Hybridization between Senecio smallii and S. tomentosus (Compositae) on the granitic flatrocks of the Southeastern United States
Senecio smallii andS. tomentosus occasionally hybridize on the granitic flatrocks of the Piedmont of the Southeastern United States. These hybrids are intersectional crosses between sectionsAurei andTomentosi. The parental species have different chromosome numbers withn = 22 inS. smallii andn = 23 inS. tomentosus. Meiosis in the hybrids is abnormal with 22ns and li or 2iis and 3is, and the percentage of stainable pollen is greatly reduced over that of the parents. The hybrid pollen grains are structurally abortive as demonstrated by the scanning electron microscope. Origin of the natural hybrids was documented by the production of artificial hybrids including F1’s, F2’s, and BC’s. Very few of the F2 seeds germinated, and all of the F2 seedlings later died, indicating hybrid breakdown. Seeds of the F1 and BC generations germinated well, and the seedlings were vigorous. Population samples indicate that there is some variation within the natural population systems; however, no evidence of introgression was detected. The isolating mechanisms and possible evolutionary effects of this natural hybridization are discussed.
The role of small populations for the differentiation in plants.
When trying to answer the question whether the effects of genetic drift have evolutionary significance, we must keep in mind that a local population is normally a dynamic system, changing in size, splitting into minor entities, or merging into larger complexes. Most populations have certainly been small for longer or shorter periods, and hence the role of genetic drift must not be neglected. The main role of genetic drift in the complex processes involved in the evolution of plants may be (1) the introduction of genetic factors, which are primarily non-adaptive, (2) the rapid fixation of genetic changes, and (3) the breaking down of rigid gene blocks. Generally speaking a population may be said to be in a selective phase when it is large and in a creative phase when it is small.
Marie Curie's doctoral thesis: Prelude to a Nobel Prize
This paper is a critical account for the most significant body of Marie Sklodowska Curie's scientific work, presented in the hopes that she be remembered for more of the "right" reasons. It also offers some new insights into her methods and results.