Sympatric speciation involves reproductive isolation evolving without geographic barriers, requiring strong disruptive selection and assortative mating. Polyploidy causes instant reproductive isolation in plants. Cichlid fish in lakes provide compelling examples of sympatric diversification through sexual selection and ecological specialization.
From your study of speciation, you know that new species arise when populations become reproductively isolated and diverge genetically. The most intuitive mechanism is allopatric speciation, where a geographic barrier — a mountain range, a river, an ocean — physically separates populations and prevents gene flow. Sympatric speciation asks a harder question: can a single population, living in the same place with no physical barriers, split into two reproductively isolated species? The answer is yes, but the conditions are demanding.
The fundamental problem is gene flow. When individuals in a population can freely interbreed, any genetic divergence between subgroups gets blended away each generation. For sympatric speciation to work, something must counteract this homogenizing force. The two main mechanisms are disruptive selection and assortative mating, and they typically must act together. Disruptive selection favors individuals at the extremes of a trait distribution over those in the middle — for example, birds with very large or very small beaks might feed more efficiently on different seed sizes than birds with medium beaks. If individuals also preferentially mate with others who share their extreme phenotype (assortative mating), the population can begin to split into two non-interbreeding groups even without any geographic separation.
The clearest and most dramatic mechanism of sympatric speciation is polyploidy in plants — a whole-genome duplication that creates an individual with twice the normal chromosome number. A tetraploid plant (4n) is immediately reproductively isolated from its diploid (2n) ancestors because crosses between them produce triploid (3n) offspring that are usually sterile, just as a mule (horse × donkey cross) is sterile due to mismatched chromosome numbers. Polyploidy can generate a new species in a single generation, making it the fastest known speciation mechanism. It is remarkably common in plants: estimates suggest that 30-80% of flowering plant species have polyploid origins.
The cichlid fishes of the African Great Lakes provide the most celebrated animal examples. In Lake Victoria alone, over 500 species evolved from a common ancestor in perhaps 15,000 years — far too many species in too small and uniform a lake for geographic isolation to explain. Instead, sexual selection on male coloration combined with ecological specialization on different food sources appears to have driven divergence. Females prefer males of particular colors, creating assortative mating, while competition for resources drives ecological divergence. The lesson of sympatric speciation is that reproductive isolation does not require mountains or oceans — it requires that selection and mating preferences be strong enough to overcome the blending power of gene flow within a shared habitat.