UV-Resistance Can Evolve by Natural Selection The postulates are satisfied: –Postulate 1: Daphnia from different ponds differ in their tolerance of UVR. –Postulate 2: Daphnia clones display similar tolerance to UVR. –Postulate 3: Daphnia from more transparent ponds have higher UVR tolerance (survive better under UVR). Brooks tested postulate 3 by isolating animals from different ponds, growing them in a common garden and exposing them to different levels of UVR. Visible UV-A UV-B Question: What do you think is the basis for the differences in tolerance?

Natural Selection Lecture Outline Artificial selection Case studies of natural selection Dr. Pangloss and some caveats Summary

The Best of All Possible Worlds? Darwin’s original insight was that natural selection not only led to phenotypic change, but also adaptation. Evolutionary biologists Lewontin and Gould were critical of what they called the “adaptationist program”: –Natural selection is the only important evolutionary force –Selection acts on “atomized” traits –Selection optimizes (“Panglossian Paradigm”) Their critique of these views: –Other evolutionary forces (e.g., drift) are important in evolutionary change. –Selection works on integrated phenotypes, not atomized pieces. –Direction of selection is always constrained by history (or phylogenetic constraints). They argued that many biologists were guilty of adaptive storytelling. Richard Lewontin Stephen Jay Gould Voltaire “All is for the best in the best of all possible worlds”

Caveats about Selection Selection is one evolutionary force, a set including drift, migration and mutation. Phenotypic change need not be attributed to the operation of selection (e.g., in small populations, drift can strongly influence phenotypic change over time). Organisms are integrated and selection for one trait (e.g., tameness) can alter other traits (e.g., tail, coat, and ears). Selection is constrained by history (e.g., all tetrapods go through an embryonic stage with five-digit limbs). These caveats are important; nonetheless selection can be a powerful force. To demonstrate it most fully, one would: –Confirm that the postulates hold. –Study the genetics/development of phenotype. –Study the reasons for fitness differences.

Natural Selection Lecture Outline Artificial selection Case studies of natural selection Dr. Pangloss and some caveats Summary

Darwin suggested that natural selection could play a part in the emergence of new species as well as the adaptation of organisms to their environment. The process of selection requires heritable phenotypic variation with fitness consequences (“Darwin’s postulates”). Darwin made his case for natural selection through discussion of artificial selection: humans have dramatically changed animals and plants, sometimes in short periods (e.g., foxes). The postulates have been confirmed in a number of natural systems as well (e.g., finches, mice, and daphnids). While a powerful force, selection is not solely responsible for all evolutionary change and always works with constraints.