Question No. 21
- Does the process of speciation occur today, and can one observe it?
- Answered 6 March 2008.
Question author: Alena.
Asked 2 December 2007.
Briefly: Non-viral organisms speciate at a rate of about one new species per year per entire biosphere. This is too small a number to be observed. It is impossible to observe the speciation process due to the limited sensitivity of the existing monitoring techniques.
Such a situation is common in science, when some well established laws and data predict that a process must be occurring, yet it is impossible to observe the process due to the limited sensitivity of the measurements techniques.
1. Evolution is going on
It is established that the species life span does not depend on body size and population size (species abundance). It is on average the same for small unicellular species with huge populations and for macroscopic multicellular species of, e.g., mammals, birds, etc., who have population sizes by many orders of magnitude smaller
How does one know that evolution proceeds at the above rate, one new species per biosphere per year? This follows from two firmly established facts:
1) the existence of the absolute time scale as proved by the data on the decay of radioactive isotopes,
2) the existence of paleodata, i.e. excavated remnants of extinct species.
There are no other data that would be indicative of evolution. According to the paleoevidence, all species persisted morphologically unchanged in the paleorecord for a mean time period of about T = 3 x 106 years (three million years) and then became extinct. (Namely this relative morphological constancy allows one to unambiguously ascribe particular remnants to a particular species.) It is established that the species life span does not depend on body size and population size (species abundance). It is on average the same for small unicellular species with huge populations and for macroscopic multicellular species of, e.g., mammals, birds, etc., who have population sizes by many orders of magnitude smaller. Total number of species N inhabiting the biosphere at any give moment of time, despite considerable fluctuations, did not change its order of magnitude. Currently it is about several million species (3-10 million). This means that extinction of old species is invariably accompanied by the appearance of new ones. From this the rate of evolution can be estimated by the ratio N/T ~ 1-3 species per year.
2. Speciation and intraspecific genetic variability are unrelated
Evolution implies change of the species-specific genetic information, which is called species genome. Genetic information of species is written in DNA molecules, which represent linear sequences of genetic letters - nucleotide pairs. Quantitative difference between genomes of related species is approximately the same between any two evolutionarily close species. It does not depend on body size or species abundance (population numbers). Mutation rate (the number of nucleotide substitutions occurring per unit time) is dictated by the molecular structure of DNA molecules and is, hence, universal in all individuals of a species. The greater the total number of individuals in a species, the greater amount of mutations will accumulate per unit time in the global population of this species. If such intraspecific mutations had determined the rate of evolution (i.e. one of the accumulated mutations occasionally initiated the speciation process), then, obviously, the rate of evolution would have been proportional to the number of individuals of a species. However, such a possibility is unambiguously ruled out by the paleoevidence. Consequently, the observed random accumulation of intraspecific mutations in individual genomes is not the driving force of the speciation process.
Intraspecific genetic variability is limited by natural stabilizing selection. If the number of errors in the genetic information of the individual exceeds the limit of sensitivity of the process of competitive interaction, such individuals are perceived by the others as defective, are outcompeted and expelled from the population in one way or another. Individuals with the permissible number of genetic errors not exceeding the sensitivity limit are perceived as normal. Due to the random nature of errors, in different populations they affect different regions of the species genome. Occasionally, some errors can be fixed in the genomes of all members of the population and be repeatedly transmitted from generation to generation. In this case one witnesses the formation of subspecies and races of a given species. Although in different races and subspecies different genetic errors are fixed, the genomes of subspecies and races remain equally close to the normal species genome, being distanced from it by no more than the above-discussed limit of sensitivity of stabilizing selection. Therefore, all races and subspecies are genetically equally valuable. Mutational process and stabilizing selection result in the appearance of errors in some places of the genome and their disappearance, in an equal number, from some other regions. Such genetic drift of random mutational substitutions does not move the average population genome away from the normal genome and does not change the main species-specific characteristics of individuals, which make it possible to unambiguously ascribe the individual to a particular extant or extinct species.
Artificial selection, unlike random genetic drift in natural populations, consists in a directional selection of defective individuals which may feature properties valuable for humans.
Thus, all the observed intraspecific mutations, including any types of artificial selection, hybridization and genetic modification, are not related to speciation.
3. External factor of the speciation process: RNA viruses
One is therefore led to conclude that evolution is driven externally, by means of incorporation into the species genome of external fragments of functionally sensible genetic fragments. This novel sensible information should be delivered to all species of the biosphere at an equal mean rate, over equal mean periods of time of the order of three million years. Consequently, there should be a single source of this information, universal for the entire biosphere.
What is the nature of this universal source of extra-specific genetic information?
In order to initiate protein synthesis and all other cell processes, the relevant DNA information is transcribed piecewise onto a large number of short RNA molecules, which serves as protein-synthesis templates. DNA has a built-in program of repairing mutations (fixing problems) that arise during copying of DNA molecules in the course of cell division. This reduces the frequency of DNA mutations million-fold compared to the frequency of RNA mutations, for which there is no program of reparation.
In nature there exist RNA-viruses (called retroviruses) which species-specific genetic information is written not in DNA, but in RNA molecules. Therefore, during reproduction such viruses mutate million times more frequently than any other organisms of the biosphere that store their genetic information in the form of DNA. Outside the cell, viruses are dead. They only become active within the cell of their host. Human immunodeficiency virus (HIV), which causes AIDS, belongs to retroviruses. It was first observed in the 1980-s. There are all grounds to assert that this virus had not existed before and that its appearance is an observed example of the evolutionary process of speciation, but in viruses, not in living organisms. This is an example of the evolutionary appearance of a novel genetic program of killing individuals of an extant species. HIV attacks the normal genetic program of Homo sapiens.
If the civilization had no ways of resisting the virus, it could in principle infect all humans possessing normal genetic program and thus kill the majority of the population. The few survivors would be those individuals whose genetic program proved to be resistant to the virus, as it deviated in one way or another from the normal one. Such deviant genetic programs are defective in terms of meaningful species-specific information. However, since the virus, "tuned" for the normal genome, would be unable to function within the organisms with such deviant genetic programs, after all normal programs had been destroyed, the virus itself would go extinct and disappear from the biosphere. The pandemia would stop.
The population remnants that survive the viral attack will be composed of individuals with deviant (defective) genetic programs. However, such a population will rapidly restore the normal genome and the original species-specific genetic information. This is possible due to the mechanism of genetic recombination: the genome of the offspring is randomly assorted from the two sets of parental genomes. In the result, some offspring occasionally carry a smaller number of mutational substitutions than their parents do, while others are borne with an even greater number of defects. In the course of competitive interaction within the population, the most defective individuals are precluded from leaving offspring, i.e. their genetic material is discarded. In the result, the average number of mutational substitutions per genome in the population will be continuously decreasing. Over a relatively short period of time all mutational substitutions that were 'favored' by the virus and accumulated during the pandemia, will disappear from the population. The initial high level of genetic orderliness of the population will be restored. Thanks to the existence of such recovery mechanisms, viral attacks and pandemia do not pose a threat to the species existence. They do, however, impose limitations on the global population numbers of the infected species. The discovery of HIV is an indication that the appearance of novel harmful viral agents is a common phenomenon repeatedly occurring during the many million of years of species' existence.
Once in several million years RNA viruses, that are rapidly evolving themselves, are able to prepare a novel meaningful progressive genetic fragment for the host species. Occasional incorporation of such a fragment into the genomes of one or a few infected individuals radically increases their competitiveness (as the definition of the word 'progressive' implies). Offspring of such genetically modified competitive individuals outcompete individuals whose genomes lack the progressive fragment; such individuals are forced out from the population. The number of individuals carrying the novel meaningful fragments grows exponentially. This results in a burst-like appearance of a novel species and extinction of the original one. Propagation of the progressive genetic information in the population occurs in very much the same manner as infection propagates during an epidemic. The only difference is that in the former case there are no individuals that could oppose spread of the more progressive novel information, while during epidemic some individuals are, as discussed above, resistant to the infection.
RNA viruses change and evolve in a random manner. They cannot 'specialize' on preparing novel genetic information for a particular species or taxon. For this reason the probability of novel genetic information to arise is one and the same for all species of the biosphere. Namely this fact explains the observed approximate uniformity of evolutionary rates in all lineages and their independence of body size and population numbers. This statement completes our response to the question posed.
4. Conventional approaches to speciation and evolution; creationists and evolutionists
Let us now consider how the same question could be answered by professional biologists-evolutionists. The brief answer would be similar to the one given above: evolution is going on at approximately the same rate as ever before (about one new species per biosphere per year), however, it is impossible to monitor such rare speciation events. However, as for the causes of speciation, the opinions would differ.
There is an on-going debate, which periodically intensifies or damps down, on the issue of whether speciation is gradual or intermittent with periods of stasis ('punctuated'). In reality, as is well-known in physics, continuity or discreteness depends on the considered spatial or temporal scale of the process (phenomenon) in question. For example, such characteristics of gas like temperature and pressure are continuous when considered at a linear scale greatly exceeding intermolecular distances; but all characteristics of gas become discrete when viewed on a scale comparable to or less than the mean distance between the neighboring molecules. When viewed on a time scale greatly exceeding the mean time of species existence, evolution can be taken for continuous. Indeed, apparently, the genomes of humans and chimpanzees carry some fundamentally different pieces of genetic information. However, what these differences are on the molecular level and where exactly they reside in the genome has not been possible to determine so far. If one randomly picks up two individual genomes from the human population and compares their nucleotide sequences, the observed quantitative differences between the two humans will not be drastically different from the differences between two randomly picked up chimpanzee genomes, or between a human and a chimpanzee genome. At the molecular level the discreteness of the evolutionary process so far remains unresolved.
All species in the biosphere, including closely related ones, are discrete and strictly delimitated from each other. No continuous "bridges" (transitional forms) between species are observed either for the extant species or for extinct species, as testified by the paleodata. The fact of species discreteness is the essense of species existence. Had continuous transitional forms existed, it would be altogether impossible to speak of species as entities.
The process of speciation (i.e. formation of novel species from the old one) cannot be resolved in the available paleoevidence. It is only known that this process occurs on a time scale much shorter than the average species life span (3 x 106 years) and takes no more than ten thousand years. Relatively speaking, speciation is burst-like, the new species arises nearly instantaneously, in an eye blink which lasts no more than a few tenths of per cent of the total species life span. This observation is in perfect agreement with the mechanism of "progressive infection", when the old species incorporates novel genetic fragments prepared by RNA viruses that attack it. Temporal discreteness of the speciation process on a time scale of 104 years << T ~ 3 x 106 years and its continuity on a time scale exceeding T ~ 3 x 106 years are the two most important observed properties of evolution.
The continuity of evolution is alledgedly explained by the Darwinian theory of natural selection. Questioning the continuity of evolution is commonly interpreted as questioning or abolishment of the darwinism itself. The species discreteness argument has been often invoked by creationists — people who interpret evolution as being driven by an external, extra-biospheric agent ("creator") — to deny evolution. What creationists essentially assert is in certain ways equivalent to the above made statement that the driver of evolution (RNA viruses) is external with respect to the species that evolves. Implicit and explicit pressure imposed by the proponents of the Darwinian theory of evolution (which, in the light of recently available genetic knowledge, got the name "neo-darwinism") on colleagues-scientists who, based on paleoevidence, speak in favor of the discrete nature of speciation ("punctuated equilibrium") make the latter if not renounce but gradually reformulate their views.
Charles Darwin's theory of natural selection obviated the need to invoke an external force (a creator from outside the biosphere) for the explanation of the origin of species. It became clear that evolution is driven by some processes within life itself. This statement, which is Darwin's important contribution, remains valid and free from controversy. At the same time Darwin believed that the natural speciation process occurred at the expense of the observed intraspecific variability in a manner similar to the appearance of new artificially selected breeds of animals and sorts of plants. However, in Darwin's time the absolute chronological time scale remained unknown, as did the aforementioned independence of species duration on body size and population size. Darwin could not either see or evaluate the fundamental differences between natural and artificial selection, as the necessary evidence became available much later. Therefore, Darwin's concept, which describes the evolutionary process as continuous, remains valid on a time scale greatly exceeding the mean time of species existence, and can be interpreted as implicitly comprising the discrete nature of evolutionary changes at a shorter time scale not resolvable in Darwin's time. The overlooked difference between the artificial selection process based on intraspecific variability and the speciation process based on acquisition of extraspecific genetic material is, however, the critical shortcoming of the concept, which eventually makes modern Darwin-based evolutionary theory vulnerable to the attacks of the creationists' kind.
As a concluding remark we note that Linnaeus who had formulated the principles of evolutionary systematics a hundred years prior to Darwin, essentially found that species from one and the same genus differ less from each other than species from different genera, or even more so, from different orders or classes. Linnaeus wrote that species are created by an external force (which is true, as species origin cannot occur without extraspecific genetic information) and that they remain as they are (which is true as well, as species do not change for millions of years). It is highly unlikely that Linnaeus could not appreciate that congeneric species arose from a common ancestor, i.e. that the acts of their "creation" were correlated (otherwise they would not have been so similar). The existence of the evolutionary process followed unambiguously from Linnaeus' systematics independently of Darwin's concept. If new species were designed by an extra-biospheric "creator" instead of evolving from their predecessors, then any species would have been equally similar to, and equally different from, the others. No Linnaeus' systematics could then be built. Most probably, Linnaeus as a distinguished thinker understood that quite well. However, by acknowledging the creation of further unchanged species by the external actor, Linnaeus might have been skillfully able to see his works recognized during his lifetime and have escaped prosecution by the dominant public opinion, which did not considerably differ from the views of modern creationists. It is only now that one can identify the candidate for the external evolutionary driver and this is RNA viruses. This driver, which is external with respect to the evolving species, remains, however, within the biosphere; it does not involve an external "creator" or conflict with the major statement of Darwin's concept.
- Evolutionary theory attacked: a debate in need of quantitative arguments? (on whether the responses of evolutionists to creationists are persuasive)
- Makarieva A.M., Gorshkov V.G. (2004) On the dependence of speciation rates on species abundance and characteristic population size. Journal of Biosciences, 29, 119-128. Abstract. PDF (0.1 Mb). Copyright Indian Academy of Sciences.