The Nature of Selection

The Nature of Selection E3: Lecture 2

Darwin’s Voyages The voyage of the Beagle: At the age of 22, Darwin set out on a trip around the world aboard the HMS Beagle. During this five year voyage, Darwin collected many specimens and studied geological formations. Contributions to Darwin’s later ideas: Reading Lyell’s Principles of Geology Discovery of fossils similar to known species Observation of geographical patterns in groups of species The ensuing intellectual voyage: Further contributions to Darwin’s theory: Discovery that some of his collected “variants” were actually different species Reading Malthus’ Principle of Population amount time “What fine opportunities for geology and studying the infinite host of living beings: Is this not a prospect to keep up the most flagging spirit?" Malthusian Principle population intense comp. food supply

The Origin What was Darwin trying to explain? The collection of living forms on this planet is diverse: There is variation in form within species. There is variation in form between species. There are hierarchical connections across species. Darwin postulated that the variation within species was the raw material for variation between species: In many cases, organisms are well-adapted to their environment. Darwin proposed a mechanistic process by which a lineage could change over time, accounting for adaptation and the origin of new species: Evolution by natural selection. kangaroo rat wallaby mouse hummingbird “I look at varieties which are in any degree distinct and permanent, as steps leading to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to species.”

The Nature of Selection Evolution by natural selection occurs when: There is variation in phenotypic traits among individuals within a population. At least some of the variation is heritable. There is variation in the ability to survive and reproduce (fitness), which is correlated to phenotype. OR… Natural selection occurs when there is heritable phenotypic variation correlated with fitness Darwin’s postulates offer evolutionary biologists a set of conditions to explore. Do natural populations satisfy all three postulates? If so, why? Darwin’s postulates spell out criteria for animal and plant breeders. phenotype average fitness 3 1 0

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

Darwin’s Analogy Breeding explicitly involves a realization of Darwin’s third postulate: By allowing animals and plants with desired phenotypes to breed and those with undesired phenotypes not to breed Darwin himself was a pigeon breeder and he spends several pages in his first chapter discussing the morphological and behavioral pecularities of the different breeds. All pigeons were known to be (1) descendents of the rock pigeon and (2) inter-fertile. Darwin claimed an ornithologist “would certainly [rank these breeds] as well-defined species.” Here Darwin is building his case for the origin of new species through natural selection through an analogy with the origin of new breeds through artificial selection: pouter tumbler rock fantail trumpeter …there will be variation in the ability to survive and reproduce (fitness), which is correlated to phenotype. The Birmingham roller “I felt fully as much difficulty in believing that since they had been domesticated they had all proceeded from a common parent, as any naturalist could in coming to a similar conclusion in regard to the many species of finches, or other groups of birds, in nature.”

Domestication as Selection Experiments Human agriculture and domestication involve a series of selection experiments. Maize, which accounts for over 20% of human nutrition, was domesticated from the Mexican grass teosinte 9000 years ago. A Mediterranean plant is ancestor of a set of vegetables (cabbage, cauliflower, broccoli, kale, etc.) via selection over 7000 years. All breeds of dogs derive from the gray wolf via domestication 10,000-100,000 years ago. teosinte maize GO UW!

The Fox Farm Experiment Clearly Darwin’s first (variability) and third (fitness differences) postulates hold. Question: How would you establish Darwin’s second postulate (heritability) experimentally?

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

Classic Study of Natural Selection During his voyage aboard the Beagle, Darwin visited an archipelago of volcanic islands 600 miles off the west coast of South America: the Galápagos. • These islands are home to a set of interesting organisms as well as interesting research. • Darwin collected finches from some of the islands, finding out later that these belonged to different species. • Coming full circle, Rosemary and Peter Grant have studied natural selection in Galápagos finch populations under natural settings. • Here, we focus on beak evolution in the medium ground finch.

The Nature of Selection