The theory of isolation is a post Darwinian theory put forth by Moritz Wagner. Wagner believed that Darwin was not able to explain the concept of speciation completely.
Wagner stated that there is separation of the same species in the population due to several barriers like geographical barriers like waterbody, mountain, continents split, etc which prevents interbreeding in the individuals because there is absence of genetic exchange between two or more groups. In order to develop genetic diversity which is followed by the origin of new species it is important that the physical separation of two or more groups takes place in a population.
The populations that are geographically isolated are exposed to several environmental factors leading to mutation in the populations. Mutations lead to variations and the variations depending on whether they are deleterious or beneficial are further selected. Only the beneficial or the harmless mutations are processed by natural selection. When the two or more groups are isolated the chance of an individual to reproduce with an individual of another group comes to zero. Every group adapts and changes as a result of its surroundings, which might result in certain mutations and variations. Just one group can have these mutations and variations; they cannot exist in other groups. The more isolated the groups are, the more they diverge from one another and become a new species.
Geographic isolation can lead to genetically divergent groups due to mutation and variation in accordance to the environment. As they have developed differently, the gene pools of these genetically distinct populations cannot be mixed. Due to the phenotypic differences that result in reproductive isolation, it is difficult for members of these two groups to interbreed if they come together.
Examples of isolation
- Wagner’s idea was first studied on the flightless beetles Pimelia and Melasoma. This population of beetles was divided into certain groups wherein they were confined to a certain geographical area. The river served as a geographic barrier, dividing this population of beetles into distinct groups that were each restricted to a particular region. After crossing the river a different species which is closely related appears.
- An illustration of reproductive isolation is seen in Darwin’s finches. The groups had unique gene pools and were geographically separated. Owing to their geographical isolation, it was challenging for birds to fly across islands, and even if they could, the genetic makeup, behavioural traits, and mating rituals of the various species prevented interbreeding.
- The reticulated giraffe also known as the Somali giraffe and the common giraffe are the two groups that came from the same population. The two groups were separated because of the Tana river in Kenya which led to the isolation of several generations along with the desired mutations eventually forming into two different species of giraffes.
- Australia’s kangaroos and koalas have been isolated from the outside world for many generations. These species are distinct from others found elsewhere in the globe as a result of isolation.
According to Moritz Wagner, a crucial element in speciation is a group’s separation from its original population.
Darwinists including Alfred Russel Wallace, August Weismann, Ernst Haeckel, and Charles Darwin criticised Wagner’s theory of isolation. Darwin’s Origin of Species contains the majority of Wagner’s views on geographic isolation, and Darwin even stated that the majority of Wagner’s arguments were not well substantiated. By emphasising geographic isolation in the emergence of new species, Wagner went on to fully disregard the significance of natural selection.
Geographic isolation is not the only process that causes speciation; natural selection also plays a significant role.
Sulloway, F. J. (1979). Geographic isolation in Darwin’s thinking: the vicissitudes of a crucial idea. Studies in history of biology, 3, 23-65.
Weissman, C. (2010). The origins of species: the debate between August Weismann and Moritz Wagner. Journal of the History of Biology, 727-766.New England Complex Systems Institute.