HYBRID FITNESS SEEMS NOT TO BE AN EXPLANATION FOR THE PARTIAL REPRODUCTIVE ISOLATION BETWEEN ECOTYPES OF GALICIAN LITTORINA SAXATILIS

Partial reproductive isolation between closely related groupsof organisms is suggested to be of central importance duringspeciation. Galician populations of Littorina saxatilis arephenotypically differentiated into an upper-shore and a lower-shoremorph. These mate assortatively in the mid-shore zone of overlap,and genetic assessment shows an impeded gene flow between theparental morphs. The traditional explanation as to why assortativemating occurs is that reproductive isolation is reinforced dueto hybrid unfitness. Earlier studies have, however, not foundhybrids to be less viable. Likewise, growth and migratory behavioursare merely intermediate between those of the parental morphs.In the present study we compared male and female fertilitycomponents of the parental morphs and the hybrids to test hypothesesof decreased hybrid fertility. The results showed that hybridmales were as fertile as other males, and hybrid females didnot produce fewer embryos, nor aborted embryos at a higher rate,than the parental morphs. ^* To whom correspondence should be addressed. (Received 19 March 1999; accepted 7 July 1999)     

THE CHILD WHO DOES NOT TALK

The majority of normal children will have developed some speech by the age of two years. Significant delay in speech development may be the result of (1) deafness, (2) mental retardation, (3) faulty innervation of the speech organs, (4) aphasia, (5) autism, (6) a family trait, (7) hospitalism, (8) parental neurosis, or (9) some combination of these factors. Each nonspeaking child needs an individually planned study for precise diagnosis and recommendation for treatment.