The Origin of Species

THE ORIGIN OF SPECIES Introduction Darwin recognized that the young Galapagos Islands were a place for the genesis of new species. The central fact that crystallized this view was the many plants and animals that existed nowhere else. Evolutionary theory must also explain macroevolution, the origin of new taxonomic groups (new species, new genera, new families, new kingdoms). Speciation is the keystone process in the origination of diversity of higher taxon. The fossil record chronicles two patterns of speciation: anagenesis and cladogenesis. Anagenesis is the accumulation of changes associated with the transformation of one species into another. Cladogenesis, branching evolution, is the budding of one or more new species from a parent species. Cladogenesis promotes biological diversity by increasing the number of species. A. What Is a Species? Species is a Latin word meaning “kind” or “appearance.” Traditionally, morphological differences have been used to distinguish species. Today, differences in body function, biochemistry, behavior, and genetic makeup are also used to differentiate species. 1. The biological species concept emphasizes reproductive isolation In 1942 Ernst Mayr enunciated the biological species concept to address biological diversity. A species is a population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species. A biological species is the largest set of populations in which genetic exchange is possible and that is genetically isolated from other populations. Species are based on interfertility, not physical similarity. For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species. In contrast, humans have considerable diversity, but we all belong to the same species because of our capacity to interbreed. 2. Prezygotic and postzygotic barriers isolate the gene pools of biological species No single barrier may be completely impenetrable to genetic exchange, but many species are genetically sequestered by multiple barriers. Typically, these barriers are intrinsic to the organisms, not simple geographic separation. Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges overlap. Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes. Prezygotic barriers impede mating between species or hinder fertilization of ova if members of different species attempt to mate. These barriers include habitat isolation, behavioral isolation, temporal isolation, mechanical isolation, and gametic isolation. Habitat isolation. Two organisms that use different habitats even in the same geographic area are unlikely to encounter each other to even attempt mating. This is exemplified by the two species of garter snakes, in the genus Thamnophis, that occur in the same areas but because one lives mainly in water and the other is primarily terrestrial, they rarely encounter each other. Behavioral isolation. Many species use elaborate behaviors unique to a species to attract mates. For example, female fireflies only flash back and attract males who first signaled to them with a species-specific rhythm of light signals. In many species, elaborate courtship displays identify potential mates of the correct species and synchronize gonadal maturation. Temporal isolation. Two species that breed during different times of day, different seasons, or different years cannot mix gametes. For example, while the geographic ranges of the western spotted skunk and the eastern spotted skunk overlap, they do not interbreed because the former mates in late summer and the latter in late winter. Mechanical isolation. Closely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible. To illustrate, mechanical barriers contribute to the reproductive isolation of flowering plants that are pollinated by insects or other animals. With many insects the male and female copulatory organs of closely related species do not fit together, preventing sperm transfer. Gametic isolation occurs when gametes of two species do not form a zygote because of incompatibilities preventing fusion or other mechanisms. In species with internal fertilization, the environment of the female reproductive tract may not be conducive to the survival of sperm from other species. For species with external fertilization, gamete recognition may rely on the presence of specific molecules on the egg’s coat, which adhere only to specific molecules on sperm cells of the same species. A similar molecular recognition mechanism enables a flower to discriminate between pollen of the same species and pollen of a different species. If a sperm from one species does fertilize the ovum of another, postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult. These barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown. Reduced hybrid viability. Genetic incompatibility between the two species may abort the development of the hybrid at some embryonic stage or produce frail offspring. This is true for the occasional hybrids between frogs in the genus Rana, which do not complete development and those that do are frail. Reduced hybrid fertility. Even if the hybrid offspring are vigorous, the hybrids may be infertile and the hybrid cannot backbreed with either parental species. This infertility may be due to problems in meiosis because of differences in chromosome number or structure. For example, while a mule, the hybrid product of mating between a horse and donkey, is a robust organism, it cannot mate (except very rarely) with either horses or donkeys. Hybrid breakdown. In some cases, first generation hybrids are viable and fertile. However, when they mate with either parent species or with each other, the next generation is feeble or sterile. To illustrate this, we know that different cotton species can produce fertile hybrids, but breakdown occurs in the next generation when offspring of hybrids die as seeds or grow into weak and defective plants. Reproductive barriers can occur before mating, between mating and fertilization, or after fertilization. 3. The biological species concept has some major limitations While the biological species concept has had important impacts on evolutionary theory, it is limited when applied to species in nature. For example, one cannot test the reproductive isolation of morphologically similar fossils, which are separated into species based on morphology. Even for living species, we often lack the information on interbreeding to apply the biological species concept. In addition, many species (e.g., bacteria) reproduce entirely asexually and are assigned to species based mainly on structural and biochemical characteristics. 4. Evolutionary biologists have proposed several alternative concepts of species Several alternative species concepts emphasize the processes that unite the members of a species. The ecological species concept defines a species in terms of its ecological niche, the set of environmental resources that a species uses and its role in a biological community. As an example, a species that is a parasite may be defined in part by its adaptations to a specific organism. The pluralistic species concept may invoke reproductive isolation or adaptation to an ecological niche, or use both in maintaining distinctive, cohesive groups of individuals. The biological, ecological, and pluralistic species concepts are all “explanatory” concepts—attempts to explain the very existence of a species as discrete units in the diversity of life. The morphological species concept, the oldest and still most practical, defines a species by a unique set of structural features. A more recent proposal, the genealogical species concept, defines a species as a set of organisms with a unique genetic history—one tip of the branching tree of life. The sequences of nucleic acids and proteins provide data that are used to define species by unique genetic markers. Each species has its utility, depending on the situation and the types of questions that we are asking. B. Modes of Speciation Two general modes of speciation are distinguished by the way gene flow among populations is initially interrupted. In allopatric speciation, geographic separation of populations restricts gene flow. In sympatric speciation, speciation occurs in geographically overlapping populations when biological factors, such as chromosomal changes and nonrandom mating, reduce gene flow. 1. Allopatric speciation: Geographic barriers can lead to the origin of species Several geological processes can fragment a population into two or more isolated populations. Mountain ranges, glaciers, land bridges, or splintering of lakes may divide one population into isolated groups. Alternatively, some individuals may colonize a new, geographically remote area and become isolated from the parent population. For example, mainland organisms that colonized the Galapagos Islands were isolated from mainland populations. How significant a barrier must be to limit gene exchange depends on the ability of organisms to move about. A geological feature that is only a minor hindrance to one species may be an impassible barrier to another. The valley of the Grand Canyon is a significant barrier for ground squirrels that have speciated on opposite sides, but birds that can move freely have no barrier. The likelihood of allopatric speciation increases when a population is both small and isolated. A small, isolated population is more likely to have its gene pool changed substantially by genetic drift and natural selection. For example, less than 2 million years ago, small populations of stray plants and animals from the South American mainland colonized the Galapagos Islands and gave rise to the species that now inhabit the islands. However, very few small, isolated populations will develop into new species; most will simply perish in their new environment. A question about allopatric speciation is whether the separated populations have become different enough that they can no longer interbreed and produce fertile offspring when they come back in contact. Ring species provide examples of what seem to be various stages in the gradual divergence of new species from common ancestors. In ring species, populations are distributed around some geographic barrier, with populations that have diverged the most in their evolution meeting where the ring closes. Some populations are capable of interbreeding, others cannot. One example of a ring species is the salamander, Ensatina escholtzii, which probably expanded south from Oregon to California, USA. The California pioneers split into one chain of interbreeding populations along the coastal mountains and another along the inland mountains (Sierra Nevada range). They form a ring around California’s Central Valley. Salamanders of the different populations contrast in coloration and exhibit more and more genetic differences the farther south the comparison is made. At the northern end of the ring, the coastal and inland populations interbreed and produce viable offspring. In this area they appear to be a single biological species. At the southern end of the ring, the coastal and inland populations do not interbreed even when they overlap. In this area they appear to be two separate species. Flurries of allopatric speciation occur on island chains where organisms that were dispersed from parent populations have founded new populations in isolation. Organisms may be carried to these new habitats by their own locomotion, through the movements of other organisms, or through physical forces such as ocean currents or winds. In many cases, individuals of one island species may reach neighboring islands, permitting other speciation episodes. For example: a single dispersal event may have carried a small population of mainland finches to one Galapagos Island. Later, individuals may have reached neighboring islands, where geographic isolation permitted additional speciation episodes. The evolution of many diversely adapted species from a common ancestor is called an adaptive radiation. The Hawaiian Archipelago, 3,500 miles from the nearest continent and composed of “young” volcanic islands, has experienced several examples of adaptive radiations by colonists. Individuals were carried by ocean currents and winds from distant continents and islands or older islands in the archipelago to colonize the very diverse habitats on each new island as it appeared. Multiple invasions and allopatric speciation have ignited an explosion of adaptive radiation, leading to thousands of species that live nowhere else. In contrast, the Florida Keys lack indigenous species because they are apparently too close to the mainland to isolate their gene pools from parent populations. While geographic isolation does prevent interbreeding between allopatric populations, it does not by itself constitute reproductive isolation. True reproductive barriers are intrinsic to the species and prevent interbreeding, even in the absence of geographic isolation. Also, speciation is not due to a drive to erect reproductive barriers, but results from natural selection and genetic drift as the allopatric populations evolve separately. Diane Dodd has demonstrated the ability of prezygotic reproductive barriers to develop as a byproduct of adaptive divergence by allopatric populations in fruitflies, Drosophila pseudoobscura. She divided a sample of fruit flies into several laboratory populations that were cultured for several generations on media containing starch or containing maltose. Through natural selection acting over several generations, the population raised on starch improved their efficiency at starch digestion, while the “maltose” populations improved their efficiency at malt sugar digestion. Females from populations raised on a starch medium preferred males from a similar nurturing environment over males raised in a maltose medium after several generations of isolation, demonstrating a prezygotic barrier to interbreeding. Similarly, the ability of postzygotic reproductive barriers to develop in allopatric populations has been demonstrated by Robert Vickery in the monkey flower, Mimulus glabratus (a plant that has a very large range throughout the Americas). He cross-pollinated plants from different regions in his greenhouse and planted the hybrid seeds to see if they developed into fertile plants. When plants from different regions were crossed, the proportion of fertile offspring decreased as distance from the source populations increased. Some hybrids were almost sterile, demonstrating hybrid breakdown, a postzygotic barrier. In summary, in allopatric speciation, new species form when geographically isolated populations evolve reproductive barriers as a byproduct of genetic drift and natural selection to its new environment. These barriers prevent interbreeding even if populations come back into contact. These barriers include prezygotic barriers that reduce the likelihood of fertilization and postzygotic barriers that reduce the fitness of hybrids. 2. Sympatric speciation: A new species can originate in the geographic midst of the parent species In sympatric speciation, new species arise within the range of the parent populations. Here reproductive barriers must evolve between sympatric populations In plants, sympatric speciation can result from accidents during cell division that result in extra sets of chromosomes, a mutant condition known as polyploidy. In animals, it may result from gene-based shifts in habitat or mate preference. An individual can have more than two sets of chromosomes from a single species if a failure in meiosis results in a tetraploid (4n) individual. This autopolyploid mutant can reproduce with itself (self-pollination) or with other tetraploids. It cannot mate with diploids from the original population, because of abnormal meiosis by the triploid hybrids In the early 1900s, botanist Hugo de Vries produced a new primrose species, the tetraploid Oenotheria gigas, from the diploid Oenothera lamarckiana. This plant could not interbreed with the diploid species. Another mechanism of producing polyploid individuals occurs when individuals are produced by the matings of two different species, an allopolyploid. While the hybrids are usually sterile, they may be quite vigorous and propagate asexually. Various mechanisms can transform a sterile hybrid into a fertile polyploid. These polyploid hybrids are fertile with each other but cannot interbreed with either parent species. One mechanism for allopolyploid speciation in plants involves several cross-pollination events between two species of their offspring and perhaps a failure of meiotic disjunction to a viable fertile hybrid whose chromosome number is the sum of the chromosomes in the two parent species. The origin of polypoid species is common and rapid enough that scientists have documented several such speciations in historical times. For example, two new species of plants, called goatsbeard (Tragopodon), appeared in Idaho and Washington. They are the results of allopolyploidy events between pairs of introduced European Tragopodon species. Many plants important for agriculture are the products of polyploidy. For example, oats, cotton, potatoes, tobacco, and wheat are polyploid. Plant geneticists now hydridize plants and use chemicals that induce meiotic and mitotic errors to create new polyploids with special qualities. Example: artificial hybrids combine the high yield of wheat with the ability of rye to resist disease. While polyploid speciation does occur in animals, other mechanisms also contribute to sympatric speciation in animals. Sympatric speciation can result when genetic factors cause individuals to be fixed on resources not used by the parent. These may include genetic switches from one breeding habitat to another or those that produce different mate preferences. Sympatric speciation is one mechanism that has been proposed for the explosive adaptive radiation of almost 200 species of cichlid fishes in Lake Victoria, Africa. While these species are clearly specialized for exploiting different food resources and other resources, non-random mating in which females select males based on a certain appearance has probably contributed too. Individuals of two closely related sympatric cichlid species will not mate under normal light because females have specific color preferences and males differ in color. However, under light conditions that de-emphasize color differences, females will mate with males of the other species and this results in viable, fertile offspring. The lack of postzygotic barriers would indicate that speciation occurred relatively recently. Sympatric speciation requires the emergence of some reproductive barrier that isolates a subset of the population without geographic separation from the parent population. In plants, the most common mechanism is hybridization between species or errors in cell division that lead to polyploid individuals. In animals, sympatric speciation may occur when a subset of the population is reproductively isolated by a switch in resources or mating preferences. 3. The punctuated equilibrium model has stimulated research on the tempo of speciation Traditional evolutionary trees diagram the diversification of species as a gradual divergence over long spans of time. These trees assume that big changes occur as the accumulation of many small ones, the gradualism model. In the fossil record, many species appear as new forms rather suddenly (in geologic terms), persist essentially unchanged, and then disappear from the fossil record. Darwin noted this when he remarked that species appear to undergo modifications during relatively short periods of their total existence and then remained essentially unchanged. The apparently sudden appearance of species in the fossil record may reflect allopatric speciation. If a new species arose in allopatry and then extended its range into that of the ancestral species, it would appear in the fossil record as the sudden appearance of a new species in a locale where there are also fossils of the ancestral species. Whether the new species coexists with the ancestor or not, the new species will not appear until I has diverged in form during its period of geographic separation. In the punctuated equilibrium model, the tempo of speciation is not constant. Species undergo most morphological modifications when they first bud from their parent population. After establishing themselves as separate species, they remain static for the vast majority of their existence. Under this model, changes may occur both rapidly and gradually during the few thousands of generations necessary to establish a unique genetic identity. On a time scale that can generally be determined in fossil strata, the species will appear suddenly in rocks of a certain age. Stabilizing selection may then operate to maintain the species relatively the same for tens to hundreds of thousands of additional generations until it finally goes extinct. While the external morphology that is typically recorded in fossils may appear to remain unchanged for long periods, changes in behavior, physiology, or even internal anatomy may be changing during this interval. C. From Speciation To Macroevolution Speciation is at the boundary between microevolution and macroevolution. Microevolution is a change over the generations in a population’s allele frequencies, mainly by genetic drift and natural selection. Speciation occurs when a population’s genetic divergence from its ancestral population results in reproductive isolation. While the changes after any speciation event may be subtle, the cumulative change over millions of speciation episodes must account for macroevolution, the scale of changes seen in the fossil record. 1. Most evolutionary novelties are modified versions of older structures The Darwinian concept of “descent with modification” can account for the major morphological transformations of macroevolution. It may be difficult to believe that a complex organ like the human eye could be the product of gradual evolution, rather than a finished design created specially for humans. However, the key to remember is that that eyes do not need to be as complicated as the human eye to be useful to an animal. The simplest eyes are just clusters of photoreceptors, pigmented cells sensitive to light. Flatworms (Planaria) have a slightly more sophisticated structure with the photoreceptors cells in a cup-shaped indentation. This structure does not allow flatworms to focus an image, but it enables flatworms to distinguish light from dark. Flatworms move away from light, probably reducing their risk of predation. Complex eyes have evolved several times independently in the animal kingdom. Examples of various levels of complexity, from clusters of photoreceptors to camera-like eyes, can be seen in mollusks. The most complex types did not evolve in one quantum leap, but by incremental adaptation of organs that worked and benefited their owners at each stage in this macroevolution. The range of the eye complexity in mollusks includes: (a) a simple patch of photoreceptors found in some limpets, (b) photoreceptors in an eye-cup, (c) a pinhole-camera-type eye in Nautilus, (d) an eye with a primitive lens in some marine snails, and (e) a complex camera-type eye in squid. Evolutionary novelties can also arise by gradual refinement of existing structures for new functions. Structures that evolve in one context, but become co-opted for another function are exaptations. Natural selection can only improve a structure in the context of its current utility, not in anticipation of the future. An example of an exaptation is the changing function of lightweight, honeycombed bones of birds. The fossil record indicates that light bones predated flight. Therefore, they must have had some function on the ground, perhaps as a light frame for agile, bipedal dinosaurs. Once flight became an advantage, natural selection would have remodeled the skeleton to better fit their additional function. 2. “Evo-devo”: Genes that control development play a major role in evolution “Evo-devo” is a field of interdisciplinary research that examines how slight genetic divergences can become magnified into major morphological differences between species. A particular focus is on genes that program development by controlling the rate, timing, and spatial pattern of changes in form as an organism develops from a zygote to an adult. Allometric growth tracks how proportions of structures change due to different growth rates during development. Change the relative rates of growth even slightly, and you can change the adult form substantially. Different allometric patterns contribute to the contrast of adult skull shapes between humans and chimpanzees, which both developed from fairly similar fetal skulls. Evolution of morphology by modification of allometric growth is an example of heterochrony, an evolutionary change in the rate or timing of developmental events. Heterochrony appears to be responsible for differences in the feet of tree-dwelling versus ground-dwelling salamanders. The feet of the tree-dwellers with shorter digits and more webbing may have evolved from a mutation in the alleles that control the timing of foot development. These stunted feet may result if regulatory genes switched off foot growth early. Thus, a relatively small genetic change can be amplified into substantial morphological change. Another form of heterochrony is concerned with the relative timing of reproductive development and somatic development. If the rate of reproductive development accelerates compared to somatic development, then a sexually mature stage can retain juvenile structures - a process called paedomorphosis. This axolotl salamander has the typical external gills and flattened tail of an aquatic juvenile but has functioning gonads. Macroevolution can also result from changes in genes that control the placement and spatial organization of body parts. For example, genes called homeotic genes determine such basic features as where a pair of wings and a pair of legs will develop on a bird or how a plant’s flower parts are arranged. One class of homeotic genes, Hox genes, provides positional information in an animal embryo. Their information prompts cells to develop into structures appropriate for a particular location. One major transition in the evolution of vertebrates is the development of the walking legs of tetrapods from the fins of fishes. The fish fin which lacks external skeletal support evolved into the tetrapod limb that extends skeletal supports (digits) to the tip of the limb. This may be the result of changes in the positional information provided by Hox genes during limb development. Key events in the origin of vertebrates from invertebrates are associated with changes in Hox genes. While most invertebrates have a single Hox cluster, molecular evidence indicates that this cluster was duplicated about 520 million years ago in the lineage that produced vertebrates. The duplicate genes could then take on entirely new roles, such as directing the development of a backbone. A second duplication of the two Hox clusters about 425 million years ago may have allowed for even more structural complexity. 3. An evolutionary trend does not mean that evolution is goal oriented The fossil record seems to reveal trends in the evolution of many species and lineages. For example, the evolution of the modern horse can be interpreted to have been a steady series of changes from a small, browsing ancestor (Hyracotherium) with four toes to modern horses (Equus) with only one toe per foot and teeth modified for grazing on grasses. It is possible to arrange a succession of animals intermediate between Hyracotherium and modern horses that shows trends toward increased size, reduced number of toes, and modifications of teeth for grazing. If we look at all fossil horses, the illusion of coherent, progressive evolution leading directly to modern horses vanishes. Equus is the only surviving twig of an evolutionary bush that included several adaptive radiations among both grazers and browsers. Differences among species in survival can also produce a macroevolutionary trend. In the species selection model, developed by Steven Stanley, species are analogous to individuals. Speciation is their birth, extinction is their death, and new species are their offspring. The species that endure the longest and generate the greatest number of new species determine the direction of major evolutionary trends. To the extent that speciation rates and species longevity reflect success, the analogy to natural selection is even stronger. However, qualities unrelated to the overall success of organisms in specific environments may be equally important in species selection. As an example, the ability of a species to disperse to new locations may contribute to its giving rise to a large number of “daughter species.” The appearance of an evolutionary trend does not imply some intrinsic drive toward a preordained state of being. Evolution is a response to interactions between organisms and their current environments, leading to changes in evolutionary trends as conditions change.