Which process can produce new inheritable characteristics within a multicelular species

Mutations

Explanation:

Random modifications within the genome may arise during the cycle of cell division, called mutations. These are errors which occur as copies of the DNA are produced within the cell; mutations may range from small modifications called single nucleotide polymorphisms to large-scale deletions and multi-gene additions.  These are transmissible across several generations, and are said to be heritable.

These mutations create variants which, within a community, become permanent, resulting in the creation of different, genetically distinct populations called species.  Mutations can even accumulate over time in a group, changing the distribution of alleles or various gene forms-this is called genetic drift.

the oldest known insects are believed to live around 400 millions ago as suggested by the fossil records.   some plants having woody stems also evolved during this time.

after the evolution of insects the first four legged species or the tetrapods evolved in shallow freshwater habitats from the intermediate species approximately 397 million years ago. the tetrapods went on conquering the land giving rise to amphibians, reptiles, mammals and birds.

the major split in tetrapods first occurred 340 million years ago branching off from amphibians.

363 million years ago when the carboniferous period began the insects started roaming on land and taking flights, following which the four limbed tetrapods adapted in the terrestrial habitat.