Swine are vulnerable to infection by bird flu virus and human flu virus, which can both be present in an individual pig at the same time. When this occurs, it is possible for genes from bird flu virus and human flu virus to be combined, thereby producing a genetically distinctive virus, which can subsequently cause widespread disease. The production of new types of flu virus in the manner described above is most similar to the phenomenon of

Population evolution is the process by which gene frequencies change within a gene pool. Evolution is the theory that explains how populations of organisms change over time.  Evolution is a change in the inherited traits of a population over successive generations. It is not directed towards a goal, nor is it solely dependent on natural selection.  The theory of evolution explains the similarities and differences within life's organisms and processes. According to this theory, every organism shares a common ancestor.  The two major mechanisms that drive evolution are natural selection and... Show more

Population evolution is the process by which gene frequencies change within a gene pool. Evolution is the theory that explains how populations of organisms change over time. 

Evolution is a change in the inherited traits of a population over successive generations. It is not directed towards a goal, nor is it solely dependent on natural selection. 
The theory of evolution explains the similarities and differences within life's organisms and processes. According to this theory, every organism shares a common ancestor. 
The two major mechanisms that drive evolution are natural selection and genetic drift. 

Other mechanisms include:
Non-random mating
Allele (gene) flow
Mutation
Migration
Competition
Speciation 

Natural selection can only take place if there is variation, or differences, among individuals in a population. These differences must have some genetic basis. 
Genetic drift is the evolutionary mechanism that causes random fluctuation in allele frequencies across generations. 

It can be caused by:
Population bottlenecks: A sharp reduction in the size of a population
The founder effect: When a small portion of individuals leave a population 

Related Test: Basic Biology Practice Test: Darwin's Theories

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