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8/3/2009 Lecture #21 Date ________ • Chapter 24 ~ The Origin of Species Macroevolution: the origin of new taxonomic groups • Macroevolution refers to evolutionary change above the species level • Speciation: the origin of new species – Ex: Monkey flower (Lewis v. Cardinal hybrids) speciation has shown mutations in as few as 8 genes can result in speciation • Two basic patterns of evolutionary change can be distinguished – 1- Anagenesis (phyletic evolution): accumulation of heritable changes – 2- Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (basis of biological diversity) Macroevolution 1 8/3/2009 What is a species? • Biological species concept (Mayr): a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring (genetic exchange is possible and that is genetically isolated from other populations) Reproductive Isolation (isolation of gene pools), I • • • • • • Prezygotic barriers: impede mating between species or hinder the fertilization of the ova Habitat (snakes; water/terrestrial) Behavioral (fireflies; mate signaling) Temporal (salmon; seasonal mating) Mechanical (flowers; pollination anatomy) Gametic (frogs; egg coat receptors) Reproductive Isolation, II • • • • Postzygotic barriers: fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity) Reduced hybrid fertility (mule; horse x donkey; cannot backbreed) Hybrid breakdown (cotton; 2nd generation hybrids are sterile) 2 8/3/2009 Modes of speciation (based on how gene flow is interrupted) • “Modes of speciation” address the origins of reproductive isolation. • Allopatric: – populations segregated by a geographical barrier – genetic divergence arises largely thru natural selection, mutation, and genetic drift – believed to be the most common mode of speciation – may result in adaptive radiation (island species) • Sympatric: – reproductively isolated subpopulation in the midst of its parent population (change in genome) – Genetic divergence arises largely through diversifying selection and polyploidy. – polyploidy in plants; cichlid fishes Parapatric: – no specific extrinsic barrier to gene flow – population is continuous but the population does not mate randomly – form of speciation that occurs due to variations in the mating habits of a population within a continuous geographical area. Peripatric: – reproductive isolation evolves in a small population, isolated from its parent population. – is a special version of the allopatric speciation mode and happens when one of the isolated populations has very few individuals – also known as founder effect speciation. – genetic divergence arises largely through genetic drift and natural selection. – proposed explanation for the rapid speciation of Hawaiian Drosophila. Hawaiian fruit fly Drosophila setosimentum • An autopolyploid is an individual that has more than two chromosome sets, all derived from a single species Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue. 2n = 6 Gametes produced by flowers on this branch will be diploid. Offspring with tetraploid karyotypes may be viable and fertile—a new biological species. 2n 4n 4n = 12 Figure 24.8 • An allopolyploid is a species with multiple sets of chromosomes derived from different species • Ex: Triticale (has 6 chr. sets) is a hybrid of wheat (4 chr. sets) and rye (2 chr. sets) Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Viable fertile hybrid (allopolyploid) Species A Meiotic error; 2n = 4 chromosome number not reduced from 2n to n 2n = 10 Normal gamete n= 3 Normal gamete n= 3 Species B 2n = 6 Figure 24.9 3 8/3/2009 Triangle of U: Overview of genetic relationships between various members of the genus Brassica. (a type of mustard plant) Chromosomes from each of the genomes A, B and C are represented by different colours. The cartoon shows the origin of the AABB, AACC and BBCC species which have chromosome sets from their AA, BB and CC ancestors. • In cichlid fish sympatric speciation (Speciation takes place in geographically overlapping populations ) has resulted from nonrandom mating due to sexual selection EXPERIMENT Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orange light, the two species appear identical in color. The researchers then observed the mating choices of the fish in each tank. Monochromatic Normal light orange light P. pundamilia P. nyererei RESULTS Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile. CONCLUSION The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate. Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory, the genetic divergence between the species is likely to be small. This suggests that speciation in nature has occurred relatively recently. Punctuated equilibria • Tempo of speciation: – gradual vs. divergence in rapid bursts – Niles Eldredge and Stephen Jay Gould (1972) helped explain the non-gradual appearance of species in the fossil record 4