The Tangled Bank: An Introduction to Evolution

By Carl Zimmer

Publication date (2nd ed.): 2013 according to publisher (my copy says 2014), 452 pg.

Roberts and Company Publishers. ISBN: 978-1-936221-44-8.

Author: Carl Zimmer is one of the best science writers in the business. You can keep up with him on his blog, which is part of the National Geographic “science salon” called Phenomena, a collection blogs by Carl Zimmer, Brian Switek, Ed Yong, Virginia Hughes, and Nadia Drake, all of whom are experienced science writers with a talent for accuracy and clarity. They cover everything from dinosaurs to DNA to dark matter and are the first place I go to in the morning for interesting science news. If it sounds like I am selling them, I am in order to convince you that a book by Carl Zimmer is both more accurate than the current textbook you are using and better written. Zimmer and the others are not just authorial guns for hire, they care about science communication and they do it well. My first introduction to Carl Zimmer was a book called “Parasite Rex“. You probably are thinking that a book about parasites would not be the most interesting of books, but you would be wrong. Read it and it will open up a whole new (albiet disturbing) world for you.

The name of the book is derived from the opening line of the last paragraph in The Origin of Species, by Charles Darwin, a fitting name for a book introducing evolutionary topics. While I have a few complaints, none are major and I highly recommend the book. My chief complaints are that I always want more, but there is only so much one can put into a book, especially an introductory text.

The book is filled with high quality pictures and graphs that break up the text, but whereas many books have flashy graphics that serve little purpose other than to distract from the text, all the figures in the book clearly relate to the topic at hand without excessively cluttering up the book. They also provide data that get the reader to go beyond the “because I told you” format so many books use and actually look at some of the data supporting the scientific concepts (and serve as a great way to integrate math, geography, and art standards into the science). Each chapter also have a list of resources for further reading and an extensive bibliography, so anyone can check the data presented in the primary and peer-reviewed literature for themselves.

One thing that might make some teachers and students a little annoyed is that important terms are not in bold font, nor does it have problem sets. However, it explains all the terms as they come up, it does not require flipping to the end of the book for every new term, although there is also a glossary for those that need it. The book is designed to be read, not just skimmed through while one picks out the bold words, like so often happens. However, there is also a study guide for the book written by Dr. Alison Perkins, which includes all the learning objectives, questions, activities, and pedagogical suggestions that teachers are looking for.

The book begins with a detailed discussion of whale evolution as an example to introduce several general concepts of evolution and various ways in which evolution may be studied. It covers fossils, placing them into phylogenetic and geologic context, DNA studies, embryology, and ecology from their earliest beginnings to today. Zimmer doesn’t go into the disputes that arose about whale origins, instead just focusing on what has become the consensual understanding, which I find a bit disappointing, but perfectly understandable for the context of this book and especially this introductory chapter. Nevertheless, I like presenting disputes because it shows the dynamic nature of science as an exploration, not just a book of facts. He presents the exploration through a discussion of the fossils being discovered and how they were interpreted, he just cleans up the historical path and makes it neater than it really was.

Chapter 2 brings a history of evolutionary thought, starting in the 1600s and the development of evolutionary concepts before Darwin. Zimmer correctly explains that Darwin was not the first to conclude that organisms evolved, but he did provide a plausible mechanism for how it happened. He then continues with a discussion of the changes and additions to evolutionary theory in the decades since Darwin. He tackles the important misconceptions of evolution, including the common misunderstanding of what a scientific theory really means, which form the basis of most people’s arguments against evolution.

Chapter 3 presents geological data, including how radioactive decay is used to date rocks and biomarkers to detect traces of life within rocks. He tells us how fossils tell us about the past, followed by a brief overview of the major transitions in life from the dawn of life to today.

One of the many phylogenetic trees in the book showing the evolution of, in this case, birds from dinosaurs.

Chapter 4 is probably one of the most important chapters that is left out of many introductory biology texts. Zimmer tells us what phylogeny is and how to read a phylogenetic tree to understand evolutionary relationships. It is particularly disturbing so many books skip this step because it is vital to understanding much of what comes after. Misunderstandings here reverberate throughout one’s ability to understand evolutionary theory, yet reading phylogenetic trees is not as intuitive as most teachers think.  He talks about homology and how that affects our understanding of evolution. After he introduces the concepts, he demonstrates the concepts through a series of phylogenetic trees, such as early mammals, dinosaurs, and hominids.

Chapter 5 talks about DNA and how variation is introduced. Zimmer does a great job of discussing the various types of mutations and showing the typical view of point mutations is but the smallest way of introducing variation. His discussion of the role of sexual selection in creating diversity is short, although his description of Mendel’s experiment with peas helps somewhat. He also gives short shrift to lateral (aka horizontal) gene transfer, in which genes are transferred not through descendants but sometimes through completely unrelated organisms by, for instance, viruses. Zimmer also completely ignores endosymbiosis, which helped create mitochondria and chloroplasts, and hybridization, which makes this chapter not as satisfying for me.

Chapter 6 covers the role of genetic drift and selection well, although he leaves out a discussion of gene flow from one population to another. I like that he talks about fitness in terms of more than one gene, showing that what may be good for one gene is not necessarily good for another in terms of fitness, so that evolution is limited by the interplay between genes that each have their own optimal conditions. This would have been a good place to address the misunderstanding of “survival of the fittest,” which is commonly viewed as a tautology (the fittest survive, but how do you determine who is fittest? The ones that survive) but he does not mention it. This is a very common misconception. First, the phrase was never used by Darwin and is incorrectly and second, it is being incorrectly interpreted. It is not the overall fitness of a particular organism that matters, but a measure of how many offspring successfully survive and reproduce. It doesn’t matter evolutionarily if you are the toughest guy on the block if you don’t breed and produce successful offspring.

Chapter 7 discusses molecular phylogenies, figuring out evolutionary relationships from their DNA or protein sequences. One complaint I have here is that he talks about how successful the molecular clock is, how you can tell time using the amount of mutations separating species. In all actuality, the molecular clock has some serious issues, as in, it doesn’t work very well. Fortunately, he does discuss some of the challenges of the molecular clock (genes don’t mutate at the same rate either between each other or within different parts of the same gene, or through time, it requires fossils to calibrate and then tries to claim better results than the fossil data, etc.). The problems with the molecular clock mean that its usefulness and accuracy are limited and requires statistical manipulation of the data to try to take into account the known issues. Unfortunately, the figures lead one to believe the molecular clock actually acts clock-like, reducing the impact of the text describing its problems and the examples in the text downplay the problems. A bonus to this chapter is that he brings back the topic of horizontal gene transfer and shows its importance in a box at the end of the chapter. I might have put this in the last chapter and discussed it more, but it could be that Zimmer thought it might confuse people by introducing too much complexity at once and wanted the readers to develop a bit more understanding before throwing another wrench in the works.

Chapter 8 gives a great discussion of adaptation, taking it from the gene to species evolution. I particularly like his discussions showing how gene duplications and rewiring without the need for further point mutations can make huge differences. This is a really important concept to understand, that variation is more than just the single point mutations most people think about. He ends the chapter with a discussion of the limits of evolution based on physical limits and baggage from previous evolutionary steps, although I would have liked to see a brief mention at least of the constraints imposed by having genes with different optimal conditions that all have to be balanced.

Remember when I said chapter 5 gave short shrift to variation through sexual reproduction? That is because chapter 9 is completely devoted to the topic. Here he goes into several aspects of sexual selection, including trade-offs that may limit evolution in any one particular direction. Trade-offs in this case refer to the fact that improving one thing takes away from another. The genes with different optimal conditions are an example of this. Improve and you hurt another until a balance is achieved.

Formation of the Chicxulub crater formed at the end of the Cretaceous.

Chapter 10 defines what a species is (which is nowhere near as easy as it sounds) and how species evolve into other species. Chapter 11 extends that to evolution on a grand scale, showing the development of global biodiversity through time. I would have preferred to see a discussion of the difficulties in determining fossil biodiversity, such as the relationship between the amount of outcrops of a particular time and the number of species known, but there is only so much one can put into a textbook. Inevitably, the chapter discusses the major extinctions of the world, although he only talks about two of them, the Permo-Triassic and the Cretaceous-Paleocene extinctions, probably because they are better known by far than the others. His discussion of the Permian extinction doesn’t mention that the reason the volcanoes at the time put out so much carbon dioxide was that they apparently burned through huge coal deposits, which pumped up the carbon dioxide way beyond what the volcanoes would have done alone, but he gets the gist of the cause of the extinction. He also discusses briefly the debate in why the extinction occurred at the end of the Cretaceous, which is good. The chapter ends with a discussion of the current mass extinction taking place and the causes for it.

Chapter 12 discusses coevolution, both mutualistic and antagonistic. Here Zimmer finally discusses endosymbiosis and the important role it played in evolutionary history. Chapter 13 is an interesting discussion about the evolution of behavior in both plants and animals.

Chapter 14 will of course be the most controversial chapter because it deals with human evolution. Zimmer does a good job with this chapter, although I would have preferred a clearer statement that hominids and apes both evolved from a common ancestor, but where our ancestors became adapted for savanna life, the apes evolved more towards forest life. He talks about the interbreeding that happened between neanderthals and Homo sapiens, as well as with the Denivans, according to the genetic research published recently, which will make a few people uncomfortable, but is the truth nevertheless. The chapter wraps up with a discussion of some evolutionary psychology, which is highly controversial, but the parts he discusses are well supported by experimental evidence.

The last chapter is arguably the most important one in the book. Here Zimmer discusses the role of evolution in medicine, with examples of disease progression, vaccines, antibiotics, and cancer. If, by the time people have worked their way through the book and are still asking themselves why it is important they understand evolution, this chapter is a sledgehammer wake-up call. One cannot finish the book without having a strong understanding of the importance of this concept of evolution and why biologists consider it the central tenet of all biology. As Dobzhansky said, “Nothing in biology makes sense except in the light of evolution.”