4.2 Natural and Artificial Selection

2A: Natural Selection

Natural Selection

Natural selection is the process that results when the characteristics of a population of organisms change over many generations. The criteria that must be present for natural selection to occur are:

1. Individuals with certain inherited traits survive specific local environmental conditions in order to pass their genes to their offspring.

2. There must be diversity within the species.

Selective Pressure

Remember the bacteria Staphylococcus aureus from Evolution Lesson One? Individual members of Staphylococcus aureus were selected for by their environment, as they survived the application of an antibiotic. Due to their resistance to antibiotics, some were able to survive to reproduce and pass on their genetic information - including the gene that had the antibiotic resistance allele. An abiotic (non-living) environmental condition can be said to select for specific variations in some individuals, and select against different variations in other individuals. This is how the environment can exert a selective pressure on a population. Selective pressure may also arrive from living (biotic) factors, such as parasites, predators and competition for resources.

An example of selective pressure is the plants found in Southern California in the United States. This is an arid (dry) region that receives very little rain, especially in the summer months. Plants need to be able to survive this dry, hot weather in order to survive to reproduce. Overtime, plants that were able to store more water, and prevent water loss, such as cacti, or plants that lose their leaves when conditions become too dry were able to survive to reproduce over other plant species. The selective pressure of a hot, dry environment has determined what plant populations could survive.

The cacti in the image above have thick, succulent stems that are able to store water, even when environmental conditions become very dry and hot. The environmental selective pressure of Southern California has helped this population of cacti develop specific adaptations that help it survive to reproduce in this environment.

Natural Selection is Situational

Natural selection does not have a purpose or a direction, and therefore, it does not anticipate change in an environment. A trait that at one time in one type of environmental condition seems to have no apparent advantage for survival, may be the trait that in a different time and situation will help individuals survive to reproduce. The alleles for this trait will then be inherited by the offspring of the survivors. As a result, over many generations, there will be more and more individuals that inherit the allele for the trait that helps them to survive the change in their environment. Those individuals form a population that is better adapted to their environment.

Fitness

When referring to natural selection, the term fitness refers to the contribution an individual makes to the gene pool of the next generation by producing offspring that survive long enough to reproduce. A high degree of fitness means that an organism will survive and reproduce, therefore passing it's advantageous genes onto the next generation, and those offspring will also survive to reproduce. Fitness is often described as the the number of reproductively viable offspring that an organism produces in the next generation. An organism with many viable offspring has high fitness, whereas an organism with few or no viable offspring has low fitness. High and low reproductive rates are relative to the typical number of offspring for the species.

2B: Artificial Selection

Artificial Selection

Human beings have been artificially selecting organisms for particular traits for thousands of years. Selective breeding is a form of artificial selection. Selective breeding and artificial selection are forms of biotechnology. Biotechnology is the use of technology and organisms to produce useful products. Artificial selection has had a large impact on human survival. Most of the food we eat has come from species that have been selectively bred, including milk, eggs, fruits, vegetables, meat and grains.

The image above is of a Holstein cow. Holstein cows have been bred selectively over generations to produce more milk. Notice how large the udder is on this individual?
Initially cows that produced more milk than others would have been bred to bulls who had mothers who produced more milk. This process occurred over and over by farmers until the Holstein breed was established.

Artificial selection also occurs in breeding animals to be pets, such as dogs. All domestic dogs are all the same species, and have been repeatedly artificially selected for to create specific breeds. Over many generations, breeders can change how a particular dog breed looks.

Looking at the image above, it is hard to imagine that all of these breeds of dogs are the same species! It is believed that domesticated dogs evolved from wild wolves, who were attracted to human settlements by leftover waste and garbage. Over approximately 15 000 years, they have evolved into a separate species.

The key difference between natural and artificial selection is that in natural selection, the environment selects for traits, whereas in artificial selection, humans select for traits.

Artificial Selection and Food Crops

The food crops that we rely on for most of our diet - rice, corn, wheat and vegetables - are the result of selective breeding.

The image above is of vegetables in the cabbage family. These all evolved from one plant - wild mustard (Brassica oleraccea) - which has been modified to create many common food crops including broccoli, cauliflower and brussel sprouts. Plant breeders began to modify the traits of Brassica oleraccea over 4000 years ago in Europe and Asia, where wild mustard evolved. The traits of the artificially selected varieties differ from the original plant, but are members of the same species, and can reproduce to create viable offspring.

Humans breed crops to increase their nutritional value and harvest yield. We also breed them to be drought resistant and pest resistant. It does seem like artificial selection provides us with endless possibilities, but it does have it's limitations. Most crop breeders understand that selective breeding must be balanced to maintain genetic variation within the crops and the ability of the plants to respond to the conditions of environmental change.

Consequences of Artificial Selection

Artificial selection, like all technologies, has negative as well as positive consequences. For example, English bulldogs are selectively bred for different traits such as their flat faces. Unfortunately, this desirable trait results in severe respiratory problems. When animals are selectively bred, particularly if it is to a close relative, they end up getting more homozygous recessive disorders. If animals with different traits are bred, they will have more heterozygous traits, which will make them healthier, but they will not have the same consistent phenotypes of a purebred.

The image above is of a sleeping English Bulldog. It has been artificially bred to have a flat face, however, this leads to severe respiratory disorders.

Some artificial selection techniques have been used to introduce new genetic information into domesticated organisms. However, one of the goals of domestication is to produce organisms that are all similar. To make plants similar, it is necessary to reduce genetic diversity. Plants are specialized through domestic breeding to produce qualities that growers want, so plants that have been selectively bred lack genetic diversity.

The previously mentioned factors that affect artificially selected crops - decreased variation and decreased ability to respond to conditions of environmental change - are not always important to plant breeders, particularly when they are selecting for economically important agricultural crops. Modern conventional farming centers around economics, and makes use of large scale monocultures (growing only one plant in a large area). While this creates large amounts of food and makes it easy to harvest, it also decreases genetic diversity.

Gene banks

In order to protect against disasters that may arise from a lack of diversity in our food crops, gene banks have been established. Gene banks contain populations of early ancestors of modern plants --our most important food plants came from wild ancestors with genetic combinations that allowed them to survive and reproduce in their environment. These specimens have been collected in the wild and from prehistoric archaeological sites Seeds can survive for long periods of time, so they can be recovered from early settlements. By preserving these organisms, their genetic diversity is available for introduction into modern plants if the need arises.