Natural Selection and the Sex Ratio

2017 ◽  
pp. 139-149
Author(s):  
W. F. Bodmer ◽  
A. W. F. Edwards
Keyword(s):  
Nature ◽  
1960 ◽  
Vol 188 (4754) ◽  
pp. 960-961 ◽  
Author(s):  
A. W. F. EDWARDS
Keyword(s):  

1997 ◽  
Vol 75 (4) ◽  
pp. 648-650 ◽  
Author(s):  
Eigil Reimers ◽  
Dag Lenvik

Theory suggests that a male in good condition at the end of the period of parental investment will outproduce a sister in similar condition, while she will outproduce him if both are in poor condition. Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. As maternal condition declines from good to poor, the fetal sex ratio should decline from a high proportion of males to a high proportion of females. Data from 1525 domestic reindeer (Rangifer tarandus) in six different herds slaughtered during December and January in 1976 through 1979 do not support a relationship between the sex of the fetus and either the condition of the mother or her age.


2018 ◽  
Author(s):  
Lysanne Snijders ◽  
Ralf H. J. M. Kurvers ◽  
Stefan Krause ◽  
Indar W. Ramnarine ◽  
Jens Krause

AbstractIndividual foraging is under strong natural selection. Yet, whether individuals differ consistently in their foraging success across environments, and which individual and population-level traits might drive such differences, is largely unknown. We addressed this question in a field experiment, conducting over 1,100 foraging trials with nine subpopulations of guppies, Poecilia reticulata, translocating them across environments in the wild. A-priori, we determined the individual social phenotypes. We show that individuals consistently differed in reaching food, but not control, patches across environments. Social individuals reached more food patches than less social ones and males reached more food patches than females. Overall, individuals were, however, more likely to join females at patches than males, which explains why individuals in subpopulations with relatively more females reached, on average, more food patches. Our results provide rare evidence for individual differences in foraging success across environments, driven by individual and population level (sex ratio) traits.


2020 ◽  
Vol 287 (1934) ◽  
pp. 20201377 ◽  
Author(s):  
Jaco M. Greeff ◽  
Karina Pentz ◽  
Marié Warren

Ever since Darwin's discovery of natural selection, we expect traits to evolve to increase organisms' fitness. As a result, we can use optimization models to make a priori predictions of phenotypic variation, even when selection is frequency-dependent. A notable example is the prediction of female-biased sex ratios resulting from local mate competition (LMC) and inbreeding. LMC models incorporate the effects of LMC and inbreeding. Fig wasp sex ratio adjustments fit LMC predictions well. However, the appropriateness of LMC models to fig wasps has been questioned, and the role that a coincidental by-product plays in creating the apparent fit has been clearly illustrated. Here, we show that the sex ratio adjustments of a fig wasp are the result of a dual mechanism. It consists of a standard facultative LMC response favoured by natural selection, as well as a mechanism that may be the result of selection, but that could also be a coincidental by-product. If it is a by-product, the fitness increase is coincidental and natural selection's role was limited to fine-tuning it for higher fitness returns. We further document a case of an apparent fitness-reducing sex ratio adjustment. We conclude that the use of the adaptationist approach demands that our understanding of traits must be remodelled continually to rectify spurious assumptions.


2020 ◽  
Vol 287 (1921) ◽  
pp. 20192849 ◽  
Author(s):  
Brendan P. Zietsch ◽  
Hasse Walum ◽  
Paul Lichtenstein ◽  
Karin J. H. Verweij ◽  
Ralf Kuja-Halkola

The ratio of males to females among an individual's offspring at birth (offspring sex ratio) has long been of great interest to evolutionary biologists. The human offspring sex ratio is around 1 : 1 and is understood primarily in terms of Fisher's principle (R. A. Fisher, The genetical theory of natural selection , 1930), which is based on the insight that in a population with an unequal sex ratio, each individual of the rarer sex will on average have greater reproductive value than each individual of the more common sex. Accordingly, individuals genetically predisposed to produce the rarer sex will tend to have greater fitness and thus genes predisposing to bearing that sex will increase in frequency until the population sex ratio approaches 1 : 1. An assumption of this perspective is that individuals' offspring sex ratio is heritable. However, the heritability in humans remains remarkably uncertain, with inconsistent findings and important power limitations of existing studies. To address this persistent uncertainty, we used data from the entire Swedish-born population born 1932 or later, including 3 543 243 individuals and their 4 753 269 children. To investigate whether offspring sex ratio is influenced by genetic variation, we tested the association between individuals' offspring's sex and their siblings' offspring's sex ( n pairs = 14 015 421). We estimated that the heritability for offspring sex ratio was zero, with an upper 95% confidence interval of 0.002, rendering Fisher's principle and several other existing hypotheses untenable as frameworks for understanding human offspring sex ratio.


1983 ◽  
Vol 33 (2) ◽  
pp. 130-136 ◽  
Author(s):  
A.V. Millard ◽  
E.A. Berlin
Keyword(s):  

Genetics ◽  
1998 ◽  
Vol 148 (2) ◽  
pp. 719-731
Author(s):  
Antonio Bernardo Carvalho ◽  
Michelle Cristina Sampaio ◽  
Flavia Roque Varandas ◽  
Louis Bernard Klaczko

Abstract Most sexually reproducing species have sexual proportions around 1:1. This major biological phenomenon remained unexplained until 1930, when Fisher proposed that it results from a mechanism of natural selection. Here we report the first experimental test of his model that obeys all its assumptions. We used a naturally occurring X-Y meiotic drive system—the sex-ratio trait of Drosophila mediopunctata—to generate female-biased experimental populations. As predicted by Fisher, these populations evolved toward equal sex proportions due to natural selection, by accumulation of autosomal alleles that direct the parental reproductive effort toward the rare sex. Classical Fisherian evolution is a rather slow mechanism: despite a very large amount of genetic variability, the experimental populations evolved from 16% of males to 32% of males in 49 generations and would take 330 generations (29 years) to reach 49%. This slowness has important implications for species potentially endangered by skewed sexual proportions, such as reptiles with temperature sex determination.


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