Aquilegia formosa. Aquilegia pubescens. Due different pollinator preferences in flower color and morphology, cross-pollination between species of Aquilegia is relatively rare. Data from Hodges and Arnold Behavioral isolation occurs when individuals from different populations do not recognize each other as potential mates. For example, populations may differ in species-recognition mechanisms, where changes in a particular signal e.
Behavioral isolation is evident in many species of birds especially based on song , fishes especially based on coloration , and insects especially based on chemical signals. Heliconius butterflies are an example of behavioral isolation. Sympatric species are able to produce fertile offspring, but they typically do not because of assortative mating preferences. Males of the sympatric H.
Hence, the existence of species-specific signals and matching mating preferences can significantly reduce the probability of gene flow among closely related species. Heliconius cydno. Heliconius melpomene. Photo: Charles J. Males of both species have assortative mating preferences and almost exclusively court females of their own species. Data from Jiggins et al. Mechanical isolation occurs when individuals of two populations meet, recognize each other as potential mates, and attempt to copulate, but copulations are unsuccessful because of some mechanical incompatibility.
Mechanical incompatibilities can be caused by size-mismatches between populations, or because of genital incompatibilities that arise through genital coevolution. Coevolution of male and female genital traits can arise through a variety of mechanisms, including sexual conflict see Chapter As consequence, genitals in many animal group evolve and diversify rapidly, with stark differences even among closely related species.
Accordingly, heterospecific matings would not lead to successful sperm transfer and fertilization even when copulation is attempted. Mechanical isolation can also occur in plants, when closely related species have stamens and styles in different configurations. For example, black sage Salvia mellifera have stamens and style positioned along the upper lip of the flower; hence, pollen is transferred from flower to flower on the backs of small pollinating bees.
In contrast, the syntopic white sage S. This configuration primarily transfers pollen to the wings of larger carpenter and bumble bees. So even if the same pollinator visits flowers of the same species, successful transfer of pollen from one species to the style of the other species is unlikely. Another fascinating example of mechanical isolation comes from evolution experiments using feather lice.
These avian parasites must be small enough to fit between the feather barbs of their host to avoid being removed during preening. On the other hand, there is selection for larger body size due to the higher fecundity of larger individuals.
Since larger hosts also have a larger distance between the barbs of their feathers, there is a resulting correlation between host and parasite body size.
Scott Villa and his colleagues investigated the evolution of feather lice Columbicola columbae on domesticated pigeons Columba livia of different body size over a period of four years 60 louse generations. As expected, lice that lived on large hosts evolved a larger body size over this period of time.
Interestingly, this also led to the evolution of mechanical isolation between lice from large hosts and lice from small hosts. While males were still eager to mate with females of the other strain, size mismatched individuals were unable to copulate effectively with each other check out the videos of a mating of a size-matched pair , a male of the large strain trying to copulate with a female of the small strain , and a male of the small strain trying to copulate with a female of the large strain.
This resulted in reduced copulation time, a reduced probability of egg production, and a reduced number of offspring produced in interlineage matings Figure Hence, mechanical incompatibility has the capacity to reduce gene flow between populations adapted to different host sizes. Lice within the typical range of sexual size dimorphism exhibited longer copulation duration compared to matings between individuals with large size disparities. Data from Villa et al.
Lice within the typical range of sexual size dimorphism were more likely to produced eggs when kept in experimental vials. Lice within the typical range of sexual size dimorphism were more likely to produced offspring on a natural host. Post-mating, prezygotic isolation describes reproductive barriers that reduce the frequency or success of inter-population fertilization, if mating between members of different populations has occurred.
Post-mating prezygotic isolation mechanisms can be associated with sperm competition and cryptic female choice, or—most importantly—with a gametic incompatibilities. Gametic isolation occurs when male and female gametes cannot merge to form a zygote. Such gametic incompatibilities are frequently mediated by interactions of proteins on the surface of the egg and sperm.
Co-evolution of sperm and egg surface proteins can create a lock-and-key mechanism, just like co-evolution of male and female genitalia. Gametic isolation is particularly well-studied in aquatic broadcast spawners e. Mixing eggs and sperm from different populations clearly indicates high fertilization rates when eggs are encountering sperm from the same populations, irrespective of sperm concentration Figure Hence, chemical interaction between eggs and sperm can be critical in determining the degree of reproductive isolation between populations.
It is important to note that gametic isolation can also contribute to reproductive isolation between populations of species with other reproductive modes—including some with internal fertilization; however, mechanisms mediating gametic incompatibilities in those taxa are not so thoroughly investigated.
Data from Palumbi and Metz Post-zygotic isolation occurs when individuals from different populations mate successfully and their gametes form a zygote that starts to develop, but the resulting hybrids have reduced fitness compared to either of the parents. Reduced hybrid fitness can be caused by both intrinsic and extrinsic factors. Intrinsic hybrid incompatibilities are reductions in hybrid fitness that are independent of environmental conditions. Intrinsic barriers primarily arise due to epistatic effects, and are also known as genetic incompatibilities.
Genetic incompatibilities can be severe and cause hybrid inviability; in such cases, the development of hybrids can cease at early stages, long before such individuals are born. In other cases, genetic incompatibilities cause hybrid sterility e.
Genetic incompatibilities can arise through coevolution of different genes that contribute to the same protein complex or the sample physiological pathway. For example, most enzymes involved the mitochondrial respiratory chain are protein complexes composed of amino acid chains derived from both the nuclear and mitochondrial genomes, and different protein subunits need to work together to form a functional protein.
Evolutionary change in one subunit may consequently be accompanied by corresponding changes in other subunits. Breaking up such co-evolved gene complexes during hybridization, where an individual might inherit the mitochondria from one parent but nuclear subunits from another, can cause issues with protein complex assembly or function, ultimately leading to mitochondrial, cellular, and organismal dysfunctions.
These cases of genetic incompatibilities are also known as mito-nuclear incompatibilities. Another well-studied example of a genetic incompatibility occurs through the co-evolution of tumor and tumor-suppression genes in swordtail fishes of the genus Xiphophorus. Hybridization can disrupt the balance between tumor genes and their suppressors. Particularly, X.
Mismatches of tumor and tumor-suppressor genes in hybrids have clear consequences for hybrid phenotypes and fitness. For example, hybrids exhibit larger melanistic spots on their tails Figure In addition, the frequency of fish with a caudal spot deceases across age classes in hybrid populations, indicating that individuals with spots are selected against Figure This decrease is particularly evident in hybrid populations with a higher caudal spot frequency populations hybrid 2 and 3 in Figure Frequency of invasive melanocytes in Xiphophorus fish with different genotypes at a tumor suppressor locus on chromosome 5 and tumor locus on chromosome Only fish that are homozygous for the low activity tumor suppressor allele from X.
Data from Powell et al. Caudal spot frequencies in parental X. Caudal spots grow faster and larger in hybrids compared to X. The decrease in caudal spot frequency from juveiles to adults in two populations of Xiphophorus hybrids indicate an increased mortality of fish with a caudal spot. Extrinsic incompatibilities occur when hybrids face reduced fitness because of a mismatch between hybrid phenotype and their abiotic or biotic environment.
Such hybrids generally prove to be viable and fertile under laboratory conditions, but are ecologically inviable under natural conditions— either because they cannot tolerate the stressful environmental conditions that periodically arise in nature, or because they are competitively inferior to either parent in their respective niche. For example, some North American postglacial lakes harbor two species of stickleback Gasterosteus aculeatus : a smaller limnetic species that lives in the open water and primarily feeds on zooplankton and a larger benthic species that lives in the littoral zone and feeds on benthic macroinvertebrates Schluter Hybrids between the two species a viable and fertile.
Hence, ecological selection limits hybrid success in the natural context. Hybrids may also be behaviorally sterile. Behavioral sterility may arise by hybrids not recognizing members of either parental species as potential mates, or because they themselves are not recognized as potential mates. In other words, behavioral sterility in hybrids is a consequence mismatched signals involved in species recognition, mate choice, and associated mating preferences.
An example of reduced hybrid fitness caused by mate choice comes from the same Heliconius butterflies that we already discussed in the context of behavioral isolation. Heliconius melpomene females—and to a lesser degree those of H. Notably, however, females of both species also prefer to mate with hybrids over males from the opposite species, indicating that behavioral sterility is not complete.
Ecological selection against hybrids as evidenced in stickleback Gasterosteus aculeatus. Hybrids between benthic and limnetic stickleback have lower growth rates than either parent in their native habitat. Data from Schluter Sexual selection against hybrids as evidenced in Heliconius butterflies. Females of H. Data from Naisbit et al. It is important to note that different mechanisms of reproductive isolation are not mutually exclusive.
Between any population pair, many mechanisms may coincide such that total reproductive isolation is a composite of mechanisms that reduce the probability of mating between different populations, reduce the probability of fertilization should mating happen, and reduce the success of hybrids should fertilization happen.
More over, as we will see below, the evolution of one reproductive isolation mechanism can profoundly impact the evolution of other reproductive barriers. Hence, it many natural systems it is critical not to just examine different mechanisms in isolation, but to also consider how different mechanisms interact to keep gene pools separated from one another.
Now that we have examined what mechanisms can cause reproductive isolation between populations, we can ask how those isolating barriers actually arise. Understanding how different evolutionary forces impact the evolution of reproductive isolation ultimately sheds light on the speciation process.
We will consider three distinct scenarios: 1 Polyploidization and the evolution of instantaneous reproductive isolation, 2 allopatric speciation that includes periods of geographical separation between populations, and 3 ecological speciation, where reproductive isolation between populations can arise as a byproduct of adaptation even in the presence of ongoing gene flow. Polyploidization is a form of mutation that leads to the duplication of entire genomes, typically due to meiotic errors leading to unreduced games.
Fertilization of unreduced gametes with other unreduced gametes causes the formation of fertile polyploid lineages i. Polyploid lineages are immediately isolated from their ancestral lineage because of dysfunctional chromosome complements in crosses between individuals of different ploidy. Such auto-polyploid speciation is particularly common in plants and other organisms for which self-fertilization is possible, because the combination of two unreduced gametes is a strong limiting factor in the formation of polyploids.
Polyploid speciation can also occur through the combination of two genomes derived from different species, a process called allo-polyploid speciation Figure In this case, an unreduced gamete of one species is fertilized with a normal gamete from another, creating a hybrid with an uneven number of chromosomes.
When unreduced gametes from that hybrid are then fertilized by a normal gamete from the second species, a fertile hybrid with the full genome set of each parent arises. The common bread wheat Triticum aestivum , for example, is an allo-hexaploid that contains three distinct sets of chromosomes from three different species of grass in the genus Aegilops.
Similarly, various vegetables in the cabbage family mustard, collard greens, cauliflower, kale, bok choi, rape seed, etc. In contrast, allo-polyploids are rare among animals, but some cases have been documented in insects, fish, amphibians, and reptiles. Auto-polyploidization occurs through the combination of two unreduced gametes, which usually occirs through self-fertilization.
The resulting ployploids are instantaneously isolated from ancestral lineages with lower ploidy. Blackberries of the genus Rubus represent a polyploid species complex, including species with 2, 4, 8, 16, and 32 sets of chromosomes.
Without genetic or cytological analyses, species with different ploidy levels are almost impossible to distinguish. Allo-polyploidization oocurs through the combination of two genomes derived from different species. It is a multi-step process that first involves the production of hybrids with uneven chromosome sets. The allopatric speciation model was first introduced by Ernst Mayr in his seminal book Systematics and the Origin of Species. This model postulates that the speciation process unfolds in three distinct steps: 1 The subdivision of an initial population through a geographic barrier, 2 population divergence in isolation, and 3 secondary contract of incipient species.
In the following sections, we will examine these three steps in detail as well as pertinent examples. The initial stage of geographic isolation is central to the allopatric speciation model. Geographic isolation drives reproductive isolation and limits the spread of alleles within the separate populations. Two mechanisms can create geographically isolated populations from an initial ancestor: dispersal and vicariance.
Dispersal occurs when individuals of an original population overcome a geographic barrier and colonize a new and previously unoccupied habitat Figure Classic examples of dispersal occur during the colonization of new oceanic islands. Drosophila fruitflies that colonized the archipelago early during its formation provide a formidable example of dispersal.
For two independent clades within Drosophila , the sequence of phylogenetic divergence corresponds to the sequence in which different islands arose from the ocean. In the D. This pattern is consistent with the idea that species diversified through step-wise colonization of new habitats as they became available. Similar processes also gave rise to the diversity we observe in other oceanic archipelagos, like the Galapagos. It is important to emphasize that dispersal across geographic barriers and colonization of novel habitats is important in other ecological contexts as well.
For example, lakes are the terrestrial equivalent of oceanic islands. Species may also overcome mountain ranges that separate suitable habitats, or valleys that separate mountain tops. Furthermore, island-like situations also occur in the apparently well-connected habitats within oceans. For example, patches of coral reefs or deep-sea hydrothermal vents are separated by vast stretches of unsuitable habitats, which migrants must overcome.
Dispersal is the colonization of a new habitat across a geographic barrier. Patterns of diversification in Drosophila fruitflies are consistent with dispersal. In two clades green and orange , the sequence of species divergence corresponds to colonization of younger islands in the archipelago adopted from Obbard et al.
The inset photo shows D. Vicariance occurs when the distributional range of a species is subdivided by an emerging geographic barrier Figure For example, islands—or even entire continents—may be divided as a consequence of the rising sea levels associated with climate change; as water levels flood low laying valleys, regions with higher elevations become separated.
Similarly, volcanic activity and plate tectonics can shift land masses and create novel barriers of unsuitable habitat across which movement of organisms becomes limited.
For example, the ancestor of ratites, which includes a diverse group of flightless birds, had a Gondwanian distribution. Other classic examples of vicariance include paired species of crustaceans, fish, and other marine animals that occur in either side of the Central American land bridge i.
Vicariance is the subsetting of an original population by an emerging geographic barrier B. Patterns of diversification in ratites are a consequence of vicariance. The ancestral, contiguous range of this group on Gondwana was broken apart as a consequence of continental drift.
The second step of the allopatric speciation model is population divergence in isolation. Such population divergence was long thought to be driven primarily by mutation and genetic drift; different sets of mutations arise and drift to fixation in each of the isolated populations, ultimately causing them to diverge.
However, it is now well-recognized that natural and sexual selection also play a critical role in driving the differentiation of isolated populations.
Differences in abiotic and biotic environmental conditions can drive local adaptation and modulate patterns of selection, potentially causing substantial phenotypic and genetic differences between populations. Thus, divergent selection can accelerate population divergence, though it is not requisite for allopatric speciation to occur. The fate of populations that diverged in isolation is determined in the third stage of the allopatric speciation model. When populations come into secondary contact after the initial period of isolation, there is an entire range of potential outcomes.
Divergent populations are often referred to as incipient species at this stage. If divergence between incipient species was relatively minor and does not affect their ability to interbreed, secondary contact may simply cease the speciation process. In this case, the two lineages hybridize and fuse back together into a single panmictic population.
Alternatively, at least some degree of reproductive isolation may have accrued between incipient lineages. Consistent with the idea that drift was critical in driving population divergence, genetic incompatibilities were long thought to be a primary mechanism of reproductive isolation. In particular, secondary contact and hybridization between incipient species brings together alleles that previously drifted to fixation in the isolated lineages, and some of the new allelic combinations can be deleterious due to negative epistatic interactions Figure Such genetic incompatibilities that reduce hybrid fitness upon secondary contact are also known as Dozhansky-Muller incompatibilities.
There are two more important points to make here: 1 Dobzhansky-Muller incompatibilities do not only arise as a consequence of population differences that evolved through genetic drift.
Alternate alleles that rise to high frequencies in different populations as a consequence of selection can also be subject to negative epistatic interactions and cause reductions in hybrid fitness. So while the Dobzhansky-Muller model was primarily developed under the assumption that mutation and genetic drift are the primary drivers of population divergence, the model also applies when other evolutionary forces contribute to population differentiation.
Evolution in isolation may have inadvertent consequences on ecological, behavioral, and morphological traits, which then affect the probability of mating and successful fertilization between incipient species. In addition, though there may be no genetic incompatibilities that limit hybrid success, hybrids may still be selected against by natural or sexual selection.
Specifically, new sets of mutations arise in geographic isolation and eventually drift to fixation. Upon secondary contact, recombination of these new alleles in hybrids can be subject to negative epistatic interactions. The novel alleles a and b have no evolutionary history of interacting, and if the alleles are incompatible, hybrid fitness is reduced.
Irrespective of what mechanisms mediate reproductive isolation between incipient species, reproductive isolation is rarely complete upon secondary contract. This is evidenced hybridization being an exceedingly common phenomenon in nature. Hybridization, in these cases, is not evidence that incipient species are not species, but rather that the speciation process is still ongoing and has yet to be completed.
Ultimately, there are three alternative outcomes upon secondary contact, besides complete fusion of the the incipient species: completion of the speciation process through reinforcement, stable hybrid zones of contact, and speciation of hybrids.
Reductions of hybrid fitness upon secondary contract—whether cause by genetic incompatibilities or by natural or sexual selection against hybrids—should exert direct selection on associative mating preferences in both incipient species.
Individuals that exhibit mating preferences for potential partners from their own species and discriminate against heterospecifics ultimately have higher fitness, because they avoid producing unfit hybrid offspring.
The process of natural selection increasing the degree of reproductive isolation between incipient species is known as reinforcement. In practice, reinforcement often leads to reproductive character displacement, where differences in reproductive traits—both in terms of signals and the corresponding preferences—are accentuated whenever closely related species coexist.
A particular trait may be similar between two species in allopatry, but starkly differentiated in sympatry Figure Veuillez activer JavaScript. Por favor, active JavaScript. Bitte aktivieren Sie JavaScript. Si prega di abilitare JavaScript. Evolution of reproductive barriers and its implications for adaptive speciation. Results in Brief. Scientific advances Study reveals how cannabis-based drugs harm the brain. Scientific advances Milestone reached in geothermal deep drilling project.
Share this page. Can species be formed in ways other than geographic isolation? Evolution and Its Many Forms Today we continue a three-lecture sequence on biological, or organic, evolution.
Evolution is a unifying theme of this course, and the concept of evolution is relevant to many of our topics. The word "evolution" does not apply exclusively to biological evolution. The universe and our solar system have developed out of the explosion of matter that began our known universe. Chemical elements have evolved from simpler matter. Life has evolved from non-life, and complex organisms from simpler forms. Languages, religions, and political systems all evolve.
Hence, evolution is an appropriate theme for a course on global change. The core aspects of evolution are "change" and the role of history, in that past events have an influence over what changes occur subsequently. In biological evolution this might mean that complex organisms arise out of simpler ancestors - though be aware that this is an over-simplification not acceptable to a more advanced discussion of evolution. A full discussion of evolution requires a detailed explanation of genetics, because science has given us a good understanding of the genetic basis of evolution.
It also requires an investigation of the differences that characterize species, genera, indeed the entire tree of life, because these are the phenomena that the theory of evolution seeks to explain. We will begin with observed patterns of similarities and differences among species, because this is what Darwin knew about.
The genetic basis for evolution only began to be integrated into evolutionary theory in the 's and 's. We will add genetics into our understanding of evolution through a discussion activity. Definitions of Biological Evolution We begin with two working definitions of biological evolution, which capture these two facets of genetics and differences among life forms.
Then we will ask what is a species, and how does a species arise? Definition 1: Changes in the genetic composition of a population with the passage of each generation Definition 2: The gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity Note that the first definition emphasizes genetic change.
It commonly is referred to as microevolution. The second definition emphasizes the appearance of new, physically distinct life forms that can be grouped with similar appearing life forms in a taxonomic hierarchy. It commonly is referred to as macroevolution. A full explanation of evolution requires that we link these two levels. Can small, gradual change produce distinct species? How does it occur, and how do we decide when species are species? Hopefully you will see the connections by the end of these three lectures.
Today we will discuss how species are formed. But to do this, we need to define what we are talking about. What is a Species?
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