How do new species arise? -- Speciation -- Species: group of organisms that can interbreed and produce fertile offspring Speciation: the process by which genetically distinct species arise. Speciation: Formation of new species As adaptations and differences accumulate, the organisms become so different that they cannot interbreed (reproduce) anymore

The Unit of Evolution is a Population: A population is a group of individuals, of the same species, living in the same area, that interbreed (reproduce) Populations Have Genetic Variation (Variability) Individuals in a population are different. Small differences are determined by their genes (DNA sequences)

Speciation: Formation of New Species If a population becomes separated, or a group of individuals becomes isolated, they may change over time. They may change so much that they cannot interbreed anymore with the members of the original population. Reproductive Isolation. Reproductive Isolation is the first step towards the formation of a new species What influences reproductive isolation: Geographic Isolation Behavioral Isolation Temporal Isolation

1. Geographic Isolation A physical barrier separates (splits) the original population As time passes, the environment may selects for different traits and the 2 populations become genetically different AND can’t reproduce Barriers: Rivers, mountains, bodies of water

Kaibab and Abert Squirrels - The Kaibab squirrel (Sciurus aberti kaibabensis) became geographically isolated from the common ancestor about 10,000 years ago. Its closest relative is the Abert squirrel (Sciurus aberti aberti) - They live in opposite sides of the Grand Canyon Kaibab Abert

Example: Emus, ostriches, rheas, Cassowaries, Common ancestor lived in the continent of Gondwana (all southern continents together) When Gondwana broke up, the populations separated and changed over time. All these birds are closely related but live far apart.

Wrasses: Original population split up when the Wrasses: Original population split up when the isthmus of Panama was formed - 2 species now, one on each side of Central America

2. Temporal Isolation (Time of Reproduction) Groups of individuals reproduce at different times. The genes of the 2 groups do not mix. It is happening now with some of the migratory birds that decide not to migrate. The birds that do not migrate start mating before the migratory ones come back. Eventually they might become different enough that they may not recognize each other.

Robins usually migrate south to warmer climates. Some are not migrating. They reproduce before the migrating individuals return. Reproductive Isolation

3. Behavioral Isolation (Reproductive Behavior changes) If 2 populations have different courtship behaviors, they will not be able to interbreed Different songs (birds) or mating calls (frogs) Different scents (smells) A simple mutation can cause a change

How fast is evolution? How fast do organisms change? Gradualism Punctuated Equilibrium

Gradualism Organisms go through gradual and continuous change

Punctuated Equilibrium Organisms go through fast periods of change, followed by long periods of no change (according to fossil record)

Punctuated Equilibrium Proposed by Stephen Jay Gould (1941-2002)

Gradualism Punctuated Equilibrium

Patterns of Evolution: 1. Convergent Evolution Convergent Evolution: When 2 or more unrelated species become more similar due to similar adaptations to their environment.

Patterns of Evolution: 2. Divergent Evolution-Adaptive Radiation Speciation and Patterns p. 404-410; 435-440 Patterns of Evolution: 2. Divergent Evolution-Adaptive Radiation Divergent Evolution: when related species become more different as they adapt to different environments Divergent evolution leads to Adaptive Radiation

As species change over evolutionary time, the DNA sequences in their genes acquired slight changes. According to evolutionary theory, these changes in DNA and thus proteins accumulate over time. Species that diverged from each other long ago have more differences in their DNA, than species that diverged recently. Scientists use this degree of difference as a molecular clock to help them predict how long ago species split apart from one another. More Differences = less related, more time since split from common ancestor. Fewer Differences = more related, less time since split from common ancestor.