R"evolution" in Malawi tank or forbidden fruit

Our tanks, abounding with many different species from Lake Malawi, are sometimes places of unexpected choices of partners. Is it possible that an accidental descendant of two different individuals, is the beginning of a new, very dynamic way of evolution?

Great revolution and progress in the world of technical devices connected with our hobby, which with such a charm and easiness helps us to take care of our pets, causes that more and more frequently our tank imitating habitat from Victoria, Tanganyika or Malawi lakes, is also our first tank. No wonder that before we manage to gather necessary knowledge, our tank is a place of unexpected courtship and then some bastards appear.

Evolution - what is it?

Great African Lakes have been an object of scientific research for many years. Evolution that occurs in their waters has chosen a very specific and dynamic character. Before we start to talk about it, it is time we explained some very important words and ideas.

The idea of evolution is very often explained as a change in frequency of occurrence of alleles in a population. If we want to understand it, we should know first what alleles and population are. A population is a group of individuals of the same species living in the same area at the same time. A group is separated from similar groups with geographical or genetic barriers. Alleles decode different variants of the same trait - they are just the variants of the same gene. For example they can cause different eyes color. In somatic cells of all animals every gene is represented by two or more different or the same alleles. So, coming back to our considerations, we can talk about evolution when a frequency of occurrence of alleles changes. But why doest it happen? The answer to this question is not so easy, there can be a few explanation:

The idea of evolution is very often explained as a change in frequency of occurrence of alleles in a population. If we want to understand it, we should know first what alleles and population are. A population is a group of individuals of the same species living in the same area at the same time. A group is separated from similar groups with geographical or genetic barriers. Alleles decode different variants of the same trait - they are just the variants of the same gene. For example they can cause different eyes color. In somatic cells of all animals every gene is represented by two or more different or the same alleles. So, coming back to our considerations, we can talk about evolution when a frequency of occurrence of alleles changes. But why doest it happen? The answer to this question is not so easy, there can be a few explanation:

• A reason of such a situation can be a genetic drift. The population of progeny always differs slightly from its parental population because of genetic drift. The drift is caused by random changes in allele frequency in subsequent generations. The smaller the population, the faster the drift. Speaking more simply, it is an common chance and it is difficult to predict.
• If the number of individuals in a population changes suddenly and sizably, it is unlikely that the distribution of the alleles will remain the same after a flood, fire or an epidemic. In such a small population the gene pool can change appreciably due to the genetic drift. Such a situation happened about 10,000 years ago during the last ice age, when most of cheetahs died. Because of that, the contemporary cheetah population is genetically almost uniform. In 1999 in Malawi lake there was a terrible ecological disaster - because of poisoning the lake's water with algae, many thousand of cichlids died. Will it cause drastic changes of population? Who knows?
• When flow of genes takes place. Individuals moving from one population to another take their genes with them, changing the distribution of the alleles in both populations. Extensive migration causes the two populations to become genetically similar, which counteracts the results of genetic drift and natural selection.
• If a few organisms settle within a new territory, it is a great probability that they form a new population with different frequency of the alleles.

Every of these situations can cause a change in frequency of occurrence of alleles in a population, and can contribute to evolution. The problem is that not every such a change pushes evolution in the same way. Sometimes a change means troubles. Looking attentively at what happened to cheetahs we can quickly notice that their small genetic variability causes their low reproductive success and susceptibility of the young to diseases. It appears that the safest way of evolution it this one that is powered with natural adaptation. The way how the natural adaptation can have influence on an existence of a species is very easy to observe on the base of changes of behavior and morphological changes that appeared among Malawi cichlids. Just to satisfy hunger and to survive many species formed their own trophic adaptations. For example living in the lake predators, as Nimbochromis livingstonii and Dimidichromis compressiceps, just to be able to hunt other, smaller cichlids, formed by years of progress changes that can cause their future existence very successful. Nimbochromis livingstonii has an appearance of a lying down and rotting fish. It is able to lie down aside without any motion, waiting for an unaware victim. A strategy which it uses in order to hunt some food is an ambush. An ambush is also a hunting strategy of Dimidichromis compressiceps. This predator strongly compressed laterally, with an imperceptible, glittering, silvery color, as a place of its ambush has chosen long leaves of Valisneria. He hunts head-down in Vallisneria weed beds, keeping narrow body outline toward prey.

Two different species from Malawi Lake and two different way of hunting, but the strategy (ambush) and food the same.

What can happen if natural adaptations do not appear we can find out from one of the latest since hypothesis relating to dinosaurs extinction. After examining a bubble of air sunken in a piece of amber, it appeared that it contains approximately 30% of oxygen. Present analysis of air composition show that it consists of approximately 20% of oxygen. So it could be an environment, changing dramatically, that caused dinosaurs extinction, but it is of course one of hypothesis only. Their adaptation was not able to keep. A powerful engine which causes that in population increases a number of individuals with greater adaptation is a natural selection. Preceded with mutation and recombination of genes it permits to fix desirable features, delivering them to next generations. That was what happened to Nimbochromis livingstonii and to Dimidichromis compressiceps.

Natural selection and speciation

Natural selection, by favoring individuals with advantageous phenotypes, affects the composition of the population gene pool. Disadvantageous phenotypes, hence also the genotypes, are eliminated. Advantageous phenotypes, hence also the genotypes, are protected. Thus the number of disadvantageous alleles is reduced. They can even be completely eliminated. The number of advantageous alleles, on the other hand, increases, hence increasing the probability of their inheritance by progeny. Natural selection changes the composition of the gene pool in the direction consistent with the environmental requirements. There are three types of selection:

• Stabilizing selection favors the most common allele (and traits). Frequency of rare alleles, hence of extreme traits, does not increase.
• Directional selection increases the number of one type of alleles at the sacrifice of the others, thus shifting the probability distributions in a given direction.
• Disruptive selection is a type of the directional one, which acts in several directions. A population is divided into subgroups that are still connected. They can still reproduce, but the flow of genes is reduced.

A mechanism of natural selection is almost always the same. Individuals with an undesirable phenotype are simply those, which were not able to pass a test connected with an existence: climate and its changes, necessity of conquering food, resistance to diseases or ability in avoidance of danger, are eliminated. They are not able to grow up to an age of reproduction, they cannot find a partner or they have a small number of progeny while the individuals with a profitable phenotype have always a very large number of progeny. Such a situation we can observe also in our tanks, where more and more frequently we can find a new, albinotic variant of a very popular Ancistrus sp. If in the aquarium there are two females of that species and one of them has characteristic, dark color, that one will attract a male. Light, almost white color of body is not a very profitable one for this species. Individuals with such a color has a lot of trouble with hiding and avoiding dangers.

The rarest type of selection is the third one - disruptive selection. Let us imagine one of previously discussed species of predators from Malawi Lake. Imagine that it has a specific reason to choose as victims average sized cichlids from one of Mbuna species and it ignores too large or too small individuals. In such a situation a natural selection would cause very quickly that a population of that species would be divided into two subgroups - still connected.

However, let us return to alleles, which are the engine of all changes. If a change in frequency of occurrence of alleles in a population is connected with an isolation very often a new species appears. We call it speciation.

In the world of animals speciation is the most known way of evolution. There are many different species, which evolved from one parental generation. However, it happens sometimes that something unusual appears. It always takes a place, when two different individuals from two different species "decide" to cross. A fruit of such a breed crossing is a bastard. Such a way of evolution seems to be chosen very rarely by nature and it is almost always a man (vide: mule) who is its engine.

Bastards and a phenomenon of a first generation vigor

If we talk about breed crossing we should also talk about a phenomenon of first generation vigor. It causes that first generation after breed crossing is exceptional healthy and beautiful, it is also (talking about cichlids) unusually aggressive generation. However, those traits disappear with every following generation. In exceptionally rare situations, it is supported by natural selection, it becomes fixed and lasts with following generations that can be a beginning of a creation of a new species. Still, in most cases that phenomenon is a short-lived and bastards are sterile or they are rejected in a way of natural selection.

What does nature choose?

As it was told in the beginning, evolution in Malawi Lake has a very dynamic way and most species came into being during last 200 years. Clearly seen in many places of the lake condensation of cichlids causes that competition among individuals and among species is very intensive. Such a situation is very often a reason to accelerate speciation. That hypothesis can be confirm by Peter Smith's (University in Main, USA) researches. As he has noticed there are many distinct populations of typical Maylandia zebra in Malawi Lake. They are essentially identical, except for a single feature of a male reproductive coloration. All males of some of these populations possess a blue dorsal fin, while all males of other populations possess a red dorsal fin. Often a blue population will be within less than a kilometer from a red population. Peter Smith present an interesting question : Are populations that are geographically proximate more closely related to each other than distant populations, despite coloration? But there is also another one question: Is there any reason to such close related populations which order them not to breed cross? An answer is probably connected with natural selection or being more precise - with sexual selection which is just a special form of natural selection.

Both, allopatric speciation and sympatric speciation, are the mechanisms of such a way of evolution, that can separate populations with individuals which are best adapted to environment. This is the way of evolution the most frequently chosen by nature.

What does it happen in our tanks?

If in their natural environment our cichlids usually behave in a strict defined and almost puritan manner, why their behavior in an aquarium is so different? Why do they cross in spite of the fact that their living in the lake relatives try to avoid such a situation?

An answer to this question seems to be rather easy. One of the main reasons is just lack of knowledge and experience of many hobbyists, who enchanted with variety of species, of colors and interesting behavior, keep in very small tanks a few individuals only of many species. In this manner a natural area of geographical barrier is being disrupted. Speaking more simply in most cases species from a hobbyist tank do not live on the same area of the lake.

The next disaster is also an incorrect number of individuals of every species. Very often our tanks look like colorful meadows. There are usually, in proportion 1+3 (one male and three females), many different species. Such a situation looks a bit like that one after a great disaster, when frequency of occurrence of alleles decreases suddenly. It is impossible in that case to eliminate these alleles which are exceptionally unprofitable. Any way, what a male can a female choose if there is one only in the tank?

What is worse, buying young individuals, without characteristic sexual features, it very often appears that a breeder has in his or her tank males or females only. Such a situation, among ripe and ready to breed cichlids extorts almost a possibility of breed-crossing.

Let us repeat our first question: Is this possible that an accidental descendant of two different individuals, is the beginning of a new, very dynamic way of evolution? The answer is clear: absolutely not. It is a sad result of many mistakes, lack of knowledge and very often of unconcern of a hobbyist.

Difficult choice?

What can we do to avoid such a problem? Try to stop thinking about multicolor aquarium. A quite large (250 - 500 l.) tank with limited number of two, three species can be also beautiful and interesting and a dozen of cichlids of every species can perform not known, incredible behavior like building little communities, ritual mating and progeny protection.

If despite of these facts and knowledge we decide to set up a tank with many species, it can be useful to keep there species living on the same area in a natural environment. It can protect us from r"evolution" in our tank.

However, if despite that protection some bastards will appear one day, do not let them breed and do not give them to an inexperienced aquarist.

References

• Makala van den Bergen Ewa, "Okon, pielegnice i czlowiek", Wiedza i zycie nr 11/99
• Solomon, Berg, Martin, Villee, "Biologia", Warszawa, 1996.
• Jerzmanowski, Staron, Korczak; "Biologia", 1989.
• Smith P., "My research on the evolution of Cichlid fishes in Lake Malawi" http://nature.umesci.maine.edu/peter/cichlidresearch.php
• Fryer, G. 1959. The trophic interrelationships and ecology of some littoral communities of Lake Nyasa with especial reference to the fishes and a discussion of the evolution of a group of rock-frequenting Cichlidae. Proc. zool. Sot'. Lond.132:153-281.
• Goldschmidt Tijs, "Wymarzone jezioro Darwina - dramat w jeziorze Wiktorii", Warszawa, 1999

>Vocabulary:

• Alleles; decode different variants of the same trait. A hypothetical example: there could be three alleles of a gene encoding the color of an iris in a given population: allele decoding brown, blue and green.
• Gene; is a section of the double helix of DNA built from a certain number of nucleotide pairs. The nucleotides are arranged in a strictly defined sequence. A gene occupies a prescribed position in a chromosome - a locus. Variants of genes that occupy the same positions in homologous chromosomes are called alleles.
• Phenotype; describes the physical manifestation of that trait.
• Population; is a group of individuals of the same species living in the same area at the same time.
• Mutation; Genes are constantly being modified through a process called mutation - a change in the structure of the DNA in an individual's cells.
• Recombination; Sexual reproduction ensures that the genes in a population are rearranged in each generation, a process termed recombination.
• Speciation; is the process whereby new species are formed. Speciation can happen when a population is isolated geographically - it is called allopatric speciaton. Speciation can also occur in a population sharing the same area with other populations, but being isolated in some other way (reproductively, for example). This type of speciation, called sympatric seems quite common among plants.