- • plasticity or flexibility of organs to allow greater variety of functions. • plasticity or flexibility of organs to allow greater variety of functions.
- • permitting further improvement when needed. • permitting further improvement when needed.
- • independence or resistance to environmental changes. • independence or resistance to environmental changes.
- • of labour in body parts to enable more efficient functioning. • of labour in body parts to enable more efficient functioning.

S.W. Williston (1852-1918) proposed that in organisms having serially repeated body parts there is a tendency of reduction in the number and increase in specialisation of these parts during Anagenesis. In arthropods, crustaceans possess almost 19 pairs of serially arranged appendages which are modified for various purposes, namely, food handling, walking and swimming. But in higher arthropods such as insects, these appendages undergo high degree of specialization to produce different types of mouth parts and reduction of legs into only three pairs. Centralization of nervous system into brain rather than having segmental ganglia and increased complexity of this organ in molluscs and later in vertebrates is a typical anagenetic event. Improved and complex central nervous system gave the mammals an edge over reptiles in competition and in man it conferred the ability to change the environment. In man, Anagenesis brought about increased complexity and rationalisation of brain in ape-like ancestors which were both arboreal and terrestrial and omnivorous in diet. Terrestrial habit and group selection promoted Anagenesis and complexity of brain conferred plasticity of hands, development of speech and social life. Acquisition of homoeothermy in mammals and birds provided them with plasticity to explore areas from arctic region to deserts, which could not be conquered by their ancestors. There is no general trend in Anagenesis. Natural selection produces patterns which determine line of evolution. Sometimes regressive development and simplification of unnecessary parts is a sign of progression and improved efficiency. For example, loss of limbs in snakes, limbless lizards and Apoda is progressive specialization in relation to their habitat. Anagenesis produces degeneration in organs of sessile and parasitic animals, but degeneration takes place only in those organs that are unnecessary for parasitic life.

## Anagenetic Rates

A very rapid Anagenesis is termed Tachytely, while moderate speed of evolution in a lineage is called Horotely. A very slow rate of anagenesis which normally occurs in stable environmental conditions in isolated pockets is known as Bradytely. Evolutionary changes taking place per unit of time gives an idea how fast evolution has occurred in a lineage or in different comparable lineages.

## CLADOGENESIS – The branching evolution

Splitting of a lineage into two or more branches during evolution is called cladogenesis. This is one of the most common modes of speciation in changing environment or in a mosaic of environmental conditions. Species tend to break into populations and try to adapt to different environmental conditions thus setting off branching of the ancestral taxon into lower ranked ones.

## Tachyschizia

Rapid splitting of lineages, often due to invasion of new environment, or due to extinction of competitors, which makes new opportunities available to the population. Natural selection is relaxed in such situations and new types can be produced very fast, leading to adaptive radiation. A split in