Unit 7.2 (Natural Selection)

What is Natural Selection?

The process by which nature selects which traits are desirable by eliminating organisms that are unsuited for the challenges of their environment. Over time this leads to dramatic changes in a population and changes in the frequency of certain alleles.

3 Components Needed For Natural Selection

There are 3 basic conditions that must be met in order for the process of natural selection to act on a group or population. These 3 conditions are; there must be genetic variation among members of the group, traits or characteristics must be biologically heritable, and there must be competition for survival amongst the group.

Variation

In order for a group or species to be effected by natural selection there must be some genetic diversity among the organisms. If the group consists of a bunch of clones then the organisms will likely respond similarly to what ever environmental challenge they face, if they are adequately prepared the group will live, if they are unprepared for the challenge they may all die.

It is very important for a group to have adequate genetic diversity among the population in order to increase the chances that some members of the group will survive in the event of a catastrophic event or new environmental challenge.

In the example above the mice purely due to the natural genetic variation of the population have two colors; tan and grey. Originally the tan colored mice made up a larger percentage of the population. Over time the natural predator of the mice preyed upon the lighter colored mice. Eventually the phenotypic ratio of the population changed, the mouse population now predominantly consists of dark colored mice. The darker mice were better able to blend into their environment and avoid predation, lived longer and so were able to pass on their genes to the next generation.

Inheritance

Inheritance is a key component of natural selection. Certain traits make organisms more or less adapted for their particular environment and the challenges they will face. In order for these traits to be directed by natural selection they must be heritable traits, meaning they have to be encoded by the organism’s genes. If some advantageous trait is not genetic in origin then it cannot be inherited by the subsequent generations, and will not be controlled by the process of natural selection.

Competition

Generally speaking more organisms must be born or produced than are actually able to survive in the environment. This forces the members of a population to compete for resources to ensure their survival. Some examples of resources organisms of the same species will often compete for are; food, water, air, mating opportunities, nutrients, light, or any other resource vital for life. This is particularly apparent in organisms that will reproduce and create many offspring in a single event. Examples where this is apparent are insects, sea turtles, and any organism that may live in large groups.

Survival of Fittest

Natural selection and articles about it will often refer to “the survival of the fittest“, students may think that only the most physically strong or “fit” will survive. “Fitness” in biology refers to an organism’s ability to live and produce offspring. So the statement “The Survival of the Fittest” is actually saying that only the organisms best suited to their environment will be able to survive and pass on their genes.

Genetic Drift

Genetic Drift is when a few individuals in a population leave behind more descendants that others due to random chance and not necessarily because they were better adapted to their environment. The organisms that survived are not more fit, they just got lucky. An example of this would if you have a group of buffalo living in a valley. If a buffalo was struck by lighting and died before it could pass on it’s genes it would not mean the buffalo was “unfit“. Natural disasters are a common source of genetic drift events, things like; meteor impacts, tornados, and extinction level events can be considered sources of genetic drift.

Gene Flow

“Gene Flow” is any movement of genes from one population to another population of the same species. In nature this occurs often, an organism will find itself amongst members of its own species but in a different area than where it was born. One way this may happen is due to natural forces like a hurricane picking a fish up and throwing it into a distant lake that also contained the same specie of fish. Winds blowing pollen high in the atmosphere and carrying them to another continent where the pollen fertilizes a plant is also an example of gene flow. Gene flow regularly occurs in modern day humans because we can migrate and settle down in lands that our ancestors did not historically occupy, this provides an opportunity for groups of humans to share their “alleles” with populations that did not previously have them.

Selecting Agent

A “selecting agent” is anything in the environment that is causing death or applying pressure/difficulty to prospects of members of the group surviving. Examples of selecting agents can be; a lack of water, lack of food, predation, disease, natural disasters, UV-ray, toxic chemicals. Some selecting agents are always present regardless of what species you are examining but some are only present in unique environments. For instance, organisms present in the dark areas of the ocean floor do not face the threat of over-exposure to the sun’s UV-rays, but they do have to deal with regulating their body temperature in such cold conditions.

Artificial Selection

If humans are picking what traits in a population are desirable and therefore deciding which organisms will die and which will reproduce this is called “Artificial Selection“. Humans have done this for thousands of years, as a result we have created most of the the fruits and vegetables in your grocery market. Artificial selection/selective breeding has been used to take grey wolves and create all the breeds of dog that we have today.

The condiment we know as “mustard” is made from a small weed called the mustard plant. Thousands of years ago humans selectively bred the mustards with specific traits they wanted, like broader leaves, larger root bulbs etc. They repeated this process until we had vegetables like; onions, kale, kohlrabi, lettuce, broccoli and a few more.

Sexual Selection

Sexual selection is a type of natural selection that effects an organism’s ability to successfully obtain a mate or the opportunity to sexually reproduce. Among higher level vertebrates like mammals and birds the males will often engage in many display behaviors to attract and win mates. Conversely, females in these groups tend to be more selective and picky in regards to who they will mate with. This pattern is quite common throughout the animal kingdom and makes sense when you take into account the cost benefit analysis for mating for males vs females. Females have the burden of rearing for, carrying, providing for, or sharing resources with the new life they will form. While males have significantly less cost towards mating since they do not have to give birth or share their body’s nutrients with potential offspring.

Males may engage in intense physical competition, perform complicated rituals, or exhibit risky ornate body decorations to attract female attention. The traits the males display often increase their chances of dying so why do they do it?

If the male was able to make it to reproductive age successfully this may indicate the male must have well suited genes to allow them to survive so long. The females tend to be choosy because she must pay a significant price if she chooses wrong. If the female chooses a male with poor genetic health she may go through all the effort of having a child, feeding it, and taking care of it only for it to die off young because it did not have the appropriate genetic traits to allow it to thrive.