Species complex
A group of closely related species with unclear boundaries.
A species complex is a group of closely related organisms whose boundaries are often unclear due to morphological similarity and the ability to hybridize. Such complexes may include cryptic species hidden under a single species name, sibling species that are each other's closest relatives, or a species flock of related species sharing a habitat. Recognizing these complexes is important for accurate biodiversity assessment, disease and pest control, and conservation biology.
Quick Facts
- Field
- Biology
- Known for
- Group of closely related, morphologically similar species with blurred boundaries
Facts from the source article.
Definition
A species complex is a group of closely related but distinct species, and the idea is closely linked to how we define a species itself. Today, biologists see a species as a separately evolving lineage of populations, but they recognize that the rules for telling species apart can vary depending on the group. Because of this, many species originally defined only by how they look have turned out to be multiple distinct species once genetic differences or reproductive isolation are examined. In a narrower sense, the term is used for groups where hybridization is happening or has happened, creating intermediate forms and fuzzy boundaries. The informal term superspecies applies here, like the grizzled skipper butterfly, which is a superspecies split into three subspecies. Some researchers also use species complex for a single species with internal variation, such as ring species or subspecies, where it is often unclear if they should be considered separate species. Several other terms are used interchangeably, though some have slightly different meanings. In zoology and bacteriology, there are no official ranks between subgenus and species, but botany has four ranks below subgenus. Cryptic species, or sibling species, are lineages that look identical but are genetically distinct, often identified through DNA barcoding. They can result from true morphological similarity or because high variation within a species hides the differences between species.
Identification
To tell closely related species apart, scientists often look for tiny differences. These might be in physical form, visible only with a microscope, but sometimes species look identical. Then researchers turn to other clues like behavior, life cycle, or genetics. For instance, treecreepers are identified by their unique songs, and fungi are often tested by seeing if they can mate. DNA analysis is now a common tool, sometimes the only one that works. Methods like DNA barcoding and molecular phylogenetics help uncover hidden species, such as the fly agaric mushroom, water fleas, and African elephants.
Evolution and ecology
Species forming a complex have typically diverged recently, sometimes allowing retracing of the speciation process. Species with differentiated populations, such as ring species, are sometimes seen as early, ongoing speciation—a species complex in formation. However, similar but distinct species have sometimes been isolated for a long time without evolving differences, a phenomenon called morphological stasis; for example, the Amazonian frog Pristimantis ockendeni comprises at least three species that diverged over 5 million years ago. Stabilizing selection may maintain similarity in species complexes, especially when adapted to special environments like hosts or extreme conditions, constraining possible evolutionary directions. Asexual reproduction, such as apomixis in plants, can separate lineages without great morphological differentiation. A species complex is usually monophyletic, but closer examination can disprove that; for instance, yellow-spotted fire salamanders formerly classified as one species are not monophyletic, with similarity arising from convergent evolution. Hybrid speciation can lead to unclear boundaries through reticulate evolution, where hybrid species have two parent species as most recent common ancestors, as seen in Heliconius butterflies and various insects, fungi, and plants.
Practical implications
Cryptic species complexes are very common in the marine environment, a finding confirmed by detailed analysis using DNA sequence data. The increased use of DNA sequencing in phylogeography and DNA barcoding has led to discovery of many cryptic species complexes in all habitats. In the marine bryozoan Celleporella hyalina, detailed morphological analyses and mating compatibility tests confirmed more than 10 ecologically distinct species that had diverged for many millions of years. Pests, disease-causing species, and their vectors have direct human importance; when found to be cryptic species complexes, the ecology and virulence of each species must be re-evaluated for appropriate control strategies, as diversity increases capacity for more dangerous strains. Examples include cryptic species in the malaria vector Anopheles, fungi causing cryptococcosis, and sister species of Bactrocera tryoni, where only B. tryoni inflicts widespread damage to Australian fruit crops. When a species is found to be several phylogenetically distinct species, each typically has smaller distribution ranges and population sizes than previously reckoned, and they may differ in ecology such as breeding strategies or habitat requirements. For example, giraffe populations and subspecies differ genetically to such an extent that they may be considered species; while the giraffe as a whole is not threatened, each cryptic species considered separately faces a much higher level of threat.
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