SPECIES, EVOLUTION AND THE PROCESS OF SPECIATION We usually think of species as a collection of individuals which share a common phenotype including morphology, behavior and ecological relationships with their environment. This view is based on what is known as the typological species concept wherein species represent a static type defined by the possession of an essential, nonvarying set of characters. Variable characters within a species were considered to be accidental (nonessential) and of no importance. Today this view has been replaced by the biological species concept (BSC) which represents a radical departure in thinking about species necessitated by the concept of evolution. According to the BSC, developed by Ernst Mayr of Harvard, species are dynamic entities which interact both with their environment and with other species and are defined as a population or group of populations which reproduce among themselves, but not with other species. The BSC is the product of a shift in the late 1950s from typological thinking to populational thinking wherein individuals belonging to a particular species are viewed not as possessing a fixed set of characters, but rather fit into a range of variation described in terms of a statistical mean with variance (standard deviation). Rather than being nonessential, accidental and unimportant, individual variation is the key to the dynamic nature of species and enables them to vary over both time and space. Thinking of species in terms of variable populations rather than fixed types was the major conceptual advance that gave rise to evolutionary thought. If species aren't types which can be described in terms of phenotypic characters, what are they? Mayr views species in relational terms rather than in absolute terms. Individuals belong to a particular species because they are bonded to their conspecifics by a relationship (reproduction) and not by the possession of a set of unique characteristics. Hence, the term "species" represents a relationship, not a type, and so is similar to the terms "sister" and "brother" which also describe a relationship between individuals rather than the possession of a set of absolute properties. You cannot know whether or not someone is a brother or sister simply by looking at them. So it is with species which are real, dynamic entities that can change over time and space, yet still maintain their integrity as a unit of discontinuity in nature. The importance of reproduction among conspecifics (individuals belonging to the same species) as the basis for understanding the nature of species cannot be emphasized enough. Possession of a fixed set of characters (the basis of the typological species concept) implies no relationship among the individuals classified together and so only individuals are real; the collective category (species) is an abstraction which can apply equally well to nonbiological species, e.g., mineral and chemical species. By emphasizing reproduction, the BSC provides a basis for including individuals in the same species and separating them from other species which is not an artifact of human ability to conceptualize and generalize. The individuals in the species itself determine the category species by their reproductive behavior. What links individuals together in a species is the continuity between generations afforded by reproduction, or over longer time periods by phylogeny. Parents and offspring and ancestors and descendants are so recognized not due to any essential similarity, but because they are related by a dynamic biological process. This cannot be said for mineral and chemical species. As a consequence of interbreeding species constitute a genetic system which can vary over space and time. The term "genetic system" admittedly is vague, but it is consistent with the concept of gene pool which forms the basis of microevolutionary analysis and allows individual variation in morphological, ecological and reproductive dimensions. A change in the genetic system over time can produce phyletic evolution, and a splitting of the system to form two separate, closed systems can result from the process of speciation. Reproductive Isolating Mechanisms Species exist as populations spread out in time and space and so individuals are classified as belonging to the same species if they are judged capable of interbreeding even though separation in time and space prevents them from actually interbreeding. The key element in the definition of species is reproductive isolation which is effected by attributes called reproductive isolating mechanisms (RIMs). A reproductive isolating mechanism is a biological attribute of the individuals belonging to the same species which is under genetic control and prevents members of different species from producing successful offspring. Since reproductive isolation is achieved by a genetically-based characteristic shared by members of the same species, geographic isolation, i.e., an extrinsic barrier between two populations, is not a reproductive isolating mechanism. RIMs are intrinsic barriers to reproduction. Most species are reproductively isolated from others by possessing several different RIMs because each species represents a different genetic system. There are eight commonly recognized isolating mechanisms which will be presented below in order of their disruption of successful reproduction. The first three mechanisms are premating mechanisms which prevent successful mating from taking place, while the last five are postmating mechanisms that prevent the production of successful offspring once mating has taken place. Premating RIMs Premating isolating mechanisms presumably evolved not to keep species from interbreeding with members of a different species but rather to adapt individuals of the same species to the environment in which they live, or to ensure successful reproduction among conspecific individuals. Since reproduction is the be all and end all of existence according to evolutionary theory, natural selection will obviously favor mechanisms which ensure the success of reproductive efforts between conspecifics. The following specific premating mechanisms have been described which emphasize the unique ecological, behavioral and morphological properties of species respectively. 1. Ecological isolation - prevents potential mates from even meeting. There are two forms of ecological isolation: habitat isolation and seasonal isolation. Habitat isolation occurs when different species choose to breed in separate areas which are ecologically different, e.g., pond vs. stream or woodland vs. meadow. Due to this physical separation individuals belonging to different species don't meet during the breeding season. Seasonal isolation occurs when members of different species breed at different times of the year even though they occupy the same habitat. 2. Ethological or behavioral isolation - involves species - specific courtship patterns or recognition cues so that even if members of two species breed in the same habitat at the same time, they will not mate with one another. Because species comprise separate genetic systems whose genes are often incompatible, any individual who mates with a member of a different species is in effect wasting its gametes and sacrificing its fitness. Natural selection will obviously favor any mechanism which enables individuals to recognize and mate only with conspecifics. Ethological isolation is a product of selection for species-specific recognition cues. Such cues could be (a) visual, e.g., the flash sequence in fireflies or the complex series of stimulus and response behaviors that characterize courtship patterns in birds (b), auditory, e.g., the species-specific calls of frogs and birds, or (c) olfactory as found in insects which release species-specific chemical attractants or pheromones. 3. Mechanical isolation - results from incompatibil- ity of sex organs between males and females from different species. This form of isolation was once considered to be the main isolating mechanism that separated species of insects when it was believed that insect genitalia operated in a key-in-lock fashion. Males of one species could not mate with females of another because they possessed the wrong key. Since that time vari- ation in genitalia within insect species has been shown to be so extensive that it minimizes the importance of this form of reproductive isola- tion. Mechanical isolation is still important, however, in some species of plants due to the location and configuration of stigmas and styles. Postmating RIMs Postmating isolating mechanisms are the by-product of genetic divergence between species and incompatibility between their distinct genetic systems. The following mechanisms prevent successful breeding and gene flow between the two systems after mating has taken place. 1. Gamete mortality - occurs when the sperm from a male of one species dies in the reproductive tract of the female of a different species before fertilization can occur. Sperm death is caused by hostile chemical secretions in the female reproductive tract, e.g., the pH of the female's secretions may not be favorable to the survival and mobility of the sperm from a different species. 2. Zygote mortality - fertilization occurs but the zygotes die before undergoing development due to the incompatibility of their gene complexes. 3. Hybrid inviability - development progresses up to a point but the hybrid dies before reproducing. 4. Hybrid sterility - the hybrid survives and may even be robust due to hybrid vigor, but it is sterile, e.g., mules which are hybrids between a horse and a donkey are physically robust but cannot reproduce. 5. Hybrid (F2) breakdown - interspecific hybrids are viable and fertile but these F1 hybrids produce inviable or sterile offspring after mating among themselves or when they backcross with either of the parental species. Recall that the process of recombination produces mosaic chromosomes - part maternal and part paternal. The F1 hybrid contains intact chromosomes from both parents, each of which contains harmonious gene combinations tested by natural selection. These adaptive gene complexes are broken up, however, through recombination when these hybrids produce gametes, and subsequent syngamy produces F2 zygotes with epigenotypes consisting of various combinations of genes from the two different species. Hence, the integrity of the two genetic systems collapses in these zygotes. The Process of Speciation Now that we have defined species as reproductively isolated units and have examined the mechanisms that result in reproductive isolation, we can address the problem of the origin of new species. A new species can arise in either of two ways: transformation of an existing species or phyletic evolution, or through the process of speciation which results in the multiplication of species. Note that in phyletic evolution the number of species is not changed, but that speciation increases the number of species. The process of phyletic evolution results from the action of natural selection operating over long time periods and species transformation occurs as the environment changes. The end result is that species A has become so modified that taxonomists recognize it as a different species (species B). At any time during this transformation only one species existed. Speciation, on the other hand, results in the production of a new species, e.g., species A gives rise to species B. Before the process of speciation takes place there is only one species (species A) but afterwards there are two species living at the same time (species A and species B). The process of speciation ultimately entails the splitting of a single genetic system into two separate systems that are intrinsically isolated through the acquisition of a reproductive isolating mechanism. For the most part, the process of speciation is gradual and takes a very long time to be accomplished - so much time that it cannot be directly observed. Consequently, our understanding of the mechanism behind this process is incomplete. By studying the genetics of species and both interspecies and interpopulation variation, however, evolutionary biologists can develop hypotheses as to how speciation occurs. The Theory of Allopatric Speciation Several hypotheses to explain how speciation occurs have been proposed and these are called modes of speciation. The process of allopatric speciation is believed to be the most common mode of speciation among animals and plants and is supported by the near universal existence of geographic variation among widely separated populations of a single species and the occurrence of examples of incipient species which give taxonomists fits. If geographic variation is extreme enough, a taxonomist will have a difficult time deciding whether individuals from allopatric populations belong to the same or different species. Two models have been proposed to explain the process of allopatric speciation: the range splitting model and the peripheral bud or peripatric model. Range splitting involves the appearance of an extrinsic barrier that separates the distribution of the species into two or more isolated clusters of populations. A river which changes course or a lava flow can effect such a barrier. The peripatric model relies on a particular population structure without the sudden intervention of a barrier. We will confine our detailed examination of the process of allopatric speciation to the peripheral bud model which is also called peripatric speciation. Geographic isolation, the essential component of both models, provides a temporary, extrinsic barrier to gene flow which acts like a RIM thus enabling the isolated population(s) to diverge genetically without the intrusion of genes from other populations of the species which would retard or prevent altogether any genetic differentiation. Model of peripatric speciation A species whose populations are contiguous will not speciate because gene flow and identical selective pressures act as a cohesive force to keep its populations from diverging genetically, and without genetic divergence (evolution) speciation cannot occur. For many species, however, all populations are not contiguous. In particular, populations located at the edge of the species range tend to be more isolated. These geographic isolates are formed by emigrants from the central populations but the opportunity for successful colonization diminishes near the edge of the range because suitable habitats are few and far between. Ernst Mayr in proposing the peripatric model noted the following differences between central and peripheral populations in a species whose population structure consisted of geographical isolates. Central vs peripheral populations 1. Density - population size is low in peripheral populations but high in central populations. This difference in density can be attributed to the harsher environmental conditions experienced at the end of the species range. The central populations occupy the habitat most favorable to the species but the further one moves away from this favorable area, the harsher and less hospitable the environment becomes. Consequently few individual variants will be able to cope with these harsher environmental conditions. Beyond the existing edge of the species' range these conditions are so severe that no individual can survive and reproduce and so a population cannot be maintained. 2. Intrapopulation variability - individual variation would be lower within peripheral than central populations because the harsher selective pressures would weed out variants that could not survive in the peripheral populations but could flourish in the more benign environment occupied by the central populations. 3. Interpopulation variability - the level of variation between geographic isolates should be much greater than between central populations. Since geographic isolates are widely separated from each other (they occupy the periphery of the entire range), they are exposed to quite different environmental conditions and selective pressures which will cause them to diverge in different directions. Central populations, on the other hand, are largely contiguous and so are kept from diverging by gene flow and similarity in selective pressures. With these differences in mind, let's examine how a geographically isolated peripheral population might diverge to the point of becoming a new species. The development of a new genetic system to the extent that it results in a new species requires a two-step process. First of all, the genetic characteristics of the old species must be removed, and secondly a genetic revolution must build a different system. Break up of the old genetic system A number of factors contribute to the erosion of genetic variation in a geographic isolate. (1) Peripheral isolates are founded by emigrants from a central population. Since these founders are few in number, they cannot possibly contain in their genomes all of the genetic variation that exists in the central population from which they migrated. This founder effect, a special case of genetic drift, results in a new population with much less genetic variation than is found in other populations. (2) Because this new population is small, the traditional or conventional action of genetic drift will further erode whatever variability exists until homozygosity exists at all loci. This erosion is hastened by the action of natural selection for two reasons. Since this population is near the edge of the species range, the same pressures which operated on the central populations will be far more extreme, e.g., temperature and rainfall. Secondly, the newly colonized area might contain new selective pressures, e.g., a different predator or competitor species. Thus, (3) harsher abiotic pressures of the same kind and (4) new biotic pressures will reduce even further the level of genetic variation. The combined result of the founder effect, genetic drift and natural selection is homozygosity - a very precarious position most often attended by extinction. For this reason most peripheral populations become extinct, but a few may escape this fate and proceed to the next step: the build up of a different level of genetic variation called by Mayr the genetic revolution. The genetic revolution Genetic variation is introduced into a population either by mutation or migration. According to the model of peripatric speciation, migration is prevented by geographic isolation so that a new system of genetic variation can only occur through mutation. The fate of these mutations, however, will be decided by natural selection. Thus, the genetic revolution involves both mutation and natural selection. Because mutations occur at random, the geographically isolated population might receive mutations not present in the other populations of the species. Furthermore, the selective value of mutants already present in the central populations will differ because of the new genetic background that characterizes the geographical isolate. Note that the phenotypic expression of an allele is not a fixed absolute but varies due to epistatic interactions with alleles at other loci. Thus, new gene combinations (different alleles for different loci coming together due to mutation) will produce new phenotypes and recurrent mutations which are weeded out by selection in the central populations might be preserved in peripheral populations. Consequently, through the interaction of mutation and selection, a different system of allelic variation can evolve. Should the population live long enough in its homozygous state until a successful genetic revolution takes place, it will have passed from one system of genetic diversity to another through a bottleneck of homozygosity or reduced genetic variation. Mayr describes this bottleneck as wiping the genetic slate clean before a new genetic system can be built. Evolution vs Speciation A successful genetic revolution does not automatic- ally make the isolated population a new species. The genetic changes described so far only constitute evolution through the agency of migration (in the founder effect), selection and genetic drift in producing homozygosity and mutation and selection in the genetic revolution producing a new system of allelic variation after the bottleneck of homozygosity has been passed. Should this new population become phenotypically distinct from all others through this process, it might be classified as a subspecies. Even so, if it expanded its range after evolving adaptations to this new environment and came in contact with other populations of its species, it could still exchange genes. A peripheral isolate will become a new species only if the genetic revolution produces a RIM. Thus, THE PROCESS OF SPECIATION REQUIRES BOTH EVOLUTION AND THE ACQUISITION OF A REPRODUCTIVE ISOLATING MECHANISM. Evolution without a RIM will only produce interpopulational variation. But, the origin of a RIM requires some degree of evolution. Why? The reason is that RIMs are intrinsic (genetically based) barriers to reproduction and cannot arise unless the gene pool has changed at least sufficiently to produce a RIM. Origin of Reproductive Isolation Because a peripheral isolate is physically separated from the rest of the species, there is no gene flow and no reason why selection should operate to reduce gene flow. Consequently, selection will not produce an isolating mechanism directly for the purpose of preventing reproduction with members of the central populations. But, selection can produce an isolating mechanism indirectly as a by-product of adaptation to the environment in which the geographical isolate finds itself. Selection for gene complexes which adapt individuals to the physical and biotic components of the environment inhabited by the peripheral population may result in postmating mechanisms if the new gene complexes differ sufficiently from those found in the central populations. Premating mechanisms may be developed as a by-product of selection for reproductive success in the new environment. Consider the following examples. 1. Successful reproduction in aquatic species may be tied to the water temperature in which the eggs develop. The peripheral population may be exposed to a different temperature regime than that found in the central populations. Consequently, selection can shift the optimum temperature for development in the peripheral population. Should individuals from this population invade the range of the central populations, they will have to breed at a different time of the year so that their eggs will develop at the same temperature to which they were adapted in the peripheral population. Thus, by adapting individuals to a new temperature regime in the peripheral locality, selection has incidentally produced a form of seasonal isolation and therefore a new species. 2. Suppose a number of flies are carried by the wind during a storm from a mainland locality to an island. The island is more exposed to severe winds than the sheltered mainland habitat and this difference in wind velocity can act as a selective pressure to modify courtship pattern in these flies. Species-specific recognition cues in flies often involve the time the males spend hovering to attract females. Variation exists both in hovering time among the males and in response to hovering time by females. In this new environment long hovering times will be selected against because the longer the male spends hovering, the greater is the risk of being swept out to sea by the wind. The most successful reproducers will be males with short hovering times and females who respond only to short hovering time. Over the span of several generations the entire population will be characterized by a courtship pattern so different from that of the mainland populations that it constitutes a premating isolating mechanism. When this occurs, the island population becomes a new species. Note that in our discussion of speciation we included a role for each factor important in microevolution: (1) migration, (2) mutation, (3) genetic drift, and (4) natural selection, and showed how these interacted to produce new species. Be sure you understand how each fits into the overall process! Only a small minority of the peripheral populations which achieve the genetic revolution will actually acquire species status. But as rare as this process of speciation is, it is the most fundamental step in macroevolution. Once a new, closed (by reproductive isolation) gene pool arises, it embarks on its own evolutionary path without any influence from its former parental species and can diverge through phyletic evolution and diversify by speciating itself. It becomes a new member of phylogeny to play the game of macroevolution by taking a chance at extending evolutionary history and beating the odds which favor extinction - the subject of our next discussion.