Macroevolution and the tree of life

Key points

• Evolution by natural selection is a core scientific model that was set out by Darwin, and has been confirmed again and again in every branch of biology.

• Creationist attempts to promote their religious beliefs, such as "intelligent design" or belief in a flat Earth, are not testable and therefore are not science.

• Speciation often occurs by the establishment of a barrier, and the isolation of part of a previously interbreeding population.

• Evolution takes place both within species lineages (phyletic gradualism) and at the time of speciation (punctuated equilibrium); the first model is commonest among asexual microorganisms that live in the open oceans, and the latter in sexual organisms that are subject to environmental and geographic barriers.

• There may be a process of species selection, acting independently of natural selection, but examples have been hard to find.

• The evolution of life may be represented by a single branching phylogenetic tree.

• Cladistics is a method of reconstructing phylogeny based on the identification of shared derived characters (homologies).

• Molecular sequencing provides additional evidence for reconstructing and dating the tree of life.

• DNA has been extracted from fossils such as woolly mammoths, but not from truly ancient fossils.

Probably all organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed. . . There is grandeur in this view of life . . . that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.

Charles Darwin (1859) On the Origin of Species

Figure 5.1 (a) Charles Darwin. (b) Branching diagram of phylogeny, the only illustration in On the Origin of Species (1859). It shows how two species, A and I, branch and radiate through time. The units I-XIV are time intervals of variable length, and the lower case letters (a, b, c) represent new species.

Figure 5.1 (a) Charles Darwin. (b) Branching diagram of phylogeny, the only illustration in On the Origin of Species (1859). It shows how two species, A and I, branch and radiate through time. The units I-XIV are time intervals of variable length, and the lower case letters (a, b, c) represent new species.

Darwin laid the framework for evolutionary biology 150 years ago. Despite millions of essays, books and web sites discussing evolution, no one has yet falsified Darwin's theory of evolution by natural selection, and so it stands as the core of modern biology and paleontology, just as Isaac Newton's laws stand at the heart of much of modern physics. And yet, surprisingly, Darwin is quoted, and misquoted, by many special-interest groups who want to use him in support of, or against, their views of politics, sociology and religion. So it is important to understand what Darwin said, how his insights affect science today, and how paleontology relies on modern evolution as its basis.

Charles Darwin's On the Origin of Species (1859) is usually remembered as the book that made the case for natural selection as the mechanism of evolution, sometimes called "survival of the fittest" (see pp. 118-19). Since the time of Darwin, evolution has been seen in action in the laboratory and in the field, and paleontologists use the principles of evolution to understand how species originate. The origin of species is also core to a second theme in Darwin's writings, namely phylogeny, or the pattern of evolution that is often represented as a branching tree diagram. Darwin's idea was that life had diversified to millions of species by the continued splitting of species from a common stem (Fig. 5.1). Indeed, he proposed that all of life, modern and ancient, could be followed back down the phylogenetic tree to a single point of origin: modern evidence confirms this remarkable insight.

Darwin's branching diagram also explained for the first time the meaning of the natural hierarchy of life that Linnaeus had discovered 100 years earlier (Box 5.1). This natural inclusive branching hierarchy is the basis of modern approaches to discovering the tree of life, the single great evolutionary tree that links all species living and extinct, from the modern biodiversity of over 10 million species, back to a single hypothetical species 3500 million years ago in the Precambrian. Paleontological aspects of evolution, such as the tree of life and studies of processes over thousands and millions of years, are sometimes called mac-roevolution ("big evolution") to distinguish them from microevolution ("small evolution"), all the smaller-scale and shorter-term processes studied by biologists and geneticists in the laboratory or in the field.

Box 5.1 Naming, describing, and classifying fossils

Life is organized in an inclusive hierarchy: small things (species) fit in larger categories, and these fit in still larger categories. Early naturalists realized that it was commonplace to be able to identify broad groups, such as wasps, bats, lizards, grasses or snails, and that within each group were many different forms, called species. Life did not consist of a random array of species. The similarity of groups of species suggested two things: first, that a classification system could be drawn up so people could identify and discuss particular forms without confusion, and second, that perhaps the inclusive hierarchy meant something.

Taxonomy is the study of the morphology and relationships of organisms. Systematics is the broader science of taxonomy and evolutionary processes, while classification refers particularly to the business of naming organisms and identifying the natural hierarchy. When a fossil is described for the first time, the author must name it. Biologists and paleontologists use a modified version of the principles established by the Swedish naturalist and scientist Carl Gustav Linnaeus (1707-1778), often regarded as the founder of systematics. Linnaeus believed that the evident hierarchical order in nature reflected the mind of God. Others at the time were to see things very differently, and to speculate about the possibility of evolution, or change through time.

In Linnaean nomenclature a species is given a genus and species name, such as Homo sapiens. These names are based on words from ancient Latin and Greek and they are printed in italics, followed by the author's name and date of publication. If, subsequently, another scientist moves a named species to another genus, perhaps because of new observations of similarity, the original author and date must then be placed in parentheses. Where several named species turn out to be the same, the subsequent names are identified as synonyms, or aliases, of the first name to have been given to the form.

When a new species is established, a type specimen is designated, and it is housed in a major institution, such as a museum or university, accessible to future investigators. The new species is defined by a short diagnosis, a few lines emphasizing the distinctive and distinguishing features of the fossil. A fuller description, supported by photographs, drawings and measurements, is also given, together with information on geographic and stratigraphic distribution.

Fossils, like living animals and plants, are classified in a hierarchical system, where species are included in genera, genera in families, and up through orders, classes, phyla, kingdoms and domains.

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