|Aug26-12, 10:31 PM||#1|
Question about evolution of animal species
Please explain something I don't understand about the evolution of animals that have sexual reproduction.
I thought that being a separate species means an animal will have sexual intercourse resulting in fertile offspring only if the partner is of the same species.
I shall cite the giraffe as an example because of an explanation about its evolution that was once taught to me.
If I understand correctly, the giraffe with a long neck probably came into existence because at one time a mutation occurred inside in a shorter-neck parent, such that the offspring had a longer neck.
Am I correct in saying that the new long neck animal will continue as a distinct species only if does not mate with short neck animals? But why? Would two animals, which are otherwise sexually attracted to each other, stop and think, "I'm unwilling to mate with you because I can see that your neck is too long (or short)"? Or did the mutation that made the offspring's neck longer also make the reproductive cells or organs physically incompatable with those of the short neck animals?
In either case, how did that long neck animal find a mate, so that the new species could survive? Did there have to be at least two mutations in the same geographical area, in both cases having the effect of causing short-neck parents to have long-neck offspring, so that the long neck animal could find a mate?
Thank you. Please keep your explanation simple because my background is physical science, not life science.
|Aug26-12, 10:46 PM||#2|
Here's how it works qualitatively:
You have giraffes with normally distributed heights. Those below a certain cutoff height always get eaten by lions because they're too short to see the lions through the grass until its too late. Those above a certain cutoff height never get eaten by lions because they can always see them in time. Now this is a huge simplification but bear with me here.
As time goes on, the short giraffes get wiped out and the tall giraffes reproduce more and more. This raises the average height of the entire population because there's no "short end" to pull down the average. In addition, as tall giraffes mate with average giraffes, their offspring might have height *between* theirs, and become the "new average".
|Aug27-12, 09:39 PM||#3|
Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales. In that case my question puzzles me. If it is a new species then it can only remain a distinct line by not mating with its neighbors, but then I don't understand how it finds a mate to permit it to reproduce.
|Aug28-12, 05:21 AM||#4|
Question about evolution of animal species
In short, speciation is always the end result of accumulating a sufficiently large number of minute changes, to the point at which the members of the emerging species no longer can or at least no longer do mate with the members of the mother or sister species. It is never the result of the same sort of drastic mutation occurring in a male and a female at the same time and the two just happening to find each other more attractive than any non-mutated specimen. The latter is, as you suggest, absurdly improbable (and I use that claim with caution, because ID proponents tend to throw it around a lot when it doesn't really apply). The key concept which you may want to read up on is exaptation.
As a caveat, I should say that I'm not entirely certain that what you described as "either/or" evolution might not sometimes occur in species which reproduce asexually. It doesn't seem implausible for the occasional drastic mutation to turn out to be useful in some way, and for the clonal offspring of the parent with that mutation to displace the offspring of all other parents or to radiate into a new ecological niche. I'm also not certain how one would go about confirming or falsifying that idea, considering the rarity of useful drastic mutations. Maybe someone more knowledgeable can run with this...
|Aug28-12, 05:47 AM||#5|
Yes, it mostly continuous. Say a species gets separated into two groups geographically and evolve continuously but differently, given their different environments. At a certain point, they drift too far from each other and can't mate anymore by the time they return to each others company. In some cases (horse and donkey) they retain sexual compatibility despite being two different species.
|Aug28-12, 06:54 AM||#6|
What happens in your scenario is the creation of a new allele (although it is unlikely that neck length is controlled by a single gene) within a population. Depending on whether the phenotype is beneficial or not, it may be carried forward to the next generation.
|Aug28-12, 07:07 AM||#7|
As others have pointed out, the key thing you seem to be missing is to understand that large scale observable changes that occur are always an accumulation of many tiny changes that you would barely notice. Individual genetic mutations that are the origin of the morphological change are never anywhere near significant enough to make that individual organism’s gametes incompatible with the gametes of another member of the same species with the opposite sex. Another example I have encountered that might help is bat wings. If you have ever seen a bat wing you will have noticed how recognisably – if spookily – it resembles any mammalian upper limb, but just with hugely elongated fingers. According to something I read some time ago, this elongation of the fingers was controlled by a group of genes that make ‘bone morphetic proteins’. The key ‘aha’ for me was to understand that the mutations to these genes did not so much say ‘grow long fingers’ as it said, ‘grow fingers at a faster rate’. It will still have taken a long and complex sequence of separate mutations for the bat’s arm to develop into the wing frame you can observe in modern bats.
|Aug28-12, 07:36 PM||#8|
As for all of the changes being gradual, thanks for correcting my misconceptions. I had visualized the mutations involved in evolution as occasionally being [I don't what word to use here] "categorical", forming definite branches on the taxonomic tree -- a warbler gets hit by a gamma ray and then its offspring is a thrush, a halibut gets hit by an alpha particle and then its offspring is a mackeral. I don't remember ever hearing this explained in school -- I'm a teacher myself so I'm vigilant about the issue of what we sometimes forget to say. Also thanks for the helpful links.
|Aug29-12, 10:47 AM||#9|
My favorite example occurs within the human population. Human beings have at least two Rh blood types: Rh positive and Rh negative. You agree that we are the same species, regardless of our Rh blood type. You agree that this is merely a small variation in the same species. Yet, there is a very slight hybridization barrier between the two populations.
There is an allele in most primates, including that contains genes that determine the blood type. If a person has two copies of either type of gene, then the baby will be the same blood type as both parents. However, a hybrid baby may be a different blood type then the mother. Then, there is a chance that the mothers immune system will attack the baby during childbirth. This is called blue-blood syndrome.
I have heard the argument that "Rh baby syndrome" is a minor problem because modern hospitals know how to handle it. Just use prenatal blood transfusions! However, it probably was close to lethal before the 17th century.
Here you see a hybridization barrier within a species (human beings) which can not be eliminated by dilution. Furthermore, it has not been eliminated by natural selection.
If an Rh woman mates with an Rh positive man, the second and third babies may be sick with Rh baby syndrome. I doubt this will lead to a new species of human now that medicine can treat it. However, it is an interesting hybridization barrier.
“Rh disease (also known as Rhesus isoimmunisation, Rh (D) disease, Rhesus incompatibility, Rhesus disease, RhD Hemolytic Disease of the Newborn, Rhesus D Hemolytic Disease of the Newborn or RhD HDN) is one of the causes of hemolytic disease of the newborn (HDN). The disease ranges from mild to severe, and typically occurs only in some second or subsequent pregnancies of Rh negative women where the fetus's father is Rh positive, leading to a Rh+ pregnancy. During birth, the mother may be exposed to the infant's blood, and this causes the development of antibodies, which may affect the health of subsequent Rh+ pregnancies. In mild cases, the fetus may have mild anaemia with reticulocytosis. In moderate or severe cases the fetus may have a more marked anaemia and erythroblastosis (erythroblastosis fetalis). When the disease is very severe it may cause haemolytic disease of the newborn (HDN), hydrops fetalis, or stillbirth.”
“The Rh factor is an inherited protein found on the surface of red blood cells. Most people have this protein and are called Rh-positive. However, some people don't have protein; they are called Rh-negative. Rh-negative pregnant women are at risk of having a baby with a potentially dangerous form of anemia called Rh disease. Fortunately, treatment usually can prevent Rh disease.”
“Diluting” a mutated gene does not destroy it. Meiosis does not destroy mutated genes, it only mixes them up. Natural selection follows copies of a mutated gene, not the individual that has the gene. Unless the mutated gene is instantly lethal, natural selection can not destroy a mutated gene in one generation.
Hybridization and reproduction barriers come in many forms. Mating behavior is only one way to keep populations separate. Other changes restrict gene flow between populations. There is a large number of such changes. “Dilution” by cross breeding can’t destroy a gene. Only natural selection can totally eliminate a gene and only after more than one generation.
These hybridization barriers are seldom toggles. One mutation does not automatically make the population different. Even using Mendelian laws, it is easy to see that genes don’t get destroyed by cross breeding populations. Most important genes have more complicated dynamics then are indicated by Mendelian laws. Even in those cases, cross breeding doesn’t eliminate the existence of a gene.
In the case of the giraffe, the first mutation that first made the neck longer didn’t need a hybridization barrier to propagate. Genes don’t get diluted continuously. If a giraffe with a slightly longer neck mates with a giraffe that doesn’t have the gene, then the gene for a longer neck will pass on to some of the offspring. The gene doesn’t have to be protected from contact with its opposite allele.
If the gene was a classic Mendelian type gene, with one major phenotype tied to one site on one chromosome chromosome, crossing can never get rid of it. An animal either has one copy, two copies or no copies. If the trait involved several alleles, then natural selection only occurs in those hybrids that have the correct gene on all alleles. The effect of these genes would then be additive. However, hybridization would still preserve the gene whether or not it is advantageous. The animal with the gene would have to be killed by natural selection to eliminate the gene.
This is especially true if the associated phenotype is recessive. The first mutation probably has a gene of only one allele, so the trait isn’t even expressed in the first mutation. The recessive gene won’t be expressed until it has already passed on to quite a few descendents. Then, the animal would need the same recessive gene in two alleles to express itself. If the trait is advantageous, then natural selection will favor animals with two genes. So the natural selection will actually act on the population, not the individual. There is a statistical favoring of the traite.
In your giraffe example, how do you know that the first giraffe with the mutation for longer neck even had a longer neck? The gene could have been recessive. Thus, it wouldn’t express itself in the first generation. If the species isn’t inbred, then it may be a few generation before the gene for a longer neck is expressed. So the selection wouldn’t act on it until long after the mutation had occurred. The population would gradually increase the fraction of animals with a gene for a longer neck. Then another mutation occurs, which could be a duplication of the allele on a chromosome. That duplicate gene could make an even longer neck. The contribution to neck length of the two alleles could add. Pretty soon one would have multiple copies of the long neck gene. The number of such multiple copies would vary in the population, but natural selection would favor more copies of the original allele.
The hybridization barrier, whatever it is, won’t be useful until a few animals already have this gene. Once the advantageous gene has spread through the population, natural selection would favor those animals which cross with those animals that have the advantageous gene. A second mutation, favoring animals which mate with those animals which show the trait, could occur randomly much later. Natural selection would then favor such animals.
Note that in your giraffe case, there is a type of geographical isolation. The short necked giraffe would find food most easily in a jungle or place with short plants. The okapi is an extant animal that lives in a jungle which is like a giraffe in many ways. However, the mutant with the slightly longer neck would find food at the edge of a forest, or any place with long trees. They would graze in the regions where they were the most comfortable. These regions would overlap but have areas outside the overlap. This would not separate the two species completely. However, it would reduce the amount of cross breeding slightly.
“The frequency of the occurrence of hybrids between Chironomus thummi thummi and Chironomus thummi piger is estimated to be 0.047% in the wild. The rare hybridization events are the consequence of the sexual isolation mechanism of different swarming behavior of thummi and piger. Under laboratory conditions hybrids are easily obtained.”
“The success or failure of interspecific crosses is vital to evolution and to agriculture, but much remains to be learned about the nature of hybridization barriers. Several mechanisms have been proposed to explain postzygotic barriers, including negative interactions between diverged sequences, global genome rearrangements, and widespread epigenetic reprogramming. Another explanation is imbalance of paternally and maternally imprinted genes in the endosperm.”
One example with marine animals. More on this mating.
“Sympatric assemblages of congeners with incomplete reproductive barriers offer the opportunity to study the roles that ecological and non-ecological factors play in reproductive isolation. While interspecific asynchrony in gamete release and gametic incompatibility are known prezygotic barriers to hybridization, the role of mating system variation has been emphasized in plants. Reproductive isolation between the sibling brown algal species Fucus spiralis, Fucus guiryi (selfing hermaphrodite) and Fucus vesiculosus (dioecious) was studied because they form hybrids in parapatry in the rocky intertidal zone, maintain species integrity over a broad geographic range, and have contrasting mating systems. We compared reproductive synchrony (spawning overlap) between the three species at several temporal scales (yearly/seasonal, semilunar/tidal, and hourly during single tides).”
Here is an article concerning a hybridization barrier based on mating preferences. Note that this is an example of a saltation. Natural selection acts on the mutation over only a few generations. This fast type of evolution is rare. They call it a saltation. Notice that even in this case, the natural selection takes more than one generation to make an effect. So even with the saltation, using a Mendelian type allele, the mutant gene spreads over more than one generation.
“Coevolution of exploiter specialization and victim mimicry can becyclic and saltational
Darwin’s Principle of Divergence explains sympatric speciation as gradual and directional. Contradicting evidence suggests that species’ traits evolve saltationally. Here, we model coevolution in exploiter-victim systems. Victims (resource population) have heritable, mutable cue phenotypes with different levels of defense. Exploiters have heritable, mutable perceptual phenotypes. Our simulations reveal coevolution of victim mimicry and exploiter specialization in a saltational and reversible cycle. Evolution is gradual and directional only in the specialization phase of the cycle thereby implying that specialization itself is saltational in such systems. Once linked to assortative mating, exploiter specialization provides conditions for speciation.”
|Aug29-12, 12:21 PM||#10|
There are other selective pressures. Male giraffes fight each other using their necks. The female takes the winner, who usually has the longer neck. Thus, there is a pressure to maintain the long neck even in areas with shorter plants. So the fighting instinct prevents long necked giraffes from mating with short necked giraffe-like animals.
There is a short necked giraffe. The okapi is a short necked giraffe-like animal that lives in jungles. There are fossils of different giraffe-like species that lived in different environments. So geography on some scale may be involved with how these different species of giraffe separated.
You can imagine a population with a mixture of short and long necks. Giraffes, like other animals, move to the areas where they can obtain the most food. So the different varieties of giraffe have a tendency to settle in different areas where they can get the most food. This reduces cross breeding.
So maybe the first long necked giraffe simply moved out to a neighborhood that was better for him.
|Aug29-12, 12:36 PM||#11|
There may be other ways to cause the split, but I can't concieve of them (I've never taken an evolution class though, I've just studied evolutionary neuroscience and worked in labs looking at the evolution of the respiratory control system).
|Aug29-12, 03:40 PM||#12|
A saltation has also been called a monster. The phenotypic change is so huge that the next generation seems almost unnatural.
My opinion is that speciation by saltation is extremely rare since saltations usually die in the first or second generation. However, I suspect that it must happen sometimes. Most of evolution is very, very gradual. However, in biology there is an exception to every rule. So there must be an exception to "evolution is gradual".
Here is an article on evolution that describes several variations of the theory of evolution, including "saltation theory".
“A number of influential biologists have seen large-scale mutations as the most probable way in which new types of organisms have emerged. An extreme form of evolution by saltation was proposed in 1940 by geneticist Richard Goldschmidt, with his theory of the ‘hopeful monster’. He held that every so often a spontaneous ‘systemic mutation’ or macromutation – a massive reorganization of the genome of an individual organism – would occur, resulting in a ‘monster’. Most of these would be unviable and perish, but occasionally a ‘hopeful monster’ would appear which would be preadapted to a new environmental niche and become a successful new species. He proposed, for instance, that at one time a reptile laid an egg and a bird was hatched from the egg. He believed that such events accounted for all the major gaps in the fossil record. Goldschmidt was excommunicated by the darwinist establishment and regarded as a lunatic for the rest of his life, though his theory did find favour with palaeontologist Otto Schindewolf, another opponent of gradualism.”
Almost all saltations are lethal in the first generation. Therefore, they probably don't contribute very much to speciation. Unfortunately, saltations are the easiest mutations to study. They show themselves when the mutation first occurs, which is in the very first generation.
So there is a lot of studies of such saltations. One of the best studied types of saltation is the homeotic saltation. This is where a big change occurs in a homeobox gene. A lot of studies have been done on homeotic saltations. Homeotic mutations have helped scientists understand a lot about heredity. However, homeotic saltations are not very likely to contribute to evolution and speciation.
There are a lot of subtler mutations which are probably more common. However, they are very hard to detect in the first or second generation. They are hard to detect even after that, since the effect is small compared to environmental of variation. These mutations have a much greater chance of surviving a few generations, and so contribute much more to evolution. An example would be a balanced chromosomal translocation.
Here is an article on chromosomal translocation. Note that there have been studies of this type of mutation. However, most of the experiments have to be done a few generations after the mutation first occurs because the detection is difficult.
“In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on cytogenetics or a karyotype of affected cells. There are two main types, reciprocal (also known as non-Robertsonian) and Robertsonian. Also, translocations can be balanced (in an even exchange of material with no genetic information extra or missing, and ideally full functionality) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes).”
|Aug29-12, 10:06 PM||#13|
Reply to Pythagorean:
Do you know of some examples how groups can become separated into two different environments? Is it realistic to say water level changing to produce a new island, or a river changing its course? What else can do it?
Reply to Darwin123:
Could it be that saltation and gradualism in the ideas of biologists have some historical or philosophical parallels to the ideas of catastrophism and uniformitarianism in the idea of geologists, respectively? The two dichotomies seems parallel to me. I hope I'm not just making a silly metaphor.
Grateful for everyone taking the time to instruct me.
|Aug29-12, 10:30 PM||#14|
For some specific examples and more theory:
An example from another page:
"An example of vicariance is the separation of marine creatures on either side of Central America when the Isthmus of Panama closed about 3 million years ago, creating a land bridge between North and South America."
|Aug30-12, 06:13 PM||#15|
I discussed “Rh baby syndrome in a separate post. I presented it as a special case of a “hybridization barrier” within a single species. The single species in this case is Homo sapien. However, I did not demonstrate that there was a geographical separation mechanism for the Rh blood types..
It turns out that the ratio between the Rh blood types in a population has a strong geographical variation. The largest percentage of Rh negatives in the world occurs among the Basque people on the border of France and Spain. This probably explains the high infant mortality that they have had.
There were legends that the Basque people were under some type of curse that caused infant mortality. Now it seems that what they have been suffering from is a high percentage of Rh positive genes. If an Rh negative mother has an Rh positive fetus, then every Rh positive fetus that follows will have severe anemia. No other region in the world has such a large problem with “Rh baby syndrome.”
Furthermore, the percentage of Rh positive genes is lowest in Asia. The population with the lowest percentage of Rh negatives is in Mongolia. They have almost no problem with “Rh baby syndrome”.
There has obviously been some geographic separation process between the two gene types. No one is sure what it is. However, there is one theory that is currently being examined.
The theory is that the Rh positive antigen provides a resistance to the infectious disease caused by the protozoa species, Taxoplasmosis gondii. There is even a theory that hybrids, people with both Rh positive and Rh negative genes, have the highest resistance to Taxoplasmosis gondii. There is a correlation between the areas with a high percentage of infection with Taxoplasmosis gondii and the Rh positive blood type. The disease is usually asymptomatic. However, there is a slightly higher percentage of mental illness and suicide among people with the disease.
Both Rh baby syndrome and symptomatic Taxoplasmosis are rare, one could say that neither has a strong fitness value. However, evolution has had hundreds of generations to work. So it is not too surprising, given evolutionary theory, that there has been a slight bias in the geographical distribution of the Rh blood types.
Here are some links to articles related to the correlation between the Rh-positive blood type, Taxoplasmosis infection, and geography.
“But then, in 1937, came the discovery of the rhesus factor, more commonly known as Rh positive or Rh negative. Basques were found to have the highest incidence of Rh negative blood of any people in the world, significantly higher than the rest of Europe, even significantly higher than neighboring regions of France and Spain. Cro-Magnon theorists point out that other places known to have been occupied by Cro-Magnon man, such as the Atlas Mountains of Morocco and the Canary Islands, also have been found to have a high incidence of Rh negative.
Twenty-seven percent of Basques have O Rh negative blood. Rh negative blood in a pregnant woman can fatally poison a fetus that has positive blood. Since World War II, intervention techniques to save the fetus have been developed, but it is probable that throughout history, the rate of miscarriage and stillborn births among the Basques was extremely high, which may be one of the reasons they remained a small population on a limited amount of land while other populations, especially in Iberia, grew rapidly.”
“If the Rh factor played the same role in all races as it does in the Caucasian, then one would expect the incidence of erythroblastosis to correspond to the frequency of the Rh-negative blood type. This expectation has apparently been fulfilled in the Mongolian race, since erythroblastosisis extremely rare among these peoples.”
“Toxoplasma and reaction time: role of toxoplasmosis in the origin, preservation and geographical distribution of Rh blood group polymorphism”
Here we demonstrated for the first time that among Toxoplasma-free subjects the RhD-negative men had faster reaction times than Rh-positive subjects and showed that heterozygous men with both the RhD plus and RhD minus alleles were protected against prolongation of reaction times caused by infection with the common protozoan parasite Toxoplasma gondii.”
|Aug30-12, 08:06 PM||#16|
Saltation has made an odd comeback in the theory called "evolution by self organized complexity". The idea seems to be that there are "islands" of fitness on the parameter landscape that can only be crossed by big sudden changes. In this theory, small changes in phenotype aren't very viable. It is only by rather improbable leaps with huge phenotypic changes that any improvement can be seen. Self organized complexity contradicts the idea of gradualism.
I am not a fan of evolution by self organized complexity. It sounds like physics, but really isn't. I am a physicist so I have an educated bias on this. Evolution of self organized complexity hasn't really been applied in a quantitative manner to data. There have been interesting processes modeled by self organized complexity. However, biological evolution is not one of them.
I see saltation and gradualism as mostly about ontogeny, not phylogeny. One is making a hypothesis in these theories about how the mutation impacts the development of an organism. If the morphology of the organism allows large changes without killing the organism, then it is very likely that evolution occurs from saltation to saltation. However, if small changes in development are less lethal than large changes, then the accumulation of small mutations is more likely to influence evolution. Thus, the real discussion for these two theories should concentrate on how organisms develop (i.e., ontogeny).
Catastrophism and uniformitarianism actually concern how the nonbiological environment affects the evolution of an organism. No hypothesis is made on the type of mutation that is probably. Catastrophism is the idea that sudden changes in environment drive evolution. The environment changes in discontinuous steps. Uniformitarianism is the idea that the environment itself changes, if at all, in small steps. Uniformitarianisms assumes that the parameters of environment are effectively continuous in time.
Catastrophism and uniformitarianism are mechanisms that were developed to explain the fossil record. Both mechanisms appear in the theory called punctuated equilibrium.
You obviously think that catastrophism and saltation are the same. I doubt that catastrophism and saltation are even consistent. I don't think catastrophism and saltation have much to do with each other.
Take the extreme case of both being true. In physical sciences, one checks out the self consistency of a theory by taking the theory to extreme limits, because the extreme limits are simpler to analyze. Suppose that evolution only occurs as a series of saltations. Also suppose that evolution can only advance shortly after a huge catastrophe.
Evolution could only take place when a big catastrophe occurs simultaneously with a saltation. If they did not take place at the same time, then the species couldn't evolve. It would simply go extinct. Yet, the hypothesis is that large catastrophes and saltations are both extremely unlikely. So the probability of both occurring at the same time is small.
Catastrophism and saltation are not equivalent as shown by the fossil record. I think that the fossil record shows that catastrophes really influence evolution. However, I think the fossil record contradicts "saltationism". If saltations were really that common, then we would see fossil evidence for them even in the periods of equilibrium. However, we see evidence of large catastrophes. Very often, we see a sudden diversification of organisms after a catastrophe. Many species of organisms diversify at the same time.
The idea of saltations and catastrophes occurring simultaneously is rather difficult to believe. I find it hard to believe that viable saltations occurred to all these species at the same time. I can see one or two species having the correct saltation at the same time as a catastrophe.
Look at it this way. The evolutionary development of a new species of organism is perhaps very slow. However, the extinction of a species can happen overnight. These are really the two hypotheses of punctuated equilibrium. So a catastrophe that occurs in a single night can change the direction of evolution for an organism that evolves very slowly. This is consistent with the fossil record.
|Aug31-12, 05:36 PM||#17|
What is important for speciation is that there has to be a change in environment between two geographical areas. The difference in environment itself vsn generate a boundary between two populations so that they become two varieties.
Allopatric peciation is when two populations are separated by a very sharp geographical barrier resulting in the eventual divergence into separate species. Hypothetically, there is no overlap between the two geographical regions. The division of a geographical area into two regions separated by a sharp boundary is called vicariance.
Sympatric speciation is where two populations diverge when there is considerable overlap between the two geographical areas.
The type of evolution that you are probably imaging is called allopatric speciation.
Definition of vicariance
“Vicariance [from Latin vicarius, derived from vicis; to change, alternate, substitute: root has calendrical associations] is a process by which the geographical range of an individual taxon, or a whole biota, is split into discontinuous parts by the formation of a physical barrier to gene flow or dispersal.
Once a species has been split by vicariance into multiple populations with little to no genetic exchange, the populations begin to drift independently. Thus vicariance is a necessary precursor to allopatric speciation.”
Definition Allopatric speciation
“Allopatric speciation (from the ancient Greek allos, "other" + Greek patra, "fatherland") or geographic speciation is speciation that occurs when biological populations of the same species become vicariant — isolated from each other to an extent that prevents or interferes with genetic interchange.”
Here is an example of allopatric speciation.
“RAPID ALLOPATRIC SPECIATION IN LOGPERCH DARTERS (PERCIDAE: PERCINA)”
“Theory predicts that clades diversifying via sympatric speciation will exhibit high diversification rates. However, the expected rate of diversification in clades characterized by allopatric speciation is less clear. Previous studies have documented significantly higher speciation rates in freshwater fish clades diversifying via sympatric versus allopatric modes, leading to suggestions that the geographic pattern of speciation can be inferred solely from knowledge of the diversification rate. We tested this prediction using an example from darters, a clade of approximately 200 species of freshwater fishes endemic to eastern North America.”
Two populations often speciate even while there is considerable oberlap between the two geographical areas of occupation. I described an example when I described “Rh baby syndrome. However, there are a lot of other examples.
Definition of sympatric speciation
“Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that they occur together at least in some places.”
This link tabulates some of the cases of sympatric specieation.
“A collection of putative cases of sympatric speciation. Listing a particular
case study does not imply that we advocate its status as a case of sympatric speciation, merely that it has been claimed as such. We therefore distinguish between cases that have been generally (though perhaps not universally) accepted, those that seem like possible candidates given further information, and ones that appear to be unlikely or have been firmly rejected. The list of rejected cases is not exhaustive.”
“Evidence is growing that not only allopatric but also sympatric speciation can be important in the evolution of species. Sympatric speciation has most convincingly been demonstrated in laboratory experiments with bacteria, but field-based evidence is limited to a few cases. The recently discovered plethora of subterranean diving beetle species in isolated aquifers in the arid interior of Australia offers a unique opportunity to evaluate alternative modes of speciation. This naturally replicated evolutionary experiment started 10-5 million years ago, when climate change forced the surface species to occupy geographically isolated subterranean aquifers.”
Theory of sympatric speciation.
“Recent theory suggests that frequency-dependent disruptive selection in combination with assortative mating can lead to the establishment of reproductive isolation in sympatry. Here we explore how temporal variation in reproduction might simultaneously generate both disruptive selection and assortative mating, and result in sympatric speciation.”
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