Chapter 8 Population genetics and natural selection

1. Chapter 8 Population genetics and natural selection • Variation within populations--Variation within plant populations Phenotypic variation: Variation responded to the environment, but not hereditary changes.

2. Example: Bonniet’s survey and experiment: Potentilla grown at higher elevation grown at lower elevation different stem length, different internode length different leaf area. Fig.8.4;

3. Kerner’s planting experiment, plants of a same species when grown at higher and lower altitudes, also show different morphological difference.Fig.8.6.

4. But further experiment presented that plants from alpine seeds and from low elevation site (180m) seeds are indistinguishable. This means that the morphological differences are morphological adjustment to environmental conditions and not hereditary changes.

5. --Genetic variation: Variation is heritable even though in a different environment. Example: Turesson’s planting experiment. Plants from nine different areas planted in the same garden show different morphology. Fig.8.8 Another example:Fig.8.10.

6. Variation within animal populations Example: Sauromalus obesus, a large herbivorous lizard of the southwestern United States. The lizard at higher elevations has larger body than that at lower elevations. Because rainfall is higher and more constant at higher elevations, and food availability is much more constant. Tracy (1999) raised juvenile lizards in a same exp. Environment of the species from six populations living at different elevations (200-890m). At the end of the experiment, lizard from higher elevations grew to a larger size. Fig.8.14.

7. Change due to chance --Relation between genetic variation/diversity and population size: Larger population generally have higher genetic diversity than smaller populations. Example: Chihuahua spruce. Chihuahua spruce is restricted to small, highly fragmented cool moist areas of subalpine environment, at elevations between 2200m and 2700m. Relationship between population size and genetic diversity is showed in fig.8.18.

8. --Isolated island populations esp. endemic island populations have lower genetic variation/diversity than mainland populations. Fig.8.19, Fig.8.20. • Natural selection Natural selection by environment can result in evolutionary change in population, result in adaptation to the environment. Example: Lizard exp. of Losos et al. (1997).

9. Losos et al. (1997) experiment: Lizards from home island (forest height 10m) 14 islands (with no lizard before) with different forest height (1-3m) 10-14 years later, morphological measurement Result: Larger differences from the forest in Staniel Cay presented larger morphological variations. Fig.8.23.

10. Estimating genetic variation in populationsMethods:(1) Transplant experiments. Example: Transplanting of Potentilla glandulosa. Plants from lowland and alpine –Mid-elev. garden Plants from lowland and mid-elev. – alpine garden. Plants from mid-elev. and alpine – lowland garden.

11. If there were no genetic differences among the populations, all plants would grow equally well in all gardens. • If there were significant differences in growth among the populations in a same garden, there were genetic differences among the populations. Fig. 8.28.

12. (2) Molecular method: a. Measuring allozymes (等位基因酶). Fig.8.18. b. Measuring numbers and lengths of DNA fragments. c. Measuring DNA sequencing. Fig.8.29