How do you understand the evolutionary tree (Part II)

T.Ryan Gregory

In Part I, we have unraveled a misconception that the order of the apexes (tops) on the evolutionary tree reflects the relationship between species on top. If we simply swapped the internal vertices without affecting the branching order (topological geometry) of the evolutionary tree, that means we don't make changes in the relationship between species. That shows that the order of vertices on the evolutionary tree is in fact arbitrary. It is important that the way species are joined together by their common ancestors.

>>> Part I

In this article, we will clearly explain the misconception of the changes that occur at the branches at the vertices, especially the section between the top and the top. This is an evolutionary tree that shows the relationship between the major vertebrate groups.

In this case, each end shows an existing species and all the inner peaks are extinct ancestors. Looking at the tree above, many people will assume that the development of modern vertebrate fish has experienced very few ' ups and downs ', while there are clearly many changes occurring in the formation phase. mammals. But is that a right concept? Is a long unbranched straight line used to represent fish mean that no change has occurred to this species since it was separated from our common ancestors? Is the outer branch (also known as the 'outer group', or less precisely, the ' base species ') the 'primitive' branch?

Before answering the questions above, let's look at the evolutionary tree below. It represents the hypothetical relationship between echinoderms, including sea lilies, starfish, sea cucumbers and sea urchins. The outermost group in this chart is human.

Looking at the tree chart above, do you think that no changes have occurred in the human development branch since separating from the common ancestors of vertebrates and echinoderms? Can you conclude that humans are 'more primitive ' than sea cucumbers?

The first tree chart has a few misleading points.

- First, the tree evolves unequally: one side is shown with a higher diversity than the other even if it does not reflect true. In fact, vertebrate fish make up half of all vertebrate species; while the first tree expression shows this with only one vertex compared to the five vertices to represent other vertebrate species.

- Second, we tend to draw conclusions based on the order of the last vertices; humans or some notable species are often located far to the right of the chart (or may be on top depending on orientation). Meanwhile, as we know, the order of the vertices is arbitrary because all the vertices can change positions.

- Third, people often misunderstand a long branch means that there is no change - this mistake is very clear when people are placed in the ' basic species ' position of the tree chart.

Therefore, it would be incorrect to read the information based on the order of the last vertices and it would be incorrect to assume that a branch without an internal peak means that no changes occur to the species being represented show. Moreover, it is simply not described as a result of incomplete sampling.

The assumption that no changes occur in short branches can also cause confusion about evolutionary relationships. Consider the following tree chart showing the relationship between some primates.

The diagram shows that humans and chimpanzees are the two closest relatives living together, both of which share a common ancestor (at the top of U) closer to any living primates. shown on the evolutionary tree. There are actually two species of chimpanzees: common chimpanzees Pan troglodytes and dwarf Pan Paniscus. The species represented at the top of U is not necessarily human or chimp. There have been many changes during the evolution of the U-to-modern species, including many ancestral species not shown in the evolutionary tree chart. Besides, there are also changes in the branch of chimpanzees to become present representatives. In short, not humans originate from chimpanzees or chimpanzees originating from humans. Chimpanzees and humans are relatives, no species is the parent of any species.

People who do not understand evolutionary concepts often ask the question: 'Why do humans come from monkeys, so why do monkeys today?' . The answer is simple: people do not evolve from modern monkeys, moreover humans and other monkeys are descendants of an extinct common ancestor. This is entirely possible because new species are often born through a process in which ancestors change into two or more descendants (this process is called ' evolutionary branching '), but not simply transforming into a new species (this process is called ' gradual change '). Therefore, humans and other monkey species may exist in the same way that Canada, Australia or New Zealand can exist.

To learn how to analyze evolutionary trees, refer to the article 'Learn about evolutionary trees', which is available online for free in Evolution: Education and Outreach.

T. Ryan Gregory is an evolutionary biologist specialized in genetic evolution at Guelph University in Guelph, Ontario, Canada.He has a special interest in evolution, genome and ecological diversity.You can learn more from his articles on personal blogs - Genomicron.His own book The Evolution of the Genome also provides more information about his research areas.

1. Subscribe to DNA and Diversity
2. Subscribe to Genomicron
3. Subscribe to Pyrenaemata