tailieunhanh - ECOTOXICOLOGY: A Comprehensive Treatment - Chapter 18

Dân số Di truyền học: Lựa chọn tự nhiên 18,1 TỔNG QUAN chọn lọc tự nhiên Lựa chọn tự nhiên hành vi độc quyền bởi việc bảo tồn và tích lũy của các biến thể, có lợi theo các điều kiện hữu cơ và vô cơ mà mỗi sinh vật đều là tiếp xúc ở tất cả các giai đoạn của cuộc sống. (Darwin ) | 18 Population Genetics Natural Selection OVERVIEW OF NATURAL SELECTION Natural Selection acts exclusively by the preservation and accumulation of variations which are beneficial under the organic and inorganic conditions to which each creature is exposed at all periods of life. Darwin 1872 General Natural selection will now be described in order to complete our discussion of pollutant-influenced evolution. More specifically natural selection resulting in microevolution will be explored. Microevolution is evolution within a species in contrast to macroevolution that focuses on evolutionary processes and trends encompassing many species. Emphasis will be placed on microevolution leading to enhanced resistance. The terms resistance and tolerance will be used interchangeably as done elsewhere Forbes and Forbes 1994 Newman 1991 1998 Weis and Weis 1989 . Some authors object to this synonymy reserving resistance to mean the enhanced ability to cope with toxicants because of genetic adaptation and tolerance to mean the enhanced ability to cope with toxicants because of physiological biochemical or some other acclimation. Natural selection is the change in relative genotype frequencies through generations resulting from differential fitnesses of the associated phenotypes. Pertinent differences in phenotype fitness can involve viability survival or reproductive aspects of an individual s life. Natural selection has the same basic qualities regardless of the life cycle component s in which it manifests. It has three required conditions and two consequences Figure as summarized by Endler 1986 . The first requisite condition is the existence of variation among individuals relative to some trait. The second is fitness differences associated with differences in that trait . differences in survival or reproductive success among phenotypes. The third condition is inheritance the trait must be heritable. Of course another implied requisite is Thomas Malthus s .