The Species Problem
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You should be familiar with the basic mechanisms of natural selection and the concept of gene flow from the previous lecture on observed selection in peppered moths and finches.
Why this matters
Defining a species is a fundamental challenge in biology because the boundaries between populations are often blurred by ongoing gene flow.
The "species problem" arises because there is no single, universally applicable definition that captures all biological realities. While the biological species concept is the most widely used in evolutionary biology, it fails for asexual organisms, fossils, and hybridising lineages. Understanding these limitations is crucial for interpreting evolutionary history, conservation biology, and the mechanisms of speciation. This lecture explores the major species concepts, their strengths and weaknesses, and how they relate to the process of reproductive isolation.
The Biological Species Concept in Plain Language
For decades, the dominant view in evolutionary biology has been the Biological Species Concept (BSC), originally articulated by Ernst Mayr. In simple terms, this concept defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. The key word here is "reproductively isolated." It means that members of one species cannot successfully mate with members of another species to produce viable, fertile offspring. This isolation can be due to pre-zygotic barriers, such as differences in mating rituals, timing of reproduction, or physical incompatibility of reproductive organs, or post-zygotic barriers, such as hybrid inviability or sterility. The BSC is powerful because it links the definition of a species directly to the evolutionary process of speciation. If two populations are reproductively isolated, they are evolving independently, and over time, they will accumulate genetic differences that make them distinct species. However, this definition has significant limitations. It cannot be applied to asexual organisms, such as bacteria, which reproduce by binary fission and do not interbreed in the sexual sense. It also struggles with fossil species, where we cannot observe mating behaviour or hybrid viability. Furthermore, many sexually reproducing organisms, such as certain plants and fish, regularly hybridise with other species, creating a continuum of gene flow that blurs the boundaries between species. In these cases, the BSC is less useful, and other concepts, such as the morphological species concept or the phylogenetic species concept, may be more appropriate. The species problem, therefore, is not just a taxonomic issue but a reflection of the complexity of evolutionary processes.
Limitations of the Biological Species Concept
While the BSC is a cornerstone of evolutionary thought, its applicability is limited by several biological realities. First, it is inapplicable to asexual organisms. Bacteria, archaea, and some eukaryotes reproduce asexually, meaning they do not exchange genetic material through mating. For these organisms, the concept of reproductive isolation is irrelevant, and species are often defined based on genetic similarity or ecological niche. Second, the BSC fails for fossil species. Since we cannot observe the mating behaviour or hybrid viability of extinct organisms, we must rely on morphological differences to define species. This can lead to misclassification, as morphological variation within a species can be as great as or greater than the differences between closely related species. Third, the BSC is problematic for hybridising species. In many plant and animal groups, hybridisation is common and can result in the formation of new species or the merging of existing ones. For example, in the genus Helianthus (sunflowers), hybridisation has played a significant role in speciation, creating new species that are reproductively isolated from their parents. In such cases, the BSC does not provide a clear boundary between species, as gene flow continues across species lines. Finally, the BSC assumes that reproductive isolation is an all-or-nothing trait, but in reality, it is often a spectrum. Populations may be partially isolated, with some gene flow occurring, making it difficult to assign them to distinct species. These limitations highlight the need for multiple species concepts, each suited to different biological contexts.
Alternative Species Concepts
To address the limitations of the BSC, biologists have developed several alternative species concepts. The morphological species concept defines a species based on observable physical characteristics, such as size, shape, and colour. This concept is widely used in taxonomy and is particularly useful for fossil species, where genetic data is unavailable. However, it can be misleading, as cryptic species—organisms that look identical but are genetically distinct—may be lumped together, while highly variable species may be split into multiple species. The phylogenetic species concept defines a species as the smallest diagnosable group of organisms that shares a unique common ancestry. This concept is based on evolutionary history and is often used in molecular phylogenetics. It is particularly useful for identifying cryptic species and understanding the relationships between lineages. However, it can be difficult to apply when there is incomplete lineage sorting or when the phylogeny is unresolved. The ecological species concept defines a species based on its ecological niche, the set of resources it uses and its role in the ecosystem. This concept is useful for understanding the adaptive significance of species and is particularly relevant in conservation biology. However, it can be difficult to define niches precisely, and closely related species may occupy similar niches. Each of these concepts has its strengths and weaknesses, and in practice, biologists often use a combination of concepts to define species. The choice of concept depends on the organism, the available data, and the research question.
Speciation and Reproductive Isolation
Speciation is the process by which new species arise, and it is driven by the accumulation of reproductive isolating mechanisms. The process can be allopatric, where populations are geographically separated, or sympatric, where speciation occurs within the same geographic area. Allopatric speciation is the most common form, as geographic barriers, such as mountains or rivers, prevent gene flow between populations, allowing them to diverge independently. Over time, genetic drift, natural selection, and mutation lead to the accumulation of differences that result in reproductive isolation. Sympatric speciation is more controversial, as it requires strong selective pressures to overcome gene flow. It is often driven by polyploidy, where an organism gains extra sets of chromosomes, making it reproductively isolated from its diploid ancestors. The result of speciation is a permanent split in the gene pool: \[ \text{Reduced gene flow} \to \text{independent divergence} \to \text{incidental reproductive isolation} \to \text{permanent split (new species)} \] This process is not instantaneous; it occurs over many generations, and the degree of reproductive isolation can vary. In some cases, hybridisation can reverse speciation, leading to the merging of species. In other cases, it can create new species, as seen in hybrid speciation. The interplay between gene flow, selection, and isolation is a dynamic process that shapes the diversity of life on Earth.
The concept
Speciation
How reproductive isolation gives rise to new species.
A common misconception is that the Biological Species Concept is the "correct" definition of a species, and that other concepts are merely approximations. In reality, the BSC is just one of several useful frameworks, and its applicability depends on the biological context. For example, in conservation biology, the phylogenetic species concept may be more relevant than the BSC, as it helps identify evolutionarily distinct lineages that warrant protection. Similarly, in microbiology, the concept of a species is often based on genetic similarity, as the BSC is inapplicable to asexual organisms. It is important to recognise that the species problem is not a failure of biology but a reflection of the complexity of evolutionary processes. Different species concepts provide different insights, and the choice of concept should be guided by the research question and the available data. C
Worked example
Problem. Consider two populations of a plant, Population A and Population B. Population A has a chromosome number of \( 2n = 10 \), and Population B has a chromosome number of \( 2n = 20 \). Assume that Population B arose from Population A through a single polyploidy event. Calculate the ratio of the chromosome number in Population B to that in Population A, and explain why this difference leads to reproductive isolation.
- Identify the chromosome numbers: Population A has \( 2n = 10 \), and Population B has \( 2n = 20 \).
- Calculate the ratio of the chromosome number in Population B to that in Population A: \( \frac{20}{10} = 2 \).
- Explain the mechanism of polyploidy: Population B is a tetraploid (\( 4n = 20 \)), having gained an extra set of chromosomes from Population A.
- Discuss reproductive isolation: When a tetraploid (\( 4n \)) mates with a diploid (\( 2n \)), the offspring is a triploid (\( 3n \)), which is typically sterile due to irregular meiosis.
- Conclude that the chromosome number difference creates a post-zygotic barrier, preventing gene flow between the two populations and leading to speciation.
Units check. The ratio is dimensionless, as it is a comparison of two chromosome numbers.
Three quick tests
- Why is the Biological Species Concept inapplicable to bacteria?
Answer
Bacteria reproduce asexually and do not interbreed, so the concept of reproductive isolation is irrelevant. - What is a cryptic species?
Answer
A cryptic species is a group of organisms that look identical but are genetically distinct and reproductively isolated. - How does polyploidy contribute to sympatric speciation?
Answer
Polyploidy creates a new chromosome number that is reproductively isolated from the parent population, allowing speciation to occur within the same geographic area.
Back to BU-104 →
Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-019 · Threads: evolution