Speciation

In the last column, I began a discussion of the different concepts of species used by biologists. How do we know what a “good species” is? The column ended with the Biological Species Concept (BSC, for short), in which the ability to interbreed determines if two populations should be combined into one species.

Determining species limits is tricky because hybrids often occur when two presumed species overlap. Orioles provide a nice example of the changing winds of bird taxonomy. Prior to 1983, birders could see Baltimore Orioles in eastern North America and Bullock’s Orioles in the west. Based on the fact that these two species interbreed at their region of overlap in the Great Plains, the AOU Check-list Committee combined the two species into one, called the Northern Oriole. Subsequent work revealed that the two species have very different molting patterns. DNA comparisons indicate that Baltimore Oriole and Bullock’s Oriole are not even their closest relatives. So the Check-list Committee reversed itself and split Northern Oriole back to two species.

Our current taxonomy has some inconsistencies. The American Black Duck and Mallard interbreed freely. Waterfowl biologists are concerned that hybridization with Mallards may be swamping the genetic constitution of the black ducks. Similarly, the Golden-winged Warbler is declining throughout much of its range, due in part to extensive hybridization with the Blue-winged Warbler (a species that is rapidly increasing its range). So despite the ability of these two pairs of species to interbreed freely, we recognize each pair as two separate species.

Compare that situation to the Yellow-rumped Warbler, one of the most familiar warblers nesting in Maine and indeed throughout North America. Two forms of this species are recognized, our eastern Myrtle Warbler and the western Audubon’s Warbler. Male Myrtle Warblers have white throats and male Audubon’s have yellow throats. The two types of yellow-rumps interbreed in a very limited area in the southern Canadian Rockies. Despite the more limited interbreeding than the two species pairs above, the two distinct forms are combined into a single species.

This discussion leads us to consideration of the last species concept, the Phylogenetic Species Concept (PSC, for short). Advocates of the PSC argue that the ability to interbreed is a misguided approach. Rather, these advocates define a species as a group of individuals that all share at least one characteristic found uniquely in that group. If individuals sometimes interbreed with other species but still maintain those unique characteristics, it’s no big deal. Each species has its own history (or phylogeny). Use of the PSC would cause the Myrtle Warbler and Audubon’s Warblers to be split into two species. The limited interbreeding does not alter that fact that the two types of yellow-rumps have unique throat colors.

Unique songs or calls can be used to define a species under this approach. From the work of Jeff Groth and Tom Hahn, we know that there are at least nine populations of Red Crossbills with distinctive flight calls. The different call types do show some morphological differences and differences in choice of cones they feed on. The PSC would result in these nine call types being split into nine species.

Genetic differences can be used to define species using the PSC. If all individuals in a population have a unique sequence of DNA, that information can be used to define a species. Of course, a DNA sequencer is a bit unwieldy for birders to carry in the field!

One upshot of using the PSC is that the number of recognized species of birds will increase. I have seen one estimate that the number of bird species could be doubled to about 20,000. That is one way to work on your life list.

Regardless of the species definition chosen, biologists will continue to be confused for some species or groups of species. Populations change and diverge through time in response to changing climate, increases in competition or changes in predator numbers. To drive this point home, let us end with Herring Gulls.

Herring Gulls occur around the globe in higher latitudes of the northern hemisphere. European Herring Gulls are the lightest in color. They interbreed with the slightly darker American Herring Gulls. American Herring Gulls from Alaska interbreed with even darker Vega Herring Gulls in Siberia. The Vega gulls interbreed with the darker yet Herring Gulls in central Russia and they interbreed with Lesser Black-backed Gulls in Europe. But at the end of this ring, the Lesser Black-backed Gulls and European Herring Gulls can’t interbreed! Where do we draw the species line?

[Originally published on December 13, 2009]

In the last column, I wrote about the scientific classification of birds.  I pointed out that the families and orders into which birds are classified are pigeonholes defined by scientists.  The only taxonomic unit that is defined by nature is the species.  Recognizing species is not as easy as one might think.  We’ll begin to explore this prickly subject today.

A recent scientific review showed that there are over 60 definitions of species.  I will collapse this list to three basic concepts and discuss the strengths and weaknesses of each.

The morphological species concept is based on the idea that all members of a species should be similar.  Of course similar is a subjective adjective.  I like to show my introductory biology class a portrait of the basketball player Wilt Chamberlain next to the jockey Willie Shoemaker.  An alien would scarcely suspect that these two mean, differing so much in height and skin color, belong to the same species.  Strike one for the morphological species concept.

Sibling species, species that are deceptively similar, also pose a problem for the morphological species concept.  Prior to 1973, certain flycatchers with eye-rings and wing-bars were identified as Traill’s Flycatchers.  Fieldwork showed that some male Traill’s Flycatchers gave a sneezy fitz-bew song and others gave a fee-bee-o song.  Researchers found that some Traill’s females responded only to the fitz-bew songs and others only to the fee-bee-o songs.  Traill’s Flycatcher is actually two very similar species: the Willow Flycatcher and the Alder Flycatcher.  Distinguishing the two species in the field is nearly impossible and even in-hand identification is not always possible.  Strike two for the morphological species concept.

Ernst Mayr, one of the greatest biologists of the 20th century, developed the biological species concept, defining a species as a population of interbreeding organisms that are reproductively isolated from other populations.  This concept is widely held and is the predominant view of the members of the American Ornithologists Union Check-list Committee that rules on taxonomic changes for the birds of the Americas.

The critical aspect of this concept is the ability to breed with other individuals to produce viable offspring.  Different species should be unable to hybridize; they should be reproductively isolated even if they are found in the same area.  Isolating mechanisms include songs, displays and infertility that prevent members of different species from mating or producing viable young.

Two problems arise with his concept.  First, how do you treat populations that are geographically separate from each other?  Consider the scrub jay complex.  Scrub Jays are very common birds west of the Rocky Mountains, essentially occupying the ecological role of our Blue Jays.  But there is a population of Scrub Jays in south Florida.  Also, the Scrub Jays on Catalina Island, off the coast of Southern California, look a bit different from western Scrub Jays.  How can you apply a test of the ability to interbreed for populations that are not in contact with each other?  The answer is that it becomes a judgment call.  Formerly, these jays were all treated as one species, the Scrub Jay.  More recent work, based in part on DNA differences, has split the scrub jays into the Florida Scrub Jay, Western Scrub Jay and Island Scrub Jay.  We still don’t know if these different populations can interbreed.  Strike one against the biological species concept.

Another problem is that a lot of birds do hybridize.  More than 10% of bird species care capable of creating viable offspring with members of other species.  Ducks are the most notorious birds in this regard with over 400 hybrid combinations known.  Mallards hybridize with over 50 species of ducks and geese.  Wood Ducks hybridize with over 40 species.  Over 10% of the American Black Ducks and Mallards in New England are hybrids.

Hybrids occur regularly in wood warblers as well.  Lawrence’s Warbler is a well-known hybrid between a Golden-winged Warbler and a Blue-winged Warbler.  The Cincinnati Warbler described by Audubon is now known to be a cross between a Blue-winged Warbler and a more distantly related species, the Kentucky Warbler.

Even though hybrids often are capable of reproduction, in some cases their mixed-parentage offspring are not fertile.   Some hybrids capable of reproduction may have more difficulty in securing a mate than pure-breed individuals.

But many hybrids can secure mates and produce offspring that are fertile.  In such cases, ornithologists measure the proportions of hybrid to “pure” offspring where two populations overlap.  The two populations are judged to be the same species if hybrids are common.  In the next column, we will see this rule in action.  I will also present a third, more controversial, species concept.

Taxonomy is the branch of biological science that is concerned with the classification of organisms. To use birds as an example, all birds are classified into the Class Aves within the Subphylum Vertebrata (which includes all animals with backbones) and the Phylum Chordata (which includes sea squirts as well as vertebrates).

The Class Aves is divided into a number of orders. For example, the Order Gaviiformes includes the loons while the Passeriformes includes all the perching birds. In turn, each order is broken into a number of families. The Corvidae (jays and crows) and the Vireonidae (the vireos) are but two of the many families in the Order Passeriformes. Families are broken down into genera (the singular is genus). Finally, each genus contains one or more species. Following the convention developed by the Swedish taxonomist, Carolus Linnaeus, every species is referred to by its genus and species name. The scientific name of the Black-billed Cuckoo is Coccyzus erythropthalmus. It’s a good thing we have standardized common names for our birds!

Ideally, the taxonomic system should reflect the relatedness of the species within a group of organisms. So, all species that belong to a particular genus are more closely related to each other than they are to other members of their family that are in different genera. Let’s consider the sparrow family, the Emberizidae. Within this family, the White-throated Sparrow and White-crowned Sparrow both belong to the genus Zonotrichia. Our classification suggests these two species are more closely related to each other than either is to the Chipping Sparrow, classified into the genus Spizella. But all the sparrows, classified into many different genera, are all considered more closely related to each other than to any member of another family like the finches (Fringillidae) or tanagers (Thraupidae).

Of all the different levels of a taxonomic system, the species is the only one that is defined by nature. We have techniques that allow us to determine if a population of organisms constitutes a species. All of the other levels are defined by humans. There is no formal definition of what constitutes a genus or a family or an order.

Taxonomy operates on a system of priority. The first classification of a group of organisms is the one that is usually followed. Revisions to taxonomy can be published to reflect better understanding and new discoveries but such revisions have to be reviewed and approved by other taxonomists before an older taxonomy can be changed. As an example, barnacles used to be classified in the Phylum Mollusca, the group that contains snails, clams and squids. Like most mollusks, barnacles have a calcareous shell around the body. Careful examination of the internal structures of barnacles showed them to be crustaceans, more closely related to lobsters, shrimp and crabs. Barnacles were re-classified into the Phylum Arthropoda. The rapid increase in our knowledge of DNA sequences of many organisms has caused us to revise many taxonomic classifications.

Different groups of organisms were originally classified by different taxonomists. Each taxonomist had a subjective notion of how similar species should be to allow them to be classified into the same genus. There are 500 species of marine tropical snails called cone shells. All 500 of these species are classified into the genus Conus. The frog genus Pristimantis contains over 400 species of frogs native to South America and northern Central America. Different taxonomists might have defined each genus more narrowly and might have established ten or more genera for the same groups of species. At the other extreme, some species are considered so distinctive that they are put into their own genus. The Gray Hawk is the only species in the genus Asturina. The distinctive Shoebill from Africa is not only the sole species in its genus but the only member of the family Balaenicipitidae. The Black Skimmer is one of only three species in the family Ryncopidae. Birds are split into smaller genera than many other groups of organisms.

In the end, taxonomy is a way of organizing earth’s biodiversity. A taxonomic system is really a collection of pigeonholes, nested within larger pigeonholes. It is up to each taxonomist to decide how large a genus pigeonhole or a family pigeonhole should be. In other words, all taxonomic units above the level of the species are artificial groupings created by taxonomists.

We recognize the species level as the base of all taxonomy. Species are not artificial units created by scientists but groups determined by the biology of the organisms. However, recognizing a species is tricky and often controversial. We’ll jump into that fray in the next column.

[Originally published on November 15, 2009]