The Meanings of Evolution

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Introduction

Evolution is one of the most controversial and misunderstood topics in biology and it is well to ask ourselves at the out set what the word evolution means. The uses of the word in English are quite interesting. Evolution, according to the Oxford English Dictionary 2nd Edition, comes from the Latin evolvere which literally means to roll out, e meaning out and volvere meaning to roll. The sense then of many of the definitions is given by the following: to unfold, unroll, open, expand. By the early 19th century the word took on a new meaning, namely to develop by natural processes from a more rudimentary to a more highly organized state by gradual modification from earlier forms. This latter definition appealed to the notion of orderly progress that was characteristic of the Victorian Era.

Darwin and the Meaning of Evolution

Curiously, Darwin rarely used the word evolution and did not refer to his theory as the theory of evolution. Instead Darwin referred to Modification with Descent as a better description of his theory. The reason is that Darwin, unlike some of his popularizers was an astute and systematic scientist who realized that Modification with Descent need not be progressive from more rudimentary(simpler) forms to more complex or organized forms. Rather Descent with Modification via natural selection is perfectly capable of going in the opposite direction, say from birds with flight to flightless birds, from free living forms to parasitic forms, from fish with fully functional eyes to cave fish with only vestigial eyes.

"Finally, I believe that many lowly organized forms now exist throughout the world, from various causes. In some cases variations or individual differences of a favorable nature may never have arisen for natural selection to act on and accumulate. In no case, probably, has time sufficed for the utmost possible amount of development. In some few cases there has been what we must call retrogression of organization. But the main cause lies in the fact that under very simple conditions of life a high organization would be of no service, possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured." Origin of Species 6th Edition.

So to Darwin, the term evolution did not describe what he meant at all. Today though our use of the word has caught up to Darwin and we too have given up the viewpoint that evolution is only in the direction of simpler to more complex organisms. Evolution now means modification with descent is now the basic meaning understood by biologists.

Modern Meanings

Today there is yet another meaning for the word evolution, and one not really understood by Darwin because the necessary concept had not been developed and that is evolution meaning a change in gene frequencies in a population. As you probably know we inherit physical characteristics from our parents, and the basic units of inheritance are contain the genes that make up at least part of the DNA in your cells. This DNA with the genes are passed on from parents to offspring, influencing and in some cases completely determining the form of the off spring with respect to certain characteristics. You are familiar with this idea when someone says to you that you have your mother's eyes, or dare I say dimples.

Now a population is simply a group of individuals of the same species living together and interacting in the same area. Just as an individual has a set of genes, a population can be thought of as a pool or collection of all the genes in the individuals of a population. This set of all the genes in a population is called the "gene pool". This may seem to be a rather abstract idea but as we will see later on it is critical for understanding how scientists today believe evolution operates, so learn the concept now and we will save time later.So we see that there has been a gradual shift in the meaning of the word "evolution" from one meaning an unfolding, to one related to an orderly progression, one meaning Modification with Descent to our modern definition related to changes in the genetic make up of a population.

A Note on the Types of Evolution

Biologists today distinguish between three types of evolution. First of all there is microevolution, namely changes the genetic make up of a population in response to one or more agents of evolution acting on a population. For example in response to a series of cold winters natural selection, one of the agents of evolution can lead to an increase in the average size of individuals in a bird population. To the degree that this size is genetically determined biologists would say that microevolution has happened.

The second type of evolution is called speciation, This is the process by which new species arise from preexisting species. This is a complicated topic but as we will see biologists have good documentation for this process and the mechanisms by which speciation can take place. No exotic mechanisms are required.

Finally the third type of evolution is macroevolution. Macro means large and what is meant here is evolution as viewed in terms of large scale patterns as seen in the fossil record or in the distributions of related species seen today. For example, in the fossil record it can be seen that saber toothed tigers appear repeatedly and independently of of each other. On islands, repeatedly we see a group of clearly related species exploiting a wide range of niches and the most similar species to the whole group is some mainland species, perhaps thousands of mile away. This is what biologists today call adaptive radiation. The most famous example are the Galapagos finches studied by Darwin.

Sometimes macroevolution is meant to refer to the evolution of new innovations by means of a single extreme mutation, but this is not how the term is currently used. It is critical to realize that while we do not understand all the details of how macroevolution happens, there does not appear to be any clear separation between this and the other types of evolution. All biological evolution seems to boil down to the same basic idea: change in the genetic make up of a population.

In addition, scientists will often talk about non genetic types of evolution. The main one that is critical to us, is cultural evolution as human can also transmit information from one generation to the next. Some scientists such as E.O. Wilson believe that cultural evolution and biological evolution can interact in complex ways to some times to re-enforce each other and sometimes to restrain each other. The relationship between cultural and biological evolution is an area of active research today.

Paul Decelles