Chapter 18: Development and EvolutionOverview |
The history of life is marked by great events: the evolution of the first cell, a massive diversification of unicellular forms, the rise of multicellular organisms, the movement of plants and animals and fungi onto the land. Studying the fossil record and analyzing phylogenies can tell us what has happened over the past 4 billion years, and evolution by natural selection can explain why changes occurred. But what about the mechanics? How did the great events happen? Chapter 18 explores these phenomena in the context of developmental biology?
18.1 Homeotic Genes, Pattern Formation, and Diversification As the body of a multicellular organism develops, some sort of system is required to arrange cells in three-dimensional space and to specify their fate accordingly. For tissues and organs to form properly, every cell in the growing body has to be identified by its location relative to other cells and relative to time—meaning, where it is in the developmental sequence.
Refer to section 18.1 of your textbook to learn about the genes responsible for arranging multicellular organisms.
18.2 The Genetics of Homology: Limbs Among the vertebrates, limbs range from the supple wings of bats to the powerful legs of horses. Among the arthropods, limbs range from filmy fly wings to sturdy crab claws. The evolution of limbs was one of the most striking innovations in the Cambrian explosion, and diversity in limb structure and function is widely viewed as a key to the ecological and evolutionary success of arthropods and the land-dwelling vertebrates. What genes were involved in the origin and elaboration of these structures?
Refer to section 18.2 of your textbook to learn the answer to this question.
18.3 Flowers Our third example of “molecular paleontology” concerns the dominant land plants. The radiation of terrestrial life began in the Silurian period, when green algae first made the transition to land.
Turn to section 18.3 of your textbook to learn about the four major radiations of terrestrial plants since then.
18.4 Lessons from Evo-Devo Research This chapter has provided a look at three major research programs on the genetic mechanisms of evolutionary change. Our purpose was to make the case that evolutionary biologists are beginning to understand not only what major events have occurred in the history of life and why they happened in terms of fitness advantages, but also how they took place at the genetic level. The genes involved in the origin of innovations such as body segmentation, limbs, and flowers are no longer a mystery.
Refer to section 18.4 of your textbook to learn about the ongoing research into the change in structure of these genes and their regulation and how this knowledge has contributed to the field of evolution.