Standard IV Objective 2d
Artificial Selection in Animals
What is artificial selection?
Artificial selection is the intentional reproduction of individuals in a population that have desirable traits. In
organisms that reproduce sexually, two adults that possess a desired trait — such as two parent plants that are tall —
are bred together. In this example, the mechanisms of heredity dictate that the next generation will consist of more
tall plants than previous generations. If artificial selection is continued, all of the population will ultimately be tall.
Also called selective breeding, artificial selection is perhaps best understood as a contrast to natural selection, where
the random forces of nature determine which individuals survive and reproduce. In both cases, the outcome is the
same: a population changes over time, so that certain traits become more common.
Artificial Selection in Cows
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Cattle have been domesticated for several thousand years. Humans have selected animals for gentleness, meat and
milk production, and to survive in the environment. There are now several breeds. Some have thick coats and can
live in the Scottish Highlands, and some can survive in arid areas. The main breeds of dairy cattle, with high milk
yields, are Holstein-Friesian, Brown Swiss, Guernsey, Ayrshire, Jersey and Milky Shorthorn. The original wild
cattle which were first domesticated are thought to have looked like modern Chillingham White cattle. By
repeatedly selecting cows with high milk yields and allowing them to breed over many generations, humans
have artificially selected improved breeds with higher milk production. Today, breeders still practise artificial
selection. In this way, a few elite cows can produce more offspring than they would naturally
Each cow’s milk yield is measured and recorded
The progeny of bulls is tested to find which bulls have produced daughters with the highest milk yields
Only a few good-quality bulls need be kept as the sperm from one bull can fertilize many cows.
Some elite cows are given hormones to produce many eggs
The eggs are fertilized in vitro (in a test tube) and the embryos are implanted into surrogate mother.
Artificial Selection in Chickens
The chickens on the left are egg-laying hens. They have been selectively bred to lay lots of eggs, but they grow at a
normal rate. Most are still kept in battery cages, though this system was banned in England in 2012. The chickens on
the right are broiler chickens. They have been bred for meat. They grow twice as quickly and have a short life.
All of these chickens have the same common ancestor. They are descended from the jungle fowl, which can still be
found in the wild in the forests of India and South-East Asia. Chickens were first domesticated at least 3400 years
ago. Farmers have been selectively breeding chickens for thousands of years. The basic method is quite simple. If
you breed from the hen which lays the most eggs, the chances are that her daughters will also be good layers. This is
because the number of eggs which a hen lays is partly controlled by the genes which a hen inherits from her parents.
Meat chickens have been selectively bred using the same principle. Selective breeding has become a complex
scientific business. A wild jungle fowl might lay 20-30 eggs in a year. Today’s hens each lay over 300 eggs a year
on average. Eggs have become a cheap food, but at a cost to the hen. Even with good food, her bones may be
depleted by the calcium she uses to make all those egg shells. Brittle bones are a common problem for hens.. Our
demand for cheap food has caused us to make our farm animals work harder.
Other Domesticated Animals
Some consider domesticated animals to be the ultimate products of artificial selection. Thoroughbred racehorses are
one example of artificial selection of animals. The meats we eat are the result of the careful selective breeding of
cows, pigs, sheep, and chickens. Our pets are a far cry from their “wild” ancestors. Cats and dogs, which were
originally domesticated for pest control, hunting, or shepherding, eventually were bred to become companion
animals. A glance at a group of dogs — all of the species Canis familiaris— reveals an astounding diversity of body
type, size, and coloration.
There can be a down side to artificial selection. Because this process essentially removes variation in a population,
selectively bred organisms can be especially susceptible to diseases or changes in the environment that would not be
a problem for a natural population. Inbreeding — the mating of closely related individuals — is also a problem. In
dogs, this has resulted in breeds that have health issues ranging from decreased life span to hip dysplasia.
Artificial Selection in Plants
What is artificial selection?
Artificial selection is the intentional reproduction of individuals in a population that have desirable traits. In
organisms that reproduce sexually, two adults that possess a desired trait — such as two parent plants that are tall —
are bred together. In this example, the rules of heredity tell us that the next generation will have more tall plants
than previous generations. If artificial selection is continued, all of the population will ultimately be tall. Also called
selective breeding, artificial selection is perhaps best understood as a contrast to natural selection, where the random
forces of nature determine which individuals survive and reproduce. In both cases, the outcome is the same: a
population changes over time, so that certain traits become more common.
What are some examples of artificial selection?
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Artificial selection has generated untold diversity in both plants and animals. In agriculture, superior strains of corn,
wheat, and soybeans have resulted from careful breeding. The Brassicas are great examples of artificial selection.
Cabbage, broccoli, cauliflower, Brussels sprouts, collards, and kale are all members of the same species, Brassica
oleracea. Gardeners have cultivated flowers such as roses and orchids, carefully manipulating heredity to produce
the “perfect” hybrid.
Teosinte (left) and its modern
A variety of vegetables of the
descendent, corn (right), a product
Brassica oleracea species
of artificial selection
For centuries, humans have been using artificial selection to change the appearance of plants. Most of the time, these
changes are meant to produce some sort of change in the plant that is pleasing to look at. For instance, flower color
is a large portion of artificially selecting for the plant’s traits. Brides planning their wedding day have a special color
scheme in mind and flowers that match that scheme are important to bringing their imagination to life. Florists and
flower producers can use artificial selection to create blends of colors, different color patterns, and even leaf
coloring patterns on their stems to get the desired results.
Around Christmas time, poinsettia plants are popular decorations. The colors of poinsettias can range from a deep
red or burgundy, to a more traditional bright red for Christmas, to white, or a mixture of any of those. The colored
part of the poinsettia is actually a leaf and not a flower, but artificial selection is still used to get the desired color for
any given plant.
What is a Tangelo?
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It is interesting to consider that vegetation adapts and evolves just as mammals, insects, and amphibians do. A
cactus, for example, is a plant that has adapted to live in the harsh environment of the desert. It greedily stores
moisture in its flesh which is guarded by a thick skin and further protected by external needles meant to prick the
mouth and gums of any hungry animal. Citrus fruits have also adapted, with the tangelo being one product of that
adaptation.
Consider, now, the citrus family. There are lemons, limes, grapefruits, oranges, tangerines, and clementines, to name
a few of the most common members of the family. Clearly, over centuries these different fruits have adapted to
respond to different natural elements. The tangelo, however, is an interesting member of the citrus family. It is, in
fact, a hybrid of two plants. Tangelos are believed to have originated ins Southeast Asia some 3,000 years ago. They
are a lovely hybrid of the tangerine (also known as the mandarin orange) and the grapefruit (closely related to the
pomelo).
A tangelo looks much like an orange except for the fact that it is not perfectly round. Rather, a tangelo has a more
oblong shape. In terms of size, a tangelo is between a tangerine and a grapefruit. In general, a tangelo is the size of a
medium to large orange. This member of the citrus family is known for the juice that it provides. A ripe tangelo is
filled with much more juice than pulp. In terms of flavor, a tangelo tastes much like a tangerine.
Although hybrid fruits and new breeds of fruits are often borne from Mother Nature and Father Time, it is possible
for human intervention to create new fruits. In order to ensure bountiful harvests of enjoyable food, governments
sometimes step in and use science to engineer food-bearing plants. The United States Department of Agriculture has
worked to produce specific breeds of tangelos that yield bountiful harvests and provide enjoyable flavors. There are
now different kinds of tangelos just as there are different types of apples. The two main “breeds” of tangelos are
the Minneola tangelo, which was created in 1931, and the Orlando tangelo, which was created in 1911. Each of
these “breeds” is the hybrid of one specific type of tangerine and one specific type of grapefruit. Thus, the tangelos
that you will find in the grocery store today are products of both a natural occurrence and scientific intervention.