Species Concepts, Reproductive Isolating Mechanisms, and
Geographic Patterns of Variation
Bruce G. Stewart
General Objectives and Study Guide
Your objectives for these Notes on the Web and associated readings and exercises are:
Related Textbook Readings:
Species Concepts
Why complicate matters by studying more than one species concept?
The world of life is far more interesting and complex than many people will ever realize. Some think it would be nice if everything fit into a nice little box with its own definition that was universally applicable. (I think the world would be quite boring if it was so predictable!) Such is the case with one of the most fundamental concepts in biology, the species. Just what is a species? The biology textbook (Johnson and Losos 2008) doesn't elaborate much at all on this important concept. The general zoology textbook (Hickman, et al. 2004) does a nice job of expressing species concepts. Some avoid the issue altogether as a "problem" while others embrace the reality of the biological world and use species concepts to reveal some of its processes, complexity, order, and beauty.
Chung (2004) wrote that "Just labeling something a problem does not necessarily make it one. Why should it be a problem that there are a host of species definitions and no consensus among biologists?" He goes on to ask, "What is the relevance of the species problem to the biology classroom?" And he points out that "...there are compelling reasons to thoughtfully and deliberately incorporate some elements of the species problem into introductory courses." Finally, Chung (2004) echos the reason that I have long taught the species concept issue in my introductory courses at Murray State College, "...challenging students to grapple with some of the complexities inherent in the species problem will result in a more sophisticated conception of species and that this will help students understand other key concepts in biology."
There are many species concepts, but I have chosen four to discuss here in your Notes on the Web. They are presented below.
Biological Species Concept.
This concept was popularized by Ernst Mayr (Click Here for a PBS Photograph of Dr. Ernst Mayr). It has surely been the dominant concept of a species within vertebrate systematics for many years and is precisely the "general biology textbook" concept most often cited. It is defined in part as "groups of interbreeding natural populations that are reproductively isolated from other such groups." (Mayr 1969, p. 26). Much more is presented about the concept of "reproductive isolation" later in your Notes on the Web.
Remember there is still not a single, universally applicable species concept. This is not a weakness in biological science, but rather is due to the diversity of natural histories of organisms and the diversity of views by biologists as to what information the species concept should convey. Please visit the following link to read briefly about the role that Ernst Mayr has played in the "modern synthesis" whereby genetics, species concepts, and the like were brought together in the 20th century. ---> Link to PBS page on Ernst Mayr and the Evolutionary Synthesis
Morphological (=Typological or Phenetic) Species Concept
This concept is based on a more-or-less subjective determination as to whether two populations are "similar enough" to be called a single species. Linnaeus and countless others before and since have used this concept. Although there are subjectivities involved (e.g. how similar do two organisms need to be to be called a single species), there are modern mathematical techniques that are used to quantitate the degrees of similarity. This is a method that can be applied by knowledgeable biologists when little is known about a species' life history, such as reproductive behavior. Plus, it can apply to asexual species (those that reproduce without sex) whereas the Biological Species Concept cannot.
Phylogenetic Species Concept
This species concept, articulated well by Joel Cracraft (1983, 1987) and Rosen (1978, 1979) views a "species" as the evolutionary unit as identified by evidence for patterns of ancestry and descent. Complete reproductive isolation is not required (Tony Echelle, pers. comm.). A formal definition is "A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent." (Cracraft, 1983, 1987). The Phylogenetic Species Concept is based on shared character distributions (e.g. apomorphies and synapomorphies) within and between populations. A population (or group of populations) that possess, for example, a unique allele (form of a particular gene) or other heritable character would be considered a separate species from populations that do not possess that allele or character. In other words, the population is an evolving unit as indicated by this evidence, and this is precisely what a species category should represent according to this approach. The Phylogenetics Species Concept uses the cladistics method (covered earlier in our section on taxonomic techniques) as a tool to reveal species patterns and relationships.
Use of the Phylogenetic Species Concept, according to some detractors, would greatly increase the number of named species, but Cracraft (1983, 1987) and others dispute this and point out that convenience should not be a criterion. One major advantage of the Phylogenetic Species Concept is that it is applicable to unusual reproductive modes such as parthenogens and hybridogens. Species that reproduce by one of these modes are unisexual (all female!) and do not cross breed even with other members of their own species (such as some species of the lizard genus Cnemodophoris in the southwestern United States)! Add to this fascinating phenomenon in nature that some species such as the cladocerans (a large group of important aquatic crustaceans), rotifers (another group of important freshwater invertebrates), some algae, and even honey bees, exhibit both "normal" sexual reproduction and parthenogenesis or other unusual modes! Thus, direct examination of genetic relationships is needed to identify evolving units (i.e. the species). Neither the Biological Species Concept nor the Morphological Species Concept can accomplish this.
Evolutionary Species Concept and Modified Evolutionary Species Concept
A formal version of the Evolutionary Species Concept was developed by E.O. Wiley of the University of Kansas. He states that an evolutionary species is "a single lineage of ancestral-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate." This is similar in some ways to the Phylogenetic Species Concept but differs in the way the lineages of related populations are determined. Specifically, the Evolutionary Species Concept uses a wider range of characters to make the species determination. Later, Frost and Hillis proposed a modified Evolutionary Species Concept which recognized different genetically distinct populations that are separated only ephemerally (in evolutionary time) as the same species. For example, two populations are sometimes separated by a relatively short-term geographic barrier may come in contact again and exhibit gene flow. Frost and Hillis stated a formal definition for an evolutionary species as the largest entity that has evolved and whose parts (i.e. populations) cannot be determined with certainty to be on separate evolutionary trajectories. This is the definition I would like you (the student) to learn for later discussion and recall. Like the Phylogenetic Species Concept, the Evolutionary Species Concepts use cladistical methods to reveal the species patterns and relationships.
Other Species Concepts
Chung (2004) cites papers that indicate there are as many as 22 species definitions! Our general biology textbook (Johnson and Losos 2008) describe one called the Ecological Species Concept. Each species concept has its applications in the biological world. The fact that so many species concepts have been proposed highlights the complex nature of life. It is a good thing that living things are known to the level that many species concepts would be necessary to represent the reality of the living world! And you may recall in our previous notes that only about 1.6 million living species have been named of the estimated 5 to 30 million that may exist on Earth. In this regard, we have only just begun, and who can say what our understanding of the species category will as more basic exploration of our biological world is achieved.
Classic Isolating Mechanisms
A. Term coined by Dobzhansky (1937 in Genetics and the Origin of Species)
B. Involves study of behavior, ecology, and genetics
C. Defined: "Isolating mechanisms are biological properties of individuals which prevent the interbreeding of populations that are actually or potentially sympatric." (Mayr 1970, p. 56, Pop. Sp. & Evolution)
1. excludes geographic or other purely extrinsic isolation
2. does not require sterility
a. example: Mallard (Anas platyrhynchos) and Northern Pintail (Anas acuta) produce fully fertile F1, F2, and F3 hybrids and any backcrosses!)
b. example: the tiger (Panthera tigris) and the lion (Panthera leo) produce a beautiful, sterile "tigon" You really should check out the links below! In the PBS site, be sure to click the "enlarge" link below the picture.
Tigon Web Site With Excellent Descriptions and Photographs
c. question becomes: "What do organisms do under natural conditions?" Is there or is there not a separation of gene pools of two populations despite the fact that there is some degree of hybridization?
D. Three Useful Terms Related to Geographic Range Relationships: Be aware of the meaning of the following terms, and how these distributional patterns can have a bearing on determining whether two species are reproductively isolated.
1. Sympatric – distributional ranges overlap
2. Parapatric – distributional ranges come into contact but do not overlap (i.e. are juxtaposed)
3. Allopatric – distributional ranges do not overlap
E. Classification of isolating mechanisms (from Mayr 1970)
1. Pre-mating Mechanisms - mechanisms that prevent interspecific crosses.
a. seasonal and/or habitat isolations - potential mates do not meet
b. ethological isolation - potential mates meet but do not mate
c. mechanical isolation - copulation is attempted but no transfer of sperm can take place
2. Post-mating Mechanisms - mechanisms that reduce full success of interspecific crosses
a. gamete mortality - sperm transfer occurs but egg is not fertilized
b. zygotic mortality - egg is fertilized but zygote dies
c. hybrid inviability - zygote produces F1 hybrid of reduced viability
d. hybrid sterility - F1 hybrid is fully viable but partially or fully sterile, or produces deficient F2's
F. Examples of Isolating Mechanisms (and some breakdowns of barriers)
1. Song and Ecology in Anurans. Male frogs and toads sing to attract mates. Females can distinguish which males are of their own species whether we can or not!. In class (even in the Internet class we will do this on Lab Day #1) we will listen to recordings of several species. But whether you hear these recordings or not, the descriptions below should help illustrate isolating mechanisms and the biological species concept. First, here are pictures of three species to get you in the mood.
First is Bufo houstonensis which is closely related to Bufo americanus. The endangered Houston Toad has a long (up to 20 seconds!), high pitched, musical trill as does its close relative, the American Toad. The songs are similar, but not identical in frequency.
Second is the Spotted Chorus Frog, Pseudacris clarki, a species of small frog that breeds in early spring in our neck of the woods. It has a song sometimes described like the sound of running your fingers down the teeth of a comb. Click on the following thumbnail image to enlarge the picture. There are related species in the genus, but again they have similar, but not identical calls. And certainly, no human observer would ever confuse the calls of this group of species with those of the two toads mentioned above.
Photograph Copyright Bruce G. Stewart
The third is a Narrow-mouth Toad, Gastrophryne carolinensis or G. olivaceous, with a male and female in amplexus (the mating grasp of frogs and toads). The male narrow-mouth toad sings a very high-pitched insect like trill. Like all anurans, they know the call of their own species. Click on the following thumbnail image to enlarge the picture.
Photograph
Copyright Bruce G. Stewart
2. Habitat and Seasonal Example in Anurans
a. Bufo americanus and Bufo woodhousei fowleri are largely isolated because the former is an early spring breeder in more wooded areas and the latter is a late spring-early summer breeder in more grassy habitats. (There are also song differences as will be demonstrated on recordings in class.)
* breakdown may occur due to habitat changes due to agriculture
* hybrids are more commonly produced during "in between" seasonal periods when the active periods of the two species overlap
3. Song and Courtship Behavior in Birds
a. There are many examples of birds in our area that are closely-related, sympatric (have overlapping geographic ranges), and sing similar, but not identical songs. Males generally do the more complex singing used to defend territories in which the females chose for nesting. Males of the same species recognize the song of their own as they battle for territories. This spacially distributes the experienced males and other behaviors distinguish them in the eyes of their females when the processes of pair bonding and nesting occur.
* The familiar night birds, the Chuck-will's-widow (Caprimulgus carolinensis) (the common almost always heard species in south central Oklahoma) and the Whip-poor-will (Caprimulgus vociferus) (the one that most people "think" they hear) have such similar calls that most people don't realize there are two different birds! One field guide (Griggs 1997) describes the calls of the two species as follows:
Chuck will's wid'-ow, repeated call. First note soft, usually not heard. The 3 loud notes have whistled quality, as in a whip-poor-will, but different accent.
Whip'-poor-will', repeated call. Clear whistle in East, with accent as indicated; burrier in West, with accent strongest on the last syllable.
4. Ethological Example #1: Gray Treefrogs
a. Hyla versicolor and Hyla chrysocelis are the gray treefrog species whose songs will be played in class. These species have different trill rates (the former has the "slow" trill and the latter has the "fast" trill). Females of each species distinguish the males songs and are attracted to the males of their own species.
5. Ethological Example #2: Fireflies (Lightning Beetles)
a. Fireflies (order Coleoptera) of the genera Photinus and Photurus exhibit a visual communication behavior that contributes to reproductive isolation of individuals of different species.
* males give species-specific flash patterns which females can recognize and respond to by giving a "response flash"
* males fly over low vegetation giving flashes which are unique to their species in ways such as the following
- flash duration
- flash pattern during a sequence (sort of like Morse Code!)
- frequency of flash sequences
- flight pattern during flash sequence
* females (usually flightless) give "response flashes" from an advertisement perch in the vegetation, and their responses are unique in their:
- duration
- delay time after the male's last flash sequence
b. example: Photinus tanytoxus
time---> 0 1 2 3 (sec)
male |- |- | | (0.12 - 0.16 sec blips)
female | | | |- (~0.2 sec blip)
* CLICK HERE TO SEE AN EXAMPLE OF A FIREFLY SEQUENCE
Thanks to Bill Adams, Instructional Technology Specialist at MSC for developing this Flash (no pun intended!) file for me!
The sequence you will see is similar to Photinus tanytoxus. Watch in the sky for the male flashes as he flies from left to right. Then note the female response from the ground vegetation.
Species with Polytypic Patterns in Geographic Distribution of Populationsc. we will come back to this interesting behavioral system in later discussions of alternative behaviors and sexual selection.
5. Examples of Degrees of Breakdowna. varies greatly; such as
* rare in Rana spp.(spp. means various species in the genus, and Rana is the genus of the so-called "true frogs" such as the leopard frog and bullfrog)
* widespread in Bufo spp. (the "true toads")
- Brown (1971) example with Bufo houstonensis, Bufo valliceps, and Bufo woodhousei as cited in Allison and Wilkins (2001)
Author's abstract: "Natural hybridization is reported for the Houston toad, Bufo houstonensis, with two other species, B. woodhousei and B. valliceps, in a disjunct pine forest near Bastrop, Bastrop Co., Texas. It is very difficult to morphologically distinguish B. woodhousei X B. houstonensis hybrids from the parental species. These hybrids, however, have a mating call intermediate between those of the parental species in pulse rate, dominant frequency, and duration. B. valliceps X B. houstonensis hybrids are morphologically intermediate between the parental species and have an abnormal mating call. At Bastrop all premating isolating mechanisms except mating call and body size appear to have partially broken down. Habitat destruction by man may have contributed to this breakdown. Nine relictual populations of B. houstonensis have been found in south-central Texas. At most localities the species is probably near extinction. The species seems restricted to sandy areas frequently characterized by the presence of Loblolly Pine. The destruction of these habitats is probably an important cause of the reduction in numbers of B. houstonensis, although other factors may be involved.
- Hillis, et al. (1984) as cited in Allison and Wilkins (2001) reported a less than 1% rate of hybridization of the Houston Toad with the others mentioned previously and only a single backcross. Thus, there is still solid reproductive isolation. They suggested that temporal isolation and habitat isolation were the primary isolating mechanisms.
* frequent in fishes in general (look for papers by Carl Hubbs in 1955, 1961), but mostly sterile F1 & little or no backcrossing
b. sympatric hybrid swarms - sometimes occur where "hybrid swarm" forms continuous bridge between parental types.
* The Spotted Towhee (Pipilo maculatus) (this is a native song bird species) is more or less parapatric with the Eastern Towhee (Pipilo erythrophthalum) in Oklahoma and it has been long known that they hybridize extensively across that zone. A southwestern form of the Spotted Towhee meets a population of a third species, the Collared Towhee (Pipilo ocai) where they hybridize. Throughout most of their geographic ranges, these species exhibit consistent species characteristics, but in certain areas there are hybrid populations. Thus, there have been many revisions of their taxonomy as new research data come to light.
*studies in overlap zones are considered in determining whether two forms are indeed separate species (by the Biological Species Concept). Some of these geographic forms have been "lumped" as one species due to further study, while others have been "elevated" or "split" into different species.
G. Implications of Hybridization in Defining Species
1. Self-study Activity- Consider what level of hybridization you would use to designate different populations as the same or different species. Why? What species concept would you use in your considerations? Here is a link with a table of eastern and western bird species that meet in the Great Plains.
Great Plains Birds - Stanford University Site
It has information on the degree of hybridization between the various eastern and western forms. How would you answer the question regarding whether each of these pairs is one species or two different species?
A. Includes allopatric or parapatric populations assumed to be capable of interbreeding
B. Populations may be quite different phenotypically from others
C. Those considered sufficiently distinct are often distinguished nomenclaturally as "subspecies"
1. this is a recognized taxon by International Code of Zoological Nomenclature
2. it is written as a trinomial (e.g. Acris crepitans blanchardi)
D. However, it is debatable as to whether or not a subspecies is an "evolutionary unit" or just a taxonomic convenience.
1. A notable quote by Mayr (1942, p. 106): "The taxonomist is an orderly person whose task it is to assign every specimen to a definite category (or museum drawer!). This necessary process of pigeonholing has led to the erroneous belief among non-taxonomists that subspecies are clear-cut units... Such situations exist occasionally... But subspecies intergrade almost unnoticeably in nearly all cases in which there is distributional continuity."
2. The subspecies is probably not a suitable unit for evolutionary discussions according to Mayr and many other more recent workers; however, some disagree.
E. Patterns of geographic variation that have been identified where there is much "parallel variation" among species. The can be called ecogeographic rules and here are three examples.
1. Bergmann's Rule - races from cooler climates in species of warm-blooded vertebrates tend to be larger than races of the same species living in warmer climate
* birds show only a 10-25% exception rate (often exceptions are in migratory forms
* mammals show a 20-40% exception rate (often the exceptions are in burrowing mammals)
* also works to some degree with changes in elevation
2. Allen's Rule - protruding parts (e.g. bill, tail, and ears) are shorter in cooler versus warmer climates
* exceptions include bird wings (no heat loss effect) & things related to foraging requirements
3. Gloger's Rule - pigmentation increases in warm humid versus cool dry areas in warm-blooded vertebrates - reason not clear and both nocturnal and diurnal species follow the rule. Few exceptions.
Useful Links
Acknowledgements
I would like to thank Dr. Noble Jobe for kindly taking his valuable time to provide editorial and content criticisms of these Notes on the Web. He certainly helped clean up my writing, and he suggested some thoughtful changes and additions that I have included.
Literature Cited (in progress)
Allison, Stacey and Neal Wilkins. Ecology and management of the endangered Houston Toad (Bufo houstonensis): A topical index and annotated bibliography. Department of Wildlife and Fisheries Sciences, Texas Cooperative Extension, Texas A&M University System. http://landinfo.tamu.edu/projects/houstontoad/Ready%20for%20Adobe2.pdf (accessed: 22 March 2008)
Brown, L. E. 1971. Natural hybridization and trend toward extinction in some relict Texas toad
populations. Southwestern Naturalist 16(2): 185–199.Cracraft, Joel (1983, 1987 papers)
Chung, Carl. 2004. The species problem and the value of teaching the complexities of species. The American Biology Teacher, 66(6):413-417.
Dobzhansky, T. 1937. Genetics and the origin of species.
Griggs, Jack. 1997. American Birds Conservancy's field guide to all the birds of North America: a revolutionary system based on feeding behaviors and field-recognizable features. HarperCollinsPublishers, Inc., New York, 172pp.
Hillis, D. M., A. M. Hillis, and R. F. Martin. 1984. Reproductive ecology and hybridization of
the endangered Houston toad (Bufo houstonensis). Journal of Herpetology 18(1): 56–72.Hubbs, Carl (1955, 1961 papers)
Frost, Darrel and David Hillis (one paper)
Mayr, Ernst. 1942.
Mayr. Ernst. 1970. Populations, species and evolution.
Rosen (1978, 1979)
Wiley, E. O.
© 1999, 2005, 2007 Bruce G. Stewart
Biology Homepage Bruce G. Stewart Homepage Zoology Homepage
