Earth formed about 4.6 bya. The primative atmosphere was much different than todays; there was no oxygen. The surface of the Earth began to cool, gases condensed and large amount of rain created the ocean. Lightning storms allowed the inorganic molecules to break up and rearrange themselves into organic molecules included proteins, RNA and DNA. Eventually, membrane formed that separated some of the molecules form the environment. These are thought to be the precursers of cells and are called protobionts or protocells. These developed the ability to replicate. 3.9 bya heterotrophic prokaryotes formed. A mutation lead to the development of autotrophic prokaryotes about 2.7 bya (similar to cyanobacteria) which carry out photosynthesis and introduce oxygen to the atmosphere and create the ozone layer. Eukaryotic cells are produced through the endosymbiotic theory approximately 2.1 bya. Multicelluar organisms began to develop about 1.2 bya.
Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print.
Speciation is the evolution of a new species. A species is a group of organisms that bred and produced fertile offspring in nature. Reproductive isolation has to occur in order for speciation to occur; this typically arises from geographical isolation.
Adaptive radiation is when evolution rapidly results in multiple species from one. This happens when a species is introduced to place that has multiple different environmental conditions (e.g. group of islands). This is how Darwin's finches developed and is thought to be how the different types of marsupials developed on Australia.
Prezygotic isolation can occur by temporal isolation (mate at different times), habitat isolation (do not live in the same areas), behavior isolation (not recognized as a mating partner [wrong pheromones, song, rituals]), mechanical isolation (reproductive structures do not match), or gametic isolation (male gametes can survive with females or are unrecognized).
Postzygotic isolation occurs by hybrid inviability (zygote is unable to develop), hybrid sterility (can not reproduce), or hybrid breakdown (hybrid offspring are sterile or weak).
Source: PACK, P. (2013). AP BIOLOGY. (4TH ED., PP. 73-81). BOSTON: HOUGHTON MIFFLIN HARCOURT., N.d. Photograph. Biology NotebookWeb. 22 Feb 2014. .