Orthogenesis, also called straight-line evolution, theory that successive members of an evolutionary series become increasingly modified in a single undeviating direction.
A century ago, paleontologists typically depicted the evolutionary pattern of fossil horses in North America as a linear sequence from smaller ancestor to progressively larger descendent (Fig. 2), which fit well with a concept popular during the nineteenth century called orthogenesis, or "straight-line evolution" ("ortho-," Gr., straight, direct; "-genesis," Gr., origin).
However, this argument is based on the assumption that all human behaviour is grounded in ape behaviour, and that straight-line evolution from great apes is an indisputable fact.
Although their history is not a straight line of evolution but a bushy branched pattern, there are trends toward larger body size, loss of the front teeth, longer snouts and larger eyes, longer legs and toes (reducing to just two toes fused together), and higher-crowned cheek teeth (after Scott 1913).
In general, ask students to consider in what ways the fossil record does and does not support the concept of evolution as a straight line of increasing progress or complexity.
But evolution is a squiggly path, not a straight line, and that path was obliterated Wednesday by a team that needs no growth or introspection.
Evolution in nature does not occur in a straight line.
Many have criticised the picture for depicting evolution as if it is a single process that continues in a straight line towards the humans that we are today.
Two straight lines make a straight line.
You expect a straight line.
