This is called anagenesis: species by changing, or 'phyletic evolution'.
Darwin thought most species arose directly from pre-existing species.
In turn, that means reproductive isolation is not the best definition of a species, and speciation does not require allopatry (species to be reproductively separated).
an incipient species will only [arise] when a few individuals transgress the limiting borders of their range... without a long continued separation of the colonists from the other members of their species".
Another term for geographic speciation is allopatric speciation. Volcanic islands are formed with no life, and all life has to arrive carried by wind or water.
Although the reality is much more complicated, Presumed evolution around Himalayas. For unknown reasons many asexual organisms are polyploid.
One example of evolution at work is the case of the hawthorn fly, Rhagoletis pomonella, which appears to be undergoing sympatric speciation.
Wagner's early career was as a geographer, and he published a number of geographical books about North Africa, the Middle East, and Tropical America.
He was also a keen naturalist and collector, and it is for this work he is best known among biologists.
Broadly speaking, the gulls can interbreed to some extent with their neighbours, except at the two ends of the ring. Many of the existing plant and most animal species have apparently undergone polyploidization in their evolutionary history.
The Lesser Black-backed Gulls and Herring Gulls are sufficiently different that they do not normally hybridize; thus the group of gulls forms a continuum except where the two lineages meet in Europe. Reproduction of successful polyploid species is sometimes asexual, by parthenogenesis.
A classic example of ring species is the Larus gulls' circumpolar species ring. Polyploidy has caused many rapid speciation events because offspring of, for example, tetraploid x diploid matings often result in triploid sterile progeny.