DOCSLIB.ORG

Allopatric and Sympatric Modes of Speciation: Examples from Drosophila

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

REVIEW ARTICLES Allopatric and sympatric modes of speciation: examples from Drosophila B. N. Singh* Genetics Laboratory, Department of Zoology, Institute of Science, Banaras Hindu University, Varanasi 221 005, India species, which was ‘Reproductive community of sexually Basically, there are two different ways by which new species are created: anagenesis and cladogenesis (true or cross fertilizing individuals which share in a common gene pool’7. Evolutionary species concept was suggested speciation). During anagenesis, one species is trans- 8 formed into new one so there is a phyletic change in by Simpson who considered species as a lineage (ance- 9 the course of time and there is no involvement of stral-derived relationship), but it was criticized by Mayr reproductive isolation. Cladogenesis is considered as who denied it as the definition of species, but that of true speciation during which new species originate phyletic lineage which was also applicable to incipient through splitting of pre-existing ones. Different species. There are more than twenty species concepts models have been proposed from time to time to suggested by naturalists, evolutionary biologists and taxo- explain the modes of speciation. There are two impor- nomists1,2,10. Darwin11 who proposed the natural selection tant modes of speciation which are primarily based on theory of evolution to explain the mechanisms of evolu- geographic factors: allopatric (dichopatric and peri- tion, was initially a supporter of BSC but later supported patric) and sympatric. In this article, both allopatric morphological species concept which was related to his and sympatric modes of speciation are briefly 12 13 described with the help of suitable examples from theory of natural selection . Mayr introduced the con- Drosophila. cept of sibling species for those species which were morphologically similar but reproductively isolated. In Drosophila, a large number of sibling species (pairs or Keywords: Allopatric, Drosophila, speciation, sympa- groups) showing evolutionary significance have been re- tric. ported14. Cook15 coined the term speciation during which new species of organisms arise. Different evolutionary SPECIES is a taxonomic rank and it is also the basic unit biologists have suggested different modes of specia- of binomial classification. There are a large number of tion3,4,13,16–21. The terms used in this article are explained, concepts of species which define them in different which are: Allopatry – species living in different geo- ways1,2. Linnaeus (1707–1778), a prominent taxonomist graphical areas; Sympatry – species living in the same suggested the binomial nomenclature and was a strong geographical area; Genetic drift – random fluctuation in supporter of typological species concept (essentialism). allele frequency in small populations; Founder effect – Mayr and Ashlock3 have described four species concepts: when a daughter species arises from a few individuals (in typological, nominalistic, biological and evolutionary extreme case a single gravid female), there will be a larger species concepts in their book Principle of Systematic founder effect; Asymmetrical mode of sexual isolation – Zoology. The most widely accepted concept is biological between two species when sexual isolation is one-sided, species concept (BSC) which has been elaborated in de- it is called asymmetrical isolation, and when sexual isola- tail by Jordan, Mayr and Dobzhansky2. Different species tion is two-sided, it is called symmetrical isolation; Ance- concepts and modes of speciation have also been dis- stral-derived species – when the original species giving cussed by Singh2. Coyne and Orr4 suggested that defini- rise to another (new) species, the former is ancestral spe- tions of species have been historically controversial but cies and the new one is derived species; Stabilizing, cen- one of the most widely accepted concept is BSC. Accord- tripetal or normalizing selection – it signifies selection ing to Castillo and Barbash5, BSC works well for Droso- for intermediate phenotypes and elimination of extreme phila for the study of evolution of reproductive isolation, phenotypes, which leads to reduction in the degree of but other species concepts may have more utility for taxo- genetic variability; Disruptive selection or diversifying nomic and systematic studies. Mayr6 defined a species as selection is, when a population is subjected within a ‘Group of actually or potentially interbreeding natural single generation to various environments to which populations which are reproductively isolated from other different genotypes are most suited and this leads to sus- such groups’. Dobzhansky was a population geneticist tain the genetic variability. Different factors have been and introduced the term gene pool to the definition of considered important in the process of speciation such as mechanisms generating genetic variability, origin of *e-mail: [email protected] reproductive isolating mechanisms and geographical 56 CURRENT SCIENCE, VOL. 121, NO. 1, 10 JULY 2021 REVIEW ARTICLES factors. On the basis of these factors, White16 proposed between them. Both subpopulations which are allopatric different models to explain the modes of cladogenesis in distribution, evolve independently due to the action of (true speciation) like: (i) Strict allopatry without a narrow different evolutionary forces such as mutation, natural population bottleneck, (ii) Strict allopatry with a narrow selection, genetic drift, migration, etc. and accumulate population bottleneck (founder effect), (iii) Extinction of micro-evolutionary changes in the long course of time. intermediate populations in a chain of races, (iv) Clinal Because of geographic isolation, gene flow between them speciation, (v) Area effect speciation (primarily genic), is completely prevented and they become genetically dif- (vi) Stasipatric speciation (primarily chromosomal), and ferent. In the long run, they develop genetically based (vii) Sympatric speciation. White16 considered polyploidy mechanisms of reproductive isolation and acquire the sta- which is basically a chromosomal model separately, and tus of two separate allopatric species. If the geographic comes under sympatric mode of speciation that common- barrier is removed, they become two sympatric species ly occurs in plants. Mayr13 gave much emphasis on geo- but interbreeding is prevented between them since they graphic speciation (allopatric) which is an important would have already developed reproductive isolating mode of speciation. Thus, two speciations are primarily mechanisms. Thus, allopatric species become sympatric important namely, allopatric and sympatric, during which species due to removal of geographical barrier, and still new species of organisms arise due to origin of reproduc- maintain their own gene pool due to prevention of inter- tive isolating mechanisms which are considered a breeding. Thus, allopatric species originating through pre-requisite for speciation22. Sexual isolation which is a dichopatric mode of speciation (dumbbell model) become prezygotic (premating) isolation has been extensively sympatric because of the developing zone of contact studied in Drosophila, and the asymmetrical mode of resulting from the removal of geographical barrier. One mating preference is considered to discuss ancestral– example of dumbbell model of allopatric speciation is derived relationship between species which are primarily provided in Hawaiian Drosophila when the population based on allopatric and sympatric modes of specia- existing at low elevation had been split into two by the tion23,24. Nanda and Singh25 discussed the behavioural re- separation of two islands Molokai from Maui due to rise productive isolation and speciation in different species in sea level16. groups of Drosophila including Hawaiian species along In the allopatric mode of speciation (with a narrow with origin of sexual isolation, genetics of premating iso- population bottleneck), which is also known as peripatric lation, asymmetrical mode of mating preference, and the mode of speciation, a small number of individuals (in direction of evolution. In this article, both allopatric and extreme case a single gravid female) migrate from the sympatric modes of speciation are discussed briefly with original population to a new area and evolve into a new suitable examples from Drosophila which is considered species in the long course of time. It becomes genetically as an interesting and important biological model. different from the original population as it passes through population bottleneck/founder effect/flush crash cycle. It has also been suggested that the rate of evolution is faster Allopatric mode of speciation in such cases because of the action of genetic drift and genetic differences. This is the basic difference between Based on geographical distribution of species/popu- dichopatric and peripatric modes of speciation although lations, different terms are used such as sympatric, broadly both come under allopatric mode. Carson26–29 did allopatric, parapatric and peripatric. To explain the extensive work on Hawaiian species of Drosophila and mechanism of speciation, geographical component is very explained their evolution by peripatric mode of speciation important. It may range from absence (sympatric) to involving population bottleneck, founder principle, ran- complete separation (allopatric). Allopatric mode of spe- dom genetic drift and flush crash cycles. In the Hawaiian ciation was initially
Recommended publications
  • Hered Master 7..Hered 317 .. Page642

    Hered Master 7..Hered 317 .. Page642

    Heredity 80 (1998) 642–650 Received 3 September 1997 Inheritance of behavioural differences between two interfertile, sympatric species, Drosophila silvestris and D. heteroneura CHRISTINE R. B. BOAKE*, DONALD K. PRICE% & DEBRA K. ANDREADIS Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville, TN 37996, U.S.A. The Hawaiian fly species, Drosophila silvestris and D. heteroneura, are sympatric and interfertile but show strong behavioural isolation and major differences in male aggressive behaviour and the associated morphology. As a first step in elucidating the genetic control of the differences between these species, we examined the mating and aggressive behaviour of their reciprocal F1 hybrids. The latency to the first wing vibration and the latency to copulate did not differ significantly between the parental species. However, D. heteroneura females had a very low tendency to copulate with D. silvestris males, rarely mating during the observation period. The duration of copulation also differed significantly: same-species pairs of D. silvestris had copula- tions that lasted about 50% longer than those of same-species pairs of D. heteroneura. The hybrids were intermediate between the parental species for both the tendency to copulate with D. silvestris males and the duration of copulation, suggesting codominance or polygenic inherit- ance for those traits. The aggression traits that we scored were the leg posture and wing extension during early aggression, and the leg posture and head position during escalated aggression. The parental species showed clear differences for each of these traits. The F1 hybrids resembled one parent or the other, without showing intermediate values, suggesting single-gene dominance or threshold expression of many genes for those traits.
  • Allopatric Speciation with Little Niche Divergence Is Common Among

    Allopatric Speciation with Little Niche Divergence Is Common Among

    Journal of Biogeography (J. Biogeogr.) (2016) 43, 591–602 ORIGINAL Allopatric speciation with little niche ARTICLE divergence is common among alpine Primulaceae Florian C. Boucher1*, Niklaus E. Zimmermann2,3 and Elena Conti1 1Institute of Systematic Botany, University of ABSTRACT Zurich,€ 8008 Zurich,€ Switzerland, 2Dynamic Aim Despite the accumulation of cases describing fast radiations of alpine Macroecology, Swiss Federal Research plants, we still have limited understanding of the drivers of speciation in alpine Institute WSL, 8903 Birmensdorf, Switzerland, 3Department of Environmental floras and of the precise the timing of their diversification. Here, we investi- Systems Science, Swiss Federal Institute of gated spatial and temporal patterns of speciation in three groups of alpine Technology ETH, CH-8092 Zurich,€ Primulaceae. Switzerland Location Mountains of the European Alpine System. Methods We built a new phylogeny of Primulaceae including all species in three focal groups: Androsace sect. Aretia, Primula sect. Auricula and Soldanella. Combining phylogenetic information with a detailed climatic data set, we investigated patterns of range and ecological overlap between sister-species using an approach that takes phylogenetic uncertainty into account. Finally, we investigated temporal trajectories of diversification in the three focal groups. Results We found that a large majority of sister-species pairs in the three groups are strictly allopatric and show little differences in substrate and cli- matic preferences, a result that was robust to phylogenetic uncertainty. While rates of diversification have remained constant in Soldanella, both Androsace sect. Aretia and Primula sect. Auricula showed decreased diversification rates in the Pleistocene compared to previous geological epochs. Main conclusions Allopatric speciation with little niche divergence appears to have been by far the most common mode of speciation across the three groups studied.
  • Speciation in Geographical Setting

    Speciation in Geographical Setting

    Speciation Speciation 2019 2019 The degree of reproductive isolation Substantial variation exists in among geographical sets of species - anagenesis populations within an actively 1859 1859 evolving species complex is often Achillea - yarrow tested by crossing experiments — as in the tidy tips of California 100K bp 100K bp back in time back in time back Rubus parviforus K = 4 mean population assignment 2 mya 2 mya ID CO WI_Door 5 mya BC_Hixton BC_MtRob WI_BruleS 5 mya BC_McLeod CA_Klamath MI_Windigo WA_Cascade WA_BridgeCr SD_BlackHills OR_Willamette MI_Drummond Speciation Speciation 2019 Reproductive isolation will ultimately stop all Although simple in concept, the recognition of species and thus the definition genetic connections among sets of populations of what are species have been controversial — more than likely due to the – cladogenesis or speciation continuum nature of the pattern resulting from the process of speciation 1859 Example: mechanical isolation via floral shape changes and pollinators between two parapatric species of California Salvia (sage) 100K bp back in time back 2 mya S. mellifera 5 mya Salvia apiana 1 Speciation Speciation Although simple in concept, the recognition of species and thus the definition Animal examples of speciation often show of what are species have been controversial — more than likely due to the clear reproductive barriers - hence zoologists continuum nature of the pattern resulting from the process of speciation preference (as opposed to botanists) for the Reproductive isolating Biological
  • Diversification in the Hawaiian Drosophila

    Diversification in the Hawaiian Drosophila

    Diversification in the Hawaiian Drosophila By Richard Thomas Lapoint A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Environmental Science, Policy and Management in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Patrick M. O’Grady, Chair Professor George K. Roderick Professor Craig Moritz Spring 2011 ! Diversification in the Hawaiian Drosophila Copyright 2011 By Richard Thomas Lapoint ! Abstract Diversification in the Hawaiian Drosophila by Richard Thomas Lapoint Doctor of Philosophy in Environmental Science, Policy and Management University of California, Berkeley Professor Patrick M. O’Grady, Chair The Hawaiian Islands have been recognized as an ideal place to study evolutionary processes due to their remote location, multitude of ecological niches and diverse biota. As the oldest and largest radiation in the Hawaiian Islands the Hawaiian Drosophilidae have been the focus of decades of evolutionary research and subsequently the basis for understanding how much of the diversity within these islands and other island systems have been generated. This dissertation revolves around the diversification of a large clade of Hawaiian Drosophila, and examines the molecular evolution of this group at several different temporal scales. The antopocerus, modified tarsus, ciliated tarsus (AMC) clade is a group of 90 described Drosophila species that utilize decaying leafs as a host substrate and are characterized by a set of diagnostic secondary sexual characters: modifications in either antennal or tarsal morphologies. This research uses both phylogenetic and population genetic methods to study how this clade has evolved at increasingly finer evolutionary scales, from lineage to population level.
  • L22-Speciation Announcements

    L22-Speciation Announcements

    L22-Speciation Announcements 1st Drafts for papers due Oct 29th -DO NOT INCLUDE YOUR NAME TITLE OF PAPER by --first and last initials and ZS1234 last four-digits of student ID --include the recitation date and time as well. Announcements Supplemental materials on speciation posted to Carmen (will be in exam 3) PollEverywhere msg that “maximum responses reached”...don’t worry! THINK-PAIR-SHARE (90 sec) If 'things' look alike, what would qualify them as being of the same species? _________ speciation follows subdivision of a population due to physical barriers. A. parapatric B. peripatric C. sympatric D. allopatric Low relative genetic diversity is a consequence of the founder effect in peripatric speciation. A. True B. False THINK-PAIR-SHARE (90 sec) Why are there so many unusual species on the Galapagos Islands or in Madagascar? What kind of speciation might explain this phenomenon? Modes of speciation: Parapatric speciation A gradient or cline causes adjacent populations to experience different selective conditions -but the populations can still mate, generating hybrids Hybrids may lack traits that facilitate success in any part of the cline, causing them to be outcompeted by nonhybrids Modes of speciation: Parapatric speciation A gradient or cline causes adjacent populations to experience different selective conditions -but the populations can still mate, generating hybrids Bounded hybrid superiority suggests that hybrids occupying the HZ harbor unique traits exclusive of the progenitors that make them well-suited to environmental conditions
  • Speciation and Recessive Mutations for BMC Biology

    Speciation and Recessive Mutations for BMC Biology

    Essay: On the close relationship between speciation, inbreeding and recessive mutations. Etienne Joly, [email protected], Toulouse, September 2010 All the ideas developed in this essay are relatively simple, and most of them are related to many Foreword previously published works. So much work, however, This past year, 2009, was the Darwin year, celebrating has already been published on evolution and speciation the 200th anniversary of Charles Darwin's birth, and 150 that an autodidactic newcomer such as myself could years since the publication of his fabulous milestone not hope to read, let alone understand and remember all book, ‘The Origin of Species’ (to which I will the primary papers published previously on evolution subsequently refer to as ‘The Origin’). At the start of and speciation. If I have failed to acknowledge 2009, I was inhabited by a nagging ethical concern : how previous works developing ideas related to those put would humans deal with a situation where a group of forward here, the reader can be assured that this was individuals found themselves fertile among one another, not done maliciously but simply as a result of my but with limited fertility with the rest of the human race ? relative naivety on the subject. I do, however, hold the In other words, could speciation occur within the human firm conviction that, if some of the ideas developed in race ? This concern sprouted from the idea that this essay prove to be correct and relatively novel, it chromosomal rearrangements seemed to me like a very was only possible because of this naivety.
  • Allopatric Speciation

    Allopatric Speciation

    Lecture 21 Speciation “These facts seemed to me to throw some light on the origin of species — that mystery of mysteries”. C. Darwin – The Origin What is speciation? • in Darwin’s words, speciation is the “multiplication of species”. What is speciation? • in Darwin’s words, speciation is the “multiplication of species”. • according to the BSC, speciation occurs when populations evolve reproductive isolating mechanisms. What is speciation? • in Darwin’s words, speciation is the “multiplication of species”. • according to the BSC, speciation occurs when populations evolve reproductive isolating mechanisms. • these barriers may act to prevent fertilization – this is prezygotic isolation. What is speciation? • in Darwin’s words, speciation is the “multiplication of species”. • according to the BSC, speciation occurs when populations evolve reproductive isolating mechanisms. • these barriers may act to prevent fertilization – this is prezygotic isolation. • may involve changes in location or timing of breeding, or courtship. What is speciation? • in Darwin’s words, speciation is the “multiplication of species”. • according to the BSC, speciation occurs when populations evolve reproductive isolating mechanisms. • these barriers may act to prevent fertilization – this is prezygotic isolation. • may involve changes in location or timing of breeding, or courtship. • barriers also occur if hybrids are inviable or sterile – this is postzygotic isolation. Modes of Speciation Modes of Speciation 1. Allopatric speciation 2. Peripatric speciation 3. Parapatric speciation 4. Sympatric speciation Modes of Speciation 1. Allopatric speciation 2. Peripatric speciation 3. Parapatric speciation 4. Sympatric speciation Modes of Speciation 1. Allopatric speciation Allopatric Speciation ‘‘The phenomenon of disjunction, or complete geographic isolation, is of considerable interest because it is almost universally believed to be a fundamental requirement for speciation.’’ Endler (1977) Modes of Speciation 1.
  • Introduction Speciation Is a Burning Issue in Evolutionary Biology, but It

    Introduction Speciation Is a Burning Issue in Evolutionary Biology, but It

    Introduction Speciation is a burning issue in evolutionary biology, but it is both fascinating and frustrating. Defining speciation depends on one’s species concept viz., typological, biological, evolutionary, recognition etc. In its simplest form, speciation is lineage splitting (ancestor-descendent sequence of populations); the resulting lineages are genetically isolated and ecologically distinct. Speciation is the process of evolutionary mechanism by which new biological species (or taxa) arise. There are two ways of new species (or taxa) origin from the pre-existing one:- i. by splitting of the parent species into two or more species (by the splitting of phylogenetic lineage) and ii. by transformation of the old species into a new one in due course of time. The Biologist O.F. Cook (1906) seems to have been the first to coin the term ‘speciation’ for the splitting of lineages (cladogenesis).The process of evolutionary mechanism by which new biological plant species (or taxa) arise, is known as plant speciation. General Mechanism of Speciation operating in nature: The mechanism of speciation is a two- staged process in which reproductive isolating mechanisms (RIM's) arise between groups of populations. Stage 1 • gene flow is interrupted between two populations. • absence of gene flow allows two populations to become genetically distinct as a result of their adaptation to different local conditions (genetic drift plays an important role here). • as populations differentiate, RIMs appear because different gene pools are not mutually coadapted. • reproductive isolation appears primarily in the form of postzygotic RIMs: hybrid failure. • these early RIMs are a byproduct of genetic differentiation, not directly promoted by natural selection.
  • Introduction and Bibliography

    Introduction and Bibliography

    Pacific Science (1988), vol. 42, nos. 1-2 © 1988 by the University of Hawaii Press. All rights reserved Introduction and Bibliography Hampton Carson first came to Hawaii in LITERATURE CITED June 1963 at the urging of Elmo Hardy and Wilson Stone. That year saw the first major CARSON, H. L. 1980. Hypotheses that blur gathering in Honolulu of scientists from and grow. Pages 383-384 in E. Mayr and many specialties in the interdisciplinary and W. B. Provine, eds. The evolutionary syn­ cooperative pattern that has proved so pro­ thesis: Perspectives on the unification of ductive in the study of Hawaiian Drosophila biology. Harvard Univ. Press, Cambridge, on "the Project" (Spieth 1980). Carson came Mass . with Harrison Stalker from Washington SPIETH, H. T. 1980. Hawaiian Drosophila University in St. Louis. Together (following Project. Proc., Hawaiian Entomol. Soc. Dobzhansky), they had developed a power­ 23(2) :275-291. ful method of population studies based on detailed examination of the distribution of inversions in the polytene chromosomes of Bibliography ofHampton Lawrence Carson Drosophila robusta and other species of the 1934-1986 mesic forests of the central and eastern United States. 1. CARSON, H. L. 1934. Labrador quarry. Carson's cytological approach can be traced General Mag. 37(1) :97-104. to the influence of McClung, and especially 2. CARSON, H. L. 1935. Use of medicinal of Metz, during his graduate studies at the herbs among the Labrador Eskimo. University of Pennsylvania in the late 1930s General Mag. 37(4):436-439. and early 1940s (Carson 1980). These studies 3. CARSON, H . L.
  • Taxonomic Relationships Within the Endemic Hawaiian Drosophilidae (Insecta: Diptera)

    Taxonomic Relationships Within the Endemic Hawaiian Drosophilidae (Insecta: Diptera)

    Records of the Hawaii Biological Survey for 2008. Edited by Neal L. Evenhuis & Lucius G. Eldredge. Bishop Museum Occasional Papers 108: 1–34 (2010) Taxonomic relationships within the endemic Hawaiian Drosophilidae (Insecta: Diptera) P.M. O’Grady, K.N. MaGNacca & r.T. LaPOiNT (University of california, Berkeley, department of Environmental Science, Policy & Management, 117 Hilgard Hall, Berkeley, california 94720, USa; email: [email protected]) The Hawaiian drosophilidae are an incredibly diverse group which currently consists of 559 described species and several hundred species that are known, yet await description (Kane - shiro, 1997; O’Grady, 2002). Early morphological studies identified two main groups of Hawaiian drosophilidae, the “drosophiloids” and the “scaptomyzoids” (Throckmorton, 1966), all the members of which are now referred to as Hawaiian Drosophila and Scapto­- myza, respectively. although one morphological study suggests that these two lineages are not closely related (Grimaldi, 1990), all other analyses to date support the monophyly of the Hawaiian drosophilidae (deSalle, 1992; Thomas & Hunt, 1993; O’Grady, 1998; remsen & deSalle, 1998; remsen & O’Grady, 2002). Furthermore, comprehensive sampling within Scaptomyza suggests that the continental members of this genus are derived from the Hawaiian taxa (O’Grady & deSalle, 2008). The Hawaiian Drosophila lineage was initially composed of members of the genus Drosophila endemic to Hawai‘i, as well as several endemic Hawaiian genera including Nudidrosophila, Antopocerus, and Ateledrosophila. These three genera were subsequently synonymized with Drosophila and are now considered to be of species group rank within a larger clade of endemic Hawaiian Drosophila (Kaneshiro, 1976). The Hawaiian Drosophila are currently divided into the following species groups (Table 1), based on morphological characters: antopocerus, ateledrosophila, haleakalae, picture wing, modified mouthpart, modi­fied­ tarsus, nudidrosophila, and rustica.
  • The Temporal Dimension of Marine Speciation

    The Temporal Dimension of Marine Speciation

    Evol Ecol DOI 10.1007/s10682-011-9488-4 ORIGINAL PAPER The temporal dimension of marine speciation Richard D. Norris • Pincelli M. Hull Received: 2 September 2010 / Accepted: 7 May 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Speciation is a process that occurs over time and, as such, can only be fully understood in an explicitly temporal context. Here we discuss three major consequences of speciation’s extended duration. First, the dynamism of environmental change indicates that nascent species may experience repeated changes in population size, genetic diversity, and geographic distribution during their evolution. The present characteristics of species therefore represents a static snapshot of a single time point in a species’ highly dynamic history, and impedes inferences about the strength of selection or the geography of spe- ciation. Second, the process of speciation is open ended—ecological divergence may evolve in the space of a few generations while the fixation of genetic differences and traits that limit outcrossing may require thousands to millions of years to occur. As a result, speciation is only fully recognized long after it occurs, and short-lived species are difficult to discern. Third, the extinction of species or of clades provides a simple, under-appre- ciated, mechanism for the genetic, biogeographic, and behavioral ‘gaps’ between extant species. Extinction also leads to the systematic underestimation of the frequency of spe- ciation and the overestimation of the duration of species formation. Hence, it is no surprise that a full understanding of speciation has been difficult to achieve. The modern synthe- sis—which united genetics, development, ecology, biogeography, and paleontology— greatly advanced the study of evolution.
  • Chromosomal Inversions and the Reproductive Isolation of Species

    Chromosomal Inversions and the Reproductive Isolation of Species

    Chromosomal inversions and the reproductive isolation of species Mohamed A. F. Noor*, Katherine L. Grams, Lisa A. Bertucci, and Jane Reiland Department of Biological Sciences, Life Sciences Building, Louisiana State University, Baton Rouge, LA 70803 Edited by Wyatt W. Anderson, University of Georgia, Athens, GA, and approved August 15, 2001 (received for review June 1, 2001) Recent genetic studies have suggested that many genes contribute sophila persimilis. These species differ by several inversions (see to differences between closely related species that prevent gene Materials and Methods). The two species hybridize, albeit rarely, exchange, particularly hybrid male sterility and female species in nature, and gene flow has been detected at the sequence level preferences. We have examined the genetic basis of hybrid sterility (15, 16). Natural selection seems to have strengthened the and female species preferences in Drosophila pseudoobscura and mate discrimination exercised by these females to prevent mal- Drosophila persimilis, two occasionally hybridizing North American adaptive hybridization (17), because hybrid male offspring are species. Contrary to findings in other species groups, very few sterile. These species seem to have separated initially before regions of the genome were associated with these characters, and the split between the D. melanogaster races or the split be- these regions are associated also with fixed arrangement differ- tween D. simulans and D. mauritiana; thus genetic divergence ences (inversions) between these species. From our results, we between them should be greater on average in the absence of propose a preliminary genic model whereby inversions may con- introgression. tribute to the speciation process, thereby explaining the abun- Previous studies of these species have identified the genetic dance of arrangement differences between closely related species basis of their differences in cuticular hydrocarbon profile (18) that co-occur geographically.