ISOLATING MECHANISMS AND SPECIES FORMATION 



different times, and so do not have the opportunity to cross. But the occa- 

 sional hybrid trees can be explained easily by the fact that some trees 

 of an early variety may pollinate later than usual, and some of a late 

 variety may pollinate earlier than usual. Anderson has shown that such 

 seasonal differences are important in isolating some species of the Iris. 

 The same has been demonstrated for some other genera, and it seems 

 probable that seasonal isolation is common among plants. 



Seasonal isolation is by no means unknown among animals. Generally 

 speaking, the breeding season of warm-blooded vertebrates is rather long, 

 and the seasons of many members of the same group coincide or overlap 

 broadly. Nonetheless, Mayr cites several instances in which seasonal iso- 

 lation appears to be effective among birds. But among the cold-blooded 

 vertebrates and among the invertebrates, breeding seasons may be quite 

 restricted, and so seasonal isolation may be highly effective. In northeast- 

 ern United States Rana clamitans, R. pipiens, and R. stjlvatica may all 

 breed in the same pond. But generally sylvatica will begin breeding before 

 the others appear in the ponds. And clamitans may not begin breeding 

 until the others are through. Water temperature is the decisive factor in 

 determining when each species begins breeding. R. sylvatica begins to 

 croak at water temperatures as low as 44° F in southern Michigan, and 

 probably breeds at temperatures only slightly higher. R. pipiens begins 

 to breed when the water temperature reaches 55° F, or perhaps somewhat 

 higher. R. clamitans does not appear in the ponds until the water temper- 

 ature exceeds 60°, and the minimum temperature for breeding has not 

 been determined for this species. Actually, crosses between these three 

 species fail in the early embryonic stages, but the seasonal separation 

 may have preceded the sterility barrier and made possible its develop- 

 ment. A similar situation applies to the salamanders, Amhystoma tigrinum 

 and A. maculatum in the same area. A. tigrinum begins breeding very soon 

 after the spring thaws, and is usually through breeding by the end of 

 March, But A. maculatum does not begin breeding until late in March 

 or early in April. Blair has published comparable data for the toads of 

 the genus Bujo. 



There are many instances known in which behavior differences, par- 

 ticularly in regard to courtship, restrict random mating between members 

 of different species. Among plants, mating is usually based upon more or 

 less mechanical (from the viewpoint of the plant) situations, such as wind 

 pollination or insect pollination. But among animals, some preparation 

 usually precedes mating. This may be very slight for some marine animals 

 like the sea urchin in which discharge of gametes, together with some sex 

 stimulating substance, by a single individual may induce all or most of 

 a large colony to shed their sex cells. Gametes are then mixed by the 

 water currents, and fertilization follows. At the other extreme, elaborate 

 courtship, sometimes lasting for many days, may precede mating. Mayr 

 has pointed out that in many birds pair formation may precede copulation 

 by periods up to several weeks. Among such species, he finds that inter- 

 specific hybrids are very rare, while they may be rather common between 

 closely related species which do not have such an "engagement" period. 



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