Sympatric Speciation

Sympatric speciation (from the Ancient Greek sýn, "together," and patrís, "fatherland") is the divergence of a single ancestral interbreeding population into two or more reproductively isolated daughter species within the same geographic range, in the complete absence of extrinsic physical barriers to dispersal or mating. In spatial population-genetics terms, whereas allopatric speciation begins with zero initial gene flow (m = 0) enforced by a geographic barrier, sympatric speciation begins in a panmictic population (m = 0.5) where every individual has the spatial opportunity to encounter and mate with any member of the opposite sex.

For over a century following Charles Darwin's 1859 formulation of his "principle of divergence," the plausibility of sympatric speciation was one of the most contentious debates in evolutionary biology. While Ernst Mayr (1963) argued that homogenizing gene flow and meiotic recombination would almost always swamp nascent sympatric polymorphism in sexually reproducing animals, late-20th and 21st-century mathematical modeling and genomic investigations established two rigorous pathways by which sympatric speciation proceeds in nature: (1) instantaneous chromosomal polyploidization (particularly prevalent in vascular plants) and (2) ecological disruptive selection coupled with non-random assortative mating (verified across host-specific phytophagous insects, crater-lake cichlid fishes, and oceanic island palms).

Felsenstein's (1981) Two-Allele vs. One-Allele Barrier

The central theoretical obstacle to sympatric speciation via gradual divergence was formalized by Joseph Felsenstein in 1981. Suppose a population inhabits a heterogeneous environment containing two distinct ecological niches (Niche 1 and Niche 2). Disruptive natural selection favors homozygotes BB in Niche 1 and homozygotes bb in Niche 2, while selecting against intermediate ecological heterozygotes Bb (underdominance of ecological fitness). By Maynard Smith's (1966) Levene soft-selection model, disruptive selection can maintain a balanced polymorphism at the B/b locus if niche fitness differentials are sufficiently strong.

However, maintaining a polymorphism is not speciation; true reproductive isolation requires evolving prezygotic assortative mating so that BB individuals mate exclusively with BB and bb individuals mate exclusively with bb. Felsenstein distinguished two genetic architectures for achieving this coupling:

1. Two-Allele (Multi-Locus) Assortative Mating: Suppose ecological adaptation is controlled by locus B/b (and epistatic fitness modifier C/c), while mate choice is controlled by a separate, unlinked locus A/a (where AA females prefer AA males and aa females prefer aa males). For sympatric speciation to occur, the population must build strong linkage disequilibrium (D) between the ecological loci and the mating locus, dividing into two isolated gametic haplotypes: ABC (Niche 1 specialists that mate with A) and abc (Niche 2 specialists that mate with a). However, every time an occasional ABC × abc cross occurs, meiotic recombination (at crossover rate r up to 0.5) shuffles the chromosomes into recombinant gametes (Abc, aBC), decaying linkage disequilibrium by ΔD = −r D per generation. Because recombination acts explicitly against the nascent species barrier, two-allele sympatric speciation fails unless disruptive selection s is exceptionally strong relative to recombination (s > r).
2. One-Allele Assortative Mating and "Magic Traits": Recombination ceases to oppose speciation under two conditions:

• One-Allele Mechanism: If a single new allele M (fixing in both nascent ecological subpopulations) instructs its bearer to "mate at the microhabitat where you developed" or "mate with individuals matching your own morphology," recombination between the two incipient species does not break down assortative mating because both subpopulations carry the identical MM genotype at the mating locus.
• Pleiotropic "Magic Traits" (Gavrilets 2004): Coined by Sergey Gavrilets, a magic trait is a phenotypic character that is simultaneously subject to disruptive ecological selection and directly mediates non-random assortative mating (effectively r = 0 because adaptation and mating are pleiotropic expressions of the same locus). Examples include host-plant preference in phytophagous insects that court and copulate directly on their larval food plant, body size or jaw morphology in fishes that also serves as a visual mate-choice cue, foraging depth along light-attenuated water columns coupled to visual opsin sensitivity, and flowering time (allochrony) in plants adapted to different soil types.

The most rapid and unambiguous mechanism of sympatric speciation—generating near-complete postzygotic reproductive isolation in a single generation—is polyploidization (whole-genome duplication). Quantitative cytogenetic analyses by Sarah P. Otto and Jeannette Whitton (2000) and loren Rieseberg (2009) estimate that polyploidy directly accounts for ~15% of all speciation events in angiosperms (flowering plants) and ~31% of speciation events in leptosporangiate ferns.

1. Autopolyploidy: Arises within a single diploid species (2*n*) when mitotic non-disjunction or meiotic failure (unreduced 2*n* gametes) produces a tetraploid (4*n*) offspring. If the newly formed 4*n* individual self-fertilizes or mates with a sibling 4*n* individual, it establishes a fertile tetraploid lineage. However, if the 4*n* tetraploid (producing 2*n* gametes) backcrosses with the surrounding 2*n* diploid parental population (producing 1*n* gametes), the resulting triploid (3*n*) F₁ hybrids suffer severe triploid block: during meiosis I, the three homologous chromosomes of each trivalent segregate randomly into unbalanced, aneuploid gametes (carrying chromosome numbers between n and 2*n*), rendering triploid hybrids sterile and isolating the 4*n* species in sympatry (e.g., the fireweed Chamerion angustifolium and the gray treefrog pair Hyla chrysoscelis [2*n* = 24] and Hyla versicolor [4*n* = 48]).
2. Allopolyploidy (Amphidiploidy): Arises when two distinct sympatric diploid species (AA with 2*n* = 2*x*₁ and BB with 2*n* = 2*x*₂) hybridize to form a sterile F₁ hybrid (AB, whose non-homologous chromosomes cannot pair during meiosis I), followed by somatic or meiotic chromosome doubling to produce a fertile allotetraploid (AABB, 2*n* = 2*x*₁ + 2*x*₂) in which every chromosome again possesses an exact homologous pairing partner. Dozens of new allopolyploid plant species have arisen in sympatry within the past 150 years, including the saltmarsh cordgrass Spartina anglica (2*n* = 120, formed in southern England ~1890 from hybridization between native S. maritima [2*n* = 60] and introduced North American S. alterniflora [2*n* = 62]) and the salsify goatsbeards Tragopogon miscellus and Tragopogon mirus (2*n* = 24, formed in Washington State in the 1920s from diploid European T. dubius, T. pratensis, and T. porrifolius [each 2*n* = 12]).

To prove that two non-polyploid sister species diverged in sympatry rather than in past allopatry followed by range overlap, Jerry A. Coyne and H. Allen Orr (2004) formulated four stringent diagnostic criteria: (1) contemporary sympatric distribution, (2) substantial genetically based reproductive isolation, (3) a reciprocally monophyletic sister-taxon relationship without evidence of historical introgression artifacts, and (4) a biogeographic setting in which an past allopatric phase is physically implausible. Three empirical systems satisfy these criteria:

Host-Plant Shift in the Apple Maggot Fly (Rhagoletis pomonella)

First noted by Benjamin D. Walsh in 1864 and investigated genetically by Guy L. Bush and Jeffrey L. Feder, the North American tephritid fruit fly Rhagoletis pomonella originally parasitized exclusively the native fruits of hawthorn (Crataegus spp.). Following the agricultural introduction of domesticated apples (Malus domestica) to the northeastern United States, a subpopulation of R. pomonella colonized apples in the Hudson River Valley by the 1860s—within ~150 to 200 years.

Two synergistic magic-trait mechanisms drove rapid sympatric reproductive isolation (reducing inter-host gene flow to m ≈ 4%–6% despite flies flying between adjacent apple and hawthorn trees):

• Habitat-Specific Assortative Mating via Olfactory Preference: Adult Rhagoletis flies court and mate exclusively on or near the surface of their host fruit, using volatile fruit esters (guided by neural peripheral olfactory receptor tuning) to locate the host tree. Consequently, a genetic shift in host-odor preference automatically translates into positive assortative mating (a classic one-allele/magic-trait coupling).
• Allochronic Isolation Driven by Pupal Diapause and Chromosomal Inversions: Domesticated apples ripen 3 to 4 weeks earlier in late summer than native hawthorns. Because larvae pupate in the soil beneath the tree, apple-race flies had to evolve shallower overwintering pupal diapause and higher pre-winter developmental thresholds (mapped to large polymorphic chromosomal inversions on chromosomes 1, 2, and 3 that suppress recombination across hundreds of metabolic loci) so that adults emerge earlier in summer in synchrony with apple fruiting.

Nicaraguan Crater-Lake Cichlids (Amphilophus) and Lord Howe Island Palms (Howea)

• Midas Cichlids in Lake Apoyo (Nicaragua): Investigated by Marta Barluenga, Axel Meyer, and colleagues (2006), Lake Apoyo is a small, uniform, bowl-shaped volcanic crater lake (~5 km diameter, maximum depth 200 m) formed ~23,000 years ago without tributary streams or historical water-level subdivisions. Colonized by a single founding lineage of the benthic Midas cichlid Amphilophus citrinellus, the population diverged in sympatry within <10,000–20,000 years into a second, reproductively isolated endemic limnetic species, the arrow cichlid Amphilophus zaliosus, characterized by an elongated fusiform body, distinct pharyngeal jaw dentition (adapted to open-water pelagic prey rather than benthic snails), and strict assortative mating in laboratory choice trials.
• Edaphic Allochrony in Howea Palms on Lord Howe Island: Investigated by Vincent Savolainen and colleagues (2006), Lord Howe Island is a tiny volcanic oceanic island (<12 km², 600 km east of Australia) too small to isolate wind-pollinated trees geographically. The endemic palms Howea belmoreana and Howea forsteriana diverged in sympatry after the ancestral H. belmoreana (growing on neutral volcanic soils) colonized mid-Pleistocene calcareous calcarenite (high-pH coral sand) deposits. Physiological adaptation to alkaline calcareous soil in H. forsteriana pleiotropically shifted its flowering phenology 6 weeks earlier than H. belmoreana, establishing prezygotic allochronic reproductive isolation across adjacent trees meters apart.

What is a "magic trait" in sympatric speciation, and why did Sergey Gavrilets give it that name?

In early mathematical models of sympatric speciation, critics objected that assuming the same gene controlled both ecological adaptation and mate choice was "cheating" or invoking "magic" to avoid Felsenstein's recombination barrier. Sergey Gavrilets (2004) termed such characters magic traits—not because they are supernatural, but because pleiotropic coupling of ecological selection and non-random mating (such as host choice in insects that mate on their food plant, or flowering time in plants adapted to different soils) removes recombination (r = 0) between niche adaptation and assortative mating, which empirical studies later showed is widespread in nature.

Why are triploid (3n) hybrids between diploid (2n) and tetraploid (4n) plants almost always sterile?

During prophase I of meiosis, homologous chromosomes must pair and segregate evenly to opposite spindle poles. In a triploid (3n) organism, every chromosome type is present in three copies (a trivalent). At anaphase I, two chromosomes of each trivalent move to one pole and one moves to the other pole independently across all chromosome sets; for a species with basic chromosome number x, the probability of producing a balanced euploid (1n or 2n) gamete is 2 × (1/2)^x (for x = 10, only 1 in 512 gametes is balanced), causing fatal embryonic aneuploidy (triploid block).

Why are small, isolated oceanic islands and volcanic crater lakes ideal natural laboratories for proving sympatric speciation?

In large continental regions or vast lakes (such as Lake Victoria or Lake Tanganyika), skeptics can always argue that two presently sympatric sister species originally diverged during an undocumented historical period of geographic isolation (such as Pleistocene lake-level drops that split a large basin into smaller puddles, or glacial refugia). On a tiny, topographically uniform 12 km² island like Lord Howe Island (for wind-pollinated palms) or inside a steep, conical 5-km volcanic crater lake like Lake Apoyo (for swimming fish), micro-allopatric barriers cannot form, ruling out historical allopatry.

1. Felsenstein, J. (1981). "Skepticism towards Santa Rosalia, or why are there so few kinds of animals?" Evolution, 35(1), 124–138. https://doi.org/10.1111/j.1558-5646.1981.tb04864.x
2. Bolnick, D. I., & Fitzpatrick, B. M. (2007). "Sympatric speciation: Models and empirical evidence." Annual Review of Ecology, Evolution, and Systematics, 38, 459–487. https://doi.org/10.1146/annurev.ecolsys.38.091206.095804
3. Barluenga, M., Stölting, K. N., Salzburger, W., Muschick, M., & Meyer, A. (2006). "Sympatric speciation in Nicaraguan crater lake cichlid fish." Nature, 439(7077), 719–723. https://doi.org/10.1038/nature04325
4. Savolainen, V., Anstett, M.-C., Lexer, C., Hutton, I., Clarkson, J. J., Norup, M. V., Powell, M. P., Springate, D., Salamin, N., & Baker, W. J. (2006). "Sympatric speciation in palms on an oceanic island." Nature, 441(7090), 210–213. https://doi.org/10.1038/nature04566