7.1 Species concepts #
7.1.1 Drawing the line #
The biological species concept, in Mayr’s form, treats species as groups of actually or potentially interbreeding natural populations, reproductively isolated from other such groups. It fits sexual animals in contact. It cannot be applied, as stated, to asexuals or to fossils. “Potentially interbreeding” is a guess about populations that never meet. Partial isolation — hybridising plants, ring species — does not give the clean break the definition hopes for.
The morphological species concept groups individuals by form. Palaeontologists and many field workers have nothing else, and it works for asexuals. Polymorphism, sexual dimorphism, and plasticity put one species in two boxes; cryptic species put two gene pools in one.
The phylogenetic species concept looks for the smallest diagnosable, and in some versions monophyletic, group. It applies where characters can be scored, and it splits more finely than the biological concept, because diagnostic differences appear before reproductive isolation is complete. De Queiroz’s useful truce is that species are separately evolving metapopulation lineages. Interbreeding, form, and monophyly are kinds of evidence that a lineage has separated, not rival essences. Early in speciation the evidence disagrees with itself. That is what a process that takes time looks like.
7.1.2 Bacteria #
Bacteria strain the biological concept because much reproduction is clonal, and gene exchange, when it happens, is transformation, transduction, or conjugation, not meiosis inside one species. Exchange can cross wide phylogenetic distances. A boundary defined as “does not exchange genes” would not match named bacterial species. Microbiologists use operational criteria, including overall sequence similarity. An average-nucleotide-identity threshold is a convention that correlates, imperfectly, with other ways of clustering strains. It is not a Mayrian species found with a sequencer.
Different genes in one bacterial genome often have different histories. A core may share a roughly common tree while accessory genes do not. The mechanisms of exchange belong to Microbiology. The evolutionary point is that isolation in bacteria is a drop in the rate of exchange, not a mating barrier, and that lineages still exist. “The species tree” is already an approximation in animals (Chapter 8). In bacteria it is a weaker one.
7.2 Where speciation happens #
The study of Where speciation happens provides fundamental principles governing cellular architecture, molecular interactions, and physiological regulation.
Allopatric Speciation Caused By Topography
7.2.1 Allopatry #
Allopatric speciation is divergence after a geographic barrier stops gene flow. It is the best-supported route because geography does the hard step first. Selection and drift then accumulate differences. Reproductive isolation is discovered later, if the populations meet again, and it has often evolved as a byproduct.
Vicariance splits an existing range. Sister species of snapping shrimp, Alpheus, on either side of the Isthmus of Panama are a classic case. The geology of final closure is still discussed, so the example is a picture of vicariance rather than a single date from which to compute a rate. Dispersal puts colonists across a barrier that already exists. Hawaiian drosophilids are speciation after dispersal. A founder effect may shift frequencies at the start. It is not required, and it is not a genetic revolution.
Peripatric speciation is allopatry with a small peripheral isolate. Small size and a range edge change the balance of drift and selection. Those are ordinary processes. The extra claim that founder events reorganise the genotype is the part that is not established.
7.2.2 Sympatry, and why polyploidy is the clean case #
Sympatric speciation is divergence without an extrinsic geographic barrier, among individuals that can meet. Recombination is the obstacle. Disruptive selection favours extremes, mating between them recreates intermediates, and recombination breaks the association between the selected traits and whatever causes assortative mating. The split needs strong disruptive selection and assortative mating strong enough to keep the extremes from recombining freely.
The apple maggot, Rhagoletis pomonella, shifted in nineteenth-century North America from native hawthorn to introduced apple. Flies mate on the host fruit, so host choice is mate choice, and the two fruits ripen at different times. Some of the genetic differences, including inversions, are older than the apple race and entered from structured populations elsewhere. The sympatric host shift is real. The raw material was not all invented on the apple. That is speciation using standing variation, under the two conditions the models demand.
Polyploid plants are the clean case because isolation need not be built by coupling preference to ecology. A cross between a new tetraploid and its diploid parent yields triploids, which are largely sterile because meiosis with an odd chromosome set is unbalanced. Isolation is immediate and postzygotic. Autopolyploidy doubles one genome. Allopolyploidy combines two species’ genomes, usually with doubling so that chromosomes can pair. Bread wheat is an allohexaploid built by such steps; the speciation consequence is the point, not the agricultural history. Polyploidy has contributed to a large fraction of plant speciation. Published percentages differ by clade and by whether ancient whole-genome duplications are counted, so none is quoted here. The route does not require a geographic barrier, and it is not how most animals speciate.
7.2.3 Parapatry, hybrid zones, and reinforcement #
Parapatric speciation is divergence between populations that stay in contact along a boundary, with some gene flow. Distance softens the sympatric problem: genes must travel through a chain of populations, so selection at each end is less easily swamped than under free migration.
Speciation By Reinforcement Schematic
A hybrid zone is where distinct populations meet and interbreed. In a tension zone, hybrids are unfit and dispersal feeds the zone with parentals; the zone sits where dispersal and selection balance, and it can move. Other zones track an environmental boundary. European fire-bellied toads, Bombina, form a long-studied zone that is stable on a human timescale and is still a site of selection against mixtures. A hybrid zone may be speciation in progress, a leak between species that have already formed, or a pair of populations that will fuse if selection relaxes.
Reinforcement is selection for stronger prezygotic isolation because hybrids are unfit. The predicted pattern is reproductive character displacement: stronger mate discrimination in sympatry than in allopatry. Drosophila pseudoobscura and D. persimilis show a version of it. The same pattern can be produced by other histories. As usual, the pattern is the clue and the process is a hypothesis.
7.3 Reproductive isolation #
7.3.1 A table of barriers #
Reproductive isolating mechanisms are biological reasons two populations exchange genes poorly. Prezygotic barriers act before a zygote forms and waste fewer gametes. Reinforcement can strengthen them. It cannot repair a hybrid that is already dead. Geography is kept off the table: a mountain is not a property of the organisms. Populations isolated only by geography have not finished speciation in the biological-species sense until intrinsic barriers exist.
| Barrier | Class | What fails |
|---|---|---|
| Habitat or host | Prezygotic | Potential mates do not meet |
| Temporal | Prezygotic | They breed at different times |
| Behavioural | Prezygotic | Courtship is not accepted |
| Mechanical | Prezygotic | Copulation or pollination does not fit |
| Gametic | Prezygotic | Gametes meet and fertilisation fails |
| Hybrid inviability | Postzygotic | The zygote dies or develops poorly |
| Hybrid sterility | Postzygotic | The hybrid does not produce functional gametes |
| Hybrid breakdown | Postzygotic | Later generations fail; the first may not |
Haldane’s rule: when one hybrid sex is absent, rare, or sterile, it is the heterogametic sex (XY males in mammals and Drosophila; ZW females in birds and butterflies). The leading explanation is dominance. Recessive incompatibilities on a sex chromosome are exposed in the heterogametic sex and masked in the other. It is a pattern, not a claim that hybrid sterility is always one recessive locus.
7.3.2 Dobzhansky–Muller incompatibilities, and ring species #
Postzygotic isolation has to evolve without either population crossing an unfit genotype, or selection would resist the substitutions. In the Dobzhansky–Muller model an ancestor aabb splits. One daughter fixes A, giving AAbb, fit on its own background. The other fixes B, giving aaBB, also fit. The hybrid meets A and B together for the first time. If that combination is inviable or sterile, neither pure population was ever unfit. The incompatibility is between loci and appears only in the cross. Real species differ at many such pairs. Isolation of this kind is a byproduct of divergence, not a device selection built in order to separate species. Reinforcement, when it happens, is a later elaboration of prezygotic barriers after hybrid unfitness already exists.
Ring species arrange a chain of populations around a barrier so that neighbours interbreed and the forms that meet at the far side do not. Ensatina salamanders around the Central Valley of California, and greenish warblers around the Tibetan plateau, are the usual examples. A chain of small differences can add up to a species-level difference. The over-reading is to treat the ring as a continuous, unbroken history. Gene flow around these rings has not always been continuous, and periods of allopatry complicate both famous cases. A ring is a useful picture of isolation by distance. It is not a clean proof of speciation entirely in sympatry.
7.4 Key terms, summary, and self-check #
Key terms. Biological species concept; morphological species concept; phylogenetic species concept; vicariance; dispersal; sympatric speciation; autopolyploidy; allopolyploidy; hybrid zone; reinforcement; prezygotic; postzygotic; Haldane’s rule; Dobzhansky–Muller incompatibility; ring species.
Summary. Species are separately evolving lineages, recognised by interbreeding, form, or diagnosable monophyly; none of these applies cleanly to bacteria that exchange genes horizontally. Allopatry, by a split range or by dispersal, is the best-supported route because geography stops gene flow first. Sympatry needs strong disruptive selection and assortative mating. Rhagoletis shows both, reusing older variation, and polyploidy isolates a plant lineage in one step. Reinforcement strengthens prezygotic barriers where hybrids are unfit. Dobzhansky–Muller incompatibilities produce unfit hybrids without a fitness valley inside either parent population. Ring species illustrate isolation accumulating with distance, and real rings are messier than the diagram.
Self-check.
1. Why can Mayr’s biological species concept not be applied to a bacterial lineage in the way he applied it to birds?
2. What does recombination threaten during sympatric speciation, and which two conditions have to beat it?
3. Why does polyploidy isolate a new plant from its diploid parent in one generation, when a host shift does not?
4. Classify habitat isolation, hybrid sterility, gametic incompatibility, and hybrid breakdown. Which class can reinforcement strengthen?
5. In the Dobzhansky–Muller model, why is neither parental population unfit? Why are ring species easy to over-read?
