The Nature of Selection The Nature of Selection

E3: Lecture 2 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

“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?"

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Malthusian Principle

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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.

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“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:1. There is variation in phenotypic traits

among individuals within a population.2. At least some of the variation is

heritable.3. 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.

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Natural Selection Natural Selection

Lecture Outline

• Artificial selection

• Case studies of natural selection

• Dr. Pangloss and some caveats

• Summary

Natural Selection 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:

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“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.”

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…there will be variation in the ability to survive and reproduce (fitness), which is correlated to phenotype.

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.

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The Fox Farm ExperimentClearly Darwin’s first (variability) and third (fitness differences) postulates hold.

Question: How would you establish Darwin’s second postulate (heritability) experimentally?

Natural Selection 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.