Answer to Question #304388 in Zoology for Vizra

Question

I need data for Assignment

Topic: Effectiveness and significance of cosmopolitan distribution

Answer 1

A species that is apparently cosmopolitan because it occurs in all oceans might in fact occupy only littoral zones, or only particular ranges of depths, or only estuaries, for example. Analogously, terrestrial species might be present only in forests, or mountainous regions, or sandy arid regions or the like. Such distributions might be patchy, or extended, but narrow. Factors of such a nature are taken widely for granted, so they seldom are mentioned explicitly in mentioning cosmopolitan distributions.                                                                                        

Cosmopolitanism of a particular species or variety should not be confused with cosmopolitanism of higher taxa. For example, the family Myrmeleontidae is cosmopolitan in the sense that every continent except Antarctica is home to some indigenous species within the Myrmeleontidae, but nonetheless no one species, nor even genus, of the Myrmeleontidae is cosmopolitan. Conversely, partly as a result of human introduction of unnatural apiculture to the New World, Apis mellifera probably is the only cosmopolitan member of its family; the rest of the family Apidae have modest distributions.

Even where a cosmopolitan population is recognised as a single species, such as indeed Apis mellifera, there generally will be variation between regional sub-populations. Such variation commonly is at the level of subspecies, varieties or morphs, whereas some variation is too slight or inconsistent for formal recognition.

For an example of subspecific variation, consider the so-called "African killer bee", which is the subspecies Apis mellifera scutellata, and the Cape bee, which is the subspecies Apis mellifera capensis; both of them are in the same cosmopolitan species Apis mellifera, but their ranges barely overlap.

Other cosmopolitan species, such as the osprey and house sparrow, present similar examples, but in yet other species there are less familiar complications: some migratory birds such as the Arctic tern occur from the Arctic to the Southern Ocean, but at any one season of the year they are likely to be largely in passage or concentrated at only one end of the range. Also, some such species breed only at one end of the range. Seen purely as an aspect of cosmopolitanism, such distributions could be seen as temporal, seasonal variations.

Other complications of cosmopolitanism on a planet too large for local populations to interbreed routinely with each other, lead to genetic effects such as ring species, for example in the Larus gulls. They also lead to the formation of clines such as in Drosophila