In evolutionary developmental biology, heterochrony is defined as a developmental change in the timing or rate of events, leading to changes in size and shape. There are two main components, namely (i) the onset and offset of a particular process, and (ii) the rate at which the process operates. A developmental process in one species can only be described as heterochronic in relation to the same process in another species, considered the basal or ancestral state, which operates with different onset and offset times, and at different rates. The concept was introduced by Ernst Haeckel in 1875
NEOTENY : The retention by adults of traits previously seen only in the young is important in developmental biology, and consequently in evolution. It occurs in two ways. In neoteny also called juvenilization the physiological (or somatic) development of an organism (typically an animal) is slowed or delayed. Neoteny is found in modern humans
n contrast, in progenesis (paedogenesis), sexual development occurs faster.[7] Both processes result in paedomorphism (or paedomorphosis), a type of heterochrony. Some authors define paedomorphism as the retention of larval traits, as seen in salamanders