Scientists strive to develop clear rules for naming and grouping living organisms. But taxonomy, the scientific study of biological classification and evolution, is often highly debated. Members of a species, the fundamental unit of taxonomy and evolution, share a common evolutionary history and a common evolutionary path to the future. Yet, it can be difficult to determine whether the evolutionary history or future of a population is sufficiently distinct to designate it as a unique species. A species is not a fixed entity—the relationship among the members of the same species is only a snapshot of a moment in time. Different populations of the same species can be in different stages in the process of species formation or dissolution. In some cases hybridization1 and introgression2 can create enormous challenges in interpreting data on genetic distinctions between groups. Hybridization is far more common in the evolutionary history of many species than previously recognized. As a result, the precise taxonomic status of an organism may be highly debated. This is the current case with the Mexican gray wolf (Canis lupus baileyi) and the red wolf (Canis rufus).
As part of the March 29, 2018 appropriations bills, the U.S. Congress directed the U.S. Fish and Wildlife Service (FWS) to obtain an independent assessment of the taxonomic validity of the Mexican gray wolf and the red wolf. As of the writing of this report, FWS considers the Mexican gray wolf a valid taxonomic subspecies and red wolf a valid taxonomic species. Both the Mexican gray wolf and the red wolf are listed as endangered under the U.S. Endangered Species Act.3
In response to the Congressional mandate, FWS requested the Board on Life Sciences of the National Academies of Sciences, Engineering, and Medicine to convene an ad hoc committee to assess the taxonomic status of the red wolf and the Mexican gray wolf. The committee’s statement of task is provided in Chapter 1.
1 The mating and production of offspring from different species.
2 The movement of gene variants or alleles from one species into another.
3 ESA; United States Public Law No. 93-205; United States Code Title 16 Section 1531 et seq.
Determining the taxonomic status of the Mexican gray wolf and the red wolf requires an understanding of the contemporary meanings of species and subspecies. All modern species concepts are united by the goal of identifying groups of organisms whose reproductive compatibility sustains genetic continuity. Among the common principles that underlie multiple species concepts are (1) some level of reproductive isolation between different species that is mediated by genetic and ecological factors, and (2) phylogenetic continuity in time that is mediated by shared evolutionary history and inheritance. These principles provide a compelling, comprehensive approach to identifying species. Most modern concepts of subspecies rely on the notion of the partial restriction of gene flow, where subspecies are groups of actually or potentially interbreeding populations phylogenetically distinguishable from, but reproductively compatible with, other such groups.
Increasingly, genomic data reveal that gene flow among taxonomic groups through hybridization is a common feature of the evolutionary history of many widely accepted species, including wolves. The complete genetic separation and absence of admixture4 is not a strict criterion for determining taxonomic status. Thus, new approaches are needed for assessing whether a given group of organisms constitutes a distinct, independently evolving lineage. The levels of genetic differentiation need not be the sole—or even primary—evidence considered. Rather, multiple data types and tools are useful for addressing questions about taxonomy. These data types fall into three broad categories: morphology, behavioral traits and ecological roles, and genetic and genomic data. Support for the validity of a taxonomic designation of species and subspecies relies upon the strength of evidence of these three data types individually and collectively. Combining data of these three types can provide a more complete picture of the taxonomy and evolutionary history of species and subspecies, generally, and of wolves specifically.
USING GENETICS AND GENOMICS TO DISCERN TAXONOMY
The nuclear DNA of canids consists of a haploid genome of approximately 2.5 billion base pairs. Therefore, the nucleus of each cell contains two copies of this haploid genome, one maternally derived and the other paternally derived. In addition, the mitochondrial genome of the cell contains a haploid genome of approximately 16,000 base pairs of DNA (mtDNA), which are usually passed on from mother to offspring. Genetic differences among individuals within and between species arise as a consequence of mutations that may replace one base with another, delete bases or insert new bases, or rearrange the order of the bases. Tracing out such genetic changes can provide insights into the past history of, as well as the current reproductive relationships among, populations and species.
Genetic techniques have become increasingly sophisticated and complicated. There are many statistical methods used for evaluating species and subspecies status. Research on the taxonomy of the Mexican gray wolf and the red wolf includes studies and analyses of individual nuclear genes, mitochondrial DNA, and whole genomes. Examining the genetic and genomic evidence on taxonomy of the Mexican gray wolf and the red wolf requires an understanding of the fundamental concepts and methods underlying these analyses.
THE TAXONOMIC STATUS OF THE MEXICAN GRAY WOLF
Gray wolves have great dispersal capabilities and generally disperse over long distances. They are habitat generalists, and they occupy a wide variety of environments. These aspects of wolf behavior have been used to argue that the recognition of subspecies of North American gray wolves,
4 The formation of novel genetic combinations through the hybridization of genetically distinct groups.
including Mexican gray wolves, cannot be justified biologically. The designation of the Mexican gray wolf as a subspecies has also been questioned because of disagreements in the application of subspecies concepts. There has been speculation about Mexican gray wolves not having sufficient morphological and genetic distinction to justify their status as a valid subspecies. There has also been speculation that the extant Mexican gray wolf population, which was derived from individuals from three captive lineages, may have included ancestry from dogs or coyotes due to previous admixture. Thus, whether the Mexican gray wolf is a valid subspecies hinges on the strength of the available evidence to answer to two questions:
- Is there evidence for distinctiveness of Mexican gray wolves from other North American Canis populations?
- Is there evidence for continuity between the historical Mexican gray wolf lineage and the present managed population?
Distinctiveness of Mexican Gray Wolves from Other North American Canis Populations
The Mexican gray wolf has, from its discovery, been considered distinct. Its size, morphology, and coloration distinguish the Mexican gray wolf from other North American wolves. Genetic and genomic analyses confirm that the Mexican gray wolf is the most genetically distinct subspecies of gray wolves in North America. Arguments against recognizing the Mexican gray wolf as a subspecies are based on a definition of subspecies that is not widely accepted in the scientific community. There is no evidence that Mexican gray wolf genomes include introgression from domestic dogs. Extant wild Mexican gray wolves behave similarly to other North American gray wolves within the confines of the human-constricted Mexican gray wolf recovery area; their wild behavior prior to their 1970 extirpation in the wild is poorly documented. The Mexican gray wolf represents a smaller form of the gray wolf and inhabits a more arid ecosystem than the gray wolf. At present, Mexican gray wolves are behaviorally and ecologically distinct.
Continuity Between the Historical Mexican Gray Wolf Lineage and the Present Managed Populations
There is morphological continuity between the historical and extant Mexican gray wolf lineages. While differences in allele frequencies and DNA sequences alone do not demonstrate the distinctiveness of a lineage, the analysis of ancient DNA reinforces the conclusion that the historical population of Mexican gray wolf represents a distinct evolutionary lineage of gray wolf. Furthermore, the extant Mexican gray wolves are direct descendants of the last remaining wild Mexican gray wolves. The known history of the extant Mexican gray wolves suggests that there is continuity between them and the historical lineage.
Synthesis of Findings
Mexican gray wolves are distinct from other North American gray wolves morphologically, paleontologically, genetically, genomically, behaviorally, and ecologically.
The Mexican gray wolf is a valid taxonomic subspecies of the gray wolf, Canis lupus, with its current classification of Canis lupus baileyi.
THE TAXONOMIC STATUS OF THE RED WOLF
The red wolf is a currently recognized species that historically inhabited much of the eastern United States. During the 20th century, populations were driven to very low numbers by predator eradication programs and by habitat loss, and the wolves were largely replaced by coyotes spreading eastwards from their original range in the western United States. A few remaining specimens from Texas and Louisiana with red wolf morphology were captured before the red wolf was declared to be extinct in the wild, and these were used to establish a breeding program. The descendants of this breeding program were reintroduced in North Carolina and are now a managed population in the wild. There has been substantial controversy regarding the species status of the red wolf. In particular, the individuals that were used to found the breeding program were captured from a region where there had already been substantial admixture between eastward-expanding coyotes and the local gray wolves or red wolves. Whether the red wolf is a valid species hinges on the strength of the evidence to answer to three questions:
- Is there evidence that the historical population of red wolves was a distinct lineage?
- Is there evidence for distinctiveness of contemporary red wolf populations from gray wolves and coyotes?
- Is there evidence for continuity between the historical red wolf population and contemporary managed populations?
The controversy about the taxonomic status of the red wolf stems primarily from the degree to which red wolves historically were a separate species or could be better categorized as either coyotes or gray wolves, and the degree to which the extant (captive and managed) population descends from the original red wolf population or, alternatively, is the result of admixture between coyotes and gray wolves.
Distinctiveness of the Historical Population of Red Wolves
Fossil evidence suggests that at least five subspecies of C. lupus were present in North America 1 million years ago. The earliest fossils attributed to C. rufus were found in Florida and dated as deriving from 10,000 years ago. Fossil evidence also indicates that coyotes (C. latrans) arose in North America and spread across the continent but disappeared from the East approximately 10,000 years ago. In the 1900s coyotes returned to eastern North America. Based on the limited set of specimens available for analyses, prior to contact with modern coyotes, populations of C. rufus could be morphologically distinguished from C. lupus using canonical discriminant analysis. Although conclusions from studies based on skull morphology differ on the issue of whether C. rufus represented a subspecies of C. lupus or a distinct species, an analysis of the anatomy of the cerebellum supports the recognition of C. rufus as a distinct species.
North American canid species are genetically very similar to each other and have substantial amounts of shared genetic variation. The mtDNA haplotypes5 from historical wolf-like canids (before their recent sympatry with coyotes) in the eastern United States cluster within the coyote clade.
Distinctiveness of Contemporary Red Wolf Populations from Gray Wolves and Coyotes
The contemporary population of red wolves in North Carolina is morphologically distinguishable from sympatric coyotes and red wolf–coyote hybrids.
5 A unique sequence of mitochondrial DNA.
The red wolf population shows evidence of past genetic contributions from populations related to gray wolves, coyotes, or both. The red wolf is genetically more closely related to coyotes than to western gray wolves. The timing of the admixture between red wolves and other canids is still unresolved, but red wolves have divergent genetic ancestry that predates European colonization.6
The red wolf has some degree of genetic ancestry not found in reference populations of western gray wolves or coyotes.
Red wolves have a social organization and reproductive behavior that are more similar to those of gray wolves than to coyotes, and when mates are available red wolves exhibit assortative mating.
Continuity Between the Historical Red Wolf Population and Contemporary Managed Populations
Morphological analyses suggest a cohesiveness among red wolf specimens from the end of the Pleistocene to the early 1900s, but it remains unclear whether this continuity is shared with the extant captive and managed populations. Genetic continuity between the managed red wolf population and the historical wolf in the eastern United States cannot be firmly established without genomic data from ancient specimens. However, the patterns of genetic variability are compatible with the hypothesis that the red wolf shares a fraction of its genetic history with a canid distinct from modern reference coyotes and gray wolves.
The social behavior of the restored populations is very similar to that reported for the natural population. The original distribution of red wolves seems to have been tied to the distribution of the eastern temperate forests. Compared with coyotes, wolves require larger home ranges to obtain sufficient prey. This requirement of larger home ranges is consistent between the original natural population and the extant managed population in North Carolina. The diet of the red wolves in the restored population includes a greater consumption of deer than the natural population, but this may be a function of prey availability and body size. Both red wolves and coyotes in North Carolina consume a similar diet in terms of types of prey, but they differ in the proportions of deer, rabbits, and other small mammals in their diets and in their seasonal consumption of these prey types.
Synthesis of Findings
The four possible taxonomic options for the red wolf are:
- it is a distinct species of wolf (C. rufus),
- it is a subspecies of gray wolves,
- it is a subspecies of coyotes, or
- it is a group of recently admixed individuals belonging to neither wolves nor coyotes.
The last option, “a group of recently admixed individuals,” can be rejected for three main reasons: (1) the estimates of deep divergent DNA in red wolves; (2) the estimates of an admixture time mostly predating the coyote expansion (even if the red wolf is considered a hybrid species between gray wolf and coyote); and (3) the presence of unique alleles in red wolves that are also found in a population of wolves on Galveston Island but not found with other reference populations. The third option, “a subspecies of coyote,” is not tenable. There are substantial morphological and behavioral differences between coyotes and red wolves. Furthermore, prezygotic7 isolation mechanisms, pos-
6 Evidence suggests that admixture in red wolves took place prior to the 1500s (before significant European colonization of what is now the eastern United States), and so is not the result of human-induced ecological changes (see Box 2-2).
7 Before the fertilization of an egg.
sibly driven by size differences, are at least partially maintained between red wolves and coyotes. Yet, the designation of red wolves as a subspecies of the gray wolf also seems inappropriate. Red wolves, historically and presently, show genetic evidence of being more closely related to coyotes than to gray wolves. The available evidence from morphology, behavior, and ecology, combined with genetic evidence of a relatively deep divergence and the maintenance of some unique genetic ancestry, suggest that the most appropriate taxonomic designation for red wolves is as a distinct species that possibly has historical admixture.
The time scales of divergence and the amount of introgression since divergence can affect taxonomic considerations. However, even a recently emerged species can be recognized as such if it is ecologically, functionally, and reproductively separated from other species. The extant red wolf seems to trace a large proportion of its genome to relatively recent admixture with coyotes. The genomes of extant red wolves might also represent much of the historical red wolf genome spread into fragments in different individuals.
Genomic DNA from historical red wolf specimens could help clarify the issue regarding continuity between historical and extant red wolves. And, more precise genetic analyses might help determine the exact proportion of the red wolf genome that has been replaced by recent admixture.
- Available evidence suggests that the historical red wolves constituted a taxonomically valid species.
- Extant red wolves are distinct from the extant gray wolves and coyotes.
- Available evidence is compatible with the hypothesis that extant red wolves trace some of their ancestry from the historical red wolves.
- Although additional genomic evidence from historical specimens could change this assessment, evidence available at present supports species status (Canis rufus) for the extant red wolf.