Dear student,
There are various methods of tracing evolution, out of which two are very common -
Tree type method - According to this method, all the organisms are originated from the similar ancestor i.e., all the organisms are homologous to each other. the only factor which leads to the differences in such organisms is the environmental conditions they are prevailing.
Another method is ladder type, according to which organisms always change in a positive sense i.e., from good to better or from better to best. The organisms keep themselves upgraded according to the prevailing conditions.
Regards.