Common misconceptions 2: Process

[gluadys]

This is the second in a series on common misconceptions of evolution. For those who missed the first it is here: http://www.christianforums.com/t5819158-common-misconceptions-1-scope.html

This section deals with an aspect of evolution often skimmed over very lightly in creationist literature and dismissed as not of particular interest. I expect one reason for this is the creationist focus on speciation as the key element of evolution. Many studies of evolutionary process do not show speciation as an observed result, so they are deemed not to show evolution. This is a conceptual error. Evolution is a process, so any study of the process is a study of evolution whether or not it shows speciation. Furthermore, the nature of the process is such that for the most part it takes place within the species. How this leads to speciation will be dealt with in another thread.

This is a much lengthier summary than the first and for easy readability it is broken into several posts.

Misunderstandings about the Process of Evolution
We have already noted that both scientists and non-scientists tend to use the term “evolution” in both broad and narrow senses. But they do so in different ways. When speaking broadly of evolution, non-scientists tend to include far more than scientists do, adding to biology various aspects of physics (big-bang), biochemistry (origin of life), and geology (age of earth), not to mention metaphysics and philosophy (subjects to be examined later). When scientists speak broadly of evolution they generally mean what Michael Ruse calls the three aspects of evolution: fact, path and cause, or what I have called here fact, history and process.

When non-scientists speak of evolution in a narrow sense, they tend to focus on the history of evolution, the pathway of evolution from a universal common ancestor to the bewildering variety of species in today’s world, and especially on the significant steps in the human lineage such as the fish->tetrapod or reptile->mammal sequences. And, of course, the immediate step from pre-sapiens to sapiens.

By contrast, when scientists speak narrowly of evolution, they tend to focus on the process or cause of evolutionary change. For the scientist, this is the heart and soul of evolution. Without a process of evolutionary change, there is no evolution.

So what is the process of evolution? There is more than one element in the process. These elements include mutation, variation, selection and speciation. But if one has to choose one of these elements as the sine qua non of evolutionary process, it is selection. Mutation and variation are necessary conditions for evolution, but in themselves they are not sufficient conditions. Selection is the factor that drives evolution. Speciation is a natural outcome of the process of evolution, but not a necessary outcome in every instance. Selection, the core of the evolutionary process happens, for the most part, within species. Failure to recognize this leads to excellent examples of natural selection being dismissed because no new species has emerged. But this does not mean that evolution has not occurred. Selection, not speciation, is the core process of evolution.

The following posts will cover these topics:

• What is selection?
• Where does selection happen?
• How does selection happen?
• Variability and fixation
• Variability and its implications for creationism

 

[gluadys]

a. What is selection?

It may seem a strange question. Is it not obvious what selection is? But, historically, it has not been easy to determine just what natural selection selects. The major advance of the theory of evolution in the 20th century was to link genetics with organic traits. A consequence of this was to redefine evolution as “a change in the frequency of alleles in a population from one generation to another.”

To understand this, we need to know

• what an allele is,
• how alleles produce variations,
• what is meant by the concept of a gene pool,
• how alleles are distributed in a gene pool and
• how that distribution can change.

I find it most helpful to define genes and alleles in terms of recipes. A gene is often looked at as a set of instructions for building a protein. What is often overlooked is the nature of this set of instructions. In many cases, a set of instructions is fairly rigid. When assembling a piece of furniture, there is a specific set of items to be assembled and only one way in which they can be put together correctly. A modification in either the items or the instructions has far reaching ramifications that affect the design of the whole item. It is very hard to envision evolution working in such tightly controlled conditions.

A recipe is also a set of instructions, but unlike the set of instructions for assembling a table or a bed, a recipe exhibits a degree of flexibility both in the items (ingredients) to be assembled, and in the manner in which they are to be handled. A typical recipe book will not contain just one recipe for eggplant parmesan or peanut butter cookies, but anywhere from two to half a dozen. And if we compare different recipe books we find still more variations in the recipes for various dishes.

For each dish a few things are standard. Eggplant parmesan will always call for eggplant and peanut butter cookies will always call for flour and peanut butter. But beyond such standard requirements, there can be many subtle variations from one recipe to another.

As a set of instructions for making a protein, a gene is much more like a recipe than directions for furniture assembly. Each gene will have standard features without which the protein cannot be made. But it can also display many variations which will affect which amino acids form the protein and the details of its chemical interactions.

The name for these differing recipes for the same basic “dish” i.e. protein, is “allele”. When a gene is modified by a mutation, without impairing its ability to produce a functioning protein, the modified gene is a new allele of that particular genetic recipe for that particular protein. Differing alleles of a gene are what produce the variations which distinguish one individual from another.

Selection, then, is essentially a selection of alleles. However, since selection cannot act directly on alleles, it operates by acting on the variations in a character trait expressed by the alleles. Since each version of a character trait is a consequence of a different version of the gene that impacts it (IOW an allele) the selection of individuals bearing certain character traits is indirectly a selection of the genetic allele.

This is the meaning of evolution as "a change in the frequency of alleles"

NOTE: in scientific papers, alleles themselves are often called “genes”. This is not an error. An allele is a gene. What distinguishes a pair of alleles from any random pair of genes is that alleles are always located in the same place on the same chromosome (its locus) and usually produce a protein with the same basic function.

Occasionally, a protein with one function will be co-opted for a new and different function and this is how some evolutionary transitions occurred. An example would be the modification of certain sweat glands to produce milk in mammalian females.

 

[gluadys]

b. Where does selection happen?

Selection happens in the gene pool. So what is the gene pool? A gene pool is the total number of inheritable genes found in one locus. Inheritable genes, of course, are found in germ cells. In a diploid species such as ourselves, each cell contains two copies of the species genome and therefore two copies of each gene. When the germ cell divides to become a sperm or egg cell, it can bequeath one or the other of these genes to each sperm or egg produced.

Since each individual has two and only two copies of the species genome (one inherited from each parent), each individual contributes two genes to the species gene pool for each genetic trait. So the total number of genes in the gene pool is double the number of the species population (or at least its reproducing population and potentially reproducing population).

Is this the same as the number of alleles?

No. Due to reproduction, most genes in the gene pool are identical copies of other genes. So what we actually find in the gene pool are a number of sets of genes. Each member of a set is identical to all other members of the set, but each set differs slightly from the other sets. The number of such sets is the number of alleles.

The number of different alleles in a gene pool can range from only one (every gene is identical to every other gene in the gene pool) to upwards of hundreds of variations. The number of different alleles in a gene pool is a measure of the variability in the species for that gene and the trait it governs.

A second item to note is that different alleles are not distributed randomly through the species. If we think of the gene pool as covering the geographical range of a species, we will note that certain alleles occur more frequently in one geographical corner than in the rest of the pool. This is to be expected. Every gene is carried by a host organism, which typically is born, lives, mates and reproduces within a specific area. So the reproduction of that particular allele will take place most often in that same area. This is what accounts for regional differences in a species.

Occasionally, we will see an allele that typically shows up in one geographical area jump to another. This is the consequence when the host organism migrates to a different locality and reproduces there. This is one way of getting gene flow from one part of a species range to another.

Finally, we need to note that different alleles do not occur with the same frequency. In a population that shows 5 different alleles, one may occur in over 60% of the population, while another shows up in only 1% and the others falling between these extremes.

Evolution is a change in the proportional frequency of various alleles in a population from one generation to another. Selection determines how many copies of each allele are passed on to the next generation. If this results in a change in the proportion of the gene pool occupied by a particular allele, this is evolution.

 

[gluadys]

c. How does selection happen?

A common creationist answer is “by death”. But death in general is not an evolutionary force. Since death comes to all, it is not selective. So "death" without qualification, cannot be a force for evolution.

Insofar as death plays a role in selection, only certain deaths count: juvenile death and death before completion of the species typical reproductive period. If a species typically goes through 10 reproductive cycles in its lifetime, but an individual dies after only two reproductive seasons, that individual will ordinarily have fewer offspring than one that reproduces in all ten cycles.

Premature death, whether by non-viability, susceptibility to disease, vulnerability to predators, etc. is one way of selecting out less fit individuals leaving the gene pool to the hardier, the more resistant, the more elusive.

However, other methods of selection don’t require the death of the unsuccessful organism before its time. The organism may have problems of infertility that interfere with reproduction. It may be less attractive as a mate. Or, on the positive side, it may be more fertile than other members of its species. If an average litter for the species is 8 and an average litter for a certain variant is 10, with no difference in the mortality rate, the variant producing the larger litter will add disproportionately to the gene pool.

All of these mechanisms lead to differential reproductive success, which is a description of selection. Some alleles will reappear in the next generation more frequently than in the current generation, and others will appear less frequently. This change in the distribution of alleles in the gene pool is evolution.

A final note: while evolution is defined as a change in the proportional distribution of alleles, we should not forget that selection of alleles does not take place apart from the organisms in which the alleles are found. The only way to select alleles is to select the individuals that carry them.

It is the pre-mature death, or infertility, or failure to attract a mate of a whole organism that prevents its alleles from multiplying in the gene pool. It is the long life of a whole organism combined with its desirability as a mate and/or its fertility that optimizes its opportunities to contribute to the gene pool of the next generation.

 

[gluadys]

d. Variability and fixation

Different genes show different numbers of alleles. In some cases a trait hardly varies at all from one individual to another. The gene which affects that trait is “highly conserved”. Selective forces do not allow for much, if any, variation. When over 95% of all genes in a gene pool are identical to each other, that allele is said to be “fixed” in the species.

Other genes show a more equal balance between available alleles. In this case, there seems to be no selective advantage to one version of the inherited trait over the other, and other things being equal, inheritance will follow Mendelian norms and maintain the balance of alleles with little change in the proportional change from one generation to another.

New mutations are the source of new alleles. The effect of mutation is to increase the variability within the species gene pool. For this reason the amount of variability can be a rough measure of how long the gene pool has existed in isolation from other gene pools. A gene pool formed very recently will have fewer new mutations and so fewer alleles than a gene pool of long standing.

Selection, by contrast, reduces the number of alleles, by favoring the reproduction of some at the expense of others. Selection pressure continuing in the same direction over a long enough period of time, will spread one particular allele to the vast majority of the individual members of the species, thus fixing that allele in the species genome. Then there can be no further selection in regard to this gene unless and until a mutation introduces a new variation on the gene.

How long a time is “long enough” will depend on the intensity of the selective pressure. A highly-effective pesticide may fix an allele for resistance in an insect population within minutes.

Evolution occurs because we have this interplay between two forces working in opposite directions. Mutations generate new alleles, increasing the variability of a gene and the number of possible variations which may be selected. Selection in turn screens the variations and in response to selective pressure chooses to favour some and eliminate others, decreasing the range of variations until new mutations occur. Both processes are simultaneously and constantly at work.

 

[gluadys]

e. Variability and its implications for creationism

Note: for another discussion of this topic see: http://www.christianforums.com/t5536798-variation-and-variability.html

The concept of the gene pool and the variability of the gene pool has implications for creationist theories on inherited variability. It is often claimed that God created the original kind with all its variability already built-in. As speciation within the kind occurred, each species received a portion of the total variability. It is sometimes asserted that the original variability was broader than that of all species within the kind today, as some has been permanently lost. It is often not noted that this theory has implications for the population size of the kind both when it was created and through its whole history.

Alleles are the measure of variability. Variability is the number of different alleles that can be found in one locus in the species genome. Each genome carries only one of the available alleles and each organism carries at most two of the available genomes, hence at most two of the available alleles. So, if we determine that a particular gene exists in 50 different versions within the population, we need a gene pool capable of accommodating 50 alleles. The gene pool itself must be much larger than 50 genes, for most genes are identical copies of other genes. Fifty alleles means there must be 50 groups, each propagating one of those alleles. We need thousands of genes in the gene pool to support 50 different versions of a gene.

But genes don’t exist in abstraction. They exist in the cells of individual organisms, and the maximum capacity per individual organism is 2 genes per cell. A gene pool of 5,000 genes requires a population of 2,500 individuals.

So the population of an original created kind endowed with all the variability it will ever display in its whole history cannot possibly be a small population. In addition, to continue accommodating that much variability, it can never suffer a serious reduction in population. If some disaster reduced a population of 2500 to only 250, the number of genes would also be cut from 5000 to only 500. And this would not be sufficient to support the same degree of variability.

Obviously, this fact raises the question of how the variability endowed on the kind at creation could have been maintained through a global flood. In fact, it simply could not have been. Nor could the present range of variability in the human population be present in an original population of two, nor in a post-flood population of eight. The present range of variability had to be added subsequent to the initial creation of the species and again after the flood.