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Time series in every group were composed of specimens of similar age stage, i.
- Similar rates of morphological evolution in domesticated and wild pigs and dogs.
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Where possible, care was taken to sample specimens equally distributed over the considered time period of that group e. Further, not the same time period could be considered in all groups e. Both sexes were included. Not all measurements could be taken in all specimens and all groups see below. The measurements for each group were either taken by the same person Table 2 ; this study , or the persons who contributed measurements were working in close collaboration with one another Table 2 ; this study and [ 32 ]; [ 26 ] and [ 27 ].
Therefore, no sampling bias is to be expected. The investigated skulls are housed in museum collections and thus no specimen was killed for this study and no live specimens were used. Only evolutionary rates from allochronic study designs reporting rates of evolution within one line, not divergence and based on phenotypic data not genetic were used for these comparisons [ 30 ]. Four domestic dog groups St. Bernard, Newfoundland, bullterrier, and Siberian husky and wolves were measured in this study Table 2.
Our sample was extended with data on eight domestic dog groups boxer, barsoi, dogue de Bordeaux. Only dentally mature specimens, i.
In the St. Bernard, Newfoundland, bullterrier, Siberian husky, and the wolf seven linear measurements were taken with calipers Table 1. Additionally, the prebasial angle Table 1 was measured in St. Bernards and bullterriers, because there is evidence that these breeds show substantial change of this trait over time [ 21 , 23 ].
The prebasial angle was measured as described in Nussbaumer [ 23 ] and Baxter and Nussbaumer [ 33 ] using a contour gauge. In the second group, which comprises the data by Geiger and Haussman [ 32 ], three linear measurements were taken with callipers: skull base length, skull length, and zygomatic breadth Table 1.
Most studied skulls except the Siberian huskies are from purebred individuals for which pedigree information and origin are known and which represent modern breeds with breeding standards [ 21 , 23 ].
All those specimens are from Swiss kennels some specimens stem from the same kennel. The here used Siberian huskies represent a local sledge dog population from the region of Sakha Yakutia in Russia and died between and [ 28 ]. They are therefore representatives of a group of dogs that lived in a time before the Siberian huskies were recognized as a breed by the American Kennel Club in the United States of America in [ 34 ] and were probably selected for functional traits [ 28 ]. Only dentally mature wild boars, i.
We quantified changes in skull measurements on specimens of the Berkshire pig based on raw data provided by two different but related studies [ 26 , 27 ] Table 2. Data of Berkshire pig specimens between about two and three years of age were used. Different investigated age stages in wild boar and Berkshire pig were not supposed to produce a bias because the age stages within each group were similar.
Evolutionary rates were calculated in darwins [ 2 ] and haldanes. Since we used continuous time series evolutionary rates were estimated using univariate least-squares regressions as described by Hendry and Kinnison [ 30 ] and Purugganan and Fuller [ 3 ]. For every measurement and every group Table 2 , regressions were computed with the natural logarithm of the measurements darwin estimates and the natural logarithm of the measurements divided by the standard deviation haldane estimates as the dependent variables.
The time in million years darwin estimates and the number of generations haldane estimates were used as independent variables [ 3 , 30 ]. These methods operate on the assumption that the evolutionary rates are linear and not changing over the investigated time period, which we could not test here. These values of life history variables were obtained from the literature; for domestic dogs, mean sexual maturity across breeds was calculated. Most wild mammals are seasonal breeders but many domestic forms breed non-seasonally e.
Therefore, the wild forms are unlikely to breed in between the breading seasons and their generation time is subsequently given in whole years, as opposed to fractions of years in domesticates. Our method contrasts with many other studies that often use only age at maturity as an estimate for generation time [ 30 ]. However, we argue that gestation time and seasonality are equally important components of generation time as sexual maturity and can add a substantial amount of time to the generation time estimate of a species.
For example, gestation time is about one third of the age at sexual maturity in wild boar and additionally, the generation time has to be rounded up from In wolves, in which a generation time of The slope of the regressions were used as evolutionary rate estimates.
The same computation of rates was used for those references that did not provide rate estimates, but raw data only. The sign of a slope plus or minus shows if a measurement tends to get smaller or larger over time, but for the interpretation of the magnitude of change only absolute values were used. For further analyses, mean and median evolutionary rates were calculated for different groups because the skull measurements are not independent of each other, see Discussion.
All analyses that are reported here were performed using Microsoft Excel , R version 3. Distributions of specimens over the investigated time periods in every group are shown in Fig. Among the domestic dog groups, only few measurements varied significantly with time, and these measurements were found in the St. No significant changes of any of the investigated skull measurements over time were found in the bullterrier and Newfoundland, where changes were expected see Background. Bernard, for which we could demonstrate a significant change and the bullterrier, in which we found no significant change Tables 4 and 5.
Historical change of maximum skull length in all of the here investigated groups. These plots illustrate the distribution of sampled specimens over the investigated time period in every group. Historical change in one of the here investigated skull variables in two domestic dog breeds. Although a change of the prebasial angle angle between the hard palate and the cranial base of the skull throughout many decades can be discerned in both breeds dorsal bending in the St.
Bernard and ventral bending in the bullterrier , a significant change of this variable could only be found in the St. Bernard see Tables 4 and 5. The prebasial angles are indicated with white bars on the depicted photographs of skulls, which are scaled to the same neurocranial length. No significant changes of the examined skull measurements could be substantiated in the Berkshire pig and the investigated geographical group of wild boar Tables 4 and 5.
The median and mean evolutionary rates of the here investigated wild boar, wolf vs. Our evolutionary rate estimates for domestic populations were significantly smaller than the ones reported by Hendry et al. Further, our evolutionary rate estimates for domestic populations were similar regarding darwins or significantly smaller regarding haldanes than the rate estimates found by Hendry et al. In sum, we found no evidence for a faster rate of evolution of skull dimensions in our sample of domestic dogs and pigs relative to wild populations regarding similar and different traits in related and unrelated wild forms.
Comparisons of our rate estimates with single traits of other domesticates from the literature revealed the following. In sum, these comparisons show that different, selected traits in domesticates have the potential to evolve both faster and slower compared to wild populations.
In an investigation on traits in plants during the early domestication process, i. These studies thus show that human intervention per se does not necessarily accelerate evolutionary rates of organisms [ 3 , 38 ]. The current study further supports this conclusion. Although evolutionary rates between wild and domestic forms appear to be similar Table 6 , we found evidence for marked differences among domestic groups and traits Tables 4 , 5 and 6 , as has also been shown previously for improvement of production in livestock [ 45 ].
For some domestic dog breeds, it is well known that there have been changes of breeding standards or interpretations thereof and subsequent changes of skull shape and size were expected and could be confirmed with our data: the here reported skull shape changes in the St. Bernard corroborate the descriptions of changes given previously [ 21 , 22 , 23 , 24 ]. The skulls of the St.
We used partially the same sample as these studies, so this similarity of results is not surprising. However, not in all cases could reported skull dimension changes be quantitatively substantiated: 1 the bullterrier has been described to show a prominent decrease of the basicranial angle, i. We expanded the sample used by Nussbaumer [ 23 ] by incorporating more specimens until the year Table 2 , but we as well did not find a statistical significant decrease of the basicranial angle through time Fig.
In some domestic dogs, no marked changes were expected, as for example in the Siberian husky group that was probably not subject to artificial selection for aesthetic requirements; in this case in fact we did not find directed changes of skull dimensions Tables 4 and 5. Selection for functional requirements might influence skull shape in these dogs, but there is no evidence for such a change in the sample at hand.
Although the here observed inter-breed differences in the magnitude of changes over time might be the result of differential selection pressures, i.
Gene-to-phenotype models and complex trait genetics
Further, differences in relatedness among the examined domestic dog groups might have an influence on the observed changes or lack thereof. However, due to the reticulate nature of dog evolution, especially in modern breeds [ 46 ], predicting how relatedness of breeds among one another and the wolf influence evolutionary rates is difficult if not impossible. The comparison of linear measurements has some limitations and would profit from complementation by advanced morphometric analysis, such as geometric morphometrics [ 47 ].
Such analyses have already been used for the investigation of craniodental shape variation in the domestication of dogs [ 21 , 48 , 49 , 50 , 51 , 52 ] and pigs [ 53 , 54 , 55 ]. First, complex shape changes, e. Second, the different linear skull dimensions are not independent from one another and parts of the skull function as non-independent, integrated modules [ 56 , 57 ].
It is therefore to be expected that some of the linear dimensions of the skull change in a correlated fashion.
Plant Breeding Reviews, Part 1: Long-term Selection: Maize, Volume 24
No significant changes of skull dimensions could be found in the Berkshire pigs, although such changes have been described previously [ 4 , 26 ] Tables 4 and 5. In contrast, comparatively fast evolutionary rates were reported for production traits, such as weight gain, in other domestic pig breeds [ 14 ] Table 6.
These contrasts reflect the history of pig breeding, in which performance, and not appearance, was and still is the prime concern of the breeders [ 4 ]. In contrast to the wild boar group that was examined in this study, for which no significant changes of skull traits could be shown over time, more change occurred in the here studied wolf group.
Our study shows that artificial selection in domesticated dogs and pigs does not necessarily result in evolutionary rates of skull dimensions significantly higher than those recorded in wild populations, although skull shape change over time has been described to be exceptionally pronounced in some dog and pig breeds. As evolutionary rates in domestication have been described as relatively fast compared to the rates under natural conditions [ 2 ], this would imply that evolution without human interventions can be faster than previously expected [ 58 , 59 ].
Or evolutionary rates in domestication are slower than previously expected. However, our investigations show that evolutionary rates may vary greatly depending on species, breed, trait, breeding aim, evolutionary rate estimate darwins or haldanes , generation time estimates, and the phase of domestication considered [ 60 ]. Further comparative studies on evolutionary rates in wild vs. Francis RC. Domesticated - evolution in a man-made world.