Why Evolution Works (and Creationism Fails) by Matt Young Paul K. Strode

Author:Matt Young, Paul K. Strode [Matt Young, Paul K. Strode]
Language: eng
Format: epub, pdf
Tags: ISBN-13:, 9780813545509
Published: 2015-04-20T16:00:00+00:00

The Arrival of the Fittest

In a 2007 article in the New York Times, Scott Gilbert, a developmental biologist at Swarthmore College, explained that while classic evolutionary biology looks at the survival of the fittest, studies in evo devo show that it may actually be “the arrival of the fittest.” Gilbert’s contemporary, University of Wisconsin–Madison biologist Sean Carroll, explains in his book Endless Forms Most Beautiful that when we look at the fossil record and the amazing diversity in animal form that has come and gone over the millennia, an understandable (but incorrect) explanation might be that new genes must have evolved to create the new body we observe. In this view, the individuals with those new traits (and the apparently new genes that code for them) deemed “most fit” by the current environment were then selected to produce more offspring than individuals without the new traits and genes. This hypothesis was promoted by Edward Lewis after he discovered the bithorax genes. When Carroll and his students investigated this hypothesis, however, they found that instead of new genes appearing over time, the same genes were being used in different ways. In other words, they provided more evidence that evolution works by modifying preexisting genes to take on new roles required by changing environments.

Carroll first found that most of the species in the phylum Arthropoda (including spiders, centipedes, lobsters, and fruit flies) share all ten Hox genes with a distant cousin, the Onychophorans. Onychophorans are small organisms (most them are no bigger than a human hand) found in the tropics and subtropics.They resemble caterpillars and they have many jointed appendages like arthropods. However, the onychophoran and arthropod lineages diverged 500 million years ago, and the two groups are now very different from each other in form. How was it possible that these very different groups of animals use the same genes to control their embryonic development? Carroll found that the organisms were simply using the same Hox genes in different ways.

During the development of the embryos of these animals, a subset of the Hox genes are turned on (actively making their protein products) in some segments of the embryo and turned off in other segments.Which genes are on and which are off in each segment have changed over evolutionary time, resulting in different arrangements of appendages and segments, and therefore the appearance of very different body forms. For example, in arthropods, the segments in which the number eight and nine Hox genes are turned on, or expressed, have legs. When gene expression is turned off in these segments and replaced by the expression of the number seven Hox gene, another type of appendage, maxillipeds, mouthparts modified for handling food, develops instead.

We now understand that the implications of this versatility of when and where master control genes are expressed goes far beyond body segments and includes hearts, fingers, eyes, and other organs. These genes are so similar among distantly related animals that they can be transplanted from one organism to another without losing functionality.

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