Is Artifical Selection Really That Different From Natural Selection?

Neolithic Revolution is the shift from hunter-gatherers to keeping animals and growing crops (cultivation). Image courtesy of http://historymadeeveryday.files.wordpress.com/2013/09/neolithic-age.jpg?w=640

Darwin’s use of artificial selection as an analogy to evolution (by natural selection) is much debated. Species evolution by human association such as plant domestication is presumed to hold strong selection pressures with rapid evolution of cultivated species in as little as a few hundred years.  Humans (Homo sapiens) began domesticating plant and animal species during the Neolithic agricultural revolution approx. 10,000-13,000 years ago. A change from hunter-gatherer into sedentary agricultural groups shows a transformation in human behavioural ecology which ultimately led to the co-evolutionary origins of domesticated crop and livestock species.

Seed crops are the most successful of domesticated plant species whose rates of evolution may be representative of other domesticated plant taxa.  Purugganan & Fuller (2010) researched the rate of evolution in phenotypic traits during the domestication process and compared them to the rates experienced by wild species under natural selection.

Archaeobotanical records provided quantitative information on ‘non-shattering’ (the retention of seed after harvesting) and grain size (which increases under domestication) from in five regions in Asia, Africa, and North America across 11 crop species that date from the Neolithic period. The evolution and fixation of the non-shattering trait is regarded as a hallmark of domestication, due to the reduced ability for natural seed dispersal, therefore detrimental in wild populations, making the cultivated species dependent on humans for continued reproduction.

It was found that phenotypic evolution in multiple crop species is significantly slower than rates observed in wild species. This indicates that the rates of evolution during the domestication process, including the strength of selection, may be similar to those measured for wild species. Domestication may be driven by unconscious selection pressures similar to that observed for natural selection;  proving artificial selection to be a valid model for the study of evolutionary change.  Some proposed reasons for the unexpected similarities are:

• The domestication process is in principle simply natural selection in an environment established by human agriculture. It could be considered as a form of animal/plant coevolution. 
• Early cultivation of proto-domesticates would have been established alongside wild plants resulting in continued gene flow, slowing the fixation of selected alleles.
• Deleterious mutations may segregate at higher frequencies in the population bottlenecks associated with the establishment of crop plants. These may lead to decreased selection efficiency and a slower pace of phenotypic evolution.

Canine Intelligence: Has Artificial Selection Created Dumb Dogs

Dog (Canis lupus familiaris) domestication and their mutually beneficial relationship with humans is one of the oldest; working, hunting and living together for over 10 000 years. During that time dogs have been selected for a wide range of physical and behavioural traits such as protection, hunting, guidance and more recently appearance and responsiveness to verbal/physical cues. Controllability and docility in temperament is assumed to be an early selection factor in the original human adoption of the ancestral dog and remains a pivotal character throughout the domestication process. Frank (1980) looked into the underlying differences between learning techniques favoured under conditions of natural and artificial selection whilst questioning the notion of animal intelligence. 

Dogs originated from the grey wolf (Canis lupus) which relies on primitive instincts in response to basic survival needs as well as a cognitive system that evolved in correlation with group hunting. Wolf behaviour is consistent in response to very specific stimulants. When comparing learning ability between wild wolves and domesticated dogs, wolves excel at observational learning, which in a natural setting add to their survival by being able to problem solve. However domesticated dogs excel at trainability or instrumental conditioning which is learning by extrinsic reinforcement in a human environment. The animal learns a response to a command that has no discernable functional connection with the feedback received. For example: a dog is trained to sit; when it completes the required response (sitting) from the cue (vocal command) it receives food (extrinsic reinforcement/feedback). In nature, a wolf does not receive food for ‘sitting’ therefore it is not bettering its survival to obey commands with no obvious function or beneficial outcome.

 

Image courtesy of: http://www.shardabakersdogworld.com/potty-training/dog-potty-training/

In the process of domestication it appears that the wolf's capacity for observational learning and automated instinctual responses has been superseded by the capacity for trainability. Artificial selection favoured docility and human intervention between the dog and the consequences of his behaviour have relaxed those pressures favouring capacity for insight. Now modern breeds have acquired a tolerance for boredom, human socialization, a reduced wariness of environmental and social stimuli and are highly dependent upon their human caregivers.

It could be argued that in training our dogs to be what we consider ‘smarter’ (by completing cue-response tasks) we have dulled their capacity to problem-solve independently and they now lack the strong ability to learn on their own through observation and trial and error as do their wild counterparts. 

Comic courtesy of: http://jsburner.edublogs.org/2010/08/04/neolitic-revolution/

Pigeon Domestication: Was It Worth It?

Feral Pigeon: Flying Rat or Urban Hero?

One of the most famous examples of domestication is Darwin and his pigeons (Freeman et. al., 2007). Darwin held much interest in the artificial selection of pigeons with elaborately shaped and coloured feathers to support his research into evolution. The domestic pigeon (Columba livia), originated 5000 years ago from Mediterranean rock doves. They were domesticated for food, fancy looks and pigeon racing. At around 350 domestic breeds there is enormous variation in size, shape, and colour. 

Fancy Pigeon Photo courtsey of John Ross. http://charlesdarwintrust.org/content/86/galleries_pigeons

 

 

Now a new generation of feral pigeons (domestic breeds released/escaped into the wild) inhabit cities around the world (Cookson, 2013). Sol (2008) examined the outcome of pigeon domestication and its flow on effects that hinder survival to its feral descendant’s populations. Artificial selection is known to displace individuals from their ‘adaptive peak’ that enhances species survival in the wild. This can be seen in the disproportionally long tarsus-metatarsus bones (leg bone) in the domestic and feral pigeon. The size of the leg bone is hereditary and influential in how an animal utilizes its environment (stride length, speed and perching stability).

 It was found that longer leg bones are a hindrance and resulted in higher mortality in feral pigeons due to inefficient locomotion required in active food searching. Surprisingly, instead of the demise of the feral pigeon, it appears re-adaption has occurred independently with feral pigeon populations reverting back to their ancestral rock dove skeletal sizes and shapes, steering away from their most direct domestic ancestors. This suggests that selective pressures of the wild environment (a phenotype–ecology association) has driven natural selection to ‘undo’ domestication and assert the size and shape phenotypes of the original, wild ancestors.

Is human interference in the evolution of species through domestication beneficial or detrimental?

As seen in the domestic pigeon, the traits selected for that are deemed ‘fit’ & ‘better’ in reality no longer allow for the species to successfully function in its native ecosystem from whence it originated. 

Will feral pigeons ever fully revert back to their ancestral physiology?

Some populations may not have had enough time yet to re-adapt OR possibly a new niche in human-made habitats has created a unique morphological optimum for feral pigeons, rather than their ancestors open rocky environment. 

Thanks to humans it could be implied that the feral pigeons (often called ‘flying rats’) presence in cities is solely “our” fault. 

Genetic "Improvement" in Maize

When we were asked to create a blog on some topic of Evolutionary Biology, I immediately thought back to one of my first year lab practicals where we demonstrated Mendelian & Population Genetics using Zea mays (maize) cobs. This got me thinking to how and where this corn came from. I found a journal article by Yamasaki et al., (2005) which explained that this maize was domesticated from teosinte (Z. mays subsp parviglumis) between 6000 and 9000 years ago in southern Mexico. Due to its adaptability and variability over a wide range of environments, this original maize, named ‘Landraces’, spread across America.

Over time, improvement of maize crops has occurred due to the selection of key morphological and agronomic traits that have been enhanced/controlled through the selection of specific alleles within the original maize genes. This resulted in loss of genetic diversity and inbred lines of maize currently used in hybrid maize production. It was found that artificial selection to improve the crops not only affected obvious morphological and developmental differences such as enhanced productivity and performance (yield, resistance to biotic and abiotic stresses) but also specific biochemical pathways (such as enzymes used in the starch synthesis pathway).

 A large-scale screening was conducted to discover the genes responsible for maize domestication and improvement. During the domestication of Zea mays, the genes that had been heavily selected for (for desirable traits) are now the ones that lack the most genetic diversity. Whereas in neutral genes (genes not modified or influenced by human interference), higher genetic diversity is retained and is expected to be reduced only by bottleneck effects, as shown by figure 1 (Yamasaki et al., 2005). By identifying the selected genes, the reconstruction of the gene selection history of maize can be ‘re-created’ with focus on candidate genes for maize improvement. In theory, artificially selected genes can be screened for in any animal or plant domesticate, although much depends upon the relative levels and patterns of gene diversity in neutral, selected and wild taxon genes. This holds potential for many future developments in reverting genetic bottlenecks and increasing genetic diversity within traits that have been heavily selected for due to domestication.