members of the subgroup may no longer respond to courtship or other signals from members of the original population. Eventually, genetic changes will become so substantial that the members of different subgroups can no longer produce viable offspring even if they do mate. In this way, existing species can continually “bud off” new species.

When tetrapods (such as this sea turtle laying its eggs on a coastal beach) evolved the ability to lay hard-shelled eggs, they no longer had to return to the water to reproduce.

Over very long periods of time, continued instances of speciation can produce organisms that are very different from their ancestors. Though each new species resembles the species from which it arose, a succession of new species can diverge more and more from an ancestral form. This divergence from an ancestral form can be especially dramatic when an evolutionary change enables a group of organisms to occupy a new habitat or make use of resources in a novel way.

Consider, for example, the continued evolution of the tetrapods after limbed animals began living on land. As new species of plants evolved and covered the Earth, new species of tetrapods appeared with features that enabled them to take advantage of these new environments. The early tetrapods were amphibians that spent part of their lives on land but continued to lay their eggs in the water or in moist environments. The evolution about 340 million years ago of amniotic eggs, which have structures such as hard or leathery shells

The last common ancestor of the four-legged animals living today gave rise to amphibians and was the predecessor of reptiles. Birds and mammals evolved from different lineages of ancient reptiles.