Sympatric Speciation
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You should understand the basic mechanism of allopatric speciation, including how geographic barriers reduce gene flow, and be familiar with the concept of reproductive isolation as the defining criterion for species formation.
Why this matters
Sympatric speciation demonstrates that new species can arise without physical separation, challenging the assumption that geography is the primary driver of evolution.
While allopatric speciation is often considered the dominant mode of speciation in animals, sympatric speciation provides a crucial counter-example, particularly in plants and some invertebrates. It highlights the power of disruptive selection and assortative mating to drive divergence within a single, interbreeding population. Understanding this process is essential for a complete picture of evolutionary dynamics, as it shows that ecological differentiation alone can be sufficient to break down gene flow, leading to the formation of new species in the same geographic area.
The Core Concept: Divergence in Place
Imagine a population of fish living in a single lake. In allopatric speciation, a mountain range might rise, splitting the population into two groups that can no longer meet. Over time, they evolve separately and become different species. Sympatric speciation is different because there is no mountain. The population remains in one place, yet it still splits into two distinct species. How is this possible? The key lies in the behaviour of the individuals and the specific resources they use. If some individuals in the population prefer a particular food source or habitat, and they tend to mate with others who have the same preference, a feedback loop is created. This is known as assortative mating. Because they mate with similar individuals, they exchange genes primarily within their own group. Even though they live in the same lake, the "preference" group and the "generalist" group effectively become isolated from each other genetically. Over many generations, natural selection favours traits that improve efficiency in using the preferred resource. This leads to physical and behavioural differences between the two groups. Eventually, the differences become so significant that they no longer recognise each other as potential mates, or they cannot interbreed successfully. Thus, two species have formed from one, without any physical barrier. This process relies heavily on strong selection pressures and non-random mating to overcome the homogenizing effect of gene flow.
Disruptive Selection and Assortative Mating
The engine of sympatric speciation is disruptive selection. Unlike stabilising selection, which favours intermediate phenotypes, disruptive selection favours extreme phenotypes at both ends of the distribution. For example, consider a bird population feeding on seeds. If small seeds and large seeds are abundant, but medium seeds are scarce, birds with small beaks are efficient at eating small seeds, and birds with large beaks are efficient at eating large seeds. Birds with medium beaks are inefficient at both. If mating is linked to beak size (assortative mating), small-beaked birds mate with small-beaked birds, and large-beaked birds mate with large-beaked birds. This reduces gene flow between the two extremes. The result is a bimodal distribution of beak sizes. Over time, genetic differences accumulate between the two groups. The critical factor here is that the trait under selection (beak size) must be correlated with mate choice. If small-beaked birds mated randomly with large-beaked birds, gene flow would mix the genes, and the population would remain a single, generalist species. Therefore, the strength of assortative mating is a prerequisite for sympatric speciation to proceed. Without it, the homogenizing power of random mating will overwhelm the divergent pressure of selection.
Genetic Architecture and Polyploidy
In many animal species, sympatric speciation is difficult because recombination breaks up the genetic combinations that favour one extreme or the other. However, in plants, a unique mechanism called polyploidy can drive sympatric speciation almost instantaneously. Polyploidy occurs when an organism has more than two sets of chromosomes. This can happen due to errors in cell division, such as meiosis or mitosis, resulting in a diploid cell becoming tetraploid (four sets of chromosomes). A tetraploid plant can self-fertilize or cross with other tetraploids to produce fertile offspring. However, if a tetraploid plant crosses with a normal diploid plant, the offspring will be triploid (three sets of chromosomes). Triploids are usually sterile because their chromosomes cannot pair correctly during meiosis. Thus, the tetraploid lineage is immediately reproductively isolated from the diploid parent population. This is a form of sympatric speciation because it occurs within the same geographic area. The new species is distinct from the start, with no gradual divergence required. This mechanism is common in plants like wheat and cotton, where many species are polyploid. It provides a clear example of how genetic changes can create instant reproductive barriers, bypassing the need for long-term selection and assortative mating.
Challenges and Evidence
Critics of sympatric speciation argue that it is rare because gene flow is usually too strong to allow divergence. For gene flow to be overcome, selection must be very strong, and assortative mating must be very precise. In most natural populations, individuals mate randomly, which mixes genes and prevents divergence. However, evidence for sympatric speciation is growing. Among the strongest examples are cichlid fishes in small crater lakes, such as Barombi Mbo in Cameroon and Lake Apoyo in Nicaragua, where several species appear to have arisen within one lake that offers no barriers, differing in colour, diet and depth preference. Studies show that these species exhibit strong assortative mating based on colour. Another example is the apple maggot fly. Originally, these flies fed on hawthorn fruit. Some began feeding on apple trees, which were introduced to North America. Apple-feeding flies now mate on apple trees, while hawthorn-feeding flies mate on hawthorn trees. This host-based assortative mating has led to genetic divergence between the two groups. Although they are not yet fully separate species, they are on the path to sympatric speciation. These examples demonstrate that when ecological niches are distinct and mate choice is tied to those niches, sympatric speciation is possible. It is not the default mode, but it is a valid and documented evolutionary process.
The concept
Speciation
How reproductive isolation gives rise to new species.
A common misconception is that sympatric speciation requires no gene flow at all. In reality, some gene flow may still occur, but it must be low enough that selection and assortative mating can maintain divergence. If gene flow is too high, the populations will merge back into one. The key is the balance between the rate of divergence and the rate of gene flow. If divergence is faster than gene flow, speciation proceeds; if gene flow is faster, it fails. This dynamic is often modelled using quantitative genetics, where the strength of selection and the strength of assortative mating are compared to the migration rate. C
Worked example
Problem. A population of insects has a gene frequency for a trait linked to host preference. Initially, 50% of the population prefers Host A and 50% prefers Host B. Assortative mating is perfect (individuals only mate with those of the same preference). Selection favours Host A preference with a fitness of \( w_A = 1.2 \) and Host B preference with a fitness of \( w_B = 1.0 \). Calculate the frequency of the Host A allele after one generation of selection and mating.
- Define the initial frequencies: \( p_0 = 0.5 \) for Host A, \( q_0 = 0.5 \) for Host B.
- Calculate the mean fitness of the population: \( \bar{w} = p_0 w_A + q_0 w_B = (0.5)(1.2) + (0.5)(1.0) = 0.6 + 0.5 = 1.1 \).
- Calculate the new frequency of the Host A allele after selection: \( p_1 = \frac{p_0 w_A}{\bar{w}} = \frac{0.5 \times 1.2}{1.1} = \frac{0.6}{1.1} \approx 0.545 \).
- Since assortative mating is perfect, the mating pool consists of only Host A and Host B individuals. The frequency of Host A in the next generation remains \( p_1 \) because there is no mixing with Host B.
- Thus, the frequency of the Host A preference increases from 0.5 to approximately 0.545 after one generation.
Units check. The frequency is a dimensionless ratio, which is consistent with the definition of allele frequency.
Three quick tests
- What is the primary mechanism that allows sympatric speciation to overcome gene flow?
Answer
Assortative mating, where individuals preferentially mate with those of similar phenotypes, reducing gene flow between diverging groups. - How does polyploidy contribute to sympatric speciation in plants?
Answer
Polyploidy creates immediate reproductive isolation because polyploid individuals are often sterile when crossed with diploid parents, preventing gene flow between the two groups. - Why is disruptive selection important for sympatric speciation?
Answer
Disruptive selection favours extreme phenotypes, creating distinct ecological niches that can drive divergence and support assortative mating based on those traits.
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Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-019 · Threads: evolution