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What is a Species?

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Introduction and Goals

The term "species" refers to a fundamental taxonomic unit. All living organisms have a species classification, and most of us are pretty comfortable using the term "species." However, actually pinpointing what makes one group of organisms distinct from another group can be difficult.

New species primarily arise from a subpopulation of an existing species. In this process known as cladogenesis, a new species arises and the new and parental species may eventually coexist. Thus, biological diversity increases because where there once was one species, there now are two. However, at what point has the new group diverged enough from the existing group (speciation) to form a population that we can comfortably refer to as a new species?

Evolutionary biologists are still debating about how to accurately define a species. As you will learn, the problem is that not all definitions work in every situation (and perhaps this statement itself provides insight into the diversity of life). We will explore a variety of species concepts. By the end of this tutorial you should have a basic understanding of:

  • The three major species concepts
  • The difference between prezygotic and postzygotic reproductive isolation, and how both keep species distinct
  • The conditions that favor speciation
  • The distinction between allopatric and sympatric speciation

Performance Objectives:

  • Compare and contrast the biological, morphological and phylogenetic species concepts
  • Describe the ways that reproductive isolation can be maintained, both prezygotic and postzygotic
  • Discuss the conditions required for allopatric speciation to occur
  • Explain how polyploidy can lead to sympatric speciation
  • Summarize the process of adaptive radiation and why it is often seen in organisms that live on island groups.

What is a Species?

Numerous concepts have been proposed to define a species (a list of 26 possible species concepts can be found here), however, only three concepts will be described in this tutorial. None are perfect, but the biological species concept is the most easily applicable to sexually reproducing organisms.

The biological species concept defines a species as a group of individuals who interbreed, or have the potential to interbreed, in nature. Notice that this definition is very similar to the definition of a population. Remember that microevolution works at the population level. According to the biological species concept, when individuals of a population become sufficiently different, so as to lose the capacity to interbreed, then a new species has arisen. However, this definition is not very useful for those that do not undergo sexual reproduction. It also is not useful in defining groups that are extinct, and can prove problematic in defining groups of bacteria that undergo conjugation (the exchange of small amounts of genetic information).



The morphological species concept (also sometimes called the typological species concept) defines a species as a group of organisms that are consistently and persistently distinct from other groups of organisms and distinguishable by their morphology (shape).  In other words, if they appear different, then they are different species. The advantage of this definition is that it works for extinct species and for those that reproduce asexually. However, it can be insufficient. For example, if you were not familiar with dogs and came upon a group of Chihuahua dogs, you might define dogs as "small, four-legged, warm-blooded animals with prominent canine teeth." If you then came upon a group of Great Danes, you could easily classify them as a second species due to their large size.

The phylogenetic species concept  groups organisms based on shared (unique) evolutionary history; the smallest group of organisms that share a common ancestor and can be distinguished form other groups that do not, essentially an evolutionary unit . According to this definition, two species might actually be able to interbreed, yet because they have different evolutionary histories they are considered different species. Also, this species concept results in more species being recognized, because if two populations have the potential to interbreed but don’t because of their geographic location, they are considered to be different species. As you has probably realized, it can be difficult to establish at what point genetically distinct populations become their own species, and in some cases the designation of a species can be somewhat arbitrary.

Most biologists use the biological species concept when it is practical; when not, they turn to one of the other concepts, although with the availability of genetic information for many populations, the phylogenetic species concept is becoming more widely used.

Figure 1. Despite the difference in size, Speedy the Chihuahua and Lucky the lab mix are the same species. (Click image to enlarge)


Reproductive Isolation - Prezygotic Isolation

If a species is defined as a group of potentially interbreeding populations, then it is necessary to determine what prevents certain groups of organisms from interbreeding. Is it a different anatomy, behavior, time available for mating, or some other barrier that separates populations into species? Ultimately, some form of reproductive isolation is involved. Reproductive isolation can be categorized according to whether the barrier blocks fertilization (a prezygotic barrier) or prevents complete development after a hybrid (an individual derived from two species) zygote has been formed (a postzygotic barrier). Figure 2 summarizes how different isolation mechanisms can prevent interbreeding.

Figure 2. Isolation Mechanisms Can Prevent Interbreeding. (Click image to enlarge)

Prezygotic barriers to reproduction include habitat, timing, behavior, and gametic isolation; any barrier that functions before a zygote is formed. In the case of habitat isolation, two species may occupy an overlapping territory but use different habitats (this is also called ecological isolation). For example, seven lizard species from a single genus live in close proximity within forested areas of the Dominican Republic. However, one of these lizards (Anolis distichus) prefers sunny perches close to the ground; another (A. cybotes) perches in shady, mid-level tree branches (Figure 3). Occupying these separate microhabitats prevents them from interacting physically, therefore, hybrids are rarely produced. Habitat isolation is also used in breeding programs; cat fanciers who raise and sell Himalayans would not want their female cats in the same room with male Abyssinian cats.

Two species that are receptive to mating at different times of the year, or even day, exhibit temporal isolation. For example, the eastern spotted skunk and the western spotted skunk have overlapping ranges, but they do not produce hybrids because the western variety mates in the summer and the eastern variety mates in the winter. Similarly, there are many flower species that don't interbreed because they flower at different times of the day, season, or year, thus they cannot cross-pollinate.

Figure 3. Prezygotic Barriers to Reproduction - Habitat Isolation between two species of Anolis lizards in the Dominican Republic. (Click image to enlarge)

Groups of organisms that do not interbreed because they exhibit different behaviors exhibit behavioral isolation. Eastern (Figure 4) and western (Figure 5) meadowlarks are difficult to tell apart based on size, shape, and color, however, their calls are quite distinct. Presumably this difference serves to allow the females to distinguish males from the two different species. Click on the icon beside each bird to hear its song. Can you hear a difference? Behavioral isolation may also prevent different firefly populations from mating because different species have their own pattern of light pulses. Other examples include mating dances and various courtship rituals.


Figure 4. Eastern meadowlark

Figure 5. Western meadowlark

Eastern meadowlark call

Western meadowlark call

In mechanical isolation, differences in size and shape of genitalia (or flower structures) prevent successful mating.  This is commonly seen in insects, where the genitals function like a “lock and key”, so that the intromittent organ of a male will only fit into the opening of the reproductive tract of a female that is a member of the same species.

Gametic isolation is prezygotic isolation in the most literal sense. Here, gametes do not form a zygote; for any number of reasons (morphological, chemical, or environmental), the fusion of gametes does not occur.



Reproductive Isolation - Postzygotic Isolation

In the case of postzygotic reproductive isolation, individuals from two species are capable of producing a zygote, however, these offspring (or their offspring) are incapable of normal growth and/or reproduction. Postzygotic barriers to hybridization include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown (Figure 6).

In reduced hybrid viability, hybrids between two species either fails to develop or do develop but lack vigor and rarely, if ever, reach sexual maturity. These offspring are considered unviable because ultimately, they are unable to reproduce.

In reduced hybrid fertility, interbreeding between species occurs and hybrids are formed, however, the hybrids themselves are usually sterile. Reduced fertility is observed in mules, which are sterile hybrids of donkeys and horses.

In hybrid breakdown, hybrids are capable of reproducing but their offspring have either reduced fertility or reduced viability so a hybrid population cannot persist.

Figure 6. Isolation Mechanisms Can Prevent Interbreeding. (Click image to enlarge)

If all of these isolating mechanisms do not function, there are a number of possible outcomes. If the hybrids are able to produce fertile offspring among themselves and with the parent species and the offspring have the same fitness, the two parent species may ultimately coalesce back into a single species.  The hybrids may persist in a stable hybrid zone (geographic area) between the two parent species (there are many example this in the scientific literature; Figure 7).  If the hybrids are more well adapted to the environment than either parent species, the two parent species could ultimately disappear and be replace by the hybrid (this has been documented but is very rare).


Figure 7. In Eastern Europe there is a stable hybrid zone (pink) between two species of mice, Mus musculus and Mus domesticus.  Information for museum specimens shows that this hybrid zone has been stable for hundreds of years (;



Conditions that Favor Speciation

Now that we have addressed several mechanisms that keep members of different species from interbreeding, we will focus on some theories about how new species arise (speciation). Essentially, gene flow between closely related populations must be interrupted. This can happen in several ways; we will discuss two of them. Allopatric speciation occurs when populations become physically isolated due to some sort of geographical barrier. Sympatric speciation occurs when populations become genetically isolated, even though they have not been physically separated from each other .

Allopatric speciation involves some sort of geographical isolation that physically blocks migration of individuals (and therefore gene flow) between populations. Geographical isolation may arise as a result of changes in the path of a river, the uplift of a mountain range, the formation of a canyon, or other landmass changes. A good example of allopatric speciation involves two species of antelope squirrels whose populations are separated by the Grand Canyon. Presumably they evolved from once-interbreeding populations that were isolated on either rim as the canyon was formed by the Colorado River. 


Sympatric Speciation in Plants

Sympatirc speciation is speciation in the absence of geographic separation.  As you can probably predict, this occurs more rarely than allopatric speciation.  However, sympatric speciation is fairly common in plants, although it also has been documented in a few animals.  It is not unusual for errors in cell division (meiosis) to produce gametes with an extra set of chromosomes if the homologous pairs are not separated in Meiosis I. If a plant is able to self-fertilize, the union of two of these unreduced gametes results is a condition called polyploidy.  For example, if a diploid plant produces diploid instead of haploid gametes, then undergoes self-fertilization, the offspring would be tetraploid, with four sets of chromosomes. These tetraploid individuals cannot interbreed successfully with diploid plants of the parent population, thus resulting in speciation. In this manner, speciation can occur quite rapidly (in one generation) within a sympatric population of plants.


Adaptive Radiations

Adaptive radiation refers to the relatively rapid evolution of many new species from a single common ancestor into diverse habitats. Adaptive radiations are most often observed on island groups where new opportunities exist for immigrant species. The immigrant arrives on the island, and over time, as it moves into new habitats, the different populations become adapted to the different environments within those habitats.  Genetics diversification occurs, and eventually the populations becomes reproductively isolated from each other. Examples include the speciation of Darwin's finches in the Galapagos Islands (Fig. 8a) , the diversification of honeycreepers (another group of birds) and fruit flies in the Hawaiian islands, and the radiation of Anolis lizards on different islands in the Caribbean (Fig. 8b). You can use this app to investigate the different “ecomorphs” of Anolis found on the different islands (

Figure 8.   A comparison of the adaptives radiations of finches on the Galapagos Islands (a) and Anolis is the West Indies (b), showing their different ecological niches. (Click image to enlarge)

Test Your Understanding:  Darwin's Finches

During Darwin’s historic voyage, he spent time on the Galapagos Islands where he studied 13 species of finches (shown in the image below). It is known that all 13 species found on the various islands of the Galapagos are descendents of a single species of finch found on the mainland of Ecuador.

A 14th species of finch is found on Cocos Island. While the Galapagos is a group of islands, Cocos island is one single island and it is found approximately 400 miles from the mainland of Ecuador.

  • Use what you know about speciation to explain why 13 different types of finches evolved on the Galapagos while only one species of finch is found on Cocos Island.

Darwin's finches on the Galapagos. The finches numbered 1–7 are ground finches. They seek their food on the ground or in low shrubs. Those numbered 8–13 are tree finches. They live primarily on insects.

1. Large cactus finch (Geospiza conirostris)
2. Large ground finch (Geospiza magnirostris)
3. Medium ground finch (Geospiza fortis)
4. Cactus finch (Geospiza scandens)
5. Sharp-beaked ground finch (Geospiza difficilis)
6. Small ground finch (Geospiza fuliginosa)
7. Woodpecker finch (Cactospiza pallida)
8. Vegetarian tree finch (Platyspiza crassirostris)
9. Medium tree finch (Camarhynchus pauper)
10. Large tree finch (Camarhynchus psittacula)
11. Small tree finch (Camarhynchus parvulus)
12. Warbler finch (Certhidia olivacea)
13. Mangrove finch (Cactospiza heliobates)





The 14th Darwin’s finch
Pinaroloxias inornata – the Cocos Island


From BSCS, Biological Science: Molecules to Man, Houghton Mifflin Co., 1963




This tutorial explored the formation and maintenance of species. The species level is the fundamental taxonomic unit, but how this unit is defined depends upon the biology of the organisms being studied and the information available. Paradoxically, the diversity of life itself leads to imprecision in defining exactly what constitutes a species; what works well for one group may be impractical for another. At one level this argument may seem like splitting hairs. After all, a dog is a dog, and a cat is a cat; you don't have to be a trained biologist to recognize the difference. However, asexually reproducing organisms, or those for which we have only fossil remains, require different species concepts.  The available data indicate that most new species arise from existing species, in a process known as cladogenesis. Cladogenesis can be thought of as the "budding" of one species from another. Basically, a small group undergoes sufficient genetic changes to become a new species and may even coexist, at least for awhile, with the parental species. How much does a group have to change in order for it to be categorized as a "new" species? The answer depends on how one defines a species.

Although biologists have yet to come up with the best definition for a species, there are three major definitions in use today: the biological species concept, the morphological species concept, and the phylogenetic species concept. The biological species concept is probably the most widely used of the three, but it cannot always be applied; the other two can be more applicable in certain instances.

The biological species concept is useful in defining a species among contemporary groups that reproduce sexually. In thinking about how speciation occurs, a critical component is reproductive isolation. If two populations of the same species become separated over time, microevolutionary changes will occur and eventually these populations will become reproductively isolated. Typically this happens when a small group becomes isolated from a larger group (a founder event). Once isolated, gene flow between the two groups ceases. Recall, small populations are highly susceptible to microevolutionary changes, especially genetic drift, which is probably why cladogenesis seems to be a common feature of speciation. There are a number of ways that two groups can become reproductively isolated. These reproductive isolation events can be classified as prezygotic (habitat, behavioral, temporal, mechanical, and gametic isolation) or postzygotic (hybrid viability, hybrid fertility, and hybrid breakdown). Geographic isolation can play a significant role in isolating one population of a species from another. When speciation occurs in separate geographical locations, the pattern is described as allopatric speciation. However, there are instances when reproductive isolation takes place in the same geographic locale; in these cases, the event is termed sympatric speciation. Sympatric speciation is fairly common in plants and relatively rare in animals. 



After reading this tutorial, you should have a working knowledge of the following terms:

  • adaptive radiation
  • allopatric speciation
  • cladogenesis
  • gametic isolation
  • hybrid
  • hybrid breakdown
  • polyploidy
  • postzygotic barrier
  • prezygotic barrier
  • reproductive isolation
  • speciation
  • sympatric speciation
  • temporal isolation

Questions?  Either send your instructor a message through ANGEL or attend a weekly review session (the times and places are posted on ANGEL).