behavior tliat are nut stimulating to tlic sexes of tlie 

 otlier. 



I'".thological isolating ineciianisins (Spietii 1958), 

 effective muier natural conditions, often lose efficacy 

 when those conditions are disturbed : by so nuich they 

 are difficult to work with experimentally. Where 

 their home ranges overlap, two species of mice, Pcro- 

 ntysciis Icucopiis and P. ijossypiniis, frequently occur 

 in the same habitat. Yet, very few hybrids have ever 

 been found. When brought into the laboratory, how- 

 ever, the two species not only hybridize freely, but 

 produce fertile offspring. It appears that ethological 

 factors keep them separated under natural conditions 

 (Dice 194(|). 



Two closely related budworm species, Choristo- 

 ui'iira jiiniifcrana found on balsam fir, and C. pinus 

 found on jack pine, are isolated ecologically on dif- 

 ferent host trees, and because the first species com- 

 pletes its mating season before adults of the second 

 species appear on the wing. Mthological isolation oc- 

 curs when occasionally their mating periods overlap. 

 Females will mate only with males of their own spe- 

 cies, even though males of both species attempt to 

 mate indiscriminately (Smith 1954). 



Species related but of different sizes may be un- 

 able to fit their copulatory organs together, because of 

 structural incompatibilities. Such hindrance to inter- 

 breeding is a mechanical isolation. Failure of male 

 toads to clasp females of larger or smaller species 

 results in reproductive isolation between species of 

 Microhyla (Blair 1955). Polygyrid snails of the 

 genus Stenotrema have definite behavior patterns 

 prerequisite to copulation. A careful study of several 

 species in this genus (Webb 1947) showed that dif- 

 ferences in these behavior patterns are sufficient to 

 keep some species separated, but that in other in- 

 stances it is differences in the structure of the copu- 

 latory organs that apparently prevents interbreeding. 



Genetic isolation occurs when there is inability 

 to produce offspring because of incompatibility of 

 spermatozoa and eggs, abnormalities of growth, or 

 the offspring are sterile. Sperm of the sea urchin 

 Strongylocentrottis jranciscanus sufficient to give 73 

 to 100 per cent fertilization of eggs of the same spe- 

 cies produced only to 1.5 per cent fertilization of 

 S. piirpuratiis eggs. Eggs of one species may some- 

 times be successfully fertilized by sperm of another, 

 but all sorts of disturbances may occur in the zygote, 

 such as chromosome elimination during cleavage, ar- 

 rest of gastrulation or organ formation, and death of 

 embryos in advanced stages. A well-known example 

 of a usually sterile hybrid is the mule, the result of 

 a cross between a male ass and a female horse. 



Genetic and mechanical isolation usually furnish 

 more certain reproductive separation between species 

 than do ecological or ethological isolation. The latter 

 two forms probably represent early steps in the proc- 



ess of speciation ; the former two, the culmination of 

 s|)eciation. 



(jco/irapliic isolation 



F'hysiographic barriers such as land masses, 

 mountain ranges, and bodies of water can effect com- 

 plete or nearly complete isolation of populations. 

 Population segments of a species may become geo- 

 graphically isolated when reproductive individuals 

 cross a barrier by chance. For instance, individuals 

 may be blown by storms or carried by rafts to outly- 

 ing islands. A barrier may arise subsequent to the 

 dispersal of a species, such as when species disperse 

 into a new area by way of land bridges which later 

 disappear. 



When barriers are only partially effective so that 

 gene flow between adjacent populations is hindered 

 but not stopped, and natural selection goes on inde- 

 pendently in each area, some differentiation of the 

 populations may occur but not above the level of 

 subspecies. Species that are widely dispersed over a 

 continent often display several local centers where 

 differentiation is occurring. Individuals dispersing 

 outward from these centers, however, meet and inter- 

 breed so that intergradation of characters occurs, and 

 there is gene flow from one end of the species' 

 range to the other. The rate of gene flow is directly 

 proportional to the rate at which individuals disperse 

 from birthplaces, in the face of any intervening bar- 

 riers. This is true even of birds and insects. Less 

 than 5 per cent of young pied flycatchers surviving to 

 one year of age return to their places of birth in order 

 to nest ; the other 95 per cent disperse widely. Only 

 4 .subspecies have differentiated in this species, 

 since there is widespread promiscuous interbreeding 

 between individuals. On the other hand, 63 per cent 

 of young song sparrows surviving to sexual maturity 

 return to nest in the vicinity of their birthplaces ; 

 only 37 per cent disperse elsewhere. This results in 

 a limited flow of genes from one locality to another 

 and is correlated with the development of 28 sub- 

 species (Diver 1939, Marshall 1948, Haartman 

 1949). 



When the rate of gene flow is slow, mere distance, 

 even over territory unbroken by physiographic bar- 

 riers, may permit populations to vary enough for sub- 

 speciation, or potentially even full speciation, to occur. 

 In the range from Maine to Florida, populations of 

 leopard frogs readily interbreed with adjacent 

 populations. But when individuals from Maine are 

 brought together with individuals from Florida, the 

 two cannot interbreed successfully (Moore 1946). 

 There is a progressive change from North to South 

 in several characteristics of the leopard frog, and it is 

 of interest that if populations intermediate in the 



Speciation 259 



