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Chapter 5: Speciation Mechanisms and Behavior Barry Sinervo © 1997 Chapter 5: Speciation Mechanisms and Behavior Species are groups of actually or potentially interbreeding populations, which are reproductively isolated from other such Barry Sinervo © 1997-2007 groups. Mayr 1942 BIOLOGICAL ISOLATING MECHANISMS....................................................................Bi....o..l.o..g..i.ca....l. .I..s..o..l.a..ti...ng.... .M....ec...1ha nisms Modes of Speciation Classified by Geography..............................................................................................3 Diagnosing Models of Speciation from Clinal Variation ...................................E..rn....s.t.. ..May........r.'.s. ...(1...9..4..2..).. ..fo....rmu4 lation of the biological species concept clarified a vexing question for evolutionary biologists: What is a PARAPATRIC VERSUS ALLO-PARAPATRIC SPECIATION ....................................................................................4 Ring Species in Gulls ............................................................................................s.p...eci....es....?.. .May........r'..s. ..v..i.ew..... .o..f.. .t5h e species concept has widespread acceptance A Paint Experiment in Gull Species Recognition................................................o..w...i.n..g.. .t..o.. .i.t.s.. .s..i.mp.....l..i.ci...t.y.... .N..6ev ertheless, the manner in which species arise by the process of natural selection remains controversial. The problem is Ring Species in Salamanders and the Evolution of Mimicry......................Error! Bookmark not defined. that the process of speciation takes a very long time to complete, and it SYMPATRIC SPECIATION ......................................................................................................................................6 occurs on a very large geographic scale. In addition to Mayr's definition Polyploidy ........................................................................................................................................................9 additional genetic concepts are also useful in delimiting species. Character Displacement in Mate Calling and Female Preference..............................................................9 Production of Asexual Parthenogenetic Species.................................................Hy.....bri.....d... .s..teri......l.i.ty.... .d..efi.....n..i.tel10y separates two species, although it is not the Multiple-Niche Polymorphism in Insects and Host-Plant Preference...............s.o..l..e. ...cri....t.eri....o...n... ..H...y..b..ri..d... .s1teri0 lity is a special instance of reproductive SYMPATRIC VERSUS ALLOPATRIC SPECIATION .......................................................i.s..o..l.a..ti..o...n.... .In... ..cas.....es... ..o..f. .rep... 1ro1 ductive isolation between species the barriers Assortative Mating is a Stringent Condition for Sympatric Speciation.............t.o.. ...mat......i.n..g.. ...d..o.. ..n..o...t. ..j.u..s..t 11h ave to be due to incompatibility between Cultural Inheritance of Song and Speciation in Darwin's Finches ...................g..en....o..mes....... ..T...h..e.. ..b..l.o..ck....s.. ..co1u2l d arise at various levels. These ecological, Deme Recognition Female Choice for Natal Song .............................................beha........v..i.o...ra...l..,. ....p..hy....s..i.o..l..o..g1ica3 l, morphological or genetic isolating Reinforcement and Sympatric Speciation............................................................m...echa........ni....s.m....s.,. ...w...h..i.ch.... ..s..erv14e to limit gene flow between species, are SPECIATION AND SMALL POPULATION SIZE ............................................................cat.....eg...o...ri..zed...... .i.n.. .t.er....ms.... .o...f. .p1re6- vs. post-mating mechanisms (Endler, 1977): Founder Effect Speciation ........................................................................................................................... 16 A) Premating isolating mechanisms -- prevent union of gametes that Runaway Process, Sexual Selection, and Speciation................................................................................. 16 produce a zygote STUDY QUESTIONS FOR CHAPTER 5 ................................................................................................................ 17 1. mates do not meet (seasonal or habitat isolation) 2. mates meet but do not mate (ethological or behavioral isolation) 3. mates meet but no sperm transfer occurs (mechanical isolation) B) Post-mating isolating mechanisms -- varying degrees of hybrid sterility or fitness (which are largely due to gene interactions). 1. sperm transferred but dies before fertilization 2. zygote dies during development 3. zygote produces an F1 adult that has reduced viability (survival) 4. hybrid is viable, but it is partially or completely sterile (fecundity) or the F2 is in some way deficient. 79 Asking the following questions reveals the answer to the adaptive value played a major role in promoting the process of speciation (Dobzhansky, of pre- versus post-mating isolating mechanisms. Which of the two 1941, Butlin, 1987). mating isolation mechanisms is the most effective at reducing the risk of What are the right conditions? This is a difficult question to answer. It producing low fitness offspring? Which mechanisms are most efficient requires an understanding of the field of population genetics and the in terms of time and lost reproductive opportunity? crucial parameter is how much gene flow occurs from areas where each An organism that mates with a different "semispecies" may produce low semispecies exists in its pure form. A second parameter has to do with viability offspring and this individual would have lower fitness the degree of genetic similarity between the two semispecies. compared to an organism that discriminates against such semispecies Gene flow is the movement of animals between what are referred to as and mates with members of its own semispecies. Evolving species populations of interbreeding animals. Rates of gene flow are usually discrimination mechanisms is clearly an advantage over one that only expressed as the proportion of individuals in a population that move possess post-mating isolating mechanisms. Such behavioral from one population to another generation. If gene flow between the discrimination ability should evolve rapidly in the area of incipient region of contact between the two semispecies and the rest of the speciation, under the right conditions. For these reasons, the study of semispecies ranges is too high, then the two semispecies will become behavior is central to understanding the processes of speciation. homogenized into one species even if interbreeding yields relatively low An example of premating isolation is found in the flour beetle, fitness hybrids (Endler, 1977). If gene flow is low enough, then the two Tribolium confusum). When a research pairs a female from Nigeria with pure semispecies will eventually become differentiated into two species a male from a different locality (e.g., allopatric population from owing to the evolution of isolating mechanisms. The evolution of such Croatia), viable offspring result. The same is true for the reciprocal isolating mechanisms cross. However, if a researcher simultaneously pairs the female beetle is contingent upon the Figure 5.1. Hypothetical hybrid “unfitness” resulting from an incompatibility between with a male from the same population and an allopatric population, the degree of genetic genomes of the two parental species. female preferentially sires progeny by the same-locality male. The mean similarity between the relative fitness of the sympatric male, 0.425, is much higher than the two semispecies. allopatric male (0.085). Behavioral observations indicate that the female Large differences in is less likely to become quiescent during mating attempts by allopatric genetic similarity males than when she is approached by males from her population. between semispecies Biological barriers to interbreeding undoubtedly arise during the process are more likely to of speciation because such blocks have selective value. Behavioral produce unfit hybrids isolation can be the strongest form of reproductive isolation. The than small differences clearest examples of the biological species concept in action are in similarity. If the provided when two species, which have overlapping geographic ranges, two semispecies have do not interbreed owing to differences in pre-mating behaviors. One many genetic loci that species or both species refuse to engage in the mating rituals because differ in the kind of their respective stereotyped mating rituals are too different. Species allele that they might simultaneously possess both premating and post-mating isolating possess, then those mechanisms in which case the probability of interbreeding is even more alleles might not work remote. It is these cases that are most interesting from the point of view very well together. of behavioral mechanism, as natural selection on behavior may have More importantly the way many different genetic loci work together 80 may begin to breakdown. The correct operation of one genetic locus subdivision decreases gene flow. Depending on the population size, (e.g., an enzyme) may require a particular form of the enzyme or protein genetic drift can play a major role in promoting genetic differentiation. product that is produced by a different locus (e.g., epistasis) (Whitlock If population size remains low for a Figure 5.2. Models of speciation et al
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