Anthropology 150

Evolution and Natural Selection

Charles Darwin

Evolution, Part 1: What Evolution is and is not

 Evolution is not controversial in the scientific community

 Evolution is a theory, in the scientific sense – That does not mean it is speculative

– It does mean that it is subject to revision as more data emerge

– The debates concerning evolution are about the mechanisms and form of selection, not whether it happened

Evolution, Part 2

 Evolution is not progressive. It does not have a goal, i.e., things do not get “better” through time.

 Evolution is about being biologically successful, and the traits that confer success vary with the circumstances

 Complexity isn’t better. “Higher” life forms are not better in any absolute sense. The idea of “higher” forms is a hold-over from the Great Chain of Being

Anagenesis Anagenesis, or phyletic evolution, is the accumulation of changes associated with the transformation of one species into another, e.g., if a species persists for a long period of time, it may slowly accumulate enough change so that later fossils are very different from earlier fossils. At some point, paleontologists give the new fossils a new name, thus designating a new species. Although the accumulation of changes is important, the assignment of a specific name is clearly arbitrary. In a sense, anagenesis is somewhat arbitrary.

Anagensis: The Linear Model of Evolution

The Branching Model

The Major Components of the “Modern Synthesis” of Evolution  Mutation

– The development of new genetic traits

 Inheritance

– The transmission of genetic material from one generation to the next

 Natural Selection

– The culling of each generation, so that only a portion of the population survives to reproduce

What you need to know about Mutation

 Mutations happen

Genetics – Mendelian and other

 Traits are passed from one generation to the next as discrete units (genes)

– Genetic traits don’t “blend”

 Individuals carry two expressions of a given trait. These are called alleles.

 Alleles can be dominant or recessive

– Dominant traits are expressed

– Recessive traits aren’t expressed unless both alleles are the recessive form

The Hardy-Weinberg Principle

 In a population with certain allele frequencies, the next generation will have the same allele frequencies as its predecessor if the following conditions are met:

1. The population must be sufficiently large for statistical trends to be apparent (ideally, the population is of infinite size)

2. Breeding must take place at random 3. There must be no introduction of new

genetic material 4. All individuals must have an equal chance of

surviving and reproducing.

Natural Selection

 In formulating the theory of Natural Selection, Charles Darwin made four critical observations

– There are fossil representatives of animals that no longer exist. In this he followed Lyell

– Every species produces more young than survive to maturity

– Individuals within a species show variation. They aren’t identical

– A number of species in adjoining areas show evidence of common ancestry with subsequent change occurring locally.

Evolution at Work  Mutation explains how variation is produced

 Mendelian genetics shows how variation is transmitted

 Natural selection shows how variation can lead to “descent with modification” by killing off part of the population. Selection occurs via death prior to reproduction

 Natural selection is about the frequency of alleles in a population

– Directional selection

– Stabilizing selection

Example: Darwin’s Finches

Example: The devleopment of Neandertals

Homo erectus ca. 500,000 BP Homo neandertalensis ca. 50,000 BP

Neandertals Developed as An adaptation to Ice Age conditions In western Europe Individuals that were Not stocky did not Survive to reproduce.

Directional Selection and Stabilizing Selection

Next: Reconstructing Phylogenetic Relationships and Primate Evolution