Transformation of a species into a new species in due course of time is the second mode of evolution. In this type only one species exists at a time. Say for example from adaptive zone ‘a’ species evolved into ‘b’; ‘b’ into c; and c into’d’. According to Simpson there are two types of transformation.
1. Phyletic evolution.
2. Quantum evolution.
I. Phyletic evolution
In the phyletic evolution there is a gradual change in the average characters of a species to achieve suitable adaptation to the existing environments.
This is caused either due to adaptations to a shifting environment or due to increasing specialization for a particular environment or imporved adaptations in a constant environment.
This causes origin of new species from an existing one but there is no increase in the number of species. The palaentology provides many evidences of such type of evolution.
2. Quantum evolution
It causes the evolution of higher taxonomic groups such as a new orders and classes. Here there is a rapid change in the character of a population. In fossil records, new orders and classes generally appear without intermediate stage.
Therefore, it is regarded that higher taxonomic groups originate by a different methods. The quantum evolution is macro-of mega-evolution as they are originated by macromutations, which are rare but produce widely different individuals. The individual becomes adapted to new mode of life.
The quantum evolution can be explained by conceiving preadaptation. The organism is modified in such a way that it is also by chance adapted to some other conditions. The organism, then, shifts to new adaptive zone. In this way many preadaptations may be present in a population. The organism evolves rapidly by mutation, selection and genetic drift.