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A series of free High School Biology Video Lessons.

Adaptive Radiation
Adaptive radiation is the relatively fast evolution of many species from a single common ancestor. Adaptive radiation generally occurs when an organism enters a new area and different traits affect its survival. An example of adaptive radiation is the development of mammals after the extinction of dinosaurs.

The fast evolution of species with a common ancestor.
Genetic Drift
Genetic drift results from random changes the genes and allele frequency of a group of organisms. Genetic drift is usually found in small populations because smaller random events have a bigger effect. A specific example of genetic drift is the founder effect which occurs when some organisms migrate into a new area but randomly don't match the parental population's genetic make up.

Genetic Flow
Genetic flow, also called gene migration, is the transfer of genes between different populations through migration. Immigration is when new organisms join a population, changing allele frequencies. Emigration is when members of a population leave, taking with them their genes. These change the overall balance of the gene pool of the populations these species join or leave. Gene transfer is the flow of alleles from one species to another. Gene transfer is especially common in bacteria.

How migration influences gene flow.
Reproductive Isolation - Prezygotic - Postzygotic
Reproductive isolation is a mechanism that keeps species from mating with others. Prezygotic isolation prevents the fertilization of eggs while postzygotic isolation prevents the formation of fertile offspring. Prezygotic mechanisms include habitat isolation, mating seasons, "mechanical" isolation, gamete isolation and behavioral isolation. Postzygotic mechanisms include hybrid inviability, hybrid sterility and hybrid "breakdown."

How reproductive isolation keeps species distinct.


Speciation is the formation of a new species. A species is defined as a group of organisms that can interbreed successfully with each other. In allopatric speciation, two populations can get physically separated and become two species. In sympatric speciation, on the other hand, a new species starts in the same location as the parental species due to polyploidy or hybridization.

How species are formed.

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