We call the process by which the modern life forms into what we see today from earlier predecessors evolution. This remarkable phenomenon is responsible not only for the great differences we see between species but the striking similarities they can also present.
It is a misconception that evolution seeks to create “superior” or “higher-evolved” organisms. Rather, it works by adjusting the heritable and behavioral characteristics of organisms so they are better suited, or adapted, to their specific environments
The foundation principles that allow for evolution to occur are based on the diversity of life. You may be wondering how reptiles eventually became mammals or how the pig-like Indohyus could be a close relative and ancestor of the great whales. While the ultimate transformations are nothing short of miraculous and monumental, they are based on the smallest adjustments that could barely be noticed, called genetic variations.1
Genetic variations are small alterations in the genetic codes handed down from parents to offspring. It results in genetic changes within the gene pool of any group of organisms capable of breeding. In the long term, the selective forces of nature can increase or decrease genetic variations leading to the creation of new species, called speciation.
When speaking of gene variation, the variants we describe are in the specific portions of the DNA code called alleles. An allele is a variant for a gene which is the smallest unit of the genetic code. When the genetic code is expressed, the presence of alleles ensures that the outcome will be different from the original inherited from parents.
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Genetic variation is essential because it provides offspring with unique features and special advantage in certain eventuality. For example, a population of animals will reproduce in great quantities to increase genetic variation within their genetic pool. This will lead to some offspring that are faster, taller, smaller, darker, lighter or different in any other way.
When food supplies run low or threats from their environment eliminate the unfavorable genetic variations, those with features better suited to their environment will survive and hold the keys to species survival in their genetic codes. Thus, through great diversity evolution ensures that the species are best suited to surviving their world and life will continue.2
In the following sections, we will examine a few specific mechanisms of evolution and take a closer look at how these mechanisms can act on genetic variations in the creation of new species. We call the study of these and many more mechanisms of evolution “evolutionary biology.”
Believe it or not, all life on this planet stemmed from a single common ancestor, which scientists have yet to discover. More than likely, it was some unimpressive looking bacteria that inhabited the primordial pools of our planet long ago.3
When two species share a common ancestor they are said to be related even though they may seem very different. This is very similar to the way a set of siblings may look very similar to each other and even their parents, but there are plenty of identifying factors that make each child an individual.
Common descent can be affected by geographical changes favoring a single expression of genetic variations, or any of the other mechanisms of nature that can create two separate species from a single ancestor.4
Mutations affect the gene pool of a population by introducing random changes in the chromosomes of the DNA. These random changes come in the form of increased alleles. Mutations increase genetic variations within a population and can have beneficial results, but can also be harmless or harmful.5
Sickle-cell anemia is an example of a harmful DNA mutation that exists within human beings and can be inherited from parents with the condition. Modern gene therapy hopes to rid the world of this problem. Other mutations are beneficial, for example, they could allow an organism to produce new digestive enzymes. This can make formerly inedible foods suddenly appetizing and can stave off starvation for a lucky section of a population, and life goes on.6
Migration (Gene Flow)
Also known as allele flow or gene migration, the vital exchange of alleles between different populations can greatly enhance genetic variations. This can happen when some members from one population are introduced to another compatible population and begin interbreeding. The new population will then gain some genetic diversity from the exchange.7
One such example is the way the Australian Dingo population has been all but obliterated by the introduction of the domestic dog to Australia. The hybridization between dogs and dingoes has threatened the bloodline of purebred dingoes which has existed in Australia for more than 3500 years.8
Genetic drift describes the reduction of frequency that alleles occur in a population over time. This usually occurs when a portion of a population becomes separated from their original population and gradually becomes a species all to themselves. This results in a severe reduction in genetic variation in the new population caused by limited common descent.
This can happen as a result of a natural disaster that wipes out the majority of a population or a small group becoming separated by geographic boundaries. But it will never occur in larger populations, only in smaller groups with limited options for reproduction.
As the small portion of the population begins to reproduce they lack the great genetic diversity of the total population. This means their offspring will begin to take on traits that better suit them to their environment and will eventually be very different from the common ancestor in the population to which they once belonged to.9
The American Bison is a good example of genetic drift, throughout the past few centuries, the Bison was hunted to the brink of extinction. The remaining bison alive today don’t have such an extensive genetic variation and are suffering the consequences. Purebred dogs are also the victims of limited genetic variety which leads to the concentration of hereditary diseases in some of the best show dogs. Mixed breed dogs are often favored as companions because their vast genetic variety makes them virtually impervious to hereditary issues.10
Charles Darwin, who outlined the Theory of Evolution in his famous work “The Origin of the Species,” postulated that species are in competition among themselves for the resources in their environment. Furthermore, specific traits within a genetic code will enable some individual organism to survive and breed, effectively passing these traits to future generations. This selection of favorable genetic characteristics is called natural selection.11
Giraffes are among the best examples of natural selection producing an extraordinary result. The genetic traits favored by natural selection brought about great changes in the giraffe’s skeletal structures. Longer necks meant some of the giraffes were able to reach higher food sources. This was a handy trick when food supplies were low and shorter species had wiped out all the food on the lower branches of trees. Those giraffes with shorter necks didn’t survive.
Being able to access food when food supplies were low meant some giraffes would go on to reproduce offspring with progressively longer necks. Then natural selection began to eliminate those whose necks got too long for their own good, keeping the giraffe at an elegant 20-foot height.12
Final Thoughts on the Evolutionary Biology
Scientists have made great advances in unlocking the secrets of our natural history and even made accurate predictions on the results of certain mechanisms of evolution over short terms. Even still, there is far more than we don’t know and some of the greatest secrets of life have yet to be uncovered.
Darwin was born in an age that knew nothing about DNA or gene variation and so much of what was originally theorized has had to be reexamined. If anything we may be closer to the beginning of building our full understanding of evolutionary biology than we think.13
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