Natural Selection More offspring than will survive Individuals vary in the population Individuals with certain variations will survive in their environment, pass on those variations to the next generation Overtime, offspring with certain variations make up MOST of the population

Types of Selection 2. Directional Selection Stabilizing selection 3. Disruptive Selection 3. Disruptive Selection

Populations change over time…not individuals! Aa AA aa AA

Reproductive Isolation due to… Geographical isolation Mechanical Incompatibility Behavioral isolation Seasonal isolation

Mechanical

GeographicaL Figure: 16-2 Title: original population geographical isolation ecological isolation GeographicaL genetic divergence Figure: 16-2 Title: Models of allopatric and sympatric speciation Caption: The main difference between allopatric (left column) and sympatric (right column) speciation is the mechanism by which genetic isolation arises. 1 Both cases begin with a single species (white mice) occupying a relatively homogeneous habitat. 2 In allopatric speciation, the population is separated into two isolated populations by an impassable geographical barrier (here, a river changing course). In sympatric speciation, there are no physical barriers to movement, but different parts of the population become restricted to different habitats. 3 In both cases, genetic drift and/or different environmental pressures cause the two populations to diverge genetically (tan vs. white mice). 4 The isolated populations may later come into contact. If the genetic differences between the two populations have become large enough to prevent inbreeding, then the two populations constitute separate species (brown vs. white mice). Exercise Make a list of events or processes that could cause geographical or ecological subdivision of a population. Are the items on your list sufficient to account for formation of the millions of species that have inhabited Earth? reproductive isolation

Behavioral

Seasonal

Sterility Male donkey & female horse makes a mule + =

Sterility Female donkey and male horse makes a “hinny”

Sterility Liger Zonkey