Flashcards in Artificial Selection Deck (55):
Define artificial selection
Selection by humans of a deliberately chosen trait or trait combination in a (usually captive) population. This differs from natural selection in that the criterion for survival and reproduction is the trait chosen, rather than fitness as determined by the entire genotype.
Would you expect artificial selection to work with, against or be ineffective with respect to natural selection?
The answer will depend on the example being considered (as well as the environment in which it exists). Artificial selection may oppose, amplify or be neutral in relation to natural selection.
Where does evidence come from for the domestication of the earliest animal species?
The dog is the animal with evidence for its domestication being the first and this has been found in Asia and Europe.
What was the first product that sheep were most likely to have been domesticated for?
Meat or mutton.
What other domesticated species is associated with the domestication of wheat?
Cats, and this is possibly related to their protection of stored grain from rodents, which would have been of great benefit to early humans.
What was domesticated from teosinte?
Maize or corn.
Name a domesticated species that cannot reproduce without human intervention.
Domesticated silkworms are dependent on humans for their survival as they have reduced mouthparts and require feeding and the adults cannot fly to meet mates. You may also have considered vanilla too, as it requires pollination.
Define 'breed' and 'pedigree'
Breeds are a stock of animals or plants within a species having distinctive or homogeneous appearance and/or behaviour and typically having been produced by selective breeding over time. Pedigree refers to the recorded lineage of an animal with details of its dam and sire, grand dam and grand sire and so on, back through the generations of selected breeding.
Why might it be important to identify and preserve wild stocks of otherwise domesticated species?
It is important to identify wild populations accurately in order to protect their genetic diversity. These wild ‘cousins’ may then be used to further improve domesticated species; or, of increasing importance, to protect them from disease(s) by introducing new traits into the already domesticated stock from wild sources.
Livestock and pets that are cared for by humans are provided with food and shelter and protected from most predators. How would these living conditions alter the action of natural selection in domesticated populations?
Natural selection would be weakened for these particular characters that do not affect the fitness under domestication; that is, finding, eating and digesting food, the ability to recognise and escape predators, and tolerance of severe weather conditions. Put another way, animals that would normally not survive in the wild (because of their inability to find food, escape predators, etc.) would survive under these protected conditions.
Name a wild species in which relaxation of selection for escaping from predators has happened naturally. How did the animals’ behaviour differ from that of related populations that are exposed to predators?
Svalbard reindeer have had no natural predators for many thousands of years. Like domesticated cattle, these reindeer have less inclination to run away when approached. However, unlike many domesticated animals, Svalbard reindeer retain the capacity to forage and breed in a difficult climate. This example emphasises the similarities between evolution in artificial and natural environments, driven by artificial or natural selection, respectively.
On what kind of characters would natural selection continue to act in populations of domesticated animals?
Characters that affect fecundity (breeding success), such as age at sexual maturity, litter sizes and parental behaviour.
Would domesticated animals generally produce more or fewer offspring than their wild relations?
They would produce more offspring because protection by humans would prevent natural selection against traits such as heavily pregnant mothers becoming unable to escape from predators or find enough food. One notable difference between domesticated dogs and wolves is that most dogs come into oestrus and breed twice a year. Only the most primitive (i.e. earliest) breeds, including the Australian dingo and the Basenji (an African breed), resemble their wild ancestors in breeding only once a year.
Which special form of natural selection relating to reproduction could be eliminated entirely under domestication?
Sexual selection. Humans determine the resulting offspring’s genetic make-up by choosing which adults have the opportunity to breed. Sexually receptive females may be confined and therefore unable to choose amongst males. Furthermore, in both farmed animals and captive or managed wild populations, artificial insemination (AI) with fresh or frozen sperm may be used instead of mating.
What are the likely consequences of the isolation of breeding stock over time?
The same individuals breed with each other over and over again. As a consequence, the genetic diversity decreases because of inbreeding. Modern breeds are often inbred because they mate with genetically close relatives.
How can zoos avoid causing genetic bottlenecks in captive populations of wild animals?
By managing and only allowing, as far as possible, reproductive events to take place between individuals that will maintain genetic diversity. This may involve moving animals that are not related between zoological collections for breeding purposes, although the use of frozen sperm is becoming more widespread, which for some species avoids the need for such movement. Official records are maintained through the species studbooks, an example of which was shown in Figure 1.3. Thus lessons learnt from studies of domestication also guide conservation efforts to maintain biodiversity throughout the world.
Suggest a few reasons why domesticated animals, and even wild animals that have been tamed when young, might not be the best candidates for reintroduction to the wild.
Re-introduced animals need to be able to find food, defend themselves from predators, find mates, successfully rear their young, and so on. If they had been domesticated or tamed when young, they may struggle with any or all of these activities and fail to survive.
The offspring of parents from different genetic lines. These may be from within a species, from different species (interspecific hybrid) or even different genera (intergeneric hybrid).
How are the size and shape of antlers of Svalbard reindeer different from those of the mainland subspecies?
The antlers of Svalbard reindeer are much smaller and simpler than those of the mainland subspecies.
Would dwarfism be more readily favoured by artificial selection in domesticated livestock than by natural selection in wild animals?
Yes. Provided the animals could breed successfully, issues such as impaired mobility associated with dwarfism would not reduce fecundity.
Would a dominant gene for dwarfism (or any other feature) be more easily selected than a recessive gene in domestic livestock?
Yes. Artificial selection would be much more efficient for retaining dominant genes because all animals that have the gene display the dwarf phenotype, therefore could be actively selected for with intensive breeding efforts.
Define piebald and pied.
Description of black and white patterning for domesticated animals. Body surface spot patterning of skin, hair, feathers or scales with often irregular or unsymmetrical patches of unpigmented white areas and pigmented black areas, in wild animals this is referred to as 'pied'
Description of a colour (other than black) and white patterning for domesticated animals. Body surface spot patterning of skin, hair, feathers or scales with often irregular or unsymmetrical patches of unpigmented white areas and pigmented coloured areas.
Animal now living in the wild, descended from a domesticated species.
How do herbivores physically access the nutrients contained in plant tissues?
They have to spend a long time chewing.
How could herbivorous animals deal with toxins distributed throughout the plant?
They would be unable to avoid ingesting toxins along with the nutritious tissues, so their digestive system has to prevent them being absorbed, or detoxify them and excrete the resulting waste.
How would you categorise the relationship between herbivores and their gut microbes?
The relationship between herbivores and their gut microbes is an example of mutualism (+, +), as they both benefit from the association.
What is the caecum?
A blind-ended sac extending from the junction of the small and large intestine.
What is hind-gut fermentation & how does it differ to digestion in ruminants?
Compared with ruminant forestomach fermentation, in hindgut fermenters the products of microbial digestion of cellulose cannot be absorbed unless the material re-enters the small intestine, by reflux or other means, and the bacteria themselves cannot be digested.
What is coprophagy?
The consumption of faeces, either from the same organism or from another species of animal.
Svalbard reindeer live in the far north where the vegetation is very sparse. They supplement their meagre, indigestible diet by eating the fresh faeces of barnacle geese that migrate to breed on Svalbard during the brief summer. What do these habits suggest about the efficiency of plant digestion in geese and reindeer?
Geese do not digest their food as thoroughly as reindeer can, but passage through their gut facilitates further digestion in the reindeer’s rumen.
Outline the weaning process in koalas.
Eucalyptus leaves are tough and indigestible. The single offspring is suckled for an unusually long period, and is slowly weaned onto its mother’s faeces, which supply nutrients as well as colonise the young’s gut with appropriate symbiotic microbes, before it tackles the adult diet.
Food passes through the gut more quickly in hindgut fermenters, taking 30–45 hours in a horse compared to 70–100 hours in a ruminant cow of similar body mass. Why does faster transit time provide an important advantage if a large proportion of the forage is totally indigestible?
A faster transit time means the gut contents move through and out in a shorter space of time, allowing larger volumes of food to be consumed and partially processed. The digestion might not be particularly efficient but the amount passing through is maximised.
Compare hind gut fermenters with ruminants in terms of excretion.
Because hindgut fermenters are less efficient at extracting energy from plants, they need more forage to sustain themselves. They also produce much more faeces than carnivores or ruminant herbivores. African elephants (which are generally larger than Asian elephants) defecate 14–20 times per day, producing a total of 150 kg of wet faeces, which when dried down still contains ~35 kg of dry matter.
What physical or behavioural features of the elephant can you think of that might hint of an aquatic ancestry? What evidence is there that they have aquatic roots?
Their trunk used as a snorkel when swimming and to squirt water over themselves, along with very sparse hair coverage. There is now a wealth of information to show that the elephants (Proboscidea) and the sea cows (Sirenia) must share a common ancestor. The Moeritherium, a primitive genus of the Proboscidea (40–30 million years ago) has skeletal features highly suggestive of a semiaquatic lifestyle.
Zebras also have long intestines and survive seasonal droughts by eating large quantities of dry, tough plant material and digesting it incompletely but quickly. How would this diet and digestive physiology affect the teeth?
The teeth would wear down faster because hindgut fermenters eat more food, and more of it is likely to be very abrasive grasses.
How would the differences in digestive physiology between hindgut fermenters and ruminants affect the composition of fuels circulating in the blood?
Glucose liberated from sugars and starches digested by hindgut fermenters’ own enzymes is absorbed in the small intestine before the microbes get a chance to convert it to short-chain fatty acids, so the muscles and other energy-utilising tissues of hindgut fermenters use far more glucose and less fatty acids than do those of ruminants.
What methods are employed by hind gut fermenting herbivores to combat excessive tooth wear?
The teeth of equids (horses, zebras and donkeys) withstand such use because the roots remain open, and the very long crown erupts slowly but continuously throughout life, thus replacing surface that is worn away. Elephants combat tooth wear in another way: each tooth is huge, the largest weighing several kilograms.
Why is the elephant a key stone species and in what ways is its dung essential?
The elephant makes use of approximately 40% of its diet, elephant dung is particularly rich in nutrients. Plenty of other animals take advantage of this preselected and partially digested food resource, including birds, dung beetles and dung flies. As well as rearranging the vegetation and ultimately habitat structure through their ‘gardening’ activities of uprooting trees and providing ready nutrition for other animals, their faeces is compost for the next generation of plant life.
If 35 kg of dry faeces is produced daily by an elephant from about 60 kg of dry matter eaten, what percentage has been digested?
(35/60) × 100 = 58%, indicating that only about 42% of food intake was digested well enough to be absorbed.
What are the most primitive mammals and how do they differ from other mammals?
The most primitive living mammals are the monotremes, egg-laying mammals. There are only three monotreme species: the duck-billed platypus and two species of echidna. Echidna milk is low in lactose and the mammary glands do not contain the protein alpha-lactalbumin, present in the milk of marsupials and placental mammals.
From the point of view of the mother’s physiology, what is the fundamental difference between feeding the young on milk, and bringing food to the nest, such as birds do?
Milk is a secretion, synthesised from precursors taken up from the blood; such nutrients may come directly from the diet, but milk synthesis can also be fuelled from body reserves, which is impossible for most birds. In a few kinds of birds, notably pigeons, and perhaps some penguins, both parents produce a tissue shed from the throat which is fed to the nestlings for a few days after hatching. From the point of view of nutrient storage and provision to young, this habit is comparable in function to lactation.
Comment on the likely evolution of lactation based on observations in echidna milk.
Echidna secrete a lysozyme that has both antibacterial properties (i.e. breaks glycosidic bonds) and weak activity in the lactose synthase system (i.e. forms glycosidic bonds between glucose and galactose). These observations together suggest that the milk secreted by the earliest mammary glands included a lysozyme, and that alpha-lactalbumin evolved subsequently, making lactose synthesis more efficient.
What is alpha-lactalbumin?
α-Lactalbumin is a protein present in the milk of almost all mammalian species. In primates, alpha-lactalbumin expression is upregulated in response to the hormone prolactin and increases the production of lactose.
What is lactose?
A form of carbohydrate found in milk made from glucose and galactose.
Describe the composition of milk.
A nutritious and easily digested emulsion of fats and water containing dissolved lactose (a unique disaccharide composed of glucose and galactose), proteins and various mineral salts.
Describe the composition of colostrum.
A mixture of relatively large proteins, including antibodies, secreted from mammary glands at the start of lactation, that confer immunity to the offspring.
Why does the transfer of immunity in colostrum to the neonates take such high priority?
Lactation entails prolonged, close association between the mother and offspring, thus providing the ideal conditions for her parasites and pathogens to pass to the vulnerable neonates, so they need immune defences as soon as possible.
Why is colostrum so important?
In some mammals, including humans, certain antibodies cross the placenta and reach the fetus before it is born, but postnatal transfer is still important. Babies, calves and other mammalian neonates that miss the opportunity to ingest colostrum are significantly more susceptible to infectious diseases.
Which vertebrate tissue synthesises large quantities of triacylglycerols from circulating fatty acids?
Adipose tissue. The enzymes that synthesise milk lipids in the mammary glands are very similar to those found in the adipose tissue of mammals and non-lactating vertebrates. The main difference is that the mammary enzymes are less specific than those of adipocytes: a wider range of fatty acids, including some with fewer than 10 carbon atoms, find their way into milk triacylglycerols than ever appear in mammalian adipose tissue.
What is the proposed theory for the evolution of lactation?
1. increased proximity between mother and egg/young
2. secretion from maternal glands to support the young
3. offspring utilise maternal secretion as food source
4. a gradual improvement in nutritional composition of the secretion and the means of its delivery to the offspring.
What component of milk would require new enzymes for its synthesis during the evolution of lactation?
Lactose, which is synthesised only in the mammary glands.
Detail the structure of lactose.
As a disaccharide, lactose is much more soluble than polysaccharides such as glycogen, but being a larger molecule, it exerts a smaller osmotic pressure in solution than a similar concentration (by mass) of a monosaccharide such as glucose.
When are the changes affecting the genes for lactose synthase necessary for them to contribute efficiently to milk synthesis?
The genes must be expressed in mammary glands, but only in response to the hormones and other cellular signals produced at the end of pregnancy and during lactation.