It is now thought that the frequency of new mutations in humans is about 1 for every 10, genes per generation. If this number is correct, every individual would be expected to have mutations on average. Complicating the picture is the fact that mutation rates for different genes and chromosomes apparently vary. M utations are common occurrences even in healthy people. The majority of them probably do not confer a significant advantage or disadvantage because they are point mutations that occur in non-gene coding regions of DNA molecules.
They are relatively neutral in their effect. However, some mutations are extremely serious and can result in death before birth, when an individual is still in the embryonic or early fetal stages of development. Mutations can occur naturally as a result of occasional errors in DNA replication.
They also can be caused by exposure to radiation, alcohol, lead, lithium, organic mercury, and some other chemicals.
Viruses and other microorganisms may also be responsible for them. Even some commonly prescribed drugs are thought to be potential mutagens. In this group are. M utations appear to be spontaneous in most instances. That does not mean that they occur without cause but, rather, that the specific cause is almost always unknown. Subsequently, it is usually very difficult for lawyers to prove in a court of law that a particular mutagen is responsible for causing a specific mutation in people.
With the aid of expert scientific testimony, they can often demonstrate that the mutagen can cause a particular kind of mutation. However, that is not the same thing as proving that a plaintiff's mutation was caused by that mutagen instead of some others. In order for a mutation to be subject to natural selection, it must be expressed in the phenotyp es of individual s.
Selection favors mutations that result in adaptive phenotypes and eliminates nonadaptive ones. Even when mutations produce recessive alleles that are seldom expressed in phenotypes, they become part of a vast reservoir of hidden variability that can show up in future generations.
Such potentially harmful recessive alleles add to the genetic load of a population. If so, it will become more prevalent in the next generation and spread throughout the population. In this way, natural selection guides the evolutionary process, preserving and adding up the beneficial mutations and rejecting the bad ones.
But natural selection isn't the only mechanism by which organisms evolve, she said. For example, genes can be transferred from one population to another when organisms migrate or immigrate — a process known as gene flow.
And the frequency of certain genes can also change at random, which is called genetic drift. The reason Lamarck's theory of evolution is generally wrong is that acquired characteristics don't affect the DNA of sperm and eggs.
A giraffe's gametes, for example, aren't affected by whether it stretches its neck; they simply reflect the genes the giraffe inherited from its parents. But as Quanta reported , some aspects of evolution are Lamarckian. For example, a Swedish study published in in the European Journal of Human Genetics found that the grandchildren of men who starved as children during a famine passed on better cardiovascular health to their grandchildren.
Researchers hypothesize that although experiences such as food deprivation don't change the DNA sequences in the gametes, they may result in external modifications to DNA that turn genes "on" or "off. For instance, a chemical modification called methylation can affect which genes are turned on or off.
Such epigenetic changes can be passed down to offspring. In this way, a person's experiences could affect the DNA he or she passes down, analogous to the way Lamarck thought a giraffe craning its neck would affect the neck length of its offspring.
Even though scientists could predict what early whales should look like, they lacked the fossil evidence to back up their claim. Creationists viewed this absence, not just with regard to whale evolution but more generally, as proof that evolution didn't occur, as pointed out in a Scientific American article. But since the early s, scientists have found evidence from paleontology, developmental biology and genetics to support the idea that whales evolved from land mammals. These same lines of evidence support the theory of evolution as a whole.
The critical piece of evidence was discovered in , when paleontologists found the fossilized remains of Ambulocetus natans , which means "swimming-walking whale," according to a review published in the journal Evolution: Education and Outreach. Its forelimbs had fingers and small hooves, but its hind feet were enormous relative to its size.
The animal was clearly adapted for swimming, but it was also capable of moving clumsily on land, much like a seal. When it swam, the ancient creature moved like an otter, pushing back with its hind feet and undulating its spine and tail.
Modern whales propel themselves through the water with powerful beats of their horizontal tail flukes, but A. In recent years, more and more of these transitional species, or "missing links," have been discovered, lending further support to Darwin's theory.
For example, in , a geologist discovered the fossil of an extinct aquatic mammal, called Indohyus , that was about the size of a cat and had hooves and a long tail.
Scientists think the animal belonged to a group related to cetaceans such as Ambulocetus natans. This creature is considered a "missing link" between artiodactyls — a group of hoofed mammals even-toed ungulates that includes hippos, pigs, and cows — and whales, according to the National Science Foundation.
Researchers knew that whales were related to artiodactyls, but until the discovery of this fossil, there were no known artiodactyls that shared physical characteristics with whales. After all, hippos, thought to be cetaceans' closest living relatives, are very different from whales.
Indohyus , on the other hand, was an artiodactyl, indicated by the structure of its hooves and ankles, and it also had some similarities to whales, in the structure of its ears, for example. Genetic evidence also supports the idea that whales evolved from land mammals and provides information about the exact branching of the evolutionary tree.
For instance, in , researchers reported in the journal Proceedings of the National Academy of Sciences that according to genetic analysis of " jumping gene " sequences, which copy and paste themselves into genomes, hippos were whales' closest living relatives. Before , researchers thought pigs were more closely related to whales, but this study overturned that idea, as the Associated Press reported.
In , researchers reported in the journal Science Advances about which genes within the whale genome were inactivated during the process of the creature's evolution from land mammals, as Science Friday reported.
Every cell in the body contains a set of chromosomes and genes, descended directly by a long line of cell divisions from the set originally constituted in the egg cell at fertilization. The human embryo develops into a person, rather than into, a tree or an elephant or a monstrosity, because the material carried in its chromosomes, its constellation of genes, initiates and guides a marvelously coordinated sequence of reactions that leads inevitably, under normal conditions, to the differentiation and growth of a human being.
Throughout the life of the individual, the genes, continue to exert their control over the complex chemistry of the cells and tissues of the body. As older tissue is gradually replaced by new tissue in the mature person, the food that is consumed is converted quite specifically into more of the very same individual, even though an identical diet, fed to a dog, would be transformed into more dog.
We are a long way from understanding just how genes direct the manifold activities of living systems, but we know with growing certainty that the range of possible responses of any cell or organism to the conditions it may encounter is largely gene-determined.
All the members of our species have in common the basic genetic make-up that sets us apart from other forms of life. Nevertheless, no two individuals, with the exception of identical twins, have exactly the same heredity, which is another way of saying that every person possesses a unique pattern of chromosomal genes. Differences in skin pigmentation, eye and hair color, stature, and facial features are familiar hereditary traits by which individuals and groups of individuals differ from one another.
These and the host of other inherited variations, from fingerprint patterns to blood types, are manifestations of the differences that exist in the structure and arrangement of the genic material. Some hereditary variations, such as eye color, are known to depend upon differences in the state of a single gene. This does not imply that one gene, all by itself, is responsible for the formation of blue or brown pigment in the iris of the eye.
It means that a change in this particular gene can alter the integrated functioning of the whole gene system so as to result in the production of a different kind of pigment. Other characteristics, such as height, depend upon the states of a relatively large number of genes.
Genes do not exist in a vacuum. They are always present in an environment that must be taken into account in understanding how they work. The environment within the cell and within the organism, and the more unpredictable environment outside, are intimately bound up with the functioning of genes and have varying degrees of influence upon the ultimate expression of heredity.
A trait or characteristic is not, in itself, inherited. That which is determined by genes is the capacity to produce certain traits under certain conditions. In the case of eye color, this distinction may seem unimportant, since an individual having the genetic constitution for blue eyes will have blue eyes under any environmental conditions. The Himalayan rabbit is a case in point. This rabbit has a pattern of white fur, with black fur at the extremities ears, tips of paws, tail , and this pattern is passed along from generation to generation.
If a patch of white fur from the back of such a rabbit is shaved off, and the new fur allowed to grow back while the animal is kept in a cool place, it will grow in black instead of white. Thus it is not the pattern itself that is inherited, but the capacity to produce black pigment at low temperatures and not at higher temperatures. Since the temperature at the extremities is normally lower than that of the rest of the body, the typical Himalayan pattern is obtained.
Similarly, although stature is basically under the control of genes, it can be influenced significantly by nutritional factors. Genes are remarkable not only for the way they direct the intricate pathways of metabolism and development. They have in addition, unique properties that give them special importance in biology, as the raw materials not only of evolution but probably of life itself.
Genes have the ability to organize material from their surroundings into precise copies of their own molecular configurations, and they exercise this power every time a cell divides. They are also capable of undergoing structural changes, or mutations; and once such a change has occurred, it is incorporated into the copies that the gene makes of itself.
A single unit having these properties, and having also the ability to aggregate with other such units, would possess the essential features of a living being, capable of unlimited evolution through the natural selection of variant forms and combinations most efficient in reproducing themselves. Many biologists believe that life may have originated with the accidental formation of "naked genes," organic molecules able to duplicate their own structure, and their variations in structure, from materials available in the environment.
Although the chemical nature of genes is not yet known with certainty, one of the most important recent advances in genetics is the evidence that their definitive properties can be accounted for by the theoretical structure and behavior of the molecules of compounds known as desoxyribonucleic acids, or DNA. Chromosomes contain large amounts of DNA.
Its molecules are very big, as molecules go, built up in long chains from only four kinds of simple chemical building blocks. The order in which these units occur, and the number of repetitions of similar groupings, are thought to be the basis of the specific activity of different regions of the chromosome — in other words, of genes.
The study of the properties of these molecules provides a way to explain the mechanism by which genes duplicate themselves and reproduce the variations that they may undergo. Mutations, as has already been suggested, are considered to be changes, on the molecular level, in the structure or organization of genes. A mutation in any gene is likely to be reflected in a modification of its contribution to the delicately interwoven pattern of control exercised by the whole constellation of genes, and may be detected by its effect on some physical or metabolic characteristic of the organism.
Mutations, in nature, are rather rare events, occurring usually with frequencies of from one in a thousand to one in a billion gene duplications.
They have an extremely wide range of effects, from fatal disturbances of normal development to perceptible reductions of life expectancy, from striking changes in appearance to slight alterations of metabolism that can he detected only with sensitive laboratory instruments. Mutations in man are responsible for the kinds of hereditary differences we have already discussed, and can produce, as well, such effects as early fetal death, stillbirth, diseases such as hemophilia and sickle cell anemia, color blindness and harelip.
It seems quite possible that cancer, leukemia, and other malignant diseases may originate by the occurrence of mutations in body cells other than the reproductive cells. Although the overall frequency of mutations can be increased considerably by exposure to radiations and a variety of chemicals, there is ordinarily no relation between environmental conditions and the kinds of mutations that occur.
Mutations of all sorts arise in natural populations, with low but regular frequencies, in a way that is best explained by considering them to be the consequences of accidental molecular rearrangements, occurring more or less at random in the genetic material. X rays and other kinds of high-energy radiations increase the probability that these accidents or mutations will occur, but we do not know with certainty the causes of so-called "spontaneous" mutations.
Natural radiations, such as cosmic rays, undoubtedly cause a fraction of them, but it has been estimated that the intensity of natural radiations is not sufficient to account for all the mutations that occur in plant and animal populations. Darwin believed that the inheritable variations upon which natural selection acts are caused directly by the influence of the conditions of life upon the organism, or by the effects of use and disuse of particular body parts.
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