Sympatry describes a biological state where two related species or distinct populations exist within the same geographical region, regularly encountering one another. Their ranges overlap, allowing for potential interactions between individuals from both groups. This concept highlights a shared living space and focuses solely on the current spatial distribution of populations or species.
Sympatry in Relation to Other Geographic Distributions
Understanding sympatry becomes clearer when contrasted with other ways populations are distributed across landscapes. Allopatry describes populations or species geographically separated by a physical barrier, such as a mountain range or a large body of water. For example, two groups of animals living on entirely different islands, unable to reach each other, represent allopatry.
Parapatry, in contrast, refers to populations that are adjacent but occupy distinct territories, with a narrow zone of contact. An example is a species living on a mountaintop and a related group residing in the valley below, meeting only at the base. Sympatry differs because populations share the exact same area, without a clear division or limited contact zone. Two fish species living together in the same lake exemplify sympatry.
The Process of Sympatric Speciation
Sympatric speciation is an evolutionary process where new species arise from an ancestral population within the same geographic region. This occurs without physical barriers, relying on mechanisms that lead to reproductive isolation within the shared environment. Genetic differences accumulate, preventing interbreeding and allowing distinct lineages to form.
Polyploidy
One mechanism is polyploidy, common in plants. This occurs when an organism acquires more than two sets of chromosomes, often due to errors during cell division. Such polyploid individuals are unable to reproduce with their ancestral diploid (two sets of chromosomes) population, leading to instant reproductive isolation.
Disruptive Selection
Another driver is disruptive selection, where individuals at the extremes of a trait range have higher fitness than those with intermediate traits. This can lead to ecological niche partitioning, as populations within the same area specialize on different resources or habitats. For instance, some individuals might adapt to one food source, while others adapt to another, reducing interbreeding.
Sexual Selection
Sexual selection also contributes through divergent mating preferences. If different groups within a population develop preferences for specific traits, like variations in coloration or mating calls, they may only mate with individuals exhibiting those traits. This assortative mating can effectively split a population into reproductively isolated groups, even in close proximity.
Examples of Sympatric Speciation
The apple maggot fly, Rhagoletis pomonella, provides an example of sympatric speciation driven by disruptive selection. Historically, these flies laid eggs exclusively on native hawthorn fruits. With the introduction of domesticated apple trees to North America around the mid-19th century, some flies began laying eggs on apples. This led to two distinct groups: hawthorn flies and apple flies.
Females lay eggs on the fruit type they emerged from, and males seek mates on those same fruits, resulting in assortative mating and reduced gene flow. Hawthorn and apple fruits ripen at different times, causing temporal isolation as flies emerge and mate at different periods, further solidifying their reproductive separation.
Cichlid fish in the African Great Lakes offer another instance, influenced by sexual selection. Lakes like Victoria, Malawi, and Tanganyika host hundreds of cichlid species that diversified rapidly within the same body of water. Many species differ primarily in male coloration, and female cichlids exhibit strong preferences for specific male color patterns. This female mate choice acts as a selective force, promoting reproductive isolation between groups with different color morphs and contributing to the rapid speciation observed.
Polyploidy is a common mechanism for sympatric speciation in the plant kingdom, with wheat serving as a classic illustration. Modern bread wheat, Triticum aestivum, is a hexaploid, possessing six sets of chromosomes. This species arose from natural hybridization events between different ancestral diploid and tetraploid wild grasses, followed by chromosome doubling. The resulting polyploid forms were immediately reproductively isolated from their parent species due to altered chromosome numbers, even while growing in the same areas.
The Scientific Debate Surrounding Sympatry
For a considerable period, sympatric speciation was viewed with skepticism by many evolutionary biologists. The prevailing belief was that geographic isolation, or allopatry, was almost always a prerequisite for new species to form. The difficulty in definitively proving sympatric speciation contributed to this skepticism.
Scientists must rule out any possibility of past, temporary geographic separation, even on a very small scale, known as micro-allopatry. The absence of a physical barrier makes it challenging to pinpoint the exact mechanisms driving reproductive isolation. Despite these challenges, a growing body of evidence from theoretical models and empirical studies now supports sympatric speciation as a plausible and significant evolutionary process. While it remains an active area of research, with investigations into its frequency and underlying conditions, its occurrence in nature is increasingly accepted.