This post from 2005 reflects a good summary of the state of species concepts in human origins at that time. However, some things have changed since then. We now engage with DNA evidence from fossil hominins which has highlighted the role of mixture between populations. Different perspectives on species concepts remain.
A new population that results from a speciation event is called a species. But although species result from a simple process, recognizing species in nature can be complicated. Biologists cannot travel in time to observe the speciations that resulted in today's diversity of life, so they must observe the reproduction of living organisms to determine the makeup of species. Paleontologists can find the fossil evidence of the ancestors of today's species, but they cannot observe whether those fossil organisms could reproduce with each other. Because scientists have different kinds of evidence about organisms, they use different concepts of species when testing hypotheses about their evolution.
The most obvious property that helps to define species is reproductive isolation. Biologists studying living animals often use the biological species concept, which envisions a species as a "group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups" (Mayr 1942). It is the biological species concept that primatologists use to grapple with whether chimpanzees and bonobos are different species, for example, by observing the differences in their reproductive behaviors and the strength of geographic isolation between their populations.
The biological species concept has some important limitations for paleontology. Making use of the concept depends on observing the mating behavior and interbreeding patterns of animals in their natural environments, which is not possible with fossils of organisms that lived in the past. Other kinds of observations that paleontologists might gather, such as morphological differences between fossils, have no necessary value under this concept. Another limitation is that the biological species concept does not incorporate any idea of how species may change over time. Paleontologists study fossils that may be separated by hundreds of thousands of years of time. It is difficult to imagine such widely separated individuals as part of the same reproductive community, even if they were very similar to each other. Over such time periods, evolution can transform populations substantially. The biological species concept recognizes the genetic continuity within a species caused by gene flow, but it does not incorporate a view of species existing over evolutionary time. For these reasons, paleontology requires a different kind of species concept.
Phylogenetic species concept
The phylogenetic species concept is an attempt to define species by their relationships to other species. Instead of trying to determine the reproductive boundaries of populations, scientists using the phylogenetic species concept attempt to uncover their genealogical relationships. A group of individuals that includes all the descendants of one common ancestor, leaving no descendants out, is called a monophyletic group.
Paleontologists Niles Eldredge and Joel Cracraft devised a species concept called the "Phylogenetic Species Concept," intended to apply to circumstances in which reproduction or isolation among organisms could not be observed. Under this concept, a species is "a diagnosable cluster of individuals within which there is a parental pattern of ancestry and escent, beyond which there is not, and which exhibits a pattern of phylogenetic ancestry and descent among units of like kind" (Eldredge and Cracraft 1980:92).
Key to the phylogenetic species concept is the idea that species must be "diagnosable." In other words, members of the species should share a combination of characteristics that other species lack. To look for the unique features that define a phylogenetic species, paleontologists must perform systematic comparisons with other related fossils or living species. These aspects of the concept make it widely applicable in paleontology.
But the phylogenetic species concept is not without its problems. Because the concept defines species based on morphology, without explicitly referring to populations or reproductive boundaries, it does not apply well to cases where morphologically different populations are connected by gene flow. Morphological variation among populations is not uncommon within living species. Humans today are a species with substantial morphological variation from continent to continent. Humans on different continents are not reproductively isolated, and their variation is largely distributed as clines over large geographic distances. Yet a paleontologist who had only a few fragmentary specimens from each continent would not necessarily know the pattern of variation, and many features of his specimens would appear to be unique. What would the paleontologist make of the high nose of a European specimen, the forward-facing cheeks of an Asian fossil, or the strong browridge above the eye orbits of an Australian, each taken randomly from their variable populations? By applying the phylogenetic species concept, a paleontologist would probably conclude that the different continents were homes to different human species.
Thus, because the phylogenetic species concept does not identify species based on the reproductive boundaries between them, it may have the effect of identifying populations connected by gene flow as different species. For this reason, a phylogenetic species as defined by a paleontologist may not correspond to a real prehistoric population that was the product of a speciation. Some paleontologists do not view this potential conflict as a problem, because identifying species based on unique characteristics will create as full as possible a systematization of the evolution of new features. Assuming that the number of ancient species was very large, and the number of fossils representing each of them is very small, then paleontologists can hardly hope to identify every speciation event in the past. The phylogenetic species concept may therefore provide a better approximation of the number and diversity of species that existed than other alternatives.
On the other hand, identifying populations connected by gene flow as different species can be a significant problem for paleontologists who take a greater interest in the processes of evolution than in the diversity of species in the past. Gene flow is a significant force shaping evolutionary change within populations. Moreover, evolution may cause a single species to change over time, possibly acquiring new unique features without any division of a species into separate reproductively isolated populations. Some paleontologists approach these difficulties by altering their view of the evolutionary process. If speciations can happen as a transformation of a single population in addition to the appearance of reproductive boundaries between populations, then a single evolving population may over time comprise several phylogenetic species. Or if most evolutionary change happened at the time of speciation, as asserted by the concept of punctuated equilibrium, then the phylogenetic species concept might more closely approximate the actual pattern of speciations in the past. But without such assumptions, the phylogenetic species concept's problems sometimes create a stumbling block for some paleontologists in attempting to understand the evolutionary process.
The evolutionary species concept combines the genealogical basis of the phylogenetic species concept with the genetic basis of the biological species concept. An evolutionary species is a lineage of interbreeding organisms, reproductively isolated from other lineages, that has a beginning, an end, and a distinct evolutionary trajectory (Wiley 1978). The beginning of a species' existence is a speciation, as a population becomes reproductively isolated from a parent population. The end of a species occurs either with extinction or with the branching of the species into one or more descendants.
Central to the evolutionary species concept is the idea of an evolutionary trajectory. The trajectory of a species is the evolutionary pattern of its characteristics over time. For example, one of the earliest species in the story of human evolution, Australopithecus afarensis, is represented by dozens of fossil teeth and mandibles, as well as other remains. Paleontologists hypothesize that these fossils, from several sites in East Africa, are members of a single species because of their many morphological resemblances. No very similar fossils have ever been found before 3.6 million or after 3 million years ago, dates that appear to indicate the beginning and the end of the species.
Nevertheless, the fossils do show some differences that appear over time. Although the molar teeth of the fossils do not change over time, the mandibles are thicker and more massive in more recent fossils than in the most ancient ones. As far as paleontologists can test, the mandibles form a single series evolving over time toward greater size and thickness. The evolutionary species concept infers that the fossils represent a species, beginning 3.6 million years ago and ending 3 million years ago, with an evolutionary trajectory that includes the evolution of greater mandibular thickness, without apparent changes in molar sizes.
The strength of the evolutionary species concept is that it allows paleontologists to focus on the causes of evolutionary change, whether they occur during speciations or at other times. Regarding A. afarensis, the observation that mandibles increased in size during the existence of the species may be explained by different evolutionary forces and conditions than if all the change occurred with the reproductive isolation of a new population. Although the greater mandibular thickness of later mandibles might be a unique feature, attempting to establish a new phylogenetic species for the later fossils might detract from an explanation of the overall evolutionary pattern.
But the evolutionary species concept also has its problems. Because it uses several different criteria, much more information may be necessary to define an evolutionary species. Some scientists do not view this as a drawback, since even if a scientific view of the species that once existed and their boundaries and relationships proves a challenge, it may nevertheless add to our understanding of the evolutionary process.
Yet for many paleontologists, the need to amass great numbers of fossils from different times makes the evolutionary species concept nearly impossible to implement. At the same time, if scientists always hold out the possibility that two different fossils were actually connected by gene flow, it may impede an understanding of evolutionary changes that accompany the appearance of new reproductively isolated species. If we want to have a scientific, meaning falsificationist, view of the species that have existed and their boundaries and relationships to each other, we must accept that the process will in many cases be difficult. Simply making up many species hypotheses cannot add to our knowledgeand in many cases it may detract. What is important is that we realize that our record of past species is incomplete, and our failure to substantiate the existence of many species in the past does not constitute evidence that they did not exist.
Testing species hypotheses
However species are defined, whenever scientists identify a species, they actually are stating a hypothesis about the relationships among individual organisms. Such a hypothesis may be tested using morphological, genetic, or behavioral evidence. Discovering real species that existed in the past involves predicting the morphological variability of populations, including variation that occurs among populations connected by gene flow. In the relatively small fossil samples available to paleontologists, determining the number of species in a sample is a significant problem. Researchers use a number of techniques to test species hypotheses with limited morphological samples.
Two fossil hominids: different species or not?
- What is the level of morphological difference between two or more specimens? Using a living species for comparison, scientists can determine the likelihood of sampling similar variability as the fossil sample (Miller 2000).
- What are the relative frequencies of characteristics in two samples of fossils? Statistical comparison with the differences between different populations within a living species can determine whether the differences in frequencies observed in the fossils would be likely to occur within the comparison species. Such comparisons can be extended to the differences between the sexes of a living species to test whether sexual dimorphism accounts for differences between fossils (Lee 1999).
- How do morphological features covary? If one fossil sample has a high incidence of several features that are absent or at low frequency in another sample, this supports the hypothesis that the two samples represent different species. With samples of sufficient size, say, 10 individuals or more, paleontologists can even estimate the maximum level of gene flow consistent with the morphological differences, and thereby frame a test of the hypothesis of different species in solid evolutionary terms (Hawks and Wolpoff 2001).
- Do samples represent change over time? Sometimes paleontologists can use different populations from living species to evaluate likelihood that certain kinds of changes might occur over time. The best comparisons are with large samples of fossils that represent long spans of time, however. Although the evolutionary process is in ways unique for each species, analyses of the rate and level of changes in other species provide the most powerful tests of species hypotheses available in studying the past.
Eldredge N, Cracraft J. 1980. Phylogenetic patterns and the evolutionary process: Method and theory in comparative biology. New York: Columbia University Press.
Hawks J, Wolpoff MH. 2001. The accretion model of Neandertal evolution. Evolution 55:1474-1485. PubMed
Lee SH. 1999. Evolution of human sexual dimorphism: Using assigned resampling method to estimate sexual dimorphism when individual sex is unknown. Ph.D. thesis, University of Michigan.
Mayr E. 1942. Systematics and the origin of species from the viewpoint of a zoologist. New York: Columbia University Press.
Miller JMA. 2000. Craniofacial variation in Homo habilis: An analysis of the evidence for multiple species. Am J Phys Anthropol 112:103-128. PubMed
Wiley EO. 1978. The evolutionary species concept reconsidered. Syst Zool 27:17-26.