The Role of Natural Selection in Evolution

In 1858, Charles Darwin and Alfred Wallace released a joint scientific paper which introduced the concept of evolution by means of natural selection. This paper, along with Darwin’s subsequent publication, “The Origin of Species,” changed the way science and society explain events in our natural world.

Many scientists before them had observed that organisms change, or evolve, over time. Darwin and Wallace rocked the world by describing how evolution happens; the specific mechanism by which new forms arise through natural selection.

* Difference between Evolution and Natural Selection *

1. Evolution:

In biology, evolution can be defined simply as inheritable change in a population. Genetic mutations are the raw material of evolution. A mutation is a mistake in the genetic code that may occur when a cell copies its DNA in preparation for cell division. Most mutations are either neutral or bad for the organism, but occasionally a genetic change appears that gives an organism an advantage over others in it’s population.

This inheritable change, over very large stretches of time, explains the origin of new species, occasionally the elimination of existing species, and ultimately the vast diversity of the biological world. Contemporary species are related to each other through common decent (ancestors that they share), and are products of evolution over billions of years.

Key concepts of evolution:

* Any change must be inheritable (able to be passed on to the next generation).
* These changes are regulated by natural selection.

2. Natural Selection:

This is the process in nature by which only the organisms best adapted to their environment tend to survive and transmit their genetic characteristics to the next generation, while those less well adapted tend to be eliminated. “Environment” refers to the combined biological and physical influences.

* biological influences: include things like disease, competition, and predation
* physical influences: include things like drought, fire, flood

* Genetic Diversity, Relative Fitness and Population Shift *

Three essential components of evolution via natural selection include:

1. Genetic Diversity – Populations of individuals are genetically diverse. Even members of the same species have characteristics that very from one individual to the next.

2. Fitness – In any given environment, some individuals have characteristics that put them at an advantage over individuals who do not possess those same characteristics.

3. Population Shift – In any given environment, those individuals who have advantageous characteristics will generally be healthier, live longer, and leave more offspring than individuals who do not possess those characteristics. The population will, over time, contain more and more individuals with the advantageous characteristic, and fewer individuals who do not possess the characteristic.

An example of evolution in action, over the short-term, is the growing antibiotic resistance of disease-causing bacteria. The reason that antibiotics are much more carefully prescribed than they were even ten years ago, is that people were taking antibiotics unnecessarily, for viral or other infections that had nothing to do with bacteria. In this scenario, and especially when patients do not take all of their antibiotic and stop the medication prematurely, only the weakest bacteria are killed. The strongest and most virulent (disease-causing), then repopulate. This cycle, happening over and over again, is a selective force that results in generations of bacteria that are more dangerous.

* The Power of Natural Selection over Time *

In sexually reproducing species, two organisms are considered to be within the same species if they can mate and produce fertile offspring. Over time, natural selection can make sub-populations within a species genetically different enough so that they are no longer able to reproduce with each other, creating separate species (reproductive isolation). Natural selection does not always result in new forms or species. Natural selection may also result in the elimination of species from the environment (extinction).

The “goal” of any organism is to live long enough to produce offspring. Individuals that can locate, harvest and utilize the resources from their environment most efficiently, while minimizing the influence of limiting factors acting upon them, will be the most successful in contributing their genes to the next generation.

In a nutshell: Natural selection is the process. Evolution is the outcome.

* Sources *

Brown, Bryson (2007) Evolution: A Historical Perspective. Greenwood Press.

Campbell & Reece (2005) Biology, 7th Edition. Pearson.