Sympatric speciation is the formation of new species from an ancestral species while both inhabit the same geographic region. Unlike allopatric speciation, which occurs when populations are geographically isolated, sympatric speciation happens without physical barriers like mountains or rivers. This process requires the evolution of other types of barriers that limit gene flow and interbreeding within a shared homeland.
The Barrier to Gene Flow
For sympatric speciation to occur, gene flow between diverging groups must be reduced. This is achieved through reproductive isolating mechanisms, which are traits that prevent different groups from producing viable, fertile offspring. These mechanisms create and maintain the genetic distinctness of a new species, as constant interbreeding would otherwise homogenize the population and prevent divergence.
Reproductive barriers are divided into two categories based on when they act. Prezygotic barriers function before the formation of a zygote by preventing mating or fertilization between diverging groups. Postzygotic barriers operate after a zygote has formed, and they result in the failure of the hybrid embryo to develop or the production of hybrid offspring that are sterile.
Ecological and Behavioral Divergence
Reproductive isolation can arise through ecological divergence, where subgroups adapt to different microhabitats or resources within the same area. This habitat differentiation reduces opportunities for them to interact and interbreed. For instance, if an insect species begins using a new host plant, the individuals specializing on the original plant and those on the new one will live and mate in different locations, initiating a split.
Sexual selection also drives divergence when mate choice is not random. If different female preferences for male traits, such as color or song, emerge within a population, it can split the group into distinct breeding pools. For example, if some females prefer blue males and others prefer red males, the two color forms can become reproductively isolated. As these preferences and traits become more pronounced, the feedback loop reinforces the separation, creating an effective behavioral barrier to gene flow.
Genetic Mechanisms of Isolation
Direct genetic changes can also cause reproductive isolation. A significant mechanism, especially in plants, is polyploidy—the condition of having more than two complete sets of chromosomes. Errors during meiosis can produce gametes with a diploid (2n) set of chromosomes instead of the usual haploid (1n) set. The fusion of two such gametes creates a tetraploid (4n) offspring.
This new polyploid individual is instantly reproductively isolated from the original diploid population. If a tetraploid plant breeds with a diploid plant, the triploid (3n) offspring are sterile because their chromosomes cannot pair correctly during meiosis. This postzygotic barrier means the tetraploid organism can only reproduce by self-pollinating or mating with other tetraploids, effectively forming a new species.
This process can occur as autopolyploidy, involving the doubling of chromosomes from a single species. It can also occur as allopolyploidy, where two different species hybridize and their offspring undergoes a chromosome doubling event. This creates a new species that contains the combined chromosomes of both parent species and is reproductively isolated from both. A significant portion of plant species originated through polyploidy.
The Role of Disruptive Selection
Divergence in sympatric speciation is often driven by disruptive selection. This form of natural selection favors individuals at both extremes of a trait spectrum over those with intermediate traits. Individuals with average characteristics have lower fitness, while those with extreme traits are more likely to survive and reproduce, causing the population to split into two distinct groups over time.
Consider a bird population in an environment with only very small and very large seeds. Birds with small beaks can efficiently eat small seeds, while birds with large beaks can handle large seeds. Because birds with intermediate-sized beaks are not well-suited for either food source, disruptive selection favors the individuals with the smallest and largest beaks and selects against the intermediate ones.
This selective pressure drives the ecological and behavioral divergence required for sympatric speciation. It pushes subgroups into different ecological niches or promotes distinct mating preferences. By selecting against intermediate traits, disruptive selection creates a two-peaked distribution of traits that can lead to the formation of separate species.
Observed Instances of Sympatric Speciation
An example of sympatric speciation in progress is the apple maggot fly, Rhagoletis pomonella. These flies originally laid eggs only on hawthorn fruits, but a portion of the population switched to apple trees after they were introduced to North America. Since apples and hawthorns fruit at different times, the two fly populations became temporally isolated, mating at different times. This habitat differentiation has led to genetic divergence between the two groups despite living in the same area.
The cichlid fish in Africa’s Great Lakes also demonstrate sympatric speciation driven by sexual selection. Hundreds of cichlid species evolved from a common ancestor within the same lakes, largely due to female mating preferences for male coloration. Light availability varies with water depth, affecting how male colors are perceived. Females developed preferences for males most visible in their specific light environment, leading to reproductive isolation between populations at different depths. This divergence in mating signals has resulted in a wide array of distinct species.