New species often arise when populations are separated by physical barriers. However, sympatric species emerge while still living within the same geographic area as their ancestral population. Their existence challenges traditional views of how life diversifies.
Understanding Sympatric Species
Sympatric species are distinct groups of organisms inhabiting the same geographical region but remaining reproductively isolated. This means they do not interbreed, or if they do, their offspring are not viable or fertile. Unlike allopatric speciation, which involves geographic separation, sympatry lacks a physical barrier. Instead, reproductive isolation develops through other mechanisms within their shared habitat.
Reproductive barriers prevent gene flow between diverging groups, leading to new species. These barriers are either prezygotic, preventing mating or fertilization, or postzygotic, affecting hybrid viability or fertility. The concept of sympatry and allopatry is now viewed as a continuum, with varying levels of geographic and reproductive overlap.
How Sympatric Species Arise
The emergence of new species without geographic separation is often driven by specific evolutionary mechanisms. One prominent mechanism is polyploidy, common in plants. Polyploidy occurs when an organism acquires more than two complete sets of chromosomes, often due to errors during cell division. This sudden change immediately isolates polyploid offspring reproductively from their diploid ancestors, as they cannot produce fertile offspring together. This genetic shift can lead to rapid species formation, sometimes in a single generation.
Disruptive selection also plays a significant role, favoring individuals at the extremes of a trait distribution. For instance, if a population uses two different food sources, individuals best suited for one source might experience greater survival and reproduction. This can lead to two distinct groups. Assortative mating, where individuals prefer to mate with others sharing similar traits, further reinforces this divergence. This preference accelerates reproductive isolation, as specialized individuals are more likely to mate with others sharing that specialization.
Strategies for Coexistence
Once sympatric species form, they must develop strategies to coexist without one species outcompeting the other or hybridizing. Niche partitioning is a common strategy where species reduce competition by utilizing different aspects of shared resources or habitats. This can involve consuming different food sources, foraging at different times, or occupying different microhabitats. For example, some species might specialize in eating smaller seeds, while others consume larger seeds.
Character displacement is an evolutionary change where differences between similar species are accentuated in areas where they overlap geographically. This divergence in traits, which can be morphological, ecological, behavioral, or physiological, reduces interspecific competition and promotes coexistence. It is driven by natural selection favoring individuals that compete less, leading to a shift in their characteristics. This process results in species being more ecologically dissimilar in sympatric zones compared to areas where they live apart.
Real-World Examples
Sympatric speciation and coexistence strategies are observed across various taxa, illustrating the diverse ways new species can arise and persist in shared environments. The apple maggot fly (Rhagoletis pomonella) is a well-known example of ongoing sympatric divergence. Historically, these flies laid eggs on hawthorn fruits. With the introduction of domestic apples, a new population emerged that now lays eggs on apples. Females tend to lay eggs on the fruit type they grew up in, and males seek mates on the same fruit, leading to reduced gene flow.
African cichlid fishes in lakes like Victoria and Malawi also provide compelling evidence of sympatric speciation. These fish have undergone rapid diversification, resulting in hundreds of species occupying distinct ecological niches. Their reproductive isolation is often linked to differences in mating behavior and mate choice. Palm trees on an oceanic island offer another example, where two closely related species coexist by specializing on different soil types with different pH balances. Despite airborne pollen, they do not produce viable hybrids, demonstrating reproductive isolation.