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

The chemical diversity and distribution of glucosinolates and isothiocyanates among plants

Estimates of inbreeding depression obtained from the literature were used to evaluate the association between inbreeding depression and the degree of self-fertilization in natural plant populations. Theoretical models predict that the magnitude of inbreeding depression will decrease with inbreeding as deleterious recessive alleles are expressed and purged through selection. If selection acts differentially among life history stages and deleterious effects are uncorrelated among stages, then the timing of inbreeding depression may also evolve with inbreeding. Estimates of cumulative inbreeding depression and stage-specific inbreeding depression (four stages: seed production of parent, germination, juvenile survival, and growth/reproduction) were compiled for 79 populations (using means of replicates, N = 62) comprising 54 species from 23 families of vascular plants. Where available, data on the mating system also were collected and used as a measure of inbreeding history. A significant negative correlation was found between cumulative inbreeding depression and the primary selfing rate for the combined sample of angiosperms (N = 35) and gymnosperms (N = 9); the correlation was significant for angiosperms but not gymnosperms examined separately. The average inbreeding depression in predominantly selfing species (8 = 0.23) was significantly less (43%) than that in predominantly outcrossing species (8 = 0.53). These results support the theoretical prediction that selfing reduces the magnitude of inbreeding depression. Most self-fertilizing species expressed the majority of their inbreeding de- pression late in the life cycle, at the stage of growth/reproduction (14 of 18 species), whereas outcrossing species expressed much of their inbreeding depression either early, at seed production (17 of 40 species), or late (19 species). For species with four life stages examined, selfing and outcrossing species differed in the magnitude of inbreeding depression at the stage of seed production (selfing 8 = 0.05, N = 11; outcrossing 8 = 0.32, N = 31), germination (selfing 8 = 0.02, outcrossing 8 = 0.12), and survival to reproduction (selfing 8 = 0.04, outcrossing 8 = 0.15), but not at growth and reproduction (selfing 8 = 0.21, outcrossing 8 = 0.27); inbreeding depression in selfers relative to outcrossers increased from early to late life stages. These results support the hypothesis that most early acting inbreeding depression is due to recessive lethals and can be purged through inbreeding, whereas much of the late-acting inbreeding depression is due to weakly deleterious mutations and is very difficult to purge, even under extreme inbreeding.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1996.tb04472.x

Evolution of the magnitude and timing of inbreeding depression in plants.

Variation in plant functional traits results from evolutionary and environmental drivers that operate at a variety of different scales, which makes it a challenge to differentiate among them. In this article we describe patterns of functional trait variation and trait correlations within and among habitats in relation to several environmental and trade‐off axes. We then ask whether such patterns reflect natural selection and can be considered plant strategies. In so doing we highlight evidence that demonstrates that (1) patterns of trait variation across resource and environmental gradients (light, water, nutrients, and temperature) probably reflect adaptation, (2) plant trait variation typically involves multiple‐correlated traits that arise because of inevitable trade‐offs among traits and across levels of whole‐plant integration and that must be understood from a whole‐plant perspective, and (3) such adaptation may be globally generalizable for like conditions; i.e., the set of traits (collections of t...

/pdf/the-evolution-of-plant-functional-variation-traits-spectra-1753e9n900.pdf

The evolution of plant functional variation: traits, spectra, and strategies

Control of soil-borne plant pests using glucosinolate-containing plants

viously been shown to be negatively correlated with self-fertilization, clumped pollen transfer, pollen size, and the ratio of stigma area to the pollen-bearing area on the pollinator. Here, the four correlations are shown to be consistent with evolutionary models of optimal allocation of resources to male and female functions, providing evidence that selection on sex allocation in hermaphrodites is governed by the same principles as selection on the sex ratio.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1984.tb00384.x

Population density, outcrossing rate, and heterozygote superiority in ponderosa pine.

We grew cocklebur, Xanthium strumarium L. (Compositae), in an experi- mental garden and monitored a variety of morphological, physiological, and phonological traits known to have a significant influence on plant reproductive success. We used a hierarchical series of multiple regression analyses to generate a path diagram explaining the interacting contributions of individual traits to the numbers and size of fruits produced by individual plants. Fruit production was strongly positively correlated with plant vegetative biomass, and fruit size was negatively correlated with the total number of fruits produced. Although larger plants produced more fruit, there was also a significant direct effect of the timing of anthesis on total fruit numbers; plants in this determinate species that flowered later produced more fruit. The influence of all other traits on reproductive success was indirect through their influence on plant size. Plants with higher water-use efficiencies early in the season produced leaves more quickly, while plants which emerged earlier grew more rapidly in height. These faster growing plants had a higher vegetative biomass and consequently a greater reproductive success. Latter emergence also had a direct positive influence on plant size, demonstrating that a single trait can have significant but counteracting influences on final reproductive success. These results demonstrate the utility of an inductive approach in analyzing the actual, functional pathways by which diverse traits influence growth and reproduction at the whole plant level.

Functional Interactions among Traits that Determine Reproductive Success in a Native Annual Plant

I quantified the distribution and abundance of major plant taxa on three distinct cliff systems in Minnesota and examined the effects of recreational rock climbing on that vegetation. The three cli...

The effects of rock climbing on the vegetation of three Minnesota cliff systems

We tested the hypothesis that plant loss to insects, and thus the relative fitness of an annual, was inversely related to spatial variation in the concentration of its characteristic secondary compound, methylglucosinolate, a mustard oil precursor. We found that methylyglucosinolate concentrations decreased significantly and linearly from the dry to the wet end along short soil moisture gradients in dry shortgrass prairie. Both leaf damage and capsule predation increased from the dry to the wet end. Thus, the glucosinolate appears to function defensively. Plant growth and flower production were favored at the wet end of the gradient; yet plants in the wet portion of the gradient were also more vulnerable to significant insect damage. The net result was that seed production by individual plants after predation was actually greater in the drier, harsher half of the gradient. The outcome strongly suggests that environmentally related variation in secondary compound concentration along environmental gradients can mediate and influence host-plant population abundance and distribution.

Variation in methylglucosinolate and insect damage toCleome serrulata (Capparaceae) along a natural soil moisture gradient.

The purpose of this study was to determine the potential for evolutionary change for several growth characters in a natural population of the annual plant Cleome serrulata Pursh by measuring direct...

Michael A. Farris

Papers

Population density, outcrossing rate, and heterozygote superiority in ponderosa pine.

Functional Interactions among Traits that Determine Reproductive Success in a Native Annual Plant

The effects of rock climbing on the vegetation of three Minnesota cliff systems

Variation in methylglucosinolate and insect damage toCleome serrulata (Capparaceae) along a natural soil moisture gradient.

Quantitative genetic variation and natural selection in Cleome serrulata growing along a mild soil moisture gradient