A stem cell retains the ability to divide and re-create itself while also having the ability to generate progeny capable of specializing into a more differentiated cell type.

TLS Online TPP Program

#Id: 9556

#Unit 11. Evolution and Behavior

TLS Online TPP Program

#Id: 9557

#Unit 11. Evolution and Behavior

Where, though, do these hereditary variations come from?

This was the great gap in Darwin’s theory, and he never filled it. The problem was serious because, according to the prevailing belief in blending inheritance, variation should decrease, not increase.

TLS Online TPP Program

#Id: 9558

#Unit 11. Evolution and Behavior

Orthogenesis - “Straight-line evolution,” held that the variation that arises is directed toward fixed goals.

So that a species evolves in a predetermined direction by some kind of internal drive, without the aid of natural selection.

TLS Online TPP Program

#Id: 9559

#Unit 11. Evolution and Behavior

Mutationist - Theories were advanced by some geneticists who observed that discretely different new phenotypes can arise by a process of mutation.

TLS Online TPP Program

#Id: 9560

#Unit 11. Evolution and Behavior

They supposed that such mutant forms constituted new species and thus believed that natural selection was not necessary to account for the origin of species. The last influential mutationist was Richard Goldschmidt

TLS Online TPP Program

#Id: 9561

#Unit 11. Evolution and Behavior

Ronald A. Fisher, John B. S. Haldane and Sewall Wright - which showed that mutation and natural selection together cause adaptive evolution.