Life on Earth exhibits a vast array of diverse forms, from microscopic bacteria to towering trees and complex animals. This biological diversity arises through speciation, the formation of new and distinct species from existing ones. Speciation involves populations undergoing genetic changes over time, leading to unique characteristics. This fundamental process continually enriches ecosystems and contributes to the resilience of life.
Understanding Sympatric Speciation
Sympatric speciation describes the evolution of a new species from an ancestral species while both inhabit the same geographic region. This differentiates it from allopatric speciation, which requires populations to be geographically separated by physical barriers, such as mountains or rivers, to diverge. In sympatric speciation, reproductive isolation develops within a population without such barriers. Diverging groups remain in physical contact and can potentially exchange genes.
The term “sympatric” originates from Greek words meaning “together fatherland,” emphasizing the shared habitat. This mode of speciation challenges the traditional view that geographic isolation is always necessary for species formation. For speciation to occur in sympatry, mechanisms must arise that prevent interbreeding between diverging groups, even while they coexist. Such mechanisms lead to a reduction in gene flow, allowing distinct evolutionary paths to emerge.
Key Drivers of Sympatric Speciation
Polyploidy is a primary driver of sympatric speciation, especially in plants. Polyploidy is a genetic condition where an organism possesses more than two complete sets of chromosomes, often resulting from errors during cell division. This chromosomal duplication can immediately lead to reproductive isolation, as polyploid individuals are typically unable to interbreed successfully with their parent diploid species. Offspring resulting from such interbreeding are frequently sterile due to issues with chromosome pairing during meiosis, preventing gene flow.
Disruptive selection favors extreme phenotypes over intermediate ones within a population. This selection occurs when different ecological niches or resources are available, favoring individuals specializing in one niche. For instance, if a species exploits two different food sources, individuals best adapted to one source may be less suited to the other, leading to divergence. Over time, this selective pressure can reduce gene flow between groups exploiting different niches.
Sexual selection, through divergent mate choice, can lead to sympatric speciation. If individuals within a population develop differing preferences for mates based on specific traits, such as coloration, courtship rituals, or mating calls, this can lead to assortative mating. Assortative mating means individuals preferentially mate with others sharing similar traits, reducing gene flow between groups with different preferences. This self-reinforcing process can drive the population to split into two reproductively isolated groups.
Real-World Examples of Sympatric Speciation
Polyploidy is widely observed in plants, with modern bread wheat (Triticum aestivum) as a classic example. This hexaploid species (six sets of chromosomes) arose from hybridization and polyploidization events involving ancestral wheat species. Another well-documented case is found in the plant genus Tragopogon, where new polyploid species, such as T. mirus and T. miscellus, formed rapidly in North America from introduced European diploid species. These new species were immediately reproductively isolated from their diploid parents due to their altered chromosome numbers.
The apple maggot fly (Rhagoletis pomonella) exemplifies sympatric divergence driven by disruptive selection and niche partitioning. Originally, these flies laid eggs only on hawthorn fruits. With the introduction of apple trees to North America, a new population of flies began to lay eggs on apples. These two fly populations are now partially reproductively isolated because they prefer to mate on their respective host fruits, and their life cycles have adapted to the different fruiting times of hawthorns and apples.
Cichlid fish in the African Rift Valley lakes, particularly Lake Malawi and the crater lakes of Nicaragua, offer examples where disruptive selection and sexual selection contribute to sympatric speciation. In Lake Apoyo, Nicaragua, a new elongated species, Amphilophus zaliosus, evolved from the ancestral Amphilophus citrinellus within approximately 10,000 years. These species are reproductively isolated and exhibit distinct eco-morphological differences, adapting to different niches within the same lake. Similarly, diverse cichlid species in Lake Malawi show rapid diversification, with sexual selection based on male nuptial coloration maintaining reproductive isolation.