For every organism, from the tallest tree to the noisiest cricket to the tiniest microorganism, there is a name. Naming living things is a hallmark of human communication. Names enable people to explore, classify, and interpret the world around them because names carry with them an implicit grouping of objects. The name “wolf,” for example, has meaning only to the extent that one can identify which creatures are wolves and which are not. Not all communities name organisms the same way, however. For example, a puma, a mountain lion, and a cougar are common names for the same animal; one community’s buzzard is another community’s vulture. And some widely used and accepted common names are actually quite misleading. The small mammal known as a “red panda” is not a panda at all, but a relative of the raccoon. To avoid this confusion, scientists strive to develop clear rules for naming and grouping living organisms.
Taxonomy is the scientific study of biological classification. For millennia, scientific thinkers have developed, refined, rejected, and re-developed rules to name and classify living and extinct organisms. In the 4th century BCE, Aristotle developed the first taxonomic system to divide all known organisms into two groups: plants and animals. More than 2,000 years passed before Carl Linnaeus established the binomial naming system (genus and species) still used today. The “puma” is Puma concolor. The “red panda” is Ailurus fulgens. These names reflect how the organisms are classified. The name of each organism, to which genus it is assigned, and into which larger group it is classified (e.g., family and order) all reflect scientists’ best understanding of an organism’s genetic and morphological characteristics and how they compare with those of other organisms.
The fundamental unit of taxonomy and evolution is the species. Members of the same species share a common evolutionary history and a common evolutionary path to the future. The species can also be a fundamental ecological unit whose presence or absence can change the character of an entire ecosystem (Box 1-1). Whether the goal is to study basic biological processes, preserve the diversity of species, or maintain healthy ecosystems, scientists, policy makers, and the public must
pay close attention to which sets of individuals constitute a species and, conversely, which groups of individuals represent separate species or distinct subsets of a single species, or subspecies. In particular, the appropriate taxonomic status of two types of wolf—the red wolf, Canis rufus, and the Mexican gray wolf, Canis lupus baileyi—has been the source of controversy among those who seek to devise appropriate conservation and management strategies for these animals. Assessing the evidence for the taxonomic status of these two wolves is the purpose of this report.
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 red wolf, C. rufus, and the Mexican gray wolf, C. lupus baileyi. Currently, FWS considers the red wolf to be a valid taxonomic species and the Mexican gray wolf to be a valid taxonomic subspecies. Both the red wolf and the Mexican gray wolf are listed as endangered under the U.S. Endangered Species Act (ESA; United States Public Law No. 93-205; United States Code Title 16 Sect. 1531 et seq.). However, there is an ongoing debate about the taxonomic status of both wolves.
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 (Box 1-2).
The committee was not tasked with examining conservation decisions or recommending conservation or recovery policies. The committee was also not asked to assess how red wolves and Mexican gray wolves are designated under the terms of the ESA.
To inform its task, the committee held a half-day public meeting in Washington, DC; a 1-day workshop in Irvine, California; and three webinars, each of which included multiple speakers. The public meeting, workshop, and webinars featured speakers who have conducted research on all aspects of wolf biology, from behavior to genomics, as well as scientists who have confronted similar taxonomic issues with other groups of organisms. The agendas for the open sessions of the committee meetings, the workshop, and the webinars are provided in Appendix C.
The term wolf is part of the common name for several species of dog-like animals within the taxonomic family Canidae. In addition to wolves, the canid family includes coyotes, foxes, jackals, wild dogs, and domestic dogs. Species in the canid family are classified into more than a dozen different genera.
Common names such as “wolf,” “wild dog,” and “fox” do not perfectly match the scientific names and the ways in which species are placed into genera. For example, the common name “fox” is used for species in several different genera, such as the gray fox (Urocyon cinereoargenteus) and red fox (Vulpes vulpes) of North America. The term “wolf” in the common name of a species also has a tenuous relationship with taxonomy and is more closely allied with the ecological role that a species plays. North American wolves and coyotes are classified in the genus Canis, but so are the Eurasian golden jackal (C. aureus), the African black-backed jackal (C. mesomelas), and the African side-striped jackal (C. adustus). In turn, other species that occupy an ecological niche very similar to that of the species of Canis have been placed in different genera, including the South American maned wolf (Chrysocyon brachyurus), the Asian wild dog or dhole (Cuon alpinus), and the African wild dog (Lycaon pictus).
Fossil evidence indicates that the ancestor of the genus Canis was a jackal-sized animal (jackals have a mass of about 10 kg and shoulder height of about 45 cm) that lived in North America as early as 40 million years ago (Merriam, 1911; Wang et al., 2008). About 7–8 million years ago canids crossed the Bering Strait to Eurasia, where they occupied a diverse array of habitats and eventually expanded their geographic range throughout much of Europe, Africa, and Asia (Mech, 1970). Eventually canids returned to North America. The earliest undisputed occurrence of a “wolf” in North America was the medium-sized C. edwardii, which appeared about 3 million years ago (Nowak, 1979).
Scientists have recognized three historical North American lineages of wolves, each named after one of the species in that lineage: C. dirus (dire wolf), C. lupus (gray wolf), and C. rufus (red wolf) (Martin, 1989). Since the discovery of these species, scientists have been revising their conclusions about how many species are recognized in each lineage and about the relationships among the various populations of wolves (Chambers et al., 2012). There are three reasons this process has been difficult. First, the proliferation of descendants from C. edwardii or an animal like it occurred very rapidly, which makes it difficult to trace those lineages accurately. Second, some species of
wolves have hybridized with other species, which makes delineating among species challenging (Nowak, 2002). Third, the present-day geographic ranges of wolves have changed dramatically because of predator control and habitat alteration, which makes it difficult to reconstruct their historical distributions (Hinton et al., 2017).
It is important to understand this history. One reason is that those wolves that are found together or geographically near one another now may not be the same wolves that were found together 10,000 or 100,000 years ago (Pardi and Smith, 2016). This affects which of those sets of populations might be expected to have developed mechanisms to avoid interbreeding. Another reason is that knowing which wolves were where and when can provide critical evidence about what types of wolves are most closely related, which of them are distantly related, and which wolves might resemble each other for reasons other than their sharing of a recent common ancestor.
This report is focused on four canids: the gray wolf, the Mexican gray wolf, the red wolf, and the coyote (Table 1-1). Research findings on the comparative biology and evolutionary relationships among these four canids are the core evidence used by the committee.
To appreciate why there has been debate over the taxonomic status of the red wolf and the Mexican gray wolf, it is important to understand the relationship between evolution and taxonomy. Specifically, one must appreciate how the dynamic nature of species—which are the result of ongoing evolution—can challenge those attempting to determine their taxonomy.
While biologists agree that species are real entities, they have struggled to construct a suitable definition of species. The basic reason is that the convention for naming species, which can be traced back to Linnaeus in the 18th century, was based on the presumption that species are fixed entities that do not change. This would prove false.
Darwin’s description of evolution and more than 150 years of subsequent research have shown that life is dynamic at all levels. New species are constantly forming, and existing species are constantly evolving, with genetic differences among them that grow larger over time. In other words, the relationship between members of the same species is a snapshot of a moment in time. That moment could be a point when there are only slight differences between populations or a point when different populations of the same species form well-defined, distinct groups. Nearly 100 years of research on the genetic differences between species has shown that pairs of species can be found at many levels of divergence (Travis and Baer, 2016). No single definition of a species seems capable of encompassing the diversity of relationships among populations and species.
Biologists have developed a number of what are called “species concepts” in order to define a species accurately. These concepts help scientists determine which populations represent the same species and which represent different species. The concepts have different emphases (see Chapter 2). Embedded within all of the species concepts is the principle that members of the same species will be able to mate with one another and produce offspring that can thrive in the habitats that the species occupy. When this criterion is met, the members of a species will fill a common ecological role and share a common evolutionary path to the future.
Of course, it is not always possible to ascertain directly which organisms can mate with one another or which matings will produce offspring that can thrive and which will not. In such a case one must fall back on indirect methods, and scientists use many lines of evidence to infer which populations represent the same species and which represent different species. An especially important line of evidence in the modern era concerning the potential for reproductive capability is the use of DNA sequences to determine the amount of genetic difference between two populations that may represent two species. The greater the genetic differences between two populations, the less likely it is that individuals from these two populations will recognize each other as potential mates,
TABLE 1-1 Common Names, Scientific Names, and Geographic Ranges of the Four Canid Species in North America
|Gray wolf (Canis lupus)|
|Geographic range of C. lupus includes Canada and parts of the United States (Alaska, California, Idaho, Montana, Oregon, Washington, and northwestern Wyoming)|
|SOURCE: Photos courtesy of Melba Coleman.|
|Mexican gray wolf (Canis lupus baileyi)|
|Geographic range of C. lupus baileyi includes northwestern Mexico and parts of southwestern United States (southern Arizona and New Mexico)|
|SOURCE: U.S. Fish and Wildlife Service.|
|Red wolf (Canis rufus)|
|Geographic range of C. rufus includes parts of southeastern United States (southeastern North Carolina and southeastern Texas)|
|SOURCE: U.S. Fish and Wildlife Service.|
|Coyote (Canis latrans)|
|Geographic range of C. latrans is throughout Canada, Mexico, and the United States|
|SOURCE: Photo courtesy of Melba Coleman.|
the less likely that any mating between them will produce successful offspring, and thus the more likely that these two populations represent different species.
However, DNA sequence differences are not the sole criteria for determining a species. For example, recent genetic analyses have shown that brown bears (Ursus arctos) and polar bears (Ursus maritimus) are very similar in their DNA (Liu et al., 2014), so much so that if scientists made a decision solely on the basis of genetic similarity, the two bears might be considered to be the same species. In addition, the two types of bears are capable of mating with each other (Cahill et al., 2013), which is further reason to call them a single species. In fact, the two bears are considered to be separate species. Each species has alleles not found in the other; in addition, polar bears have unique morphological and physiological features, and they are ecologically distinct from brown bears (Liu et al., 2014).
Taxonomy is shaped and reshaped by new discoveries. Thus, taxonomic designations can be considered to be working hypotheses that may be rejected when new evidence comes to light. For example, genetic analyses indicated that the species known for decades as the slimy salamander, Plethodon glutinosus, is actually a group of 14 different species (Highton and Peabody, 2000). The slimy salamander occupies forested areas in the eastern United States from upstate New York to central Florida and west to the Mississippi Valley. Morphologically, individuals are virtually indistinguishable from one end of the range to the other. However, the genetic analyses revealed substantial differences among individuals from different parts of the range. These differences are large enough that it is unlikely that salamanders from one portion of the range would breed with those from another.
Species are important, and much can depend upon their recognition. The complexity of relationships among species and the diverse lines of evidence can make the process of determining taxonomy seem arbitrary and capricious. It is not. There are, however, controversies over how best to interpret the data in hand for specific cases, and therefore some taxonomic decisions are contested. New data and innovative methods of analysis can often help resolve these controversies.
Controversies over the taxonomic validity of the Mexican gray wolf and the red wolf have arisen, in part, because different scientists may apply the criteria for defining species and subspecies differently. So, it is important to appreciate the challenges that scientists encounter when attempting to define a species or subspecies.
Defining a species requires meeting three challenges. First, taxonomy requires a variety of data. The example of polar bears and brown bears illustrates the importance of integrating genetic, morphological, and ecological data. Controversy often arises when there are insufficient data of one or more types, but decisions must be made on the basis of the data that are available.
Second, sets of populations can be in different stages in the process of species formation or, in some cases, in different stages of the process of species dissolution. This situation can make it difficult to delineate species, that is, to determine the dividing line between different species. One way to deal with this difficulty is to apply historical knowledge to the data. Such “historical knowledge” may involve reconstructing the past pattern of a species’ geographic distribution, its habitat use, and which other species were involved. And sometimes “historical knowledge” refers to a reconstruction of the evolutionary history of a species, including such details as which other species is its closest relative and which extant species most resembles the common ancestor.
The third challenge facing those attempting to define a species is hybridization, the mating and production of offspring from two different species, which has proven far more common in the evolutionary history of many species than previously thought. Hybridization and introgression, the movement of gene variants or alleles from one species into another, create enormous challenges in interpreting data on genetic distinctions between groups. In some cases, such gene movement can erode the identity of individual species and merge them into a single species (Rhymer and Simberloff, 1996). In others cases, hybridization introduces new gene variants without eroding a species identity; the ancient hybridization of Neanderthals and humans is an obvious example (Prüfer et al., 2014). In other cases, some species originated as hybrids between two other species but later evolved to be distinct from their parent species. The possibility of a hybrid origin for some species is an old idea, dating back at least to Linnaeus. Analyses of DNA sequences have produced evidence of many existing plant and animal species having a hybrid origin. Of course, a variety of factors, including anthropogenic effects, can weaken the reproductive barriers between existing species and promote the movement of genes from one species into other. It can be difficult to distinguish these cases from genetic data alone; innovative analyses of DNA are necessary to do so and, even then, are best complemented by other lines of evidence, when available.
The major challenge in defining and designating subspecies is that, given a particular species, there are many ways to define a subspecies (Haig et al., 2006). The earliest definitions of a subspecies identified sets of populations whose members shared specific variations in color patterns or morphology that were not found in other populations of the same species that were geographically separated from the population under consideration. The modern definitions of subspecies follow the spirit of these original definitions, focusing on congruent variation in many characteristics that distinguishes one geographically distinct set of populations from other sets. The various definitions generally differ on the specific quantitative criteria they use for when a subspecies designation is warranted, but all definitions emphasize geographic separation and distinctive sets of trait values.
Taxonomists who study different major groups of plants or animals tend to use different criteria for designating subspecies. This does not mean that the criteria for recognizing a distinctive unit are arbitrary and capricious. Rather, a distinctive group of populations may be considered a subspecies of a wide-ranging species by one group of taxonomists but considered to be a different species altogether by another (Isaac et al., 2004). The different frequencies of subspecies designations in different groups can reflect biologically based differences between taxa in the prevalence and patterns of distinctive geographic variation among populations, but to some extent they also reflect the different taxonomic traditions of the scientists who study different groups of species.
This report consists of four chapters in addition to this one. In Chapter 2, the committee discusses in more detail the general principles of taxonomy, species recognition, and species delineation. The chapter is designed to set the committee’s assessment of the validity of the status of Mexican gray wolves and red wolves firmly in the context of accepted general practice, and in that chapter the committee describes the subset of species concepts that are most relevant to wolves, how they are applied to individual cases, and how the committee used them to guide its interpretation of the evidence. The committee also discusses the roles of hybridization and introgression in the process of species formation and species dissolution and their influence on taxonomic designations. Genetic evidence consistent with past hybridization and introgression in Mexican gray wolves
and red wolves has provoked considerable controversy over the validity of their current taxonomic classifications. The committee discusses the nature of the evidence usually brought to bear on questions of species formation and delineation and, working from that evidence, develops a series of questions that must be answered for either species or subspecies determination.
In Chapter 3 the committee describes methods for analyzing genetic and genomic data concerning the differences among wolf taxa and their evolutionary relationships to one another. Different interpretations of the same genetic and genomic data have inspired divergent opinions on the validity of the taxonomic status of the Mexican gray wolf and red wolf. To understand how these different interpretations have arisen and to understand the committee’s interpretation of the same data, it is important to appreciate the specialized statistical methods that are used to analyze these data and to understand their assumptions, what they can reveal, and what they cannot reveal.
In Chapters 4 and 5 the committee applies the concepts from Chapter 2 and the results of analyses conducted with the methods described in Chapter 3 to the Mexican gray wolf and the red wolf, respectively. Each chapter begins with a brief review of the current taxonomy of each wolf and with specific questions that have arisen concerning their current status. Each chapter then reviews the evidence from morphology and the history of the species’ occurrence and geographic range, the evidence from genetics and genomics, and the evidence from ecology and social behavior, particularly mating behavior, that underlay the findings of the committee. These sections are followed by a synthesis of all of the evidence and a section with the committee’s conclusions. The sections reviewing the morphological, genetic, ecological, and behavioral evidence contain technical details about the available evidence, including the extent and quality of various types of data and the analyses of those data. The synthesis section summarizes the committee’s assessment of that evidence without the technical details.
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