Isotopes participate in the same chemical reactions but often at differing rates.
When isotopes are to be designated specifically, the chemical symbol is expanded to identify the mass (for example, C is not stable.
However, human beings love to see factual precision, and we want to know how old something is.
Please remember that all dating methods, even those termed "absolute," are subject to margins of error. That is a very small amount of possible error range. Modern studies almost always use two or more methods to confirm dating work and to build confidence in the results obtained.
For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.
By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.
As a result it is always undergoing natural radioactive decay while the abundances of the other isotopes are unchanged.
The 6 proton 6 neutron atoms are said to have a mass of 12 and are referred to as "carbon-12." The nuclei of the remaining one percent of carbon atoms contain not six but either seven or eight neutrons in addition to the standard six protons.
Carbon-14 reacts identically to Carbon-12 and is rapidly oxidised to form (Carbon-14)Dioxide.
Since all living organisms on Earth are made up of organic molecules that contain Carbon atoms derived from the atmosphere, they therefore contain Carbon-14 atoms.
Dating is not necessary to demonstrate that evolution is a fact.
Chronological sequence is all that is really required.