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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Lars Chittka‘s bees on Van Gogh‘s Sunflowers [email protected] Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Speciation 1 - Contents • • • • What is a species? Allopatric speciation Reproductive Isolating Mechanisms Clines and ring species Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display How many species are there? There are about 1.5 million described species of animals (of which 1 million are insects) About 400.000 species of plants About 13.000 species discovered each year Total of species including funghi, bacteria and protists: 13 to 30 million Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Felis silvestris catus Panthera tigris Panthera pardus Panthera leo Puma concolor Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The definition of biological species - a reproductively isolated community in which all individuals potentially or actually interbreed amongst themselves, but are genetically isolated from other groups (Ernst Mayr 1905 - 2005) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Archaeopteryx (about 150 million years old) (had several Dinosaur and several avian features, incl. Feathers) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Problems with the biological species concept - It cannot be applied to extinct species - It cannot be applied to asexually reproducing species (of which there are many, for example Cnemidophorus lizards, aphids (by Parthenogenesis), many plants by runners, budding in Hydra, amoebas, etc. ) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Problems with the biological species concept - It cannot be applied to extinct species / fossils - It cannot be applied to asexually reproducing species - Technically difficult to apply – can’t test 100s of 1000s of species (and populations within them (for reproductive interactions and their success) - There are many borderline cases: e.g. two North American tree species (Populus), balsam & cottonwood. Fossils show that they have been morphologically distinct for over 12 million years. Yet they also hybridise occasionally, and hybrids are also fertile. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Wolves & coyotes: another problem case - distinct entities: have evolved in separate lineages for at least 500.000 years – yet they occasionally hybridise in the wild. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Morphospecies Individuals that are similar or identical in important taxonomic traits (not just morphology, but also e.g. physiological traits). Problems – an alien might easily categorise different human (or dog) races as different species! Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display white tailed deer elk moose FIGURE 9.1 Defining Species (a) Morphospecies. Viewed today, at one moment in time, species A, C, and E are clearly distinct, demarcated by current natural discontinuities between them. (b) Paleospecies (chronospecies). Viewed historically, through time, discovered fossil intermediates (B and D) fill in the missing gaps above, giving us a more or less continuous series with no obvious discontinuities between them. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Other species definitions The evolutionary species concept: a species consists of all the individuals that share a common history. Problem: how can we decide if they actually share a common history? The recognition species concept (Paterson 1980s): “.. inclusive population of individual biparental organisms which share a common fertilisation system”. Emphasis here is on factors that keep a species as a unity, rather than those that keep species distinct. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Allopatric Speciation (the most common scenario) Four steps lead to allopatric speciation. First, a single species is an interbreeding reproductive community. Second, a barrier develops, dividing the species. Third, separated into different habitats, the divided populations become differentiated through the accumulation of differences. Fourth, so different have the separate populations become, that is when the barrier disappears and they overlap again. Interbreeding does not occur. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Note: divergence in allopatric populations can occur through adaptation to different habitat conditions OR through evolutionary chance processes (genetic drift) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cotton top Gold lion Allopatric speciation The populations of Tamarin monkeys are separated on the sides of the Amazon River. Where the river tributary is wide and individuals on opposite banks do not interbreed, the populations are diverging toward separate species. Where the river tributary is narrow, the individuals still interbreed. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Geographic isolation can occur • • • through founder events: individuals get passively displaced to islands (also called peripatric speciation) new geographical barriers: mountain ranges fold up, continents break apart, rivers or deserts form glacial barriers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Ring species—salamanders The ensatina salamander (Ensatina eschscholtzii) occurs from Canada to Southern California with interbreeding between adjacent populations through this range. The Central Valley—a dry, hot lowland area—is divided into a coastal arm and inland arm. However, where these two arms of the species meet again in Southern California, interbreeding does not occur. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 9.4 Clinal Variation In the leopard frog (Rana pipiens), tadpoles exhibit a range of temperature tolerances, generally enduring colder temperatures in higher (northern) latitudes and warm temperatures at lower (southern) latitudes. (From J. Moore) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 9.5 Reproductive Success of different pairings In the leopard frog (Rana pipiens), eggs from females in the north were fertilized with sperm from males progressively farther to the south. The degree of embryo or tadpole abnormalities was scored, from A (normal young) through progressively more abnormalities to F (high death rate). This study was done by J. Moore in 1949. Today, this study and others prompt biologists to actually divide leopard frogs into subspecies or even different species. (From J. Moore) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display FIGURE 9.6 Leopard Frogs Today-Zygotic Isolation The four groups of leopard frogs resemble one another closely in their external appearance. But early tests of interbreeding produced defective embryos in some combinations, leading biologists to suspect that these might be different subspecies or even different species. Research on males’ mating calls indicates that the various groups differ substantially, and that such prezygotic behavior separates and reproductively isolates members of each group, producing four species: (1) Rana pipiens; (2) Rana blairi; (3) Rana utricularia; (4) Rana berlandieri. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display When is there selection for reproductive isolation? • • • • When two populations become geographically isolated, they might diverge, because they are now adapting to distinct habitat conditions (or they might diverge by chance processes). When they come into contact again, they might still be “sexually compatible“ – but the offspring might be sterile (as in horses and donkeys), or less fit, for example because it is not adapted to either of the habitats of parental populations In such conditions, there are high costs to “mistakes“ (mating with the wrong partner) Thus selection will favour individuals that don‘t make such mistakes, or where the results of mistakes are sorted out early on. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Pre-zygotic Reproductive Isolation Ecological isolation, e.g. in the Malaria vectors of the genus Anopheles Anopheles labranchiae – oviposits in brackish water A. maculipennis – oviposits in running water A. messeae – oviposits in stagnant water Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Pre-zygotic Reproductive Isolation Seasonal isolation, e.g. Pinus radiata sheds pollen in February - Pinus muricata in April. Hybrids do form on Monterey peninsula but show signs of inviability and infertility. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Pre-zygotic Reproductive Isolation Ethological isolation e.g. Hawaiian picture-wing Drosophila silvestris & D. heteroneura are genetically very similar but have different mating behaviour (different wing vibration patterns – “songs”. The difference in head shape is due to a single gene with large effects Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Pre-zygotic Reproductive Isolation Ethological isolation Single genes can sometimes have large effects. That applies also to behaviour differences and sensory processing. For example, Drosophila melanogaster and D. simulans have different “songs”. Identification of species membership in different Drosophila species occurs by “song” – i.e. species specific patterns of wing vibration – that may be transmitted via the substrate. You can hear some such songs at http://www.jason.org/expeditions/jason6/people/kaneshiro.html If the two species don’t have a choice in the lab, they will mate – but will show preference for their own species when given a choice. The song durations and intervals between them are controlled by the per gene (that also controls circadian rhythms). If a transgenic fly is produced which has the per gene from the respective other species, the transgenic fly will mate with partners from the other species, showing that song and song preference are controlled by the same gene! Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Pre-zygotic Reproductive Isolation Mechanical isolation, e.g. in Partula snails on the island Moorea, where sympatric species have shells that are coiled in opposite directions, resulting in mechanical difficulty in mating. Genitalia of Culex pipiens & C. torrentium don’t fit. The species remained undetected for a long time because they are otherwise so similar morphologically. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Another case of prezygotic reproductive isolation: Mimulus flowers and and pollinators The two closely related flower species differ in multiple ways so that one species is best adapted to bumblebee pollination, the other to hummingbird pollination; differences concern morphology (including placement of reproductive organs), colour, nectar content. Several of these may be genetically linked (after Bradshaw & Schemske) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Please remember: There is a day trip Tues Next week at Baker St tube at 9.15! If you miss the coach, you will have to phone the department to get Richard Nichols’ mobile number to get instructions, then make your way to the field site by train at your own expense!