Evidence for Evolution

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THE EVOLUTION PROCESS

Evolution is the change with time of the gene pool of a species. The mechanisms of evolution are mutation, natural selection, recombination and gene flow.

Mutation provides all initial change. A mutation occurs when the DNA does not replicate perfectly. When a mutation occurs, a new allele is created. As a first approximation, these accidents (mutations) are random (can occur at any location along the DNA). The rate of these accidents is relatively constant within a given species. If the accident occurs in a critical location (believed to be less than 10% of the total in man), the result is usually disastrous. Other areas will accept change with no immediate consequence. Once made, the mutation is perpetuated and variability within the gene pool of the species is increased. Mutations add variability to the gene pool.

Natural selection occurs when the viability of an allele is tested in real life. It makes only one test. Contrary to popular opinion, evolution does not select the fittest, strongest, or most superior organism. It is instead a question of how many offspring the organism will have which in turn will reach sufficient maturity to have its own offspring. If the effect is positive, the allele will become a permanent part of the gene pool. If the effect is very successful, it will quickly become a dominant allele. If the effect is neutral or negative, the allele will not spread rapidly through the gene pool and, usually, will disappear from the gene pool. If more than one mutation is being tested at the same time, usually the case, then it is the summed effect tested. Not all good mutations make it. Some mutations would be good at one time and bad at another, depending on the environment then. A mutation that was necessary at one time may become unnecessary at another time and be consequently negated. Most of the time, the alleles removed or negated are those that harm the organism in that environment. Natural selection removes variability from the gene pool.

The environment which an organism faces and must survive is a complex one, one which is more than climate and food supply, although those are the essential elements that serve as a starting point in the study of evolution.

First of all, the mutation process is not altogether random. An intricate process called recombination developed early in sexual animals. This process serves to mix the alleles available in the two parental gene sets to provide more variability against the environment. It also results in many reproduction errors (mutations). Repair functions were developed by evolution for DNA errors to offset this error propensity. Since both the dissection means and the repair means are relatively fixed processes, then both the dissection errors and the errors in repair will follow certain patterns. When these coincide, a new allele is formed. Mutations, then, occur in clusters around particular loci not yet known or cataloged. Certain defects occur, therefore, with a given frequency, which are wholly the result of the process and not the assumption of a defective ancestral gene.

Another factor which enters into genetic change is that the product of a purely random process (and a large part of human mutations fit that description) will drift to one side or another until an outside force interferes with the drift. For example, the human is now growing larger. If this is the result of genetic drift, it will continue until some other process interferes, such as a shortage of food.

Most of the struggle in life is the struggle for enough food to avoid starvation and an ability to survive the climate. This was the entire struggle at the beginning, but as life became more complex, the selection process also became more complex. Once life began, however, other life became a part of its environment. The food chains were started.

The basic element of species survival is the ability of the individual to survive long enough to insure the survival of its offspring to the point when they also have offspring. If the offspring require no care, then the immediate death of the parent is of no consequence. In the case of the higher animals, those which require care during their maturation, the life of the caring parent must extend through that maturation period (and, of course, the parent must perform its function properly).

If an animal must endure an environment in which its population is normally controlled by predators, it is usual that the young suffer a higher death rate than the adults. In such cases the parents will usually live through several breeding seasons, to offset losses of their young. Some animals resort to large numbers of offspring, thereby feeding the predators, with enough left over to continue the species.

As animals became more complex, they themselves began to be an appreciable part of their own selection (survival) environment. Herein lies the most complex of all genetic processes, and examples abound. Sexual selection (based on an appearance which is sexually attractive) is probably (not for sure) the most common of these. There are times when sexual selection actually harms the ability of the species to survive. There are thousands of examples, but to select one, consider the Cardinal, a beautiful small bird that is quite common in North America. Somewhere back in time, the drab little hens, who had drab little roosters as soul-mates, took a liking to the color red and began choosing mates based on a hint of red in their feathers. Since they mated with roosters who had red in their makeup, their offspring tended to have red in their feathers, which suited the next generation of hens just fine. Quite quickly the rooster was a bright red, and the best target in the world for a predator. The predator, usually a hawk, could lock on to that bright red target and have a meal in no time. As a result the Cardinal rooster is quite skittish, and he should be, but without the red there is no sex and his genes end.

Recombination occurs in sexually reproducing organisms, such as the human. The parent has two sets of chromosomes in each cell, one from its father, the other from its mother. The sperm and the egg carry only one set in each. The one set carried by the sperm or egg is not a whole set from either grandparent but is a mixture of the two. Both original sets of chromosomes, in the case of each parent, are dissected and scrambled, then reformed with entirely new combinations of alleles from both grandparents. This process adds variability to the offspring and allows testing of new allele combinations. Recombination allows new combinations of the variability in the gene pool

Gene flow occurs when populations of a species that have been separated are united and the differing sets of alleles in each gene pool flow into the gene pool of the other. Our species, suddenly reunited with widespread transportation, is an excellent example of this effect. Gene flow distributes the variability in the gene pool.

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HOW EVOLUTION WORKS

Mutations are accidents in reproduction. The only place where such mutations can occur is in the production of the haploid cells (cells with a single set of chromosomes) in the sperm and egg, or in the joining of the two in conception.

A reproduction accident anywhere else in the body will affect only the cell that suffers the accident. Such accidents will not be added into the gene pool and thus are not mutations. In such an accident, the sick cell is quickly replaced by a well one and the incident is over. Yet when such an accident occurs in the sperm or egg, it will appear in every cell in the offspring. This mutation then has a 50% chance of occurring in each grandchild. If the recipient of the mutation has several children, the odds are that the mutation will join the species gene pool by way of one or more of his children.

Natural selection then determines the fate of the mutation in the species gene pool. The test is not survivability or excellence. The test is in species population growth. If the mutation aids the growth of the species population then it is successful and will remain in the gene pool. If it does not, natural selection will remove it from the gene pool (through death and hardship).

Here are a few examples concerning man and evolution to help gain understanding of the way evolution works. The effects shown are not necessarily caused by genetics, but evolution treats all conditions as if they were. Note that natural selection acts as if all genes are involved in the success or failure of the individual. Each case that reduces the expected offspring is considered a vote against each gene in the genome. Each case that equals or exceeds the expected offspring is considered a vote for each gene in the genome. The mixing of genes in recombination allow individual allele selection over the long period of time.

Effect1: The new gene shortens the life to 35 years. Natural selection would not see this defect as detrimental since the children will be old enough to fend for themselves by that time.

Effect2: The parent has too many children. If so many children were born that the resulting death or misery rate reduced the number of the children who had children, evolution would see this as detrimental. If society takes care of his children for him they will be healthy enough to raise more children and evolution would judge the condition as beneficial.

Effect3: The parent does not take good care of his children. If society does not interfere by taking care of the children for him, the suffering children are less likely to raise children of their own and evolution would judge that the condition is detrimental. If society cares for his children, evolution will judge the condition beneficial.

Effect4: The new gene lengthens life to 150 years. Evolution will not see this change as beneficial. Neither will it see later mutations that degrade it as detrimental, until the life expectancy gets so low that it affects child bearing and raising.

Effect5: The man is a murderer of children. His murder of someone else's children will affect the evaluation of the genes of their parents adversely. If the murderer has sufficient children of his own, evolution will not see anything detrimental in his lineage.

Effect6: The man is cruel and vicious with his wife. As long as he does not kill her or otherwise render her unable to care for her children, evolution will see no harm. Even if he kills her and society takes over the raising of his children, evolution will still see no harm.

Effect7: The man dies of an accident before he has children. Natural selection will see this death as detrimental.

Effect8: A young lady decides not to marry and have children. Natural selection will see this as detrimental.

Effect9: A man decides to adopt children instead of having his own. Natural selection will vote for the genes of the natural parents of the children and vote against the adoptive parent's gene set.

A great difference clearly exists between the goals of evolution and those of a compassionate culture. We are built one way, but we want to be another way. Luckily there is a large overlap where both evolution and man desire the same thing. Unfortunately, where we differ the choices are all quite painful.

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FOSSIL EVIDENCE

Fossil evidence indicates that evolution has occurred.
At its core, the case for evolution is built upon two pillars: first, evidence that natural selection can produce evolutionary change and, second, evidence from the fossil record that evolution has occurred. In addition, information from many different areas of biologyincluding fields as different as embryology, anatomy, molecular biology, and biogeography (the study of the geographic distribution of species)can only be interpreted sensibly as the outcome of evolution.

The Fossil Record

The most direct evidence that evolution has occurred is found in the fossil record. Today we have a far more complete understanding of this record than was available in Darwin's time. Fossils are the preserved remains of once-living organisms. Fossils are created when three events occur. First, the organism must become buried in sediment; then, the calcium in bone or other hard tissue must mineralize; and, finally, the surrounding sediment must eventually harden to form rock.

The process of fossilization probably occurs rarely. Usually, animal or plant remains will decay or be scavenged before the process can begin. In addition, many fossils occur in rocks that are inaccessible to scientists. When they do become available, they are often destroyed by erosion and other natural processes before they can be collected. As a result, only a fraction of the species that have ever existed (estimated by some to be as many as 500 million) are known from fossils. Nonetheless, the fossils that have been discovered are sufficient to provide detailed information on the course of evolution through time.

Dating Fossils
By dating the rocks in which fossils occur, we can get an accurate idea of how old the fossils are. In Darwin's day, rocks were dated by their position with respect to one another (relative dating); rocks in deeper strata are generally older. Knowing the relative positions of sedimentary rocks and the rates of erosion of different kinds of sedimentary rocks in different environments, geologists of the nineteenth century derived a fairly accurate idea of the relative ages of rocks.

Today, rocks are dated by measuring the degree of decay of certain radioisotopes contained in the rock (absolute dating); the older the rock, the more its isotopes have decayed. Because radioactive isotopes decay at a constant rate unaltered by temperature or pressure, the isotopes in a rock act as an internal clock, measuring the time since the rock was formed. This is a more accurate way of dating rocks and provides dates stated in millions of years, rather than relative dates.

A History of Evolutionary Change
When fossils are arrayed according to their age, from oldest to youngest, they often provide evidence of successive evolutionary change. At the largest scale, the fossil record documents the progression of life through time, from the origin of eukaryotic organisms, through the evolution of fishes, the rise of land-living organisms, the reign of the dinosaurs, and on to the origin of humans.

Today, many of the gaps in the paleontological record have been filled by the research of paleontologists. Hundreds of thousands of fossil organisms, found in well-dated rock sequences, represent successions of forms through time and manifest many evolutionary transitions. As mentioned earlier, microbial life of the simplest type was already in existence 3.5 billion years ago. The oldest evidence of more complex organisms (that is, eucaryotic cells, which are more complex than bacteria) has been discovered in fossils sealed in rocks approximately 2 billion years old. Multicellular organisms, which are the familiar fungi, plants, and animals, have been found only in younger geological strata. The following list presents the order in which increasingly complex forms of life appeared:


Life Form	Millions of Years Since First Known Appearance (Approximate)
Microbial (procaryotic cells)	3,500
Complex (eucaryotic cells)	2,000
First multicellular animals	670
Shell-bearing animals	540
Vertebrates (simple fishes)	490
Amphibians	350
Reptiles	310
Mammals	200
Nonhuman primates	60
Earliest apes	25
Australopithecine ancestors of humans	4
Modern humans	0	.15 (150,000 years)

So many intermediate forms have been discovered between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent that it often is difficult to identify categorically when the transition occurs from one to another particular species. Actually, nearly all fossils can be regarded as intermediates in some sense; they are life forms that come between the forms that preceded them and those that followed.

The fossil record thus provides consistent evidence of systematic change through time--of descent with modification. From this huge body of evidence, it can be predicted that no reversals will be found in future paleontological studies. That is, amphibians will not appear before fishes, nor mammals before reptiles, and no complex life will occur in the geological record before the oldest eucaryotic cells. This prediction has been upheld by the evidence that has accumulated until now: no reversals have been found.

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THE DISTRIBUTION OF SPECIES

Biogeography also has contributed evidence for descent from common ancestors. The diversity of life is stupendous.
Approximately 250,000 species of living plants, 100,000 species of fungi, and one million species of animals have been described and named, each occupying its own peculiar ecological setting or niche; and the census is far from complete. Some species, such as human beings and our companion the dog, can live under a wide range of environments. Others are amazingly specialized. One species of a fungus (Laboulbenia) grows exclusively on the rear portion of the covering wings of a single species of beetle (Aphaenops cronei) found only in some caves of southern France. The larvae of the fly Drosophila carcinophila can develop only in specialized grooves beneath the flaps of the third pair of oral appendages of a land crab that is found only on certain Caribbean islands.

How can we make intelligible the colossal diversity of living beings and the existence of such extraordinary, seemingly whimsical creatures as the fungus, beetle, and fly described above? And why are island groups like the Galápagos so often inhabited by forms similar to those on the nearest mainland but belonging to different species? Evolutionary theory explains that biological diversity results from the descendants of local or migrant predecessors becoming adapted to their diverse environments. This explanation can be tested by examining present species and local fossils to see whether they have similar structures, which would indicate how one is derived from the other. Also, there should be evidence that species without an established local ancestry had migrated into the locality.

Wherever such tests have been carried out, these conditions have been confirmed. A good example is provided by the mammalian populations of North and South America, where strikingly different native organisms evolved in isolation until the emergence of the isthmus of Panama approximately 3 million years ago. Thereafter, the armadillo, porcupine, and opossum--mammals of South American origin--migrated north, along with many other species of plants and animals, while the mountain lion and other North American species made their way across the isthmus to the south.

The evidence that Darwin found for the influence of geographical distribution on the evolution of organisms has become stronger with advancing knowledge. For example, approximately 2,000 species of flies belonging to the genus Drosophila are now found throughout the world. About one-quarter of them live only in Hawaii. More than a thousand species of snails and other land mollusks also are found only in Hawaii. The biological explanation for the multiplicity of related species in remote localities is that such great diversity is a consequence of their evolution from a few common ancestors that colonized an isolated environment. The Hawaiian Islands are far from any mainland or other islands, and on the basis of geological evidence they never have been attached to other lands. Thus, the few colonizers that reached the Hawaiian Islands found many available ecological niches, where they could, over numerous generations, undergo evolutionary change and diversification. No mammals other than one bat species lived in the Hawaiian Islands when the first human settlers arrived; similarly, many other kinds of plants and animals were absent.

The Hawaiian Islands are not less hospitable than other parts of the world for the absent species. For example, pigs and goats have multiplied in the wild in Hawaii, and other domestic animals also thrive there. The scientific explanation for the absence of many kinds of organisms, and the great multiplication of a few kinds, is that many sorts of organisms never reached the islands, because of their geographic isolation. Those that did reach the islands diversified over time because of the absence of related organisms that would compete for resources.

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