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In this chapter, the author compares different types of fossil genes that are found in certain species and connect them with their ancestors and their way of life. Carroll discovers various changes in species lifestyles and analyzes the genes which, in the process of evolution, have ceased to function but remained in those species’ DNA. He claims that fossil genes are preserved in DNA in the same way as they are preserved in sedimentary rock, and the text of these genes disintegrates over time. The reason for this is that if the genes are redundant and not used, they must be got rid of in the evolution process.
Fossil Opsin Genes
In the first part of the chapter, Carroll provides examples of the changes in the use of genes in different species. First, he compares the DNA of a fish called the coelacanth with its ancestors who lived millions of years ago. He states that over time the coelacanth has changed the SWS opsin gene used to see in the violet and ultraviolet spectrum and has completely got rid of the MWS/LWS opsin genes used to see in the red-green spectrum. Thus, if the opsin is inessential and not needed, the natural selection is relaxed, and no mechanism in the DNA removes mutations that disrupt the genes’ function.
Carroll claims that fossil genes evolve as a result of the continuing process of mutation when the natural selection is not active. Further on, Carroll describes the differences in opsin genes comparing nocturnal and diurnal species. The analysis of opsin genes in prosimians such as lemurs, lorises, tarsiers, and bushbabies has once again shown that the SWS opsin gene in these species has become a fossil. There is a big part of code near the gene’s beginning that is missing in each of these species. This makes the production of opsin impossible. Since this removal happens in the same place and is of the same size in each species, then the SWS opsin fossilization, for the first time, occurred in a common ancestor of the bush baby and the slow loris and transferred to these species.
Then, the author describes other cases of SWS opsin fossilization including owl monkeys, cetaceans, and the blind mole rats, and concludes that the fossilization of the gene is connected with the habitat where the species live.
Comparing Vision and Smell Genes
In the next part of the chapter, Carroll analyzes the change of a habitat influence the change in the genes that are responsible for the sense of smell. The author comes to a conclusion claiming that the situation with the genes responsible for smell is the same as with those responsible for vision. Thus, when an organ or a process is no longer needed, different genes that are in charge of this organ or process may undergo relaxed selection and, after that, fossilization.
Cause or Effect
In the next chapter, the author answers the question of whether the genes’ fossilization is a cause of evolutionary changes, or an effect, namely its byproduct. Thus, he claims that it can be both cause and effect depending on the circumstances. The author concludes the chapter by stating that the making of the fittest is not always a progressive process, and the loss of genes and the fossilization are strong arguments against the concept of “design”. Natural selection cannot make plans for the future and preserve what is being used now. The loss of genes and the fossilization evolve when the natural selection is not in effect. Over time, random mutations will accumulate and oust unused genes.
In my opinion, the quality of writing is high. Although the author sometimes uses colloquial expressions that are not peculiar to academic writing, he sounds convincing, and most of the work is written in the academic style.
Keywords: gene, DNA, opsin, fossilization, SWS, evolution, natural selection.