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Chapter 1 24: The Origin of Species Evolutionary theory must explain -microevolution - how adaptations evolve in populations and -macroevolution - origin of new species Figure 24.1 - 2 patterns of speciation: -anagenesis (phyletic evolution) - one species transforms into another -cladogenesis - branching evolution parent persists, one or more new species branch off I. What is a species? Biological species concept - population or group of pops whose members can interbreed in nature to produce viable, fertile offspring -a species is the largest unit of population where genetic exchange is possible A. Reproductive isolation barriers- isolate gene pools, contribute to speciation FIGURE 24.5 - summarized 1. Prezygotic barriers: -habitat isolation - rare encounters -behavioral isolation - courtship rituals -temporal isolation - different flowering or mating times -mechanical isolation - anatomically incompatible (I leave it to your imagination!) -gametic isolation - can't fuse to form zygotes 2. Postzygotic barriers: -reduced hybrid viability -reduced hybrid fertility -hybrid breakdown B. Other species concepts On your own - p. 449 - 450, Table 24.1 2 II. Modes of Speciation Interruption of gene flow within a population precedes speciation. Two general classes of interruptions A. Allopatric speciation - geographical barrier blocks gene flow initially (canyon, mts., etc) 1. newly isolated pops -accumulate genetic differences by microevolution 2. small peripheral isolate - already different then main - pop genetic drift 3. adaptive radiation - FIGURE 24.9 evolution of many, diversely adapted species, alike Galapagos, Hawaiian islands B. Sympatric speciation - new species arise within range of parent population. Genetic isolation without geographical isolation. How?? Genetic change - reproductive barrier - all in one generation - polyploidy requires errors in meiosis, mitosis 1. Autopolyploidy - more than 2 chromosome sets from the same species FIGURE 24.10(a) - once formed, can't breed with existing diploids offspring would be 2n + 1n = 3n - sterile triploid unpaired chromosomes = abnormal meiosis. 2. allopolyploid chromosome sets from at least 2 different species FIGURE 24.10(b) 3. 25-50% of all plant species - polyploid FIGURE 24.11 - wheat allopolyploid of 8000 years ago C. Punctuated equilibrium (PE)- the tempo of speciation 1. FIGURE 24.13 - (a) gradualism - tempo of change is slow and steady 3 (b) PE - most morphological change as species buds from parent pop. then change little. Evidence - the fossil record (few "missing links") -long periods of statis punctuated by episodes of speciation D. The origin of evolutionary novelty 1. evolutionary novelties as modified version of old structures. 2. exaptation - evolution does not anticipate a future use - new structure is co-opted for another function e.g. light, honeycombed bones of birds did not evolve in anticipation of flight - rather from light bones of agile dinosaurs "Evolution is like modifying a machine while it is running"

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