The reason for the selection is not yet known. Based on the Dobzhansky-Muller model, hybrid incompatibility should arise from the interactions of at least two genes.
In nearly all cases, we do not yet know the identities of both interacting Dobzhansky-Muller genes. In some cases, researchers have mapped the interacting partners of those hybrid incompatibility genes that have been identified to small regions of the chromosome.
In , Cornell University researchers led by Daniel Barbash reported the first case of identification of a pair of interacting genes that result in incompatibility Brideau et al. The Hybrid male rescue Hmr gene has a sequence that strongly suggests that it is a transcription factor , a gene whose protein affects the expression of other genes. The Lethal hybrid rescue Lhr gene produces a protein that is associated with heterochromatin , a region of the chromosome that is made up of repetitive DNA.
Inviability results from the combination of the D. Like Ods , the sequence data from Lhr and Hmr also suggest that positive natural selection has driven the divergence between species of these genes Brideau et al. Identification of more such Dobzhansky-Muller pairs of genes in hybrid incompatibility is underway. Recently, evolutionary geneticists have examined hybrid incompatibility as a consequence of divergence of regulatory genetic networks.
Several of the hybrid incompatibility genes thus far characterized like Ods and Hmr have regulatory functions. Given that genes interact in regulatory genetic networks, these would seem natural places for Dobzhansky-Muller incompatibilities to arise. Microarray and other techniques used to characterize gene expression have uncovered several genes that are misregulated either overexpressed or, more frequently, underexpressed in hybrids between species Oritz-Barrientos et al.
Such studies may be useful in uncovering interacting genes involved in hybrid incompatibility. To date, most fine-scale genetic studies of hybrid incompatibility have been in Drosophila , and in particular, those species most closely related to D. In other groups of organisms, either the absence of genetic tools or the lack of closely related species has limited genetic studies. This is changing, especially with the advent of genomic tools. For instance, monkeyflowers in the genus Mimulus are rapidly becoming a model system to investigate the genetics of hybrid incompatibility in plants Sweigart et al.
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Sweigart, A. A simple genetic incompatibility causes hybrid male sterility in Mimulus. Ting, C. A rapidly evolving homeobox at the site of a hybrid sterility gene. Wu, C. Haldane's rule and its legacy: Why are there so many sterile males? Trends in Ecology and Evolution 11, — Genes and speciation. Nature Reviews Genetics 5 , — link to article. Origins of New Genes and Pseudogenes. Evolutionary Adaptation in the Human Lineage.
Genetic Mutation. Negative Selection. Sexual Reproduction and the Evolution of Sex. Haldane's Rule: the Heterogametic Sex. Hybrid Incompatibility and Speciation. Hybridization and Gene Flow. Why Should We Care about Species? Johnson, Ph. Citation: Johnson, N. Nature Education 1 1 Hybrids between closely-related species are often inviable or sterile.
How does this sterility and inviability happen? Genetics helps provide insight into answering this question. Aa Aa Aa. The Dobzhansky-Muller Model. Figure 1: Dobzhansky-Muller model of hybrid incompatibility. How does genic incompatibility between species evolve without simultaneously causing defects in pure species? A popular explanation is the Dobzhansky-Muller DM model of hybrid incompatibility. This means the two populations stop interbreeding migration between them is very low.
Recently, evolutionary biologists have found that rapid reproductive isolation is more common than previously thought Culotta and Pennisi and it is often associated with what is known as sympatric speciation or speciation between populations which share the same geographical range.
Many of the mechanisms which can cause this rapid reproductive isolation are more common in plants and perhaps fungi than animals and therefore may go some way in explaining the differences we have observed in the frequency of punctuational evolution among these groups.
Looking around us, it may be that punctuational bursts of evolution are responsible for the surprising morphological diversity between some very closely related groups. Among the 80 or so species of the Andean genus Lupinus Hughes and Eastwood —a flowering plant directly related to the common garden Lupine—some are over 12 ft 3. Some are tree like; others are herbs and others yet take the form of small bushes.
Similarly, Lake Tanganyika in central Africa supports a diverse radiation of the cichlid fish of the genus Tropheus Egger et al. This radiation, beginning just one million years ago, is thought to have produced the numerous lineages, with diverse morphology, we see today. Detecting punctuational evolution at the molecular level, then, does not mean that Darwin got it wrong; likewise, it does not require a reevaluation of modern evolutionary theory. Conventional neo-Darwinian ideas such as natural selection and random drift can explain these bursts of evolution.
What is interesting about punctuational effects is that speciation itself may be an important factor in increasing the rate of evolutionary change. The evolutionary changes could still accumulate in small steps, but these steps are taken more quickly at the time of speciation. As is often the case with matters of evolution, Charles Darwin was among the first to realize that language and species might evolve in a similar way.
Researchers today generally agree that many of the principal characteristics of linguistic evolution are analogous to those of species or biological evolution. In a similar way to morphological characters and gene sequences of species, languages have heritable units that can be passed to subsequent generations and may be subject to forces similar to natural selection, mutation and genetic drift Atkinson et al.
It is possible to derive a phylogenetic tree of languages based on the similarities and differences in vocabulary. Such trees can inform us about the evolutionary history of a particular language family.
In these trees, the branches that separate the nodes are measured in units of lexical replacement acquisition of novel words ; the nodes themselves represent language-splitting events, the linguistic equivalent to a speciation event. We recently Atkinson et al. We found evidence in all cases for punctuational bursts of language evolution associated with the formation of new languages.
The rate of language evolution and factors affecting it has been discussed by linguists for many years. Kirch and Green suggested that the movement of Polynesians inhabiting Pacific islands might have caused an increased rate of language evolution owing to the many and successive founder events to which such island hopping would have led.
These founder events are the linguistic equivalent of genetic founder events. The Polynesian languages are a subset of the Austronesian tree. It seems, then, that at least some punctuational bursts of language evolution may arise from founder effects. However, language may be useful for things other than mere communication.
Linguists have long viewed language as a facilitator for group cohesion and identity. These new spellings appeared almost instantaneously and have persisted to this day.
Speciation frequently seems to act as a driving force for molecular evolution, a phenomenon that has often been overlooked by biologists. Further, the analysis of language data shows that the search for punctuational effects can also move outside biology.
What are traditionally biological techniques, including phylogenetics, are gaining momentum in other disciplines, such as anthropology and archaeology. Ford proposed that cultural change is often reflected in cultural artifacts, and he illustrated the point by noting changes in ceramic vessels over time. In principal, cultural phylogenetic trees could be studied for punctuational evolution. If the evolution of the cultural artifacts is characterised by descent with modification, such analyses would be justified.
In fact, if a meaningful phylogeny can be inferred from data derived from them, it goes some way in showing that cultural evolution often proceeds in a manner analogous to biological evolution.
The theory of punctuated equilibrium was originally proposed to explain morphological evolution observed in the fossil record. To our knowledge, there is no phylogenetic research that has demonstrated bursts of morphological evolution, but the methodology we have used is new and we look forward to investigators applying it to morphological characters in the future.
The study of punctuational evolution represents an area of great potential research which might connect disparate branches of evolutionary biology and perhaps disciplines beyond. Languages evolve in punctuational bursts. Science ; Culotta E, Pennisi E. Breakthrough of the year: evolution in action. Science ;—9.
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Proc Natl Acad Sci ;—9. Hunt G. If gene flow boundaries never emerge, speciation does not occur i. When rates of recombination are relatively low compared to selective coefficients s within niches, entire genomes will sweep to fixation before they can be shuffled by recombination.
However, recent modeling work [ 80 ] has shown that gene sweeps can occur when r is either very high or—counter-intuitively—when r is very low, but only in the presence of negative frequency-dependent selection on other loci in the genome, in addition to positive selection on an ecologically adaptive locus. Such frequency-dependent selection, liable to be common in nature, might be imposed by viral phage predation of bacteria, providing a selective advantage to rare alleles of phage receptor genes, for example [ 81 , 82 ].
Additional sampling and sequencing from natural populations will be required to assess the prevalence of gene sweeps. Some of these alleles showed evidence of natural selection, suggesting the action of gene sweeps within a single cohesive population i. These recent models [ 80 ] and empirical work [ 83 ] have made some headway in resolving the paradox of gene sweeps but also raise new questions.
How common are gene-sweeps relative to the genome-wide sweeps predicted by the Stable Ecotype Model? On what time scales do sweeps occur, and how does this affect speciation rates? More generally, can all life on Earth, including microbes and macrobes, be viewed on the same universal speciation spectrum? This genetic diversity can be neutral or selfish, consisting of mobile elements that could potentially but not necessarily be exapted for species-level adaptation.
Later stages of speciation involve divergent natural selection and barriers to gene flow. The extent to which these barriers are ecological, behavioral, physical, or genetic remains an open research question. Evidence from comparative genomics has shown that purely genetic barriers such as CRISPR may provide effective barriers over short within-species time scales [ 86 ] but not over longer evolutionary time scales [ 87 ]. Therefore, gene flow barriers will always be leaky—in both microbes and macrobes.
Here, we have argued that selection, except in special cases of sustained allopatry, is almost certainly required for the long-term success of speciation. More examples will be needed to test its generality, but our model is as follows. Selection drives speciation and is followed by genome-wide divergence, due to reduced gene flow in recombining populations or mutational divergence in clonal populations. If genome-wide divergence does not follow, speciation does not occur or is stalled at a very early stage and we are left with gene ecology.
Just how much selection on how many genes and how much divergence across the genome is needed for speciation is an open question. Another important question is, for a given sample of organisms, what fraction of the genome is shaped by selection or drift within the individual, the species, or the multispecies [ 37 ]? In asking and eventually answering this question, we begin to appreciate that not only does speciation occur along a spectrum, but species can be placed within a spectrum of biological diversity, from the molecule to the biosphere.
The funders had no role in the preparation of the article. National Center for Biotechnology Information , U. PLoS Genet.
Published online Mar Jesse Shapiro. Ivan Matic, Editor. Author information Copyright and License information Disclaimer. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
This article has been cited by other articles in PMC. Abstract Concepts and definitions of species have been debated by generations of biologists and remain controversial. A Brief History of Species Thinking Here, we consider species in the vernacular sense, as clusters of individuals that show ecological and genetic similarities. Box 1. Glossary Allopatric: a set of sampled isolates or genomes from different geographic areas, where barriers to migration and gene flow are significant.
Gene flow : exchange of genes by homologous or nonhomologous recombination Gene-specific selective sweep: the process in which a selected gene or allele spreads in a population by recombination faster than by clonal expansion. Macrobe : a multicellular eukaryote.
Open in a separate window. Fig 1. Units of species and speciation. Are Eukaryotes Fuzzy Like Bacteria? The Islands Debate Most of the initial research and theory on speciation focused on plant and animal populations, with one of the major debates centered on the relative importance of sympatric and allopatric speciation.
Fig 2. Models of speciation under different regimes of selection and recombination. Islands in Bacteria Genomic regions akin to islands of speciation have also been described in natural microbial populations reviewed in detail in [ 5 ].
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