Difficulty: Intermediate | Prerequisites: Phylogenetic trees (Ch. 47-48), natural selection, gene flow, allele frequencies.
Speciation is the process by which one species becomes two (or more) distinct species. It is the mechanism that generates biodiversity. Everything you have learned about natural selection, genetic drift, and gene flow operates within populations, but speciation is what happens when those forces push populations apart permanently.
This topic builds on phylogenetic trees: the trees you learned to read are the visual output of speciation events. You will need to understand how scientists define "species" in the first place (which turns out to be surprisingly contentious), and then how geographic separation or other barriers can split one gene pool into two.
If phylogenetics is about reading the tree, speciation is about understanding how new branches form.
There are three main ways to define a species: by reproductive isolation (biological species concept), by physical appearance (morphospecies concept), or by evolutionary history (phylogenetic species concept). New species form when populations become genetically isolated, either by geographic separation (allopatric speciation) or while living in the same area (sympatric speciation). The core requirement is that gene flow between populations stops long enough for them to diverge.
Biological species concept (BSC)
Defines a species by reproductive isolation: if two populations cannot interbreed or cannot produce viable, fertile offspring, they are separate species. Think of it as the "can they have babies together?" test.
Morphospecies concept
Defines a species by measurable physical differences in size, shape, or structure. Think of it as the "do they look different enough?" test. Used when the biological species concept cannot apply.
Phylogenetic species concept (PSC)
Defines a species as the smallest monophyletic group on the tree of life, identified by shared synapomorphies. Think of it as the "what does their evolutionary family tree say?" test.
Monophyletic group (clade)
An ancestral population plus all of its descendants. On a tree, pick any node: everything that descends from it, including the ancestor at that node, forms a monophyletic group.
Phylogenetic species
The smallest monophyletic group. The tip of a branch on the tree of life. These populations share synapomorphies that distinguish them from all other groups.
Allopatry
The condition in which populations are geographically separated from one another.
Allopatric speciation
Speciation that begins with geographic isolation. Populations are physically separated, gene flow stops, and they diverge independently.
Dispersal
A mechanism of geographic isolation where part of a population migrates to a new area, founding a separate population.
Vicariance
A mechanism of geographic isolation where a physical barrier (mountain range, river, continental drift) splits an existing population in two.
Sympatry
The condition in which populations live in the same geographic area, close enough that interbreeding is possible.
Sympatric speciation
Speciation that occurs without geographic separation. Populations diverge while living in the same area, driven by mechanisms such as disruptive selection or polyploidisation.
Disruptive selection
A form of natural selection that favours individuals at both extremes of a trait distribution over those in the middle, which can drive population divergence.
Polyploidisation
A mutation event that doubles (or further multiplies) the entire chromosome set. Polyploid individuals are often reproductively isolated from the parent population immediately.
Genetic isolation
The absence of gene flow between two populations. No exchange of alleles, no interbreeding. This is the prerequisite for genetic divergence.
Genetic divergence (genetic speciation)
The process by which isolated populations accumulate different allele frequencies and, eventually, different traits, becoming distinct species.
Biological species concept (BSC)
Species are defined by reproductive isolation: populations that do not interbreed, or that fail to produce viable fertile offspring, are separate species.
Strengths: focuses on the genetic mechanism (gene flow) that keeps species distinct.
Weaknesses: cannot be applied to asexual organisms, fossils, or geographically isolated populations where interbreeding is impossible to test.
Morphospecies concept
Species are defined by recognisable physical differences (size, shape, structure).
Strengths: simple to apply, works for asexual organisms, fossils, and geographically separated populations. Fills the gaps left by the BSC.
Weaknesses: subjective (observers may disagree on what counts as "different enough"), and can be misled by phenotypic variation within a single species (different sexes, age stages, or colour morphs of the same species may look quite different).
Phylogenetic species concept (PSC)
Species are defined as the smallest monophyletic group on the tree of life, united by shared synapomorphies.
Strengths: grounded in evolutionary history, applicable to any organism.
Weaknesses: requires detailed phylogenetic data, which is not always available.
Allopatric speciation occurs when a population is split into geographically separated groups. Gene flow stops, and the isolated populations diverge independently through natural selection, genetic drift, and mutation.
Geographic isolation arises through two mechanisms:
Dispersal: Part of the population migrates to a new location (e.g., a group of birds colonises an island). The founders are cut off from the original population.
Vicariance: A physical barrier forms and divides an existing population (e.g., a river changes course, a mountain range rises, continents drift apart).
In both cases, genetic isolation leads to independent evolution. Over time, allele frequencies shift in different directions, new mutations accumulate on each side, and the populations become distinct species.
Sympatric speciation occurs when populations diverge while living in the same area. This is rarer and more difficult to demonstrate than allopatric speciation, but two mechanisms are well documented:
Disruptive selection: Natural selection favours extreme phenotypes over intermediate ones, gradually splitting a population into two groups that eventually stop interbreeding.
Polyploidisation: A chromosome-doubling event creates individuals that are immediately reproductively isolated from the parent population, because their offspring with normal-chromosome partners are sterile or inviable. This is especially common in plants.
The central principle tying all of this together:
Genetic isolation is the absence of gene flow. Two populations stop exchanging alleles.
Without gene flow, allele frequencies in the two populations change independently (through selection, drift, and mutation). This is genetic divergence.
Genetic divergence produces synapomorphies: new traits that appear only in descendants and are absent in distant ancestors.
Gene flow, when it is present, homogenises populations and prevents speciation.
For genetic speciation to occur, there must be genetic isolation (or, rarely, sympatric speciation can override ongoing gene flow).
Allopatric speciation by dispersal explains the diversity of Darwin's finches on the Galapagos Islands: a founding population arrived from the mainland and, dispersed across different islands, diverged into multiple species adapted to different food sources.
Polyploidisation is a major force in agriculture. Many crops we eat daily, including wheat, strawberries, and bananas, are polyploid. Understanding this process is directly relevant to plant breeding and food security.
Students often think the biological species concept is the "correct" definition of a species. All three concepts are useful tools with different strengths, and scientists choose among them depending on the organisms and data available.
Students frequently assume that speciation requires geographic separation. Sympatric speciation (especially via polyploidisation) can occur without any physical barrier at all.
Students confuse "genetic isolation" with "geographic isolation." Geographic isolation is one cause of genetic isolation, but genetic isolation can also arise through other mechanisms (e.g., polyploidy, differences in mating timing) even when populations overlap geographically.
Students sometimes think gene flow always prevents speciation. Gene flow makes speciation harder, but disruptive selection or polyploidisation can override it.
⚠️ Know the strengths and weaknesses of all three species concepts. Expect a question giving you a scenario and asking which concept applies (e.g., "You find a fossil. Which species concept can you use?").
⚠️ Be able to distinguish allopatric from sympatric speciation and give an example of each.
⚠️ Know the two mechanisms of geographic isolation: dispersal vs. vicariance.
⚠️ Understand that genetic isolation is the prerequisite for speciation, and be able to explain why gene flow prevents it.
⚠️ The link between genetic divergence and synapomorphies is heavily tested: divergence creates new traits unique to descendants, which are the synapomorphies used in the phylogenetic species concept.
True or False: The biological species concept works well for classifying fossils. (False, fossils cannot be tested for reproductive isolation)
Fill in the blank: Speciation that begins with geographic separation is called __________ speciation. (allopatric)
True or False: Gene flow between populations promotes speciation. (False, gene flow homogenises populations and prevents speciation)
Fill in the blank: A physical barrier splitting an existing population in two is called __________. (vicariance)
True or False: Polyploidisation can cause instant reproductive isolation. (True)
Q: A researcher discovers two populations of lizards on opposite sides of a newly formed canyon. The populations look identical but have not interbred for 500,000 years. Under the biological species concept, are they the same species?
A: Under the BSC, the answer depends on whether they can still interbreed and produce viable fertile offspring. If the canyon were removed and they could interbreed successfully, they would be the same species under the BSC. If 500,000 years of isolation has led to enough genetic divergence that they can no longer produce viable offspring, they are separate species. The BSC has difficulty with geographically separated populations precisely because you cannot easily test reproductive compatibility.
Q: Explain why the morphospecies concept might incorrectly classify one species as two.
A: If a single species has significant phenotypic variation (e.g., males and females look very different, or juveniles look different from adults, or the species has distinct colour morphs), a researcher relying on physical appearance alone might classify those variants as separate species. The morphospecies concept is subjective: it depends on the observer's judgement about what constitutes a meaningful physical difference.
Q: A plant undergoes polyploidisation. Explain why its offspring are immediately reproductively isolated from the parent population.
A: The polyploid offspring have double the normal number of chromosomes. When they attempt to mate with normal diploid individuals, the resulting embryos have an odd number of chromosome sets, which prevents proper chromosome pairing during meiosis. The offspring are sterile or inviable, so gene flow between the polyploid and diploid populations is effectively blocked from the moment the polyploidy event occurs.
Q: Compare and contrast dispersal and vicariance as mechanisms leading to allopatric speciation.
A: Both result in geographic isolation of populations, cutting off gene flow. Dispersal is active: part of the population moves to a new location (e.g., colonising an island). Vicariance is passive: a physical barrier forms and divides an existing population (e.g., continental drift, a river changing course). In dispersal, a small founding group is separated; in vicariance, the entire population is split, often into two large groups.
Speciation is the bridge between microevolution (changes within populations) and macroevolution (the large-scale patterns seen in the history of life). The adaptive radiation and mass extinction material that follows relies heavily on speciation: adaptive radiation is rapid speciation into many niches, and mass extinction is the large-scale loss of species.
The three species concepts tie directly back to phylogenetic trees. The phylogenetic species concept uses monophyletic groups and synapomorphies, both of which you studied in the phylogenetics unit.
Speciation, species concept, biological species concept, BSC, reproductive isolation, morphospecies concept, morphological species, phylogenetic species concept, PSC, monophyletic group, clade, synapomorphy, allopatric speciation, allopatry, geographic isolation, dispersal, vicariance, sympatric speciation, sympatry, disruptive selection, polyploidy, polyploidisation, autopolyploidy, allopolyploidy, gene flow, genetic isolation, genetic divergence, allele frequency, reproductive barrier, prezygotic barrier, postzygotic barrier, BIO I, Introduction to Biology, University of Washington, Dobkowski, Ch. 480-490