INTRODUCTION

[note: “key terms” below]

The seminar began with a brief introduction that emphasized its interdisciplinary nature and the diversity of disciplines present. A summary of the main questions we will discuss throughout the seminar series is as follows:

1. • How does the scientific and evolutionary worldview connect organisms, in particular humans, to the tree of life?
   

2. • How does this worldview affect our self-perception and our behavior on the planet?
   

3. • How do the answers to these questions change the way we consider ourselves and our public policy strategies in the face of social, political, and environmental challenges?
   

SHORT SUMMARIES

Craig Moritz presented the view that species are real as lineages, and that they hold a special position in the tree of life. He points out that in nature there is no continuum of biological variation, that the phenotype we observe for a given organism is not just a step in a sequence of characteristics. Processes such as natural selection and extinction make holes in that continuum, and this results in discontinuities. Those discontinuities we call species. Although the designation of other taxonomic levels is still subjective and hard to diagnose, a species is a unique stage in the tree of life that can arise when populations become reproductively isolated. Ultimately, regardless of the concept used to diagnose them, Dr. Moritz explained that species are hypotheses of what constitutes cohesive, independently evolving lineages.

Brent Mishler proposed that species are real as lineages; however, as Linnaean ranks, he argued that they are as arbitrary as any other taxonomic level. Moreover, Dr. Mishler remarked that the potential to interbreed is a poor criterion to diagnose species. If anything, the inability to interbreed would be the better arbiter, since reproductive compatibility is the ancestral (not the derived) condition. Dr. Mishler explained that there are rank-free codes of nomenclature in development that address these problems, and that name taxa to appropriately reflect their evolutionary relationships.

Roberta Millstein acknowledged that the concerns raised about the biological species concept are well-founded. However, irrespective of the species concept, it is important to understand the processes behind cladogenesis. The potential to interbreed is just one such process. She invited us to ponder these mechanisms and to consider conservation efforts that embrace not just species lineages, but ecological relationships and ecosystems as well.

David Wake agreed with Dr. Mishler that the species level is not a privileged rank, but emphasized that species lineages are not forever monophyletic. He noted that many biologists and members of the public have a pre-Darwinian concept of species. They erroneously think that species originate, exist, and then go extinct. By contrast, Dr. Wake sees lineages as constantly evolving entities that come into being, spread out, and fall apart as segments become extinct. In this sense, even though they may be monophyletic in origin, they quickly become paraphyletic. In following, Dr. Wake noted that the species concept Darwin maintained is quite modern. He also agreed with Dr. Moritz’s notion of “species as hypotheses.”

Robert Proctor mentioned that although species may seem more real than other taxonomic ranks, it is evident that there is no clear species concept in the microbial world. Moreover, it is very different to diagnose a species of plants than it is a species of animal. Regardless of the concept used, it is important to consider what has changed in the larger political milieu that makes it possible to pose these questions. Dr. Proctor asked, “Do organisms have to be like us to deserve certain rights? Are we more likely to respect organisms that look more like us? And what should be our moral responsibility to different parts of nature?” Society tends to give more rights to organisms more closely related to us, and it is important to examine the reasons, since there is not an obvious answer. It is necessary to consider the moral repercussions and how this discussion prompts us to re-evaluate conservation priorities.

LONG SUMMARIES

Craig Moritz argued that species hold a unique position in the tree of life because they represent the transition from genetic exchange to independent evolution. He explained that in nature there is not a continuum of biological variation. That is, the phenotypes we observe are not just steps in a sequence of characteristics. There are discontinuities in the continuum, and processes such as natural selection and extinction create “holes” in it. These discontinuities are what we call species.

When Darwin wrote On the Origin of Species he was not aware of the principles of inheritance. In the 1920s, disciplines such as paleontology, systematics, genetics, and population biology began to combine principles of inheritance and quantitative models with processes such as natural selection that Darwin described. This was the neo-Darwinian synthesis, and from it arose the view that a species is the smallest inclusive independently evolving lineage. Dr. Moritz noted that although the designation of higher ranks (e.g., genera, families, etc.) is subjective, a species can be identified as the transition point from reproductive exchange to independent evolution. In this way, he explained, a species represents a unique stage in the history of life.

For example, this point can arise when populations become reproductively isolated, which is a critical part of the biological species concept. Reproductive isolation will lead to the accumulation of different mutations, and this leads to distinct lineages. Yet Dr. Moritz acknowledged that it can still be difficult to distinguish two species.

He mentioned that it is also possible to diagnose species using phylogenetic (i.e., evolutionary) methods. For example, in systematics a species can be a “tip” on a phylogeny as the smallest set of organisms that share an ancestor that is different from other such sets.

Ultimately, Dr. Moritz argued that biological- and phylogenetic species concepts describe the underlying evolutionary process. The main difference is the “grey zone” where lineages become distinct as they sort-out from influencing processes (e.g., selection and extinction). Often the group under study determines the size of the grey zone. For example, we are the sole surviving hominid lineage, but 2-4 million years ago hominids were more diverse. Sequenced DNA extracted from fossil bones shows that we hybridized with Neanderthals, and 1-4% of the genes in Europeans and Asians can be attributed to this. This hybridization makes the grey zone more evident because it connects otherwise separate lineages. However, despite genetic exchange, functional and phenotypic discontinuities show that we’re still dealing with different hominid lineages. Hybrids are less common in animals than in plants, and this contributes to why zoologists and botanists have different views of species. Different academic disciplines simply use different lenses to study and understand the same process, and the result is that they recognize species differently.

Dr. Moritz concluded by saying that species are hypotheses of what constitute cohesive, independently evolving lineages. They hold a unique position because they are the point between reticulation and independent evolution. He expressed that this is important because it influences how diversity is measured, portrayed, and applied in conservation biology. It is important to not only count and list species, but also to protect the evolutionary process that led to their origin. Species labels lead to species lists, but without an awareness of their evolutionary relationships, it is likely that we will miss species that capture a greater history and diversity of the evolutionary process.

––

Brent Mishler began with a historical perspective of species concepts and proposed that species are real as lineages, but not uniquely real (i.e., special as a rank) relative to other taxonomic levels.

He first framed the notion of speciesism by drawing parallels with racism. In the discussion about racism, a distinction is often made between racialism and racism: racialism is the proposition that races are real (note: they are not real as biological entities but exist as cultural constructs), whereas racism is the view that one race is superior to another. Racialism does not imply racism, but racism does necessitate racialism. Similarly, considering speciesism, there are three questions to consider: (1) are species real as lineages?; (2) do they have a special realism as ranks (i.e., species realism)?; and (3) is one species viewed as superior to another (i.e., speciesism)? Accepting species lineages does not imply species realism, though speciesism entails species realism.

The classification of organisms is an ancient human activity. For example, the Greeks viewed species as atomistic-like units of biodiversity. This view of species as building blocks was widely adopted in western culture. This continued into the Christian era when species became special units, different from a subspecies or genus.

Dr. Mishler explained that one of Darwin’s most original contributions in the Origin was his emphasis that speciation represents an arbitrary point in diversification. That is, the underlying reality is what pushes lineages apart (Darwin’s proposed “principle of divergence”), and the taxonomic result is a convenient label applied by humans. Dr. Mishler continued by mentioning the problems with, and exceptions to, the biological species concept. For example, clonal (asexual) organisms are often phenotypically different but do not interbreed, whereas orchids, which also maintain great phenotypic variability, do interbreed. That is, reproductive compatibility is not the best criterion to diagnose species.

Clearly, Dr. Mishler acknowledged, the biological species concept works better in vertebrates, but plants and microbes make clear that the point of contention is the size of the “grey zone” over which a lineage splits into two. Simply put, the species level represents an arbitrary slice at irregular depths across the tree of life. Taxonomists have always struggled to diagnose the cutoff for the smallest taxon, yet there are now rank-free codes of nomenclature in development that address this problem. One system is the PhyloCode, which names taxa that appropriately reflect their phylogenetic relationships.

In summary, Dr. Mishler emphasized that species are real if they are monophyletic, but that these clades do not represent a special level of biological organization. That is, the tree of life is about nested sets (i.e., “groups subordinate to groups”), and species as monophyletic groups are no more special than any other monophyletic group.

KEY TERMS (alphabetical)

Clade (i.e., monophyletic group).—a group of organisms that includes all the descendants of a common ancestor. Each descendant is more closely related to each other than to any other group of organisms. For example, egg-laying mammals (e.g., platypus), marsupials (e.g., opossum), and placental mammals (e.g., humans) form a clade called Mammalia. We are all more closely related to other mammals than to other vertebrates (e.g., reptiles).

Cladogenesis.— when one lineage splits into two.

Genus.—the rank above species in Linnaean classification. For example, the genus of humans is Homo. Another species in our genus include Homo habilis (“handy man”), the first known maker of stone tools.

Lineage.—a genealogical entity with continuity in space and time; a continuous line of descent. A series of organisms, populations, cells, or genes connected by ancestor/descendent relationships.

Linnaean classification.—The standard system of classification in which every organism is assigned a kingdom, phylum, class, order, family, genus, and species. This system groups organisms into ever smaller and smaller groups (like a series of boxes within boxes, called a nested hierarchy). However, not all Linnaean groups are clades, and they often represent segments of a lineage. That is, they only include some (but not all) descendants of a common ancestor.

Paraphyletic.—a group of organisms that includes some but not all descendants of a common ancestor.

Phylogeny.—the evolutionary relationships among organisms viewed over time; the patterns of lineage branching produced by the evolutionary history of the organisms being considered.

Subspecies.—a Linnaean grouping of organisms less inclusive than a species. The term is usually applied to groups within a species that have distinct forms and live in a restricted area.

Rank.—the relative position or level in a taxonomic hierarchy (e.g., species, genus, family, etc.).

Reticulation.—In evolution, a process of convergence (as opposed to divergence) of lineages; involves the exchange of genetic material.

Systematics.—a field of taxonomy that names and diagnoses taxa and infers how they are related in the tree of life.

Taxonomy.—the branch of biology that names and diagnoses groups of organisms based on their characteristics. Not all characteristics are evolutionary informative and not all forms of taxonomy classify based upon evolutionary relationships.

Taxon (taxa, plural).—any named group of organisms (e.g., reptiles, Felidae, beetles, Homo sapiens), whether or not it forms a clade.

SUGGESTED READINGS

Alvarez, W., 1991. The gentle art of scientific trespassing. GSA Today, 1(2), pp.29–34.

Claridge, M.F., 2009. Species are real biological entities. In F. Ayala & R. Arp, eds. Contemporary Debates in Philosophy of Biology. Wiley-Blackwell, pp. 91–109.

Mishler, B.D., 2009. Species are not uniquely real biological entities. In F. Ayala & R. Arp, eds. Contemporary Debates in Philosophy of Biology. Wiley-Blackwell, pp. 110–122.