You should have a working knowledge of the following terms:
Introduction and Goals
The term "species" (defined here) 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 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
What is a Species?
Numerous concepts have been proposed to define a species, however, only three concepts will be described in this tutorial. None are perfect, but the biological species concept is the most applicable. According to the biological species concept, a species is a group of individuals who interbreed or have the potential to interbreed in nature. By this definition, a new species exists when individuals of a population become sufficiently different and can no longer interbreed. This very useful definition does have its limitations.
The morphological species concept defines a species as a group of individuals with shared morphologies (appearances). In other words, if a group of organisms appears distinct from other groups, it constitutes a separate species. According to this definition, however, one might group all little brown birds into one species, or one might create dozens of species out of Canis familiaris, the dog.
The evolutionary species concept (also known as the phylogenetic species concept) groups organisms based on shared evolutionary history. According to this definition, two species might actually be able to interbreed, yet because they have different histories they are considered different species. The problem with this concept is that it can be difficult to know the complete history of a species. As you might gather, 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 apply the biological species concept when it is practical; when not, they turn to one of the other concepts.
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). The figure in the upper left corner summarizes how different isolation mechanisms can prevent interbreeding.
Figure. Isolation Mechanisms Can Prevent Inbreeding. (Click image to enlarge)
Prezygotic barriers to reproduction include habitat, timing, behavior, and gametic isolation. In the case of habitat isolation, two species may occupy an overlapping territory but use different habitats. 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. Occupying these separate microhabitats, as depicted in this figure, 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 alley cats. Two species that are receptive to mating at different times of the 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. Prezygotic Barriers to Reproduction - Habitat Isolation. (Click image to enlarge)
Groups of organisms that do not interbreed because they exhibit different behaviors exhibit behavioral isolation. Eastern and western meadowlarks are difficult to distinguish between based on size, shape, and color, however, their calls are quite distinct. Presumably this difference serves to distinguish mates from the different species. Click on the icon beside each bird to hear its song. Can you detect 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.
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.
Finally, in instances of mechanical isolation, differences in size and shape of genitalia (or flower structures) prevent successful mating.
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. Isolation Mechanisms Can Prevent Inbreeding. (Click image to enlarge)
In reduced hybrid viability, hybrids lack vigor and rarely, if ever, reach sexual maturity. These offspring are considered nonviable 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.
Conditions that Favor Speciation
Now that we have addressed several mechanisms that keep 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 one of two ways. 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 their ranges overlap.
As suggested, allopatric speciation involves some sort of geographical isolation that physically blocks migration of individuals (or gene flow) between populations. Geographical isolation may arise as a result of changes in water flow, volcanic uprisings, canyon formations, 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 by the formation of the canyon.
Adaptive radiation refers to the relatively rapid evolution of many new species from a single common ancestor into diverse habitats. Adaptive radiation is commonly observed on island chains where new opportunities exist for immigrant species. Examples include the speciation of Darwin's finches in the Galapagos chain of islands, the diversification of honeycreepers in the Hawaiian islands, and the radiation of cichlid fish around the globe.
Sympatric Speciation in Plants
Sympatric speciation is extremely common in plants. In plants, it is not unusual for errors in cell division (mitosis or meiosis) to produce gametes with an extra set of chromosomes. Fertilization of one of these gametes results is a condition called polyploidy. Polyploid individuals cannot interbreed successfully with diploid plants of the parent population, thus resulting in a type of gametic isolation. In this manner, speciation can occur quite rapidly (in one generation) within a sympatric population of plants.
This tutorial explored the species concept. A species is the fundamental taxonomic unit, but how this unit is defined is a bit slippery. 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, the species is a fundamental unit of evolution; how can we tell if evolution is occurring if we can't define a species? 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 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 evolutionary species concept. The biological species concept is probably the most widely used of the three, but it is not always used; the other two can be more applicable in certain instances.
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. In fact, recall 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. This definition works well for groups of individuals that reproduce sexually, but 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 defines a species as the smallest groups that are consistently and persistently distinct and distinguishable by ordinary means. 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 evolutionary species concept strives to distinguish a species based on its evolutionary history. It defines a species as a group of individuals who share at least one common diagnostic feature and are on a unique evolutionary path. Moreover, this feature can be biochemical or morphological but it must be heritable and reproducible between the members of the groups (the species). Note that in this definition, two individuals from distinct species may have the capacity to interbreed, but yet be considered distinct because they lack a critical derived character trait that defines the two species.
You might be wondering, if biologists cannot agree on what constitutes a species, how do they know speciation occurs? This is a reasonable question. The answer reveals the robust character of evolutionary theory because there are many documented cases where all three definitions apply to two groups; therefore, regardless of the definition used, biologists can reasonably conclude that speciation has, and continues, to take place.
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 parental group. Once isolated, gene flow between the two groups ceases. Recall, small populations are highly susceptible to microevolutionary changes, 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 more common in plants and relatively rare in animals.
Next, we will continue our discussion of speciation on a broader scale, by addressing the processes that govern large-scale changes among organisms.