Cospeciation

Cospeciation is an evolutionary process in which two closely associated lineages, such as a host and its obligate parasite or symbiont, diversify in tandem. When a speciation occurs in one lineage, a corresponding speciation may occur in the other, yielding congruent phylogenies. This pattern is often interpreted as evidence of a tight, coevolved relationship, though it is not universal and can be obscured by host-switching or extinction. Cospeciation is a specific form of coevolution focused on joint speciation.

Causes and scope: Cospeciation tends to arise when association is long‑lasting and diversification pressures act on both partners, with limited opportunities for host-switching. It is commonly studied in host–parasite systems and in some plant–pollinator or symbiont systems.

Inference: Detecting cospeciation involves comparing dated phylogenies of the two lineages and assessing congruence. Cophylogenetic analyses—often using event-based reconciliation approaches—evaluate whether the observed patterns result from cospeciation rather than switching, duplication, or sorting. Tools such as ParaFit, Jane, and PACo are used in the field.

Examples and complexity: Notable examples include fig trees and their pollinating wasps, and certain groups of lice and their hosts. However, many systems show partial cospeciation with varying congruence, and host-switching or extinction events can blur the signal.

Significance: Studying cospeciation helps illuminate how interdependent lineages diversify and affects interpretations of biodiversity and biogeography. Reliable inference requires robust phylogenies and careful consideration of alternative processes.