Grant P R, Grant B R
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, N.J. 08544-1003, USA.
Proc Natl Acad Sci U S A. 1997 Jul 22;94(15):7768-75. doi: 10.1073/pnas.94.15.7768.
External (environmental) factors affecting the speciation of birds are better known than the internal (genetic) factors. The opposite is true for several groups of invertebrates, Drosophila being the outstanding example. Ideas about the genetics of speciation in general trace back to Dobzhansky who worked with Drosophila. These ideas are an insufficient guide for reconstructing speciation in birds for two main reasons. First, speciation in birds proceeds with the evolution of behavioral barriers to interbreeding; postmating isolation usually evolves much later, perhaps after gene exchange has all but ceased. As a consequence of the slow evolution of postmating isolating factors the scope for reinforcement of premating isolation is small, whereas the opportunity for introgressive hybridization to influence the evolution of diverging species is large. Second, premating isolation may arise from nongenetic, cultural causes; isolation may be affected partly by song, a trait that is culturally inherited through an imprinting-like process in many, but not all, groups of birds. Thus the genetic basis to the origin of bird species is to be sought in the inheritance of adult traits that are subject to natural and sexual selection. Some of the factors involved in premating isolation (plumage, morphology, and behavior) are under single-gene control, most are under polygenic control. The genetic basis of the origin of postmating isolating factors affecting the early development of embryos (viability) and reproductive physiology (sterility) is almost completely unknown. Bird speciation is facilitated by small population size, involves few genetic changes, and occurs relatively rapidly.
影响鸟类物种形成的外部（环境）因素比内部（遗传）因素更为人所知。对于几组无脊椎动物来说情况则相反，果蝇就是一个突出的例子。关于物种形成遗传学的一般观点可以追溯到研究果蝇的杜布赞斯基。这些观点对于重建鸟类的物种形成来说并不是充分的指导，主要有两个原因。首先，鸟类的物种形成伴随着杂交行为障碍的进化；交配后隔离通常在很久之后才进化，可能是在基因交流几乎停止之后。由于交配后隔离因素进化缓慢，交配前隔离强化的范围很小，而渐渗杂交影响分化物种进化的机会则很大。其次，交配前隔离可能源于非遗传的文化原因；隔离可能部分受到鸣叫的影响，鸣叫是许多但并非所有鸟类群体中通过类似印记过程进行文化遗传的一种特征。因此，鸟类物种起源的遗传基础应在受自然选择和性选择影响的成年性状的遗传中寻找。一些参与交配前隔离的因素（羽毛、形态和行为）受单基因控制，大多数受多基因控制。影响胚胎早期发育（生存力）和生殖生理（不育）的交配后隔离因素起源的遗传基础几乎完全未知。鸟类物种形成因种群规模小而得到促进，涉及的基因变化很少，且发生得相对较快。
