1 The Origin of Species Chapter 22

“Species” must account for 2 things: 1)distinctiveness of sp. (populations) together at a single locale 2 The Nature of Species

3 2)connection that exists among diff. pop. belonging to the same sp. Subspecies –ind. in sp. that occur in diff. areas and are distinct from one another

Biological species concept: –defined by Ernst Mayr –population whose members can interbreed & produce viable, fertile offspring –reproductively compatible Western MeadowlarkEastern Meadowlark Distinct species: songs & behaviors are different enough to prevent interbreeding The Nature of Species

Song Comparisons Eastern Meadowlark YrtR2o Western Meadowlark b1cLY 5

Darwin never actually tackled how new species arose… The Nature of Species “Both in space and time, we seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on this Earth.” -Charles Darwin

7 Speciation the process by which new sp. arise by: –transformation of 1 sp. into another (anagenesis) –splitting of 1 ancestral sp. into 2 descendant sp. (cladogenesis) The Nature of Species

8 Reproductive isolation –pop. whose members don’t mate with each other or who can’t produce fertile offspring –isolating mechanisms: Geographic Ecological Behavioral Temporal Mechanical Gamete fusion Hybridization Reproductive Isolation

Isolating Mech. Poster Name mechanism Is mechanism prezygotic or postzygotic? Define/Describe how mechanism works Give 3 examples of mechanism 9

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Prezygotic Isolation 1.Geographic Isolation –sp. occur in different areas – physical barrier Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

13 Two different types of geographic isolation: 1.Allopatric speciation result of geo. isolation 2.Sympatric speciation occurs without geo. iso. (in the same geo.) Geography Isolation

2.Ecological Isolation –Species occur in same region, but occupy different habitats so rarely encounter each other 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest Prezygotic Isolation Tiglon: Cross between a male tiger and a female lion

3.Behavioral Isolation –Unique behavioral patterns & rituals isolate species – identifies members of species – attract mates of same species courtship rituals, mating calls Blue footed boobies mate only after a courtship display unique to their species Prezygotic Isolation

4. Temporal Isolation –Species that breed during different times of day, different seasons, or different years cannot mix gametes Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer Prezygotic Isolation

5. Mechanical Isolation –Morphological differences can prevent successful mating Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. Plants Prezygotic Isolation

–For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals! Damsel fly penises Animals Prezygotic Isolation

6. Gametic isolation –Sperm of one species may not be able to fertilize eggs of another species –Mechanisms: biochemical barrier so sperm cannot penetrate egg –Sperm cannot penetrate egg chemical incompatibility –sperm cannot survive in female reproductive tract Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse. Prezygotic Isolation

Hybridization Hybrid offspring don’t develop into a viable, fertile adult –reduced hybrid viability Post-zygotic Isolation zebroid Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

–reduced hybrid fertility Even if hybrids are vigorous they may be sterile chromosomes of parents differ in number or structure & meiosis in hybrids fail to produce normal gametes Mules are vigorous, but sterile Donkeys have 62 chromosomes (31 pairs) Horses have 64 chromosomes (32 pairs) Mules have 63 chromosomes! Post-zygotic Isolation

–hybrid breakdown Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species. In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species. Post-zygotic Isolation

23 Adaptive Radiation Adaptive radiation –closely related sp. ev. from a com. ancestor by adapting to different parts of the env. –caused by OR results in geographic isolation

Does speciation happen gradually or rapidly? Charles Darwin Charles Lyell Stephen Jay Gould Niles Eldredge Gradualism Punctuated Equilibrium rapid bursts of change mixed with long periods of little or no change gradual accumulation of small changes over long time The Pace of Speciation

25 The Pace of Evolution Evolution most likely includes both!

26 Speciation and Extinction Speciation, through time, has surpassed extinction –5 mass extinctions have occurred –not all groups of organisms are affected equally during extinctions

27 Consequence of extinction –previously dom. groups may perish, changing the course of ev. Speciation and Extinction

28 6 th extinction is underway –Estimates: 1/4 th of all species will become extinct in the near future rebound in sp. diversity may be slower than following previous mass extinction events large proportion of the world’s resources will be taken up by human activities Speciation and Extinction

29 The Future of Evolution Human influences on the environment affect the evolutionary processes changing patterns of NS global climate change decreased pop. sizes increase the likelihood of genetic drift geographic isolation removes effect of gene flow chemicals and radiation could increase mutation rate introduction of sp. into areas they aren’t naturally found

30 Tigers now exist in geographically isolated populations The Future of Evolution

31 Sympatric sp. avoid mating with members of the wrong sp. in a variety of ways, including differences in: –visual signals –sound production –chemical signals: pheromones –electrical signals: electroreception Biological Species Concept

32 Biological Species Concept

33 Reproductive Isolation Speciation is: –dependent upon the ev. of reproductive isolating mech. if isolating mech. have not evolved, 2 pop. will interbreed freely (no sp.) –a continuous process 2 pop. may only be partially reproductively isolated

34 –often completed by reinforcement natural selection reinforces incomplete isolating mech. until they are completely effective not inevitable –gene flow may counter reinforcement –hybrids may be inferior, but still serve as a conduit of gen. exchange which causes 2 pop. to lose their genetic distinctiveness –a race between complete reproductive isolation evolution and gene flow Reproductive Isolation

35 ex: Polyploidy individuals that have more than two sets of chromosomes plants with “extra” sets of chromosomes survive, but can’t be fertilized by diploid individuals plants can self-pollinate, creating new sp. Geography of Speciation

36 Key innovation –evolves within a sp. allowing it to use resources or other aspects of the env. that were previously inaccessible –ex: ev. of lungs in fish Adaptive Radiation

37 Character displacement –NS in each sp. favors those ind. that use resources not used by the other species –greater fitness causes sp. to develop Adaptive Radiation

38 Case 3: Lake Victoria cichlid fishes Was home to over 300 species of cichlid until recently Recent radiation: sequencing of cytochrome b gene ,000 years ago Colonized from the Nile Changes in water level encouraged species formation Lake dry down 14,000 years ago Adaptive Radiation

39 Cichlids: small, perchlike fishes Males very colorful Foraging: –Mud biters, algae scrapers, leaf chewers, snail crushers, zooplankton eaters, insect eaters, prawn eaters, fish eaters Carry a second set of functioning jaws Adaptive Radiation

40 Abrupt extinction in the last several decades 1950’s Nile perch introduced into lake 1990’s 70% cichlids extinct Adaptive Radiation

41 Adaptive Radiation

42 Case 4: New Zealand alpine buttercups Speciation in glacial habitats Periodic isolation 14 species occupy 5 distinct habitats –Snow fields: m elevation –Snowline fringe: m elevation –Stony debris: slopes at 610 to 1830 m –Sheltered: m –Boggy habitats: m elevation Adaptive Radiation

43 Adaptive Radiation

44 Adaptive Radiation