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How can human activity impact on an ecosystem?
How do the selection pressures that humans inflict on domestic dogs influence their evolutionary change?
The aim of this study is to investigate if humans manipulated the evolution of the domestic dog, through breeding them to acquire specific desired traits.
Selection pressures are factors that affect an organism’s ability to survive in its environment. These factors are what dictate whether or not the organism is suited to the environment it’s in. Within species there is variation in traits, so organisms with more advantageous traits for the specific environment will prosper, while organisms less suited to the environment will eventually be eliminated. This is a process known as natural selection, which is the basis of Darwin’s theory of evolution. Selection pressures within an ecosystem influence evolutionary change, and these selection pressures can be inflicted by one organism unto another. The process of artificial selection is when humans apply pressures on organismswith the intention of causing major changes to achieve desired traits. Investigating human impact on the manipulated evolution of the domestic dog can be helpful in understanding the interrelationships of organisms within an ecosystem. Humans have manipulated the domestic dog, canis familiaris, to be one of the most morphologically diverse mammals through selective breeding (Careau et al. 2010), with sizes ranging from a 2kg Chihuahua, to an 80kg Bull Mastiff, and builds ranging from the sleek Italian Greyhound to the chunky American Bully. So how did this one species descending from a common ancestor become so incredibly diverse?
It is hypothesised that the evolution of the domestic dog was greatly influenced by artificial selection, resulting in a vastly morphologically diverse animal due to human constructed breeding programs with the aim to achieve desired traits that differ between breeds.
Collecting and presenting data and Analysis of Data
The domestic dog is believed to be descendant of the wolf Canis lupus (Morey, 1994). This claim is backed up by the prominent similarity between dog DNA and wolf DNA (Cohn, 1997), with only 0.04% and 0.21% sequence divergence in nuclear exon and intron sequences between the two, respectively (Lindblad-Toh, et al, 2005). Over time dogs have evolved from wolves to become smaller animals, with shorter faces, steeply rising foreheads and proportionally wider cranial dimensions. Morey’s (1994, p. 343) study comparing crania from 222 modern canids – Grey Wolves, Red Wolves, Coyotes and Golden Jackals – showed that while modern dogs share snout-length proportions with similarly sized wild canids, their cranial morphology does not conform to the allometric patterns of adult wild canids, but when compared to a juvenile wolf’s cranial proportions, distinct similarities are obvious. This suggests that when humans first domesticated wolves, they favoured ones representing neotenic features, so selectively bred individuals to create a smaller, possibly more endearing animal. This led to rapid size reduction in early dogs, evident through their fossils frequently displaying crowded teeth, in jaws that weren’t big enough to accommodate the same number of teeth that their larger wolf ancestors possessed. This resulted in the dog evolving to become a paedomorfic animal, that even when fully grown is more morphologically similar to a juvenile wolf than it is to an adult wolf. The modern domestic dog is a human construct that has been sculpted by artificial selection in a bid to achieve desired traits.
A study of the changing skull shape of the St Bernard spanning 120 years (Drake and Klingenberg, 2008) showed a consistent trend of changing skull shape in the dog corresponding to breed standards. Systematic breeding of the St Bernard began in the late 1850s, with breed standards going into great deal on the desired head shape and skull morphology of the dog, stating that, “Like the whole body, (the head is) very powerful and imposing. The massive skull is wide… The muzzle is short,” (American Kennel Club, 1998, Official Standard of the Saint Bernard). Over time breeders selectively bred dogs that best met these standards in order to create the ‘perfect’ St Bernard. This resulted in significant morphological changes to the St Bernard, like a broadening of the skull, and tilting of the palate and upper jaw relative to the rest of the skull. This trend is linear throughout the period tested, and appears to be continuing in the breed today. This dramatic modification of the St Bernard skull demonstrates that artificial selection can achieve sustained and substantial change.
Selective breeding has resulted in over 400 genetically distinct breeds of dog, that display considerable variation in behavioural, physiological and morphological phenotypes (Akey, et al, 2010). Size variation between breeds surpasses that of all other living and extinct species of the canidae family (Sutter, et al, 2007), and this can be attributed to a single gene, IGF1. In Sutter, et al’s (2007) study, focusing on the vast size variation of the Portuguese Water Dog (PWD) within the breed, a genome-wide scan identified a major quantitative trait locus on chromosome 15 (CFA15), which displayed strong association with size variation within the breed. Through the use of microsatellite markers locating the gene on CFA15 it was identified that there was a single peak near the insulin-like growth factor-1 (IGF1) gene, which is known to influence size in humans and mice. It was found that 96% of PWD chromosomes carried 1 of 2 patterns of the allele IGF1, which are termed haplotypes. The haplotype associated with small dogs was ‘B’ and for large dogs it was ‘I’. The dogs homozygous for haplotype B had the smallest median skeletal size, while the dogs homozygous for haplotype I had the largest median skeletal size. Dogs that are heterozygous fell between. This made clear the effect of IGF1 in size differentiation among PWDs, but it was not certain whether this was the gene that was the determinant of size in all dogs.
Ostrander (2007) conducted a study to find out the effect of IGF1 in size differentiation among other breeds, through surveying genetic variation associated with 112 single-nucleotide polymorphisms (SNP) of chromosome 15 in 353 dogs representing 14 small breeds and 9 giant breeds. The dominance of a single unique haplotype was present in many unrelated small dog breeds, while it was near absent in giant breeds, suggesting that IGF1 is the gene likely to account for small body size in dogs. Homozygosity of the gene IGF1 was found to be dominant in smaller breeds, demonstrating the presence of artificial selection, as breeders seek to create even smaller dogs.
Dog breeds can be bred together to recreate a desired breed or to create a new breed/mixed breed. The Irish Wolfhound was a popular breed in Ireland when wolves were a pest in the late 1700s, and aided in successfully exterminating wolves by 1786 (Larson, et al, 2012), after which the demand for wolfhounds plummeted, and by 1840 the breed was extinct or all but extinct. George Augustus Graham recreated the form of the Irish Wolfhound by breeding one possible wolfhound with a Scottish Deerhound, and then through generations incorporated Borzois and Great Danes into the breed to create the modern day “Irish Wolfhound”, which retained the aesthetic form of the original breed, but not the genetic ancestry. The modern Irish Wolfhound is a complete human construct through selective breeding, which is not uncommon in present-day.
Recent genetic homogenization between breeds has occurred despite the increase in phenotypic disparity, as breeders have simultaneously closed breeding lines and selected for extreme morphological traits. Artificial selection influenced by breeders is evident through the fact that numerous widely geographically distributed dog populations possess the same identical mutations responsible for specific phenotypes; Chinese and Mexican breeds possess the same hairless gene (Drögemüller, 2008), sub-Saharan African and Thai breeds possess a ridged line of hair on their backs caused by the same gene mutation, and at least 19 different breeds possess the identical mutation for foreshortened limbs. These mutations unlikely arose across populations independently, implying significant gene flow between breeds, regardless of how morphologically diverse they appear.
All research undertaken and developed for this study was sourced from reliable platforms that provide strictly scholarly information, or that are government/education standard accredited. To ensure that no false or unreliable information was included in this report, all information used was mentioned in at least two credible sources or was supported by evidence referenced by the source. All sources used for the cultivation of this study maintained consistency on the stance that human selection impacted greatly on the evolution of the dog, while the few sources that argued against artificial selection as having an impact on the evolution of dogs were considered but not included, as their theories were easily disproved when compared to opposing studies. The evolution of the dog being influenced by artificial selection has been a topic of great interest to many scientists over time, so copious reliable research was readily available for use in this study. Some data used in this study was not recently found, so to maintain reliability and be sure this data is still relevant, dated studies were only used if their findings had been referred to in more recent studies performed by trustworthy researchers. The collection of data for this study could be improved through a more specific aim, perhaps focusing on a specific feature of the dog that has been greatly influenced by artificial selection. This study could also be improved through the inclusion of more skeletal images to support claims of changing cranial and body morphology.
The data collected and presented in this study satisfies the aim in that it successfully investigates as to whether or not humans manipulated the evolution of the domestic dog through breeding them to acquire specific desired traits. Conclusively, the study’s hypothesis was proven and the research presented evidences that human desire for specific traits in dogs is what drove the rapid evolution of the dog, resulting in a vastly morphologically diverse animal that, in spite of this, remains genetically similar across the species. Through this study it is clear that artificial selection is what caused the dog to become such a morphologically diverse animal, which provides insight into just how greatly selection pressures inflicted by humans on a species influences evolutionary change.
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