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Chapter 14 The Origin of Species Bowerbirds, native to New Guinea and Australia, are named for the structure, called a bower, that the male weaves from twigs and grasses to attract females. After building his bower, the male collects objects such as fruits, seeds, insect parts, rocks, flowers, and leaves and arranges them artfully by color and type. © 2012 Pearson Education, Inc. Figure 14.01 Females are dull colored (as are males) and tour the bowers of local males, inspecting each while its owner courts her with a song and dance. Vogelkop bowerbird photograph by Barrie Britton DEFINING SPECIES © 2012 Pearson Education, Inc. 14.1 The origin of species is the source of biological diversity Microevolution is the change in the gene pool of a population from one generation to the next. Speciation is the process by which one species splits into two or more species. – Every time speciation occurs, the diversity of life increases. – The many millions of species on Earth have all arisen from an ancestral life form that lived around 3.5 billion years ago. © 2012 Pearson Education, Inc. 14.2 There are several ways to define a species The word species is from the Latin for “kind” or “appearance.” Although the basic idea of species as distinct lifeforms seems intuitive, devising a more formal definition is not easy and raises questions. – How similar are members of the same species? – What keeps one species distinct from others? © 2012 Pearson Education, Inc. 14.2 There are several ways to define a species The biological species concept defines a species as – a group of populations, – whose members have the potential to interbreed in nature, and – produce fertile offspring. – Therefore, members of a species are similar because they reproduce with each other. © 2012 Pearson Education, Inc. 14.2 There are several ways to define a species Reproductive isolation – prevents members of different species from mating with each other, – prevents gene flow between species, and – maintains separate species. – Therefore, species are distinct from each other because they do not share the same gene pool. © 2012 Pearson Education, Inc. Figure 14.2A Similarity between two species: the eastern meadowlark (left) and western meadowlark (right). Similar looking but different songs and mating behavior Figure 14.2B Diversity within one species 14.2 There are several ways to define a species The biological species concept can be problematic. – Some pairs of clearly distinct species occasionally interbreed and produce hybrids. – For example, grizzly bears and polar bears may interbreed and produce hybrids called grolar bears. – Melting sea ice may bring these two bear species together more frequently and produce more hybrids in the wild. – Reproductive isolation cannot usually be determined for extinct organisms known only from fossils. – Reproductive isolation does not apply to prokaryotes or other organisms that reproduce only asexually. – Therefore, alternate species concepts can be useful. © 2012 Pearson Education, Inc. Figure 14.2C Hybridization between two species of bears Grizzly bear Polar bear Hybrid “grolar” bear 14.2 There are several ways to define a species The morphological species concept – classifies organisms based on observable physical traits and – can be applied to – asexual organisms and – fossils. – However, there is some subjectivity in deciding which traits to use. © 2012 Pearson Education, Inc. 14.2 There are several ways to define a species The ecological species concept – defines a species by its ecological role or niche and – focuses on unique adaptations to particular roles in a biological community. – For example, two species may be similar in appearance but distinguishable based on – what they eat or – where they live. © 2012 Pearson Education, Inc. 14.2 There are several ways to define a species The phylogenetic species concept – defines a species as the smallest group of individuals that shares a common ancestor and thus – forms one branch of the tree of life. – Biologists trace the phylogenetic history of a species by comparing its – morphology or – DNA. – However, defining the amount of difference required to distinguish separate species is a problem. © 2012 Pearson Education, Inc. 14.3 Reproductive barriers keep species separate Reproductive barriers – serve to isolate the gene pools of species and – prevent interbreeding. Depending on whether they function before or after zygotes form, reproductive barriers are categorized as – prezygotic or – postzygotic. © 2012 Pearson Education, Inc. 14.3 Reproductive barriers keep species separate Five types of prezygotic barriers prevent mating or fertilization between species. 1. In habitat isolation, two species live in the same general area but not in the same kind of place. 2. In temporal isolation, two species breed at different times (seasons, times of day, years). © 2012 Pearson Education, Inc. Figure 14.3 Habitat isolation (lack of opportunities to encounter each other) The garter snake Thamnophis atratus lives mainly in water. The garter snake Thamnophis sirtalis lives on land. Figure 14.3 Temporal isolation (breeding at different times or seasons) The eastern spotted skunk (Spilogale putorius) breeds in late winter. The western spotted skunk (Spilogale gracilis) breeds in the fall. 14.3 Reproductive barriers keep species separate Prezygotic Barriers, continued 3. In behavioral isolation, there is little or no mate recognition between females and males of different species. 4. In mechanical isolation, female and male sex organs are not compatible. 5. In gametic isolation, female and male gametes are not compatible. © 2012 Pearson Education, Inc. Figure 14.3 Behavioral isolation (different courtship rituals) The blue-footed booby (Sula nebouxii) performs an elaborate courtship dance. The masked booby (Sula dactylatra) performs a different courtship ritual. Figure 14.3 Mechanical isolation (physical incompatibility of reproductive parts) Heliconia latispatha is pollinated by hummingbirds with short, straight bills. Heliconia pogonantha is pollinated by hummingbirds with long, curved bills. Figure 14.3 Gametic isolation (molecular incompatibility of eggs and sperm or pollen and stigma) Purple sea urchin (Strongylocentrotus purpuratus) Red sea urchin (Strongylocentrotus franciscanus) 14.3 Reproductive barriers keep species separate Three types of postzygotic barriers operate after hybrid zygotes have formed. 1. In reduced hybrid viability, most hybrid offspring do not survive. 2. In reduced hybrid fertility, hybrid offspring are vigorous but sterile. 3. In hybrid breakdown, – the first-generation hybrids are viable and fertile but – the offspring of the hybrids are feeble or sterile. © 2012 Pearson Education, Inc. Figure 14.3 Reduced hybrid viability (hybrid development or survival impaired by interaction of parental genes) Some salamander species can hybridize, but their offspring do not develop fully or are frail and will not survive long enough to reproduce. Figure 14.3 Reduced hybrid fertility (vigorous hybrids that cannot produce viable offspring) A mule is the sterile hybrid offspring of a horse and a donkey. Figure 14.3 Hybrid breakdown (viable and fertile hybrids with feeble or sterile offspring) The rice hybrids on the left and right are fertile, but plants of the next generation (middle) are sterile. MECHANISMS OF SPECIATION © 2012 Pearson Education, Inc. 14.4 In allopatric speciation, geographic isolation leads to speciation In allopatric speciation, populations of the same species are geographically separated, isolating their gene pools. Isolated populations will no longer share changes in allele frequencies caused by – natural selection, – genetic drift, and/or – mutation. © 2012 Pearson Education, Inc. 14.4 In allopatric speciation, geographic isolation leads to speciation Gene flow between populations is initially prevented by a geographic barrier. For example – the Grand Canyon and Colorado River separate two species of antelope squirrels, and – the Isthmus of Panama separates 15 pairs of snapping shrimp. © 2012 Pearson Education, Inc. Figure 14.4A Allopatric speciation of geographically isolated antelope squirrels (Ammospermophilus) North rim South rim A. harrisii A. leucurus Figure 14.4B Allopatric speciation in snapping shrimp (Alpheus) A. formosus A. nuttingi ATLANTIC OCEAN Isthmus of Panama PACIFIC OCEAN A. panamensis A. millsae 14.5 Reproductive barriers can evolve as populations diverge How do reproductive barriers arise? Experiments have demonstrated that reproductive barriers can evolve as a by-product of changes in populations as they adapt to different environments. These studies have included – laboratory studies of fruit flies and – field studies of monkey flowers and their pollinators. © 2012 Pearson Education, Inc. Figure 14.5A Evolution of reproductive barriers in laboratory populations of fruit flies adapted to different food sources Initial sample of fruit flies Starch medium Maltose medium Female Starch Maltose 22 9 8 20 Number of matings in experimental groups Results Female Population Population #1 #2 Male Pop#2 Pop#1 Maltose Starch Male Mating experiments 18 15 12 15 Number of matings in starch control groups Figure 14.5B Transferring an allele between monkey flowers changes flower color Pollinator choice in and influences pollinator typical monkey flowers choice in Mimulus. Pollinator choice after color allele transfer Typical M. lewisii (pink) M. lewisii with red-color allele Typical M. cardinalis (red) M. cardinalis with pink-color allele 14.6 Sympatric speciation takes place without geographic isolation Sympatric speciation occurs when a new species arises within the same geographic area as a parent species. How can reproductive isolation develop when members of sympatric populations remain in contact with each other? Gene flow between populations may be reduced by – polyploidy, – habitat differentiation, or – sexual selection. © 2012 Pearson Education, Inc. 14.6 Sympatric speciation takes place without geographic isolation Many plant species have evolved by polyploidy in which cells have more than two complete sets of chromosomes. Sympatric speciation can result from polyploidy – within a species (by self-fertilization) or – between two species (by hybridization). © 2012 Pearson Education, Inc. Figure 14.6A Sympatric speciation by polyploidy within a single species 1 3 2 Parent species 2n = 6 Selffertilization Tetraploid cells 4n = 12 Diploid gametes 2n = 6 Viable, fertile tetraploid species 4n = 12 Figure 14.6B Sympatric speciation producing a hybrid polyploid from two different species Chromosomes cannot pair Species A 2n = 4 Gamete n=2 3 1 Sterile hybrid n=5 Species B 2n = 6 Gamete n=3 Can reproduce asexually 2 Viable, fertile hybrid species 2n = 10 14.7 EVOLUTION CONNECTION: Most plant species trace their origin to polyploid speciation Plant biologists estimate that 80% of all living plant species are descendants of ancestors that formed by polyploid speciation. Hybridization between two species accounts for most of these species. © 2012 Pearson Education, Inc. 14.7 EVOLUTION CONNECTION: Most plant species trace their origin to polyploid speciation Polyploid plants include – cotton, – plums, – oats, – apples, – potatoes, – sugarcane, – bananas, – coffee, and – peanuts, – bread wheat. – barley, © 2012 Pearson Education, Inc. 14.7 EVOLUTION CONNECTION: Most plant species trace their origin to polyploid speciation Wheat – has been domesticated for at least 10,000 years and – is the most widely cultivated plant in the world. Bread wheat, Triticum aestivum, is – a polyploid with 42 chromosomes and – the result of hybridization and polyploidy. © 2012 Pearson Education, Inc. Figure 14.7 The evolution of bread wheat, Triticum aestivum AA BB Wild Triticum (14 chromosomes) Domesticated Triticum monococcum (14 chromosomes) 1 Hybridization AB Sterile hybrid (14 chromosomes) 2 Cell division error and self-fertilization DD AABB T. turgidum Emmer wheat (28 chromosomes) Wild T. tauschii (14 chromosomes) 3 Hybridization ABD Sterile hybrid (21 chromosomes) 4 Cell division error and self-fertilization AABBDD T. aestivum Bread wheat (42 chromosomes) 14.8 Isolated islands are often showcases of speciation Most of the species on Earth are thought to have originated by allopatric speciation. Isolated island chains offer some of the best evidence of this type of speciation. Multiple speciation events are more likely to occur in island chains that have – physically diverse habitats, – islands far enough apart to permit populations to evolve in isolation, and – islands close enough to each other to allow occasional dispersions between them. © 2012 Pearson Education, Inc. 14.8 Isolated islands are often showcases of speciation The evolution of many diverse species from a common ancestor is adaptive radiation. The Galápagos Archipelago – is located about 900 km (560 miles) west of Ecuador, – is one of the world’s great showcases of adaptive radiation, – was formed naked from underwater volcanoes, – was colonized gradually from other islands and the South America mainland, and – has many species of plants and animals found nowhere else in the world. © 2012 Pearson Education, Inc. 14.8 Isolated islands are often showcases of speciation The Galápagos islands currently have 14 species of closely related finches, called Darwin’s finches, because Darwin collected them during his aroundthe-world voyage on the Beagle. These finches – share many finchlike traits, – differ in their feeding habits and their beaks, specialized for what they eat, and – arose through adaptive radiation. © 2012 Pearson Education, Inc. Figure 14.8 Cactus-seed-eater (cactus finch) Tool-using insect-eater (woodpecker finch) Seed-eater (medium ground finch) 14.10 Hybrid zones provide opportunities to study reproductive isolation What happens when separated populations of closely related species come back into contact with each other? Biologists try to answer such questions by studying hybrid zones, regions in which members of different species meet and mate to produce at least some hybrid offspring. © 2012 Pearson Education, Inc. 14.10 Hybrid zones provide opportunities to study reproductive isolation Over time in hybrid zones (Fig 14.10A) – reinforcement may strengthen barriers to reproduction, such as occurs in flycatchers (Fig. 14.10B), or – fusion may reverse the speciation process as gene flow between species increases, as may be occurring among the cichlid species in Lake Victoria (Fig. 14.10C). In stable hybrid zones, a limited number of hybrid offspring continue to be produced. © 2012 Pearson Education, Inc. Figure 14.10A Formation of a hybrid zone Newly formed species Three populations of a species 3 Hybrid zone 2 1 4 Gene flow Gene flow Population Barrier to gene flow Hybrid individual Figure 14.10B Reinforcement of reproductive barriers Allopatric populations Sympatric populations Male collared flycatcher Male pied flycatcher Pied flycatcher from allopatric population Pied flycatcher from sympatric population Figure 14.10C Fusion: males of Pundamilia nyererei and Pundamilia pundamilia contrasted with a hybrid from an area with turbid water Pundamilia nyererei Pundamilia pundamilia Hybrid: Pundamilia “turbid water” 14.11 Speciation can occur rapidly or slowly There are two models for the tempo of speciation. 1. The punctuated equilibria model draws on the fossil record, where species – change most as they arise from an ancestral species and then – experience relatively little change for the rest of their existence. 2. Other species appear to have evolved more gradually = gradualism. © 2012 Pearson Education, Inc. Figure 14.11 Two models for the tempo of speciation Punctuated pattern Gradual pattern Time 14.11 Speciation can occur rapidly or slowly What is the total length of time between speciation events (between formation of a species and subsequent divergence of that species)? – In a survey of 84 groups of plants and animals, the time ranged from 4,000 to 40 million years. – Overall, the time between speciation events averaged 6.5 million years. © 2012 Pearson Education, Inc.