Divergence without Speciation

2003 ◽  
Author(s):  
Mary Jane West-Eberhard
Keyword(s):  
Genetic Load ◽  
Theory Change ◽  
Adaptive Landscape ◽  
Isolated Populations ◽  
Evolutionary Transitions ◽  
Genetic Accommodation ◽  
Alternative Phenotypes ◽  
Shifting Balance ◽  
Monomorphic Population ◽  
Adaptive Shifts

Part II discussed the developmental origins of novelty in terms of how the phenotype is reorganized during evolution. It did not deal extensively with the problem of adaptedness during evolutionary transitions. How are we to explain transitions from one well-adapted state to another? Many still-influential discussions of adaptive shifts, such as Simpson’s (1944) treatment of quantum evolution and Wright’s (1932) discussion of shifting balance, associate change with fitness cost. Speciational theories of change depict change as dependent upon reproductively isolated populations in new environments. This chapter discusses divergence without reproductive isolation of novel forms, where the presumed cost of change is sidestepped because of the presence of adaptive options in the population undergoing change. Darwin’s solution to the problem of maladaptation during change was strict gradualism in monomorphically adapted populations. Darwin (1859 [1966]) reasoned that transitions between specialized adaptive states need not be disruptive if they were to occur by a series of small steps. Wright’s (1932) shifting balance is another solution to the same problem, but in Wright’s theory, change is initiated by a chance combination of genes that happens to suit a population to a new adaptive mode. Without a gradual adaptive change or a lucky gene combination, a shift between two peaks on Wright’s adaptive landscape would imply passing through a valley of inferior adaptedness. Alternative phenotypes offer a third kind of solution, one that requires neither strict gradualism in a monomorphic population nor chance genetic events. In species with alternative phenotypes, a recurrent novelty that happens to prove advantageous to some individuals or in some circumstances can be refined via gradual genetic accommodation as an optional trait. Since this involves developmental diversification, not transformation or loss of existing traits, the new option develops as a new specialization alongside old ones. Shapiro notes that conditional expression of alternative phenotypes is a way of having two adaptive specializations “without carrying a genetic load,” or a cost of genotypes that oblige expression of phenotypes less favorable than the fittest one.

Fisherian and Wrightian theories of speciation

Genome ◽  
1989 ◽  
Vol 31 (1) ◽  
pp. 221-227 ◽  
Author(s):  
Russell Lande
Keyword(s):  
Sexual Selection ◽  
Genetic Drift ◽  
Chromosomal Rearrangements ◽  
Adaptive Landscape ◽  
Random Genetic Drift ◽  
Premating Isolation ◽  
Sympatric Populations ◽  
Mating Preferences ◽  
Intermediate Phenotypes ◽  
Shifting Balance

Fisher's theory of sexual selection, Wright's shifting-balance theory, and recent models based on them are reviewed as mechanisms of animal speciation. The joint evolution of mating preferences and secondary sexual characters can cause rapid nonadaptive phenotypic divergence and premating isolation between geographically separated populations, or along a cline. Extensive comparative data on Drosophila species support the suggestion of R. A. Fisher and T. Dobzhansky that the evolution of mating preferences can reinforce partial postmating isolation between sympatric populations. The interaction of natural selection and random genetic drift in local populations with a small effective size can produce a rapid transition between relatively stable phenotypes separated by an adaptive valley, or between chromosomal rearrangements with a heterozygote disadvantage. Large demographic fluctuations, such as frequent random local extinction and colonization, are required for the rapid spread of new adaptations (or karyotypes) when intermediate phenotypes (or rearrangement heterozygotes) are selected against.Key words: reproductive isolation, hybridization, sexual selection, reinforcement, subdivided population, shifting balance, adaptive landscape, random genetic drift.

scholarly journals Genomic signatures of inbreeding and genetic load in a threatened rattlesnake

2021 ◽  
Author(s):  
Alexander Ochoa ◽  
H. Lisle Gibbs
Keyword(s):  
Threatened Species ◽  
Empirical Studies ◽  
Genetic Load ◽  
Population History ◽  
Deleterious Mutations ◽  
Genetic Rescue ◽  
Isolated Populations ◽  
Genome Wide ◽  
The Impact

Theory predicts that threatened species living in small populations will experience high levels of inbreeding that will increase their negative genetic load but recent work suggests that the impact of load may be minimized by purging resulting from long term population bottlenecks. Empirical studies that examine this idea using genome-wide estimates of inbreeding and genetic load in threatened species are limited. Here we use genome resequencing data to compare levels of inbreeding, levels of genetic load and population history in threatened Eastern massasauga rattlesnakes (Sistrurus catenatus) which exist in small isolated populations and closely-related yet outbred Western massasauga rattlesnakes (S. tergeminus). In terms of inbreeding, S. catenatus genomes had a greater number of ROHs of varying sizes indicating sustained inbreeding through repeated bottlenecks when compared to S. tergeminus. At the species level, outbred S. tergeminus had higher genome-wide levels of genetic load in the form of greater numbers of derived deleterious mutations compared to S. catenatus presumably due to long-term purging of deleterious mutations in S. catenatus. In contrast, mutations that escaped the “drift sieve” and were polymorphic within S. catenatus populations were more abundant and more often found in homozygote genotypes than in S. tergeminus suggesting a reduced efficiency of purifying selection in smaller S. catenatus populations. Our results support an emerging idea that the historical demography of a threatened species has a significant impact on the type of genetic load present which impacts implementation of conservation actions such as genetic rescue.

Quantitative Shifts and Correlated Change

2003 ◽  
Author(s):  
Mary Jane West-Eberhard
Keyword(s):  
Developmental Plasticity ◽  
Qualitative Change ◽  
Adaptive Plasticity ◽  
Developmental Relationships ◽  
Evolutionary Transitions ◽  
Trade Offs ◽  
New Type ◽  
Environmental Responses ◽  
Genetic And Environmental Factors ◽  
Adaptive Shifts

Preceding chapters have discussed evolutionary transitions as changes in the expression of discrete, modular traits. This chapter discusses transitions that are due to shifts in the magnitude, rather than the time, place, or repetition, of trait expression. Especially, it considers examples where environmental extremes induce quantitative change in the expression of continuously variable plastic traits. Quantitative shifts can produce novel extremes, novel combinations of extremes, or simultaneously opposite shifts due to negative correlations between traits, as in trade-offs. As pointed out by Brien (1969; see also chapter 7), correlated adaptive shifts can produce major changes in which large steps are not lethal because the usual adaptive plasticity of the organism accommodates the kind of change that occurs when “a new type of organization is born”. The same developmental plasticity that is responsible for phenotypic accommodation and homeostatic stability (see chapter 3) can produce correlated change as well. Correlations among the environmental responses of plastic traits mean that several quantitative traits can change at once, if they respond simultaneously to the same mutation or environmental factor. The two-legged goat described in chapter 3 shows how the multidimensional plasticity of the phenotype can produce a strikingly novel form that, lacking intermediates, appears to be a qualitative change—a change in kind, not merely degree. The fact that the same plasticity-mediated changes would occur whether the cause of shortened front legs were due to a mutation or to an environmental effect early in development illustrates the interchangeability of genetic and environmental factors in inducing correlated change. Raff and Kaufman (1983, p. 202) called correlated effects due to developmental relationships among continuously variable traits “relational pleiotropy.” Positive relational pleiotropy can result when numerous positively correlated traits respond in unison to a single stimulus or condition, such as variation in size. Negative relational pleiotropy can give rise to trade-offs, or negative fitness effects among traits such that an increase in the magnitude of one means a decrease in the magnitude of one or more others.

scholarly journals The mixed mating system of the sea palm kelp Postelsia palmaeformis : few costs to selfing

2010 ◽  
Vol 278 (1710) ◽  
pp. 1347-1355 ◽  
Author(s):  
Allison K. Barner ◽  
Catherine A. Pfister ◽  
J. Timothy Wootton
Keyword(s):  
Mating System ◽  
First Generation ◽  
Genetic Load ◽  
Mixed Mating ◽  
Reproductive Assurance ◽  
Isolated Populations ◽  
Mixed Mating System ◽  
Fitness Consequences ◽  
Limited Dispersal ◽  
Self Fertilization

Naturally isolated populations have conflicting selection pressures for successful reproduction and inbreeding avoidance. These species with limited seasonal reproductive opportunities may use selfing as a means of reproductive assurance. We quantified the frequency of selfing and the fitness consequences for inbred versus outcrossed progeny of an annual kelp, the sea palm ( Postelsia palmaeformis ). Using experimentally established populations and microsatellite markers to assess the extent of selfing in progeny from six founding parents, we found the frequency of selfing was higher than expected in every population, and few fitness costs were detected in selfed offspring. Despite a decline in heterozygosity of 30 per cent in the first generation of selfing, self-fertilization did not affect individual size or reproduction, and correlated only with a marginally significant decline in survival. Our results suggest both that purging of deleterious recessive alleles may have already occurred and that selfing may be key to reproductive assurance in this species with limited dispersal. Postelsia has an alteration of a free-living diploid and haploid stage, where the haploid stage may provide increased efficiency for purging the genetic load. This life history is shared by many seaweeds and may thus be an important component of mating system evolution in the sea.

scholarly journals An evolutionary perspective on contemporary genetic load in threatened species to inform future conservation efforts

2021 ◽  
Author(s):  
Samarth Mathur ◽  
John Tomeček ◽  
Luis Tarango-Arámbula ◽  
Robert Perez ◽  
Andrew DeWoody
Keyword(s):  
Population Genomics ◽  
Empirical Distribution ◽  
Demographic History ◽  
Genetic Load ◽  
Purifying Selection ◽  
Deleterious Mutations ◽  
Genetic Rescue ◽  
Isolated Populations ◽  
Potential Donor ◽  
Inbred Populations

In theory, genomic erosion can be reduced in fragile “recipient” populations by translocating individuals from genetically diverse “donor” populations. However, recent simulation studies have argued that such translocations can, in principle, serve as a conduit for new deleterious mutations to enter recipient populations. A reduction in evolutionary fitness is associated with a higher load of deleterious mutations and thus, a better understanding of evolutionary processes driving the empirical distribution of deleterious mutations is crucial. Here, we show that genetic load is evolutionarily dynamic in nature and that demographic history greatly influences the distribution of deleterious mutations over time. Our analyses, based on both demographically explicit simulations as well as whole genome sequences of potential donor-recipient pairs of Montezuma Quail (Cyrtonyx montezumae) populations, indicate that all populations tend to lose deleterious mutations during bottlenecks, but that genetic purging is pronounced in smaller populations with stronger bottlenecks. Despite carrying relatively fewer deleterious mutations, we demonstrate how small, isolated populations are more likely to suffer inbreeding depression as deleterious mutations that escape purging are homogenized due to drift, inbreeding, and ineffective purifying selection. We apply a population genomics framework to showcase how the phylogeography and historical demography of a given species can enlighten genetic rescue efforts. Our data suggest that small, inbred populations should benefit the most when assisted gene flow stems from genetically diverse donor populations that have the lowest proportion of deleterious mutations.

scholarly journals Pollen feeding in Heliconius butterflies: the singular evolution of an adaptive suite

2020 ◽  
Vol 287 (1938) ◽  
pp. 20201304
Author(s):  
Fletcher J. Young ◽  
Stephen H. Montgomery
Keyword(s):  
Life History ◽  
Current Knowledge ◽  
Adaptive Traits ◽  
Evolutionary Transitions ◽  
Complex Interactions ◽  
Pollen Feeding ◽  
Evolutionary Trajectories ◽  
Morphological And Physiological Traits ◽  
Adaptive Shifts ◽  
Potential Interactions

Major evolutionary transitions can be triggered by behavioural novelty, and are often associated with ‘adaptive suites’, which involve shifts in multiple co-adapted traits subject to complex interactions. Heliconius butterflies represent one such example, actively feeding on pollen, a behaviour unique among butterflies. Pollen feeding permits a prolonged reproductive lifespan, and co-occurs with a constellation of behavioural, neuroanatomical, life history, morphological and physiological traits that are absent in closely related, non-pollen-feeding genera. As a highly tractable system, supported by considerable ecological and genomic data, Heliconius are an excellent model for investigating how behavioural innovation can trigger a cascade of adaptive shifts in multiple diverse, but interrelated, traits. Here, we synthesize current knowledge of pollen feeding in Heliconius , and explore potential interactions between associated, putatively adaptive, traits. Currently, no physiological, morphological or molecular innovation has been explicitly linked to the origin of pollen feeding, and several hypothesized links between different aspects of Heliconius biology remain poorly tested. However, resolving these uncertainties will contribute to our understanding of how behavioural innovations evolve and subsequently alter the evolutionary trajectories of diverse traits impacting resource acquisition, life history, senescence and cognition.

scholarly journals How the tortoise beats the hare: Slow and steady adaptation in structured populations suggests a rugged fitness landscape in bacteria

2014 ◽  
Author(s):  
Joshua R. Nahum ◽  
Peter Godfrey-Smith ◽  
Brittany N. Harding ◽  
Joseph H. Marcus ◽  
Jared Carlson-Stevermer ◽  
...  
Keyword(s):  
Fitness Landscape ◽  
Structured Populations ◽  
Adaptive Landscape ◽  
Multiple Peaks ◽  
Structured Population ◽  
Shifting Balance ◽  
Genetic Epistasis ◽  
Rugged Topography ◽  
Landscape Genetic ◽  
Average Fitness

In the context of Wright's adaptive landscape, genetic epistasis can yield a multi-peaked or "rugged" topography. In an unstructured population, a lineage with selective access to multiple peaks is expected to rapidly fix on one, which may not be the highest peak. Contrarily, beneficial mutations in a population with spatially restricted migration take longer to fix, allowing distant parts of the population to explore the landscape semi-independently. Such a population can simultaneous discover multiple peaks and the genotype at the highest discovered peak is expected to fix eventually. Thus, structured populations sacrifice initial speed of adaptation for breadth of search. As in the Tortoise-Hare fable, the structured population (Tortoise) starts relatively slow, but eventually surpasses the unstructured population (Hare) in average fitness. In contrast, on single-peak landscapes (e.g., systems lacking epistasis), all uphill paths converge. Given such "smooth" topography, breadth of search is devalued, and a structured population only lags behind an unstructured population in average fitness (ultimately converging). Thus, the Tortoise-Hare pattern is an indicator of ruggedness. After verifying these predictions in simulated populations where ruggedness is manipulable, we then explore average fitness in metapopulations of Escherichia coli. Consistent with a rugged landscape topography, we find a Tortoise-Hare pattern. Further, we find that structured populations accumulate more mutations, suggesting that distant peaks are higher. This approach can be used to unveil landscape topography in other systems, and we discuss its application for antibiotic resistance, engineering problems, and elements of Wright's Shifting Balance Process.

scholarly journals Heterozygosity–Fitness Correlations Reveal Inbreeding Depression in Neonatal Body Size in a Critically Endangered Rock Iguana

2019 ◽  
Author(s):  
Jeanette B Moss ◽  
Glenn P Gerber ◽  
Mark E Welch
Keyword(s):  
Body Size ◽  
Inbreeding Depression ◽  
Genetic Load ◽  
Critically Endangered ◽  
First Year ◽  
Isolated Populations ◽  
Conditional Expression ◽  
Age Dependent ◽  
Endangered Population ◽  
Related Trait

Abstract Inbreeding depression, though challenging to identify in nature, may play an important role in regulating the dynamics of small and isolated populations. Conversely, greater expression of genetic load can enhance opportunities for natural selection. Conditional expression concentrates these opportunities for selection and may lead to failure of detection. This study investigates the possibility for age-dependent expression of inbreeding depression in a critically endangered population of rock iguanas, Cyclura nubila caymanensis. We employ heterozygote-fitness correlations to examine the contributions of individual genetic factors to body size, a fitness-related trait. Nonsignificant reductions in homozygosity (up to 7%) were detected between neonates and individuals surviving past their first year, which may reflect natural absorption of inbreeding effects by this small, fecund population. The majority of variation in neonate body size was attributed to maternal or environmental effects (i.e., clutch identity and incubation length); however, heterozygosity across 22 microsatellite loci also contributed significantly and positively to model predictions. Conversely, effects of heterozygosity on fitness were not detectable when adults were examined, suggesting that inbreeding depression in body size may be age dependent in this taxon. Overall, these findings emphasize the importance of taking holistic, cross-generational approaches to genetic monitoring of endangered populations.

scholarly journals A tortoise–hare pattern seen in adapting structured and unstructured populations suggests a rugged fitness landscape in bacteria

2015 ◽  
Vol 112 (24) ◽  
pp. 7530-7535 ◽  
Author(s):  
Joshua R. Nahum ◽  
Peter Godfrey-Smith ◽  
Brittany N. Harding ◽  
Joseph H. Marcus ◽  
Jared Carlson-Stevermer ◽  
...  
Keyword(s):  
Fitness Landscape ◽  
Structured Populations ◽  
Adaptive Landscape ◽  
Multiple Peaks ◽  
Structured Population ◽  
Shifting Balance ◽  
Genetic Epistasis ◽  
Rugged Topography ◽  
Landscape Genetic ◽  
Average Fitness

In the context of Wright’s adaptive landscape, genetic epistasis can yield a multipeaked or “rugged” topography. In an unstructured population, a lineage with selective access to multiple peaks is expected to fix rapidly on one, which may not be the highest peak. In a spatially structured population, on the other hand, beneficial mutations take longer to spread. This slowdown allows distant parts of the population to explore the landscape semiindependently. Such a population can simultaneously discover multiple peaks, and the genotype at the highest discovered peak is expected to dominate eventually. Thus, structured populations sacrifice initial speed of adaptation for breadth of search. As in the fable of the tortoise and the hare, the structured population (tortoise) starts relatively slow but eventually surpasses the unstructured population (hare) in average fitness. In contrast, on single-peak landscapes that lack epistasis, all uphill paths converge. Given such “smooth” topography, breadth of search is devalued and a structured population only lags behind an unstructured population in average fitness (ultimately converging). Thus, the tortoise–hare pattern is an indicator of ruggedness. After verifying these predictions in simulated populations where ruggedness is manipulable, we explore average fitness in metapopulations of Escherichia coli. Consistent with a rugged landscape topography, we find a tortoise–hare pattern. Further, we find that structured populations accumulate more mutations, suggesting that distant peaks are higher. This approach can be used to unveil landscape topography in other systems, and we discuss its application for antibiotic resistance, engineering problems, and elements of Wright’s shifting balance process.

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