Isolating mechanisms in evolution

Isolating mechanisms (reproductive isolating mechanisms) prevent interbreeding between species. The term, which was first used by Theodosius Dobzhansky in 1937 in his landmark book Genetics and the Origin of Species, refers to mechanisms that are genetically influenced and intrinsic. Geographic isolation can prevent interbreeding between populations, but it is an extrinsic factor and, therefore, does not qualify as an isolating mechanism. Isolating mechanisms function only between sexually reproducing species. They have no applicability to forms that reproduce only by asexual means, such as mitotic fission, stoloniferous or vegetative reproduction, or egg development without fertilization (parthenogenesis in animals). Obligatory self-fertilization in hermaphrodites (rare in animals) is a distortion of the sexual process that produces essentially the same results as asexual reproduction. Many lower animals and protists regularly employ both asexual and sexual means of reproduction, and the significance of isolating mechanisms in such forms is essentially the same as in normal sexual species.

Premating Mechanisms

Reproductive isolating mechanisms are usually classified into two main groups. Premating (prezygotic) mechanisms operate prior to mating or the release of gametes and, therefore, do not result in a wastage of the reproductive potential of the individual. Postmating (postzygotic) mechanisms come into play after mating, or the release of gametes, and could result in a loss of the genetic contribution of the individual to the next generation. This distinction is also important in the theoretical sense in that natural selection should favor genes that promote premating isolation. Those that do not presumably would be lost more often through mismatings (assuming that hybrids are not produced or are sterile or inferior), and this could lead to a reinforcement of premating isolation.

Ethological (behavioral) isolation is the most important category of premating isolation in animals. The selection of a mate and the mating process depend on the response of both partners to various sensory cues, any one of which may be species-specific. Although one kind of sensory stimulus may be emphasized, different cues may come into play at different stages of the pairing process. Visual signals provided by color, pattern, or method of display are often of particular importance in diurnal animals such as birds, many lizards, certain spiders, and fish. Sounds, like male mating calls, are often important in nocturnal breeders such as crickets or frogs but are also important in birds. Mate discrimination based on chemical signals or odors (pheromones) is of fundamental importance in many animals, especially those where visual cues or sound are not emphasized; chemical cues are also often important in aquatic animals with external fertilization. Tactile stimuli (touch) often play an important role in courtship once contact is established between the sexes. Even electrical signals appear to be utilized in some electrogenic fish.

Ecological (habitat) isolation often plays an important role. Different forms may be adapted to different habitats in the same general area and may meet only infrequently at the time of reproduction. One species of deer mouse, for example, may frequent woods, while another is found in old fields; one fish species spawns in riffles, while another spawns in still pools. This type of isolation, although frequent and widespread, is often incomplete as the different forms may come together in transitional habitats. The importance of ecological isolation, however, is attested by the fact that instances in which hybrid swarms are produced between forms that normally remain distinct have often been found to be the result of disruption of the environment, usually by humans. Mechanical isolation is a less important type of premating isolation, but it can function in some combinations. Two related animal species, for example, may be mismatched because of differences in size, proportions, or structure of genitalia.

Finally, temporal differences often contribute to premating isolation. The commonest type of temporal isolation is seasonal isolation: Species may reproduce at different times of the year. A species of toad in the eastern United States, for example, breeds in the early spring, while a related species breeds in the late spring, with only a short period of overlap. Differences can also involve the time of day, whereby one species may mate at night and another during the day. Such differences, as in the case of ecological isolation, are often incomplete but may be an important component of premating isolation.

Postmating Mechanisms

If premating mechanisms fail, postmating mechanisms can come into play. If gametes are released, fertilization may still fail (intersterility). Spermatozoa may fail to penetrate the egg, or even with penetration, there may be no fusion of the egg and sperm nucleus. Fertilization failure is almost universal between remotely related species (as from different families or above) and occasionally occurs even between closely related forms.

If fertilization does take place, other postmating mechanisms may operate. The hybrid may be inviable (F1 or zygotic inviability). Embryonic development may be abnormal, and the embryo may die at some stage, or the offspring may be defective. In other cases, development may be essentially normal, but the hybrid may be ill-adapted to survive in any available habitat or cannot compete for a mate (hybrid adaptive inferiority). Even if hybrids are produced, they may be partially to totally sterile (hybrid sterility). Hybrids between closely related forms are more likely to be fertile than those between more distantly related species, but the correlation is an inexact one. The causes for hybrid sterility are complex and can involve genetic factors, differences in gene arrangements on the chromosomes that disrupt normal chromosomal pairing and segregation at meiosis, and incompatibilities between cytoplasmic factors and the chromosomes. If the hybrids are fertile and interbreed or backcross to one of the parental forms, a more subtle phenomenon known as hybrid breakdown sometimes occurs. It takes the form of reduced fertility or reduced viability in the offspring. The basis for hybrid breakdown is poorly understood but may result from an imbalance of gene complexes contributed by the two species.

It should be emphasized that in most cases of reproductive isolation that have been carefully studied, more than one kind of isolating mechanism has been found to be present. Even though one type is clearly of paramount importance, it is usually supplemented by others, and should it fail, others may come into play. In this sense, reproductive isolation can be viewed as a fail-safe system. A striking difference in the overall pattern of reproductive isolation between animals and plants, however, is the much greater importance of premating isolation in animals and the emphasis on postmating mechanisms in plants. Ethological isolation, taken together with other premating mechanisms, is highly effective in animals, and postmating factors usually function only as a last resort.

Field Studies and Experimental Studies

Field studies have often been employed in the investigation of some types of premating isolating mechanisms. Differences in such things as breeding times, factors associated with the onset of breeding activity, and differences in habitat distribution or selection of a breeding site are all subject to direct field observation. Comparative studies of courtship behavior in the field or laboratory often provide clues as to the types of sensory signals that may be important in the separation of related species.

Mating discrimination experiments in the laboratory have often been employed to provide more precise information on the role played by different odors, colors, or patterns, courtship rituals, or sounds in mate selection. Certain pheromones, for example, which act as sexual attractants, have been shown to be highly species-specific in some insects. The presence or absence of certain colors or their presentation has been shown experimentally to be important in mate discrimination in vertebrates as diverse as fish, lizards, and birds. Call discrimination experiments, in which a receptive female is given a choice between recorded calls of males of her own and another species, have demonstrated the critical importance of mating call differences in reproductive isolation in frogs and toads. Synthetically generated calls have sometimes been used to pinpoint the precise call component responsible for the difference in response.

Studies on postmating isolating mechanisms have most often involved laboratory crosses in which the degree of intersterility, hybrid sterility, or hybrid inviability can be analyzed under controlled conditions. In instances in which artificial crosses are not feasible, natural hybrids sometimes occur and can be tested. The identification of natural backcross products can attest to incomplete postmating, as well as premating isolation. Instances of extensive natural hybridization are of special interest and have often been subjected to particularly close scrutiny. Such cases often throw light on factors that can lead to a breakdown of reproductive isolation. Also, as natural hybridization more often occurs between marginally differentiated forms in earlier stages of speciation, new insights into the process of species formation can sometimes be obtained. Finally, such studies may yield information on the evolutionary role of hybridization, including introgressive hybridization and the leakage of genes from one species into another. Morphological analysis has long been used in such cases, and chromosomal studies are sometimes appropriate. In recent years, allozyme analysis by gel electrophoresis has become a routine tool in estimates of gene exchange and molecular analysis of nuclear deoxyribonucleic acid (nDNA), or mitochondrial DNA (mtDNA), have been useful. As mitochondria are normally passed on only maternally, their DNA can also be used to identify cases in which females of only one of the two species has been involved in the breakdown of reproductive isolation.

Investigations of the role of natural selection in the development and reinforcement of reproductive isolation have employed two different approaches. One involves measuring geographic variation in the degree of difference in some signal character (call, color, or pattern, for example) thought to function in premating isolation between two species that have overlapping ranges. If the difference is consistently greater within the zone of overlap (reproductive character displacement), an argument can be made for the operation of reinforcement. Another approach involves laboratory simulations, usually with the fruit fly Drosophila, in which some type of selective pressure is exerted against offspring produced by crosses between different stocks, and measurement is made of the frequency of mismatings through successive generations. The results of such studies to this time are contradictory, and the role of selection regarding the development of reproductive isolation requires further study.

Geographic isolation may lead to the development of unique and distinct genetic and physical traits in species, forming a new subspecies. Among the most famous examples is Darwin's finches, or the Galápagos Island finches, which are speculated to have arrived in the Galápagos archipelago from hundreds of miles away in Central or South America due to wind, ocean circulation, or migration. Each of the eighteen species adapted to have unique traits as they adapted to their surroundings. Most notably, their beaks changed in shape and size to accommodate their new food sources. On Islands where the birds relied on seeds, their beaks adapted to better crack and eat seeds, while those on islands where they relied on nectar adapted to have longer, more narrow beaks. This adaptive radiation eventually led to the birds becoming distinct species.

Enhancing Reproductive Efficiency

Reproduction efficiency in most animals is enhanced immeasurably by premating isolating mechanisms. Random testing of potential mates without regard to type is unacceptable for most species in terms of reproductive capacity, time, and energy resources. Premating isolation, in this sense, is a major factor in promoting species diversity in animal communities.

Both premating and postmating isolating mechanisms are also critical to the maintenance of species diversity in that they act to protect the genetic integrity of each form: A species cannot maintain its identity without barriers that prevent the free exchange of genes with other species. Furthermore, a species functions as the primary unit of adaptation. Every species in a community has its own unique combination of adaptive features that enable it to exploit the resources of its environment and to coexist with other species with a minimum of competition. The diversity of different species that can coexist in the same area depends upon the unique “niche” that each occupies. Adaptive features that determine that niche are based on the unique genetic constitution of each species, and this genetic constitution is protected through reproductive isolation.

Developing reproductive isolating mechanisms is critical to forming new species (speciation) and ultimately developing organic diversity. The most widely accepted, objective, and theoretically operational concept for a sexual species is Ernst Mayr's biological species concept (BSC), which has served as a basis for important evolutionary research. The BSC asserts that species are single-lineage communities of similar gene pools and genetic systems that are potentially capable of interbreeding but are reproductively isolated from other species. Therefore, the origin of new species depends on the development of reproductive isolating mechanisms between populations. However, some bacterial and eukaryotic populations do not fit this definition because they may exchange DNA across species through horizontal or lateral gene transfer. A major focus of evolutionary biology and systematics research has been, and continues to be, on the various factors that influence the development of reproductive isolating mechanisms.

Principal Terms

Allozyme: One of two or more forms of an enzyme determined by different alleles of the same gene; usually analyzed by gel electrophoresis

Chloroplast Deoxyribonucleic Acid (cpDNA): A circular DNA molecule found in chloroplasts; chloroplasts are cytoplasmic organelles of green plants and some protists that carry out photosynthesis

Fission: The division of an organism into two or more essentially identical organisms; an asexual process

Hermaphrodite: An individual with both male and female organs; functions as both male and female

Hybrid: A term that in the broad sense can apply to any offspring produced by parents that differ in one or more inheritable characteristics; used to denote offspring produced by a cross between different species

Mitochondrial Deoxyribonucleic Acid (mtDNA): A circular molecule of DNA found in the mitochondria; mitochondria are cytoplasmic organelles that function in oxidative respiration

Nuclear Ribosomal Deoxyribonucleic Acid (rDNA): Nuclear DNA that codes for the ribosomal DNAs; ribosomes are small cytoplasmic particles that function in protein synthesis

Bibliography

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"Understanding Evolution: Reproductive Isolation." Berkeley, evolution.berkeley.edu/evolution-101/speciation/reproductive-isolation. Accessed 5 July 2023.