The Role of Theory in Advancing 21st-Century Biology Catalyzing Transformative Research (2008) / Chapter Skim
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9 How Much Can We Tell About the Past - and Predict About the Future - by Studying Life on Earth Today?
9 How Much Can We Tell About the Past - and Predict About the Future - by Studying Life on Earth Today?
Pages 145-156
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From page 145... ...
In addition, extinction has removed countless genetic combinations that were adapted to the environments and communities in which they arose. Thinking about the collective genetic reservoir in this way -- as a record of the past and as the starting point for future evolution -- allows one to ask some intriguing questions.
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From page 146... ...
Looking beyond the inheritance mechanisms that act within species, increased exploration of the microbial world has profound implications for our understanding of how adaptive mechanisms can be inherited and shared. As introduced in Box 3-2, microbes live in complex multispecies communities where genes can be shared between distantly related organisms.
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From page 147... ...
Petals were lost in the ancestor of the entire tribe Euphorbieae, and so the evolution of a red display to attract pollinating birds resulted from selection on available genetic variation, in this case in leaf pigmentation. Historical contingency applies equally in molecular biology.
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From page 148... ...
Whether or not the species structure of ecological assemblages may be said to contain a record of past information, it certainly has been profoundly affected by past evolutionary and environmental events. Blood-feeding bats inhabit the American tropics but not Africa, despite the abundance of mammalian prey; marine snakes have evolved in the Indo-Pacific but not the Atlantic Ocean; the ecology of tropical American rain forests is greatly influenced by the abundant water held high above ground in the leaf axils of epiphytic bromeliads, but no comparable plants have evolved in the Old World tropics.
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From page 149... ...
Although development is highly reproducible and usually stable and unidirectional, other epigenetic states are established in a stochastic manner and are plastic, resulting in significant variability between genetically identical individuals. Examples include phenotypic diversity displayed by monozygotic twins, stochastic epigenetic silencing of transposable elements that influence adjacent gene expression in plants and animals, position effect variegation in Drosophila (silencing of genes placed next to heterochromatin through translocations)
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From page 150... ...
Genomic imprinting -- allele-specific gene expression depending on whether an allele is inherited from the father or mother -- occurs in both plants and animals. In mice the mechanism involves establishment of methylation marks within specific DNA sequences in the parent (there are distinct maternal and paternal marks)
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From page 151... ...
THE CHALLENGE OF HORIZONTAL GENE TRANSFER AND SYMBIOSIS Contemporary phylogenetic inference -- inferring the genealogy of species from records stored in morphology and molecules -- is built on the assumption that life is monophyletic, so that histories of particular groups and of all life are tree-like branching structures that can be traced back to a common ancestor. As introduced in Chapter 3, evidence of wholesale and continuing lateral gene transfer within and among the three major domains of life complicates phylogenetic inferences about the earliest stages of life on Earth (true bacteria, archaea, and eukarya; see Woese, 1998; Doolittle, 1999a, b; Felsenstein, 2004)
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From page 152... ...
Lateral gene transfer early in the history of life and throughout the history of the domain of true bacteria and archaea, as well as the prevalence of symbioses in eukarya, cloud the genealogical record of biochemical pathways. Even if there remain genetrees, these phenomena of information exchange, distributed storage, and sharing complicate current methods of phylogeny reconstruction and raise the possibility that the extension of evolutionary theory will be needed to take these phenomena into account.
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From page 153... ...
can themselves be heritable is taken into consideration. For example, the explanation of sex ratios was traditionally analyzed as a problem of the evolution of sex-determining genes, but recent studies of environmental sex determination in vertebrates hint that an expanded theoretical approach that includes organism influences on environments plus feedback to both genetic and epigenetic inheritance mechanisms may be required to fully answer this unsolved problem of evolutionary theory (see Box 9-1)
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From page 154... ...
. The cycling of information between gene regulatory states and sex ratios to behavior, environ ment, and hormones and back again could result in "heritability" of the behavior variations, such as that manifested in transgenerational correlations of nest-site choice and sex ratio -- a form of "non-Mendelian" behavioral or cultural inheritance (Freedberg and Wade, 2001)
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From page 155... ...
It is known that the brain plays a role in the sex determination pathway of some TSD reptiles, as the locus of transduction of temperature into hormonal Box 9-1 signal. Aromatase, which converts testosterone to estradiol, is differentially ex pressed in the brain of the red-eared slider turtle, Trachemys scripta elegans, in a pattern that explains sex determination data in laboratory studies (Willingham et al., 2000)
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From page 156... ...
If these nongenetic adaptations, epigenetic, behavioral, or symbolic variants are to be considered true inheritance systems, part of organisms' evolutionary legacies, they must contribute to fitness differences. Major challenges to extending investigations of nongenetic inheritance to an evolutionary context include development of new experimental tools and methods to distinguish genetic from nongenetic variation, methods of measuring fitness costs and benefits, and theory development to predict and explain evolutionary dynamics when more than one inheritance system is operating.
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