The Relationship Between Phenotypic Selection and Natural Select

Phenotypic selection is a fundamental concept in evolutionary biology that plays a critical role in shaping the diversity of life on Earth. The process refers to particular traits of an organism being favored or disfavored by natural selection based on their impact on the organism's fitness in its environment. Natural selection acts on these phenotypic traits, including its physical traits, behavior, and life history, favoring those that enhance an organism's survival and reproductive success and weeding out detrimental ones. Understanding the patterns and power of phenotypic selection is essential for comprehending the mechanisms that drive the evolution of species over time. In this context, the relationship between phenotypic selection and natural selection has been widely discussed and debated among evolutionary biologists. Many studies explore the relationship between phenotypic selection and natural selection, including the different types of selection and the factors that contribute to their strength and direction.

In "Patterns and Power of Phenotypic Selection in Nature" by Joel G. Kingsolver and David W. Pfennig, the article discusses the processes of phenotypic selection in nature, focusing on the patterns and consequences of selection acting on different aspects of organismal phenotype. The beginning of the paper emphasizes that natural selection acts on different aspects of an organism's phenotype, such as morphology, behavior, and life history, and biotic and abiotic factors influence the strength and direction of selection. For example, organisms that face high levels of predation or environmental unpredictability may evolve strategies prioritizing rapid growth and reproduction. In contrast, organisms in more stable environments may invest more in long-term survival. The biotic elements in the environment that apply selection include competitors, predators, and parasites, and the abiotic elements, including weather and nutrient availability, are referred to as agents of selection. 

Most traits in organisms demonstrate continuous variation, known as quantitative traits, which are influenced by numerous genes and the environment. When selection operates on quantitative traits, three primary patterns or modes of selection can arise. The different types of selection include stabilizing selection, directional selection, and disruptive selection. The authors provide the example of amphibian populations experiencing different ecological circumstances and undergoing different modes of selection. In a study conducted by Pfennig and colleagues, the body size ​​of two species of spadefoot toads, the Mexican spadefoot toads (Spea multiplicata) and Plains spadefoot toads (Spea bombifrons), was utilized as an indicator of fitness to observe the variety of mode of selection operating on trophic morphology for different species and populations. The researchers observed that in mixed-species ponds, the largest tadpoles of S. bombifrons, the most carnivorous species, were favored by directional selection, where fitness consistently increased with the value of the trait. This observation suggests that natural selection favors a more carnivorous phenotype in S. bombifrons, potentially due to selection for resource-use phenotypes that decrease competition with the more omnivorous S. multiplicata. Conversely, stabilizing selection, where individuals with intermediate trait values have the highest fitness, favored intermediate phenotypes among S. multiplicata in the same mixed-species ponds, indicating that individuals with extreme carnivorous phenotypes were at a competitive disadvantage against S. bombifrons. Additionally, among S. multiplicata in single-species ponds, disruptive selection favored extreme trophic phenotypes. Individuals expressing phenotypes on either end of a resource-use spectrum are most likely to have fewer resources available and fewer competitors. The third mode of selection contrasts the stabilizing and directional selection seen in the mixed-species ponds where intermediates are favored, and the extreme phenotypes are selectively disfavored. Furthermore, the researchers discovered that the mode of selection governing trophic morphology varies among different species and populations. 

Another study that focuses on the relationship between phenotypic selection and natural selection is “Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta” by P.-O. Cheptou et al. The study explores the cost of dispersal and the reduction in traits related to dispersal by examining the invasion of Crepis sancta (Asteraceae), a weed, in a fragmented urban environment. Dispersal is a fundamental trait in almost all organisms and is often explained as a trade-off between costs and benefits. The benefits of dispersal include the reduction of kin competition and adapting to heterogeneous environments, while the costs associated with dispersal include the possibility of getting lost during movement. The selective factors influencing the evolution of dispersal are not fully understood, but it is believed that increasing the cost of dispersal will result in lower dispersal. Although this cost may be evident in natural populations, the strength of this selection pressure on dispersal traits is difficult to measure in the wild. Crepis sancta was particularly useful for the study because each individual generates dispersing and non-dispersing seeds. The researchers conducted the study by measuring the dispersal distance of the plant's seeds in urban and rural environments and found that the urban populations had significantly longer dispersal distances. The study also found that the urban populations had higher genetic diversity than rural populations, suggesting that urbanization has increased gene flow and facilitated the spread of beneficial traits. The results suggest that rapid evolution can occur in response to urbanization and that urban environments may act as important drivers of evolutionary change in the natural world. In addition, the results imply that species may respond quickly to changes in land use and fragmentation caused by human activity. Such selection against dispersal could aggravate population isolation and endanger the persistence of plant species in fragmented landscapes. This study highlights the remarkable ability of plant populations to adapt rapidly to human-induced changes, such as urbanization, and sheds light on the mechanisms underlying their evolutionary responses.

In conclusion, these papers highlight examples of agents of selection that apply selection, including biotic factors such as competitors and urbanization, as well as abiotic factors such as weather. Phenotypic selection is a fundamental process that drives the evolution of traits in natural populations.  Both papers illustrate that favoring certain phenotypes over others can lead to rapid evolutionary changes and the adaptation of populations to their changing environments or speciation. The first paper shows how different ecological circumstances can lead to different selection modes, ultimately leading to species' body size and lifestyle divergence. In contrast, the second paper demonstrates that fragmentation and the loss of habitats in human-altered ecosystems can cause species to respond and adapt quickly by reducing unneeded dispersal traits. By studying phenotypic selection in relation to natural selection, we can gain a deeper understanding of the mechanisms that drive evolutionary change and inform strategies for preserving biodiversity and promoting the long-term persistence of species.