Rates of Phenotypic and Genomic Evolution during the Cambrian Explosion
Current Biology, 12 September 2013
Copyright © 2013 Elsevier Ltd All rights reserved.
10.1016/j.cub.2013.07.055
Authors
Michael S.Y. Leesend email, Julien Soubrier, Gregory D. EdgecombeSee Affiliations
Highlights
The Cambrian explosion (evolution’s “big bang”) is compatible with Darwinian evolution
Anatomical and genetic evolution occurred 5 times faster during the early Cambrian
Bayesian methods can infer evolutionary rates in deep time, using living taxa
This study concerns arthropods, but the results are likely applicable to most of life
Summary
The near-simultaneous appearance of most modern animal body plans (phyla) ∼530 million years ago during the Cambrian explosion is strong evidence for a brief interval of rapid phenotypic and genetic innovation, yet the exact speed and nature of this grand adaptive radiation remain debated [1,2,3,4,5,6,7,8,9,10,11,12]. Crucially, rates of morphological evolution in the past (i.e., in ancestral lineages) can be inferred from phenotypic differences among living organisms—just as molecular evolutionary rates in ancestral lineages can be inferred from genetic divergences [13]. We here employed Bayesian [14] and maximum likelihood [15] phylogenetic clock methods on an extensive anatomical [16] and genomic [17] data set for arthropods, the most diverse phylum in the Cambrian and today. Assuming an Ediacaran origin for arthropods, phenotypic evolution was ∼4 times faster, and molecular evolution ∼5.5 times faster, during the Cambrian explosion compared to all subsequent parts of the Phanerozoic. These rapid evolutionary rates are robust to assumptions about the precise age of arthropods. Surprisingly, these fast early rates do not change substantially even if the radiation of arthropods is compressed entirely into the Cambrian (∼542 mega-annum [Ma]) or telescoped into the Cryogenian (∼650 Ma). The fastest inferred rates are still consistent with evolution by natural selection and with data from living organisms, potentially resolving “Darwin’s dilemma.” However, evolution during the Cambrian explosion was unusual (compared to the subsequent Phanerozoic) in that fast rates were present across many lineages.
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NOTA DESTE BLOGGER:
Vide uma visão diferente sobre a Explosão Cambriana de dois abalizados especialistas Douglas Erwin e James Valentine, na introdução do seu livro recente The Cambrian Explosion: The Construction of Animal Biodiversity (Roberts and Company, 2013):
Today, some two dozen major eukaryotic groups have bodies composed of more than one cell, but few have progressed beyond the stage of an association of essentially identical cell types (Buss 1987; Knoll 2011). Eukaryotes include protistan colonies and various algae that have many cells, but there is no evidence that any of these groups has ever achieved the developmental control required to produce more complex morphologic patterns. Multicellular algae and fungi have only a few cell types, whereas other eukaryotic lineages are multicellular but exhibit none of the hierarchical structure of differentiation seen in plants and animals. At least eight different groups of these multicellular eukaryotes arose well before animals finally evolved sometime more than 750 million years ago (Ma). Complex multicellularity involves a hierarchical structure of differentiated cell types, tissues, organs, and the regionally differentiated structures found in animals and vascular land plants…
Multicellularity is a generative evolutionary innovation in the sense that it provides the basis for two additional important evolutionary steps: greater body size and increased division of labor among differentiated body parts. Greater size quite literally changes the nature of the world experienced by organisms… Body size is a multiplier of inertia, and most multicellular organisms are large enough that they cross the boundary into a world where inertial forces become important. At such larger sizes, most organisms evolved new ways of locomotion and feeding, facilitated by the specialization of cells, tissues, organs, and differentiated body parts. Such division of labor is evident even in sponges, the earliest metazoan group, but becomes far more pronounced in more complex animals…
Some 120 million to 170 million years after the origin of sponges, the scrappy fossil record improved with a bang, geologically speaking. Following a prelude of a diverse suite of enigmatic, soft-bodied organisms beginning about 579 Ma, a great variety and abundance of animal fossils appear in deposits dating from a geologically brief interval between about 530 to 520 Ma, early in the Cambrian period…
The subtitle of this book, The Construction of Animal Biodiversity, captures a second theme: the importance of building the networks that mediate the interactions… Increased genetic and developmental interactions were also critical to the formation of new animal body plans. By the time a branch of advanced sponges gave rise to more complex animals, their genomes comprised genes whose products could interact with regulatory elements in a coordinated network. Network interactions were critical to the spatial and temporal patterning of gene expression, to the formation of new cell types, and to the generation of a hierarchical morphology of tissues and organs. The evolving lineages could begin to adapt to different regions within the rich mosaic of conditions they encountered across the environmental landscape, diverging and specializing to diversify into an array of body forms.
Increased genetic and developmental interactions were also critical to the formation of new animal body plans. By the time a branch of advanced sponges gave rise to more complex animals, their genomes comprised genes whose products could interact with regulatory elements in a coordinated network. Network interactions were critical to the spatial and temporal patterning of gene expression, to the formation of new cell types, and to the generation of a hierarchical morphology of tissues and organs. The evolving lineages could begin to adapt to different regions within the rich mosaic of conditions they encountered across the environmental landscape, diverging and specializing to diversify into an array of body forms.
The nature of appropriate explanations is particularly evident in the final theme of the book: the implications that the Cambrian explosion has for understanding evolution and, in particular, for the dichotomy between microevolution and macroevolution. If our theoretical notions do not explain the fossil patterns or are contradicted by them, the theory is either incorrect or is applicable only to special cases… One important concern has been whether the microevolutionary patterns commonly studied in modern organisms by evolutionary biologists are sufficient to understand and explain the events of the Cambrian or whether evolutionary theory needs to be expanded to include a more diverse set of macroevolutionary processes. We strongly hold to the latter position.…
Microevolutionary change often produces new species when different populations of a species are isolated genetically, or nearly so, such that each pursues a separate pathway of genetic change and they become distinct species; in animals, it usually means that they can no longer exchange genes. Macroevolution, by contrast, involves the study of what happens in evolution beyond the mechanisms of the formation of species. Some species, for example, are founders of major clades that encompass millions of species that occupy a wide range of ecological occupations, whereas other species are merely found in minor branches of life’s tree with rather similar ecologies or simply become extinct without issue (other patterns are not uncommon). Each of the species with those very different evolutionary outcomes arose through microevolutionary processes, yet there is obviously more to be said about their evolution, which forms the topic of macroevolution.
