recap

19.5 recap

A mutation in a gene controlling a developmental pathway can result in major changes in morphology, which provide the raw material for evolution by natural selection. The conservation of many developmental regulatory genes makes it likely that similar traits will evolve repeatedly.

learning outcomes

You should be able to:

  • Summarize how mutations in developmental genes have led to diversity in arthropod body plans.

  • Show how differences between marine and freshwater sticklebacks exemplify parallel evolution via changes in gene regulation.

Question 1

How have diverse body forms in arthropods evolved by means of mutations in developmental genes?

The developmental gene Dll controls the formation of legs in arthropods. Insects have three pairs of legs on their three thoracic segments, whereas centipedes have many legs on both thoracic and abdominal segments. In insects, the expression of Dll is repressed by the gene product of Ubx. Ubx is expressed in the abdominal segments of all arthropods, but it has different effects in different species. In centipedes, Ubx is co-expressed with Dll to promote the formation of legs. During the evolution of insects, a mutation in the Ubx gene sequence resulted in a modified Ubx protein that represses Dll expression in abdominal segments.

Question 2

How do the differences between marine and freshwater sticklebacks exemplify parallel evolution via changes in gene regulation?

Marine sticklebacks have bony structures that protect them from predatory marine fish. In freshwater sticklebacks, these structures are not prominent. The differences are due to the expression of a developmental regulatory gene, Pitx1, which encodes a transcription factor normally expressed in regions of the developing embryo that in marine sticklebacks form the head, trunk, tail, and pelvis. However, in several freshwater stickleback populations, the Pitx1 gene is no longer expressed in the pelvis, and spines do not develop. This same change in regulatory gene expression has evolved to produce similar phenotypic changes in several independent populations and is an example of parallel evolution.

Many novel traits have arisen during the course of evolution, but most of them failed to persist beyond even a single generation. Part Six of this book will examine the processes of evolution—the powerful forces that influence the survival and reproductive success of various life forms. You will see how different adaptations become prevalent in different environments, resulting in the extraordinary diversity of life on Earth today, which we will describe in further detail in Part Seven.