Isolation refers in biology to the so-called reproductive isolation , this is the interruption of the gene flow between populations of the same species. These can then no longer produce fertile offspring with members of other populations, as the biological definition of a species requires. It is therefore an essential process in the formation of new species .
Isolation comes about in different ways:
- Geographic isolation is the spatial separation of populations of the same species
- Ecological isolation is the occupation of different ecological niches by populations of the same species in the same area
Some other factors that can cause isolation are discussed in evolutionary biology, but their meaning is not yet clear. Thus, assortative mating can possibly isolate different groups within a population from one another via sexual selection and thus also cause a split into several populations without these also having to be spatially isolated.
Geographical and ecological separation isolates different populations of a species from one another, which can subsequently develop into new species under certain circumstances. However, this isolation is initially reversible (reversible) when renewed contact is established. Complete reproductive isolation, in which the separation is maintained even with direct contact, is only the result of isolation mechanisms that have evolved later.
See also: Isolation Mechanisms
Reproductive isolation arises from mutations in the generative system of an organism causing an interruption of the gene flow between the organisms of a species.
This type of isolation is particularly important in organisms that reproduce sexually. This includes many animals, plants and protists , but also bacteria that can exchange genes through conjugation .
- Mechanical isolation : mechanical incompatibility of the sexual organs (" key and lock principle ")
- Temporal and spatial isolation : Changes in the timing of reproduction: mating , rut or flowering take place at different times of the day or year. In spatial isolation, the mating locations are in different places.
Ethological isolation : changes in social triggers and behaviors in courtship , copulation, and brood care
- African widow birds (Viduinae) as breeding parasites are set as nestlings to the song of their respective host parents (see social learning and tradition ). By this song they recognize their brood hosts and their mating partners during the breeding season. In this way, depending on the different hosts, the widow birds have different lineages, which are reproductively isolated from one another.
- The males of the Western Australian seahorses ( Hippocampus subelongatus ) always mate with females of similar body size. In this way, both sexes optimize their reproductive success. It is a disadvantage for a large female to choose a smaller partner, as she cannot accommodate all of her mature eggs in the brood pouch. For a larger male, the disadvantage would be that his brood pouch would not be used optimally. So if big with big and small with small are constantly mating, this leads to a split into two types in the long term. This hypothesis could be supported on the basis of genetic data.
- Gametic isolation : chemical incompatibility of sperm and egg .
Postzygotic or metagamous are mechanisms that allow mating or fertilization, but prevent them from being successful; H. the offspring are either not viable or (severely) disadvantaged.
- The hybrid organism dies before reaching sexual maturity.
- The hybrids do not produce any germ cells capable of fertilization. This occurs, for example, when the number of chromosomes in the paternal and maternal chromosome sets differ and problems arise with the reduction division meiosis .
- The offspring of hybrids are not viable or fertile. Example: Mules are created by crossing a donkey stallion and a horse mare . Female mules cannot produce offspring with horse or donkey stallions. Mules are created by crossing a donkey mare and a horse stallion. They are mostly sterile. The water frog Rana esculenta (hybrid between sea frog and small water frog ), on the other hand, gets its vitality from backcrossing with one of the two parent species.
Different populations of a species live in the same area, but use the resources in different ways, so they have created different ecological niches . This can split the population and ultimately lead to the formation of two separate species. The different niches of sub-populations in the same habitat only lead to the emergence of new species if the reproductive ability between them is restricted or interrupted by additional changes in characteristics.
Here two populations are separated from each other by geographical barriers. If mutations occur in the various populations that lead to reproductive isolation, new species have become established. This is called allopatric speciation .
Which barriers are effective depends on how the organisms spread. The oceans represent a barrier for flightless land animals, but not for the coconut palm . Other barriers can be mountains, rivers and deserts.
Mountain formation, continental drift and ice ages are changes that are slow to take place. The gene flow between neighboring populations is hardly restricted over long periods of time. Only when the barrier is large enough can the populations develop differently.
The current geographical distribution of closely related species or the occurrence of endemic species as well as the knowledge of geological and climatological changes and the possibility of dating them allow the determination of the time periods in which new species can emerge from a parent species.
- Continental drift : Marsupials , Madagascar fauna
- Ice Age : closely related high alpine plant species in the Alps and in Scandinavia , which emerged from a stock population during the Ice Age and moved with the glaciers that receded in the warm period to the south on the one hand and to the north as organisms adapted to cold climatic conditions that lie between the Alps and Scandinavia Can't overcome the "heat barrier" at the moment.
Since geographical isolation is not a mandatory requirement for reproductive isolation, it does not always have to be accompanied by clear morphological changes:
In the USA there are sister species of Drosophila that are morphologically indistinguishable and cannot be crossed. In Hawaii , Drosophila species can be found that are geographically separated from one another, differ significantly in morphology, and yet can be crossed with one another.
This is one of the reasons why family trees based only on the interpretation of the morphological findings are ambiguous and are discussed. However, through additional investigations of the corresponding recent species, the possibilities of different pedigrees can be limited.