Changes in the rate of background extinction are analyzed as a function of changing age distributions of families. Declining background-extinction rates have previously been attributed to evolution at the family or community level. It is demonstrated, however, that if the probability of extinction for a family decreases through its own history, then overall changes in background-extinction rate are a function of a changing familial age distribution. In geologic stages where there is a preponderance of young families, extinction rates can be predicted to be higher than in times where there is a plethora of older families. In addition, it is demonstrated that a difference exists between the age distributions of families that become extinct in background- and mass-extinction intervals.

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