An ecosystem contains two types of giraffes, some with short necks and others with long necks. the distribution of this trait is equal among the population. a disease causes low-lying shrubs to die out. the long-necked giraffes can access more food, so they have a better chance of surviving to reproductive age. after several generations, all the giraffes ha

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.