Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

We report the draft genome of the black cottonwood tree, Populus trichocarpa. Integration of shotgun sequence assembly with genetic mapping enabled chromosome-scale reconstruction of the genome. More than 45,000 putative protein-coding genes were identified. Analysis of the assembled genome revealed a whole-genome duplication event; about 8000 pairs of duplicated genes from that event survived in the Populus genome. A second, older duplication event is indistinguishably coincident with the divergence of the Populus and Arabidopsis lineages. Nucleotide substitution, tandem gene duplication, and gross chromosomal rearrangement appear to proceed substantially more slowly in Populus than in Arabidopsis. Populus has more protein-coding genes than Arabidopsis, ranging on average from 1.4 to 1.6 putative Populus homologs for each Arabidopsis gene. However, the relative frequency of protein domains in the two genomes is similar. Overrepresented exceptions in Populus include genes associated with lignocellulosic wall biosynthesis, meristem development, disease resistance, and metabolite transport.

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The genome of black cottonwood, Populus trichocarpa (Torr. & Gray)

This review organizes ideas on the evolution of life histories. The key life-history traits are brood size, size of young, the age distribution of reproductive effort, the interaction of reproductive effort with adutl mortality, and the variation in these traits among an individual's progeny. The general theoretical problem is to predict which combinations of traits will evolve in organisms living in specified circumstances. First consider single traits. Theorists have made the following predictions: (1) Where adult exceeds juvenile mortality, the organism should reproduce only once in its lifetime. Where juvenile exceeds adult mortality, the organism should reproduce several times. (2) Brood size should maximize the number of young surviving to maturity, summed over the lifetime of the parent. But when optimum brood-size varies unpredictably in time, smaller broods should be favored because they decrease the chances of total failure on a given attempt. (3) In expanding populations, selection should minim...

Life-history tactics: a review of the ideas.

Summary Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, pre- diction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric

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Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?

The harmful effects of close inbreeding have been noticed for many centuries (34, 35, 165). With the rise of Mendelian genetics, it was realized that the main genetic consequence of inbreeding is homozygosis (165, Ch. 2). Two main theories were early proposed to account for inbreeding depression and its converse, heterosis (the increase in vigor observed in an F1 between two inbred lines). These are the overdominance and partial dominance hypotheses, discussed in more detail below. Research into this question has continued up to the present, and this is one of the topics that we discuss. Darwin (35, 36) was the first to point out that the evident adaptations of many plants for ensuring outcrossing could be understood in terms of the selective advantage of avoiding inbreeding depression. We review the evidence that the evolution of breeding systems of animals and plants has been significantly influenced by the occurrence of inbreeding depression. In order to do this, we consider the contemporary genetic theory of inbreeding depression and heterosis, and the experimental data concerning the strength of inbreeding depression. We emphasize data and theory relevant to natural, rather than domesticated, populations as we are chiefly concerned to evaluate the evolutionary significance of inbreeding depression. We do not attempt to give a complete bibliography of this very extensive field but try to concentrate on what seem to be the most significant findings in relation to this aim.

https://www.jstor.org/stable/pdf/2097132.pdf

Inbreeding depression and its evolutionary consequences

Numerous studies of morphological and physiological traits (reviewed in 3, 57, 95, 132, 135) as well as the more recent studies of enzyme loci (reviewed in 21, 52, 63) have generally shown the existence of significant amounts of genetic variation both within and between plant populations. These reviews (21, 52, 63) have also demonstrated considerable variation among species in intrapopulation allozyme variation. For instance, Hamrick (63) found that populations of woody plants contained higher levels of genetic variation than did populations of herbaceous species; Gottlieb (52) and Brown (21) demonstrated that predominantly outbreeding species maintain higher levels of intrapopulation genetic variation than predominantly inbreeding species. Nevo (111) found that habitat generalists and animal species with cosmopolitan or tropical distributions were typically more variable than species with specialized habitat preferences or temperate distributions. While these results are interesting and informative, they consider the influence of only a few evolutionarily important characteristics on the main-

https://www.jstor.org/stable/info/2096789

Relationships Between Life History Characteristics and Electrophoretically Detectable Genetic Variation in Plants

Allozyme polymorphisms detected in mature needle tissue of ponderosa pine

In 1974, Richard Lewontin published The Genetic Basis of Evolutionary Change, focusing enormous attention on protein variation as both a model of underlying genetic variation and a level of selection itself. In the twenty years since, scientific research has been shifted by the power of molecular biological techniques to explore the nature of variation directly at the DNA and gene levels. The "protein chapter" is coming to a close. In this book, Jeff Mitton explains the questions that geneticists hoped to answer by studying protein variation. He reviews the extensive literature on protein variation, describes the successes and failures of the research program, and evaluates the results of a rich and controversial body of research. The laboratory and field studies using protein polymorphisms revealed dynamic interactions among genotypes, fitness differentials, and fluctuating environmental conditions, and inadvertently wedded the fields of physiological ecology and population biology. Mitton's book is a useful analysis for all scientists interested in the genetic structure and evolution of populations.

Selection in Natural Populations

In the fall, sightseers take to the highways of North America to enjoy the brilliant colors that are revealed as deciduous trees recycle the dominant greens of chlorophyll. In the western United States and Canada, the most colorful tree in the fall is the quaking aspen, Populus tremuloides. Brilliant yellows, rich golds, and shimmering shades of red shine, contrasting with the various green shades of the conifers. Quaking aspen earns its name for the distinctive fluttering of its leaves, even in the most gentle breezes. Early French-Canadian trappers called the tree an aspen because of its similarity to Populus tremula, a closely related species in Europe and Asia. One of the legends attached to the aspen reflects Judeo-Christian influences. The aspen quake in fear today, according to folklore, because Jesus Christ was crucified on a cross of aspen. In addition to the esthetics of quaking leaves and brilliant fall colors, extraordinary features of the natural history and genetics of aspen lend it special appeal for naturalists and laboratory scientists. Quaking aspen merits a variety of superlatives: It is North America's most widely distributed native tree species and the second most widely distributed in the world (Barnes and

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Genetic variation and the natural history of quaking aspen

The genetic structure of a population of Pinus ponderosa was studied using seven electrophoretically-detectable protein loci. This population is composed of six groups of trees, which differ significantly from each other in their genetic constitutions. The results imply that the population is composed of groups of individuals assembled into genetically-related family units. Whenever such family clusters have been documented in other studies, marked inbreeding has also been noted as a consequence. In this population, no inbreeding is detectable. The groups also differ from one another in several characteristics which have genetic consequences within the population as a whole. These characteristics include age structures, seed output, level of infestation by woolly aphids and extent of damage by deer browsing. The population was also divided into four age classes which do not differ markedly from one another in their genetic characteristics. This suggests that genetic differentiation in time is much less marked than differentiation in space.

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Jeffry B. Mitton

Papers

Relationships Between Life History Characteristics and Electrophoretically Detectable Genetic Variation in Plants

Allozyme polymorphisms detected in mature needle tissue of ponderosa pine

Selection in Natural Populations

Genetic variation and the natural history of quaking aspen

Genetic variation in space and time in a population of ponderosa pine