An atavism is defined as the recurrence of an ancestral trait that has been missing for generations. They usually result from genetic recombination. They are claimed as evidence for universal common descent evolution however sometimes the labeling of such a trait as an atavism is based on the presupposition of universal common descent. This makes the argument a form of circular reasoning.
Two main types of features tend to fall under this category. The first is an actual reproduction of the feature in question, even if it is not fully functional, and one that may resemble something considered by evolutionists to be from the past but it is simply a malformation in the particular organism.
The first of these two results from either genetic recombination or epigenetics. In the case of epigenetics, genes that are currently are simply turned back on. In the case of genetic recombination the genes get moved back together in recombination after being separated so that they once again produce a structure. Even though these features are not generally present in the varieties of these organisms that we have today. It needs to be noted that such cases do not represent changes in genetic information but simple reactivation of what is already there.
While it is true that such cases are evidence of past structures in the kinds of animals in which they appear, they are not evidence of universal common descent. Instead, they are fully consistent with originally created kinds where, due to degeneration or simple adaptation, some features have been lost that are not essential for survival.
Sometimes malformations can occur particularly in the development of an organism that while not fatal reminds evolutionists of structures that would be from what they consider an evolutionary past. A human having a tail-like structure would be a good example. Most of the time such structures are simply the result of something going wrong while the organism is developing and may have no genetic reason for it.
In Both cases considering such structures to be evidence of universal common descent evolution is circular reasoning It requires assuming universal common descent to attribute the feature as a connection to alleged ancestral forms. Consequently, universal common descent evolution is already being assumed in the comparison.
Ultimately, atavisms cannot be considered evidence of universal common descent because even when they are genetically based, they are a result of existing genetic information. The only reason such structures can be considered evidence for universal common descent is if you are excluding a common designer left align and Biblical creation before even looking at the evidence. Consequently, they are not actually evidence for universal common descent, just evidence of the fact that population features do change over time.
