats. Therefore, while artificial selection can be a valuable tool in conservation, it must be used judiciously, with a clear understanding of the species' natural history and ecological needs.

Artificial selection significantly alters the evolutionary trajectory of domesticated plants and animals by favoring traits that are beneficial or desirable to humans, rather than those that enhance survival and reproduction in a natural environment. This human-guided evolution can lead to characteristics in domesticated species that are vastly different from their wild ancestors. For example, in plants, artificial selection has led to the development of fruits and vegetables with enhanced size, taste, and yield, often at the expense of natural defense mechanisms against pests and diseases. In animals, selective breeding has produced a wide range of breeds with specific physical and behavioral attributes, such as the various breeds of dogs, cattle, and poultry. However, this focus on human-preferred traits can also lead to reduced genetic diversity and the emergence of health issues due to inbreeding. Over generations, this form of selection can result in domesticated species that are highly adapted to human-controlled environments but may lack the genetic diversity and traits necessary to survive and thrive in the wild. Thus, while artificial selection is a powerful tool for shaping the characteristics of domesticated species, it profoundly influences their evolutionary path, often in ways that diverge significantly from natural evolutionary processes.