I'm not sure your observation, "Most of them are different..." describes why they would be difficult to spawn. Genders are different. A really big male might  run a small female ragged, but within a certain framework size, finage or color have little bearing on whether a Poeciliid (one of the popular livebearers and their relatives) can spawn with another.

In some species there are behavior cues which encourage one fish to keep company with another or to flee the scene. That works in the wild - i.e. a Mexican river system with several, sometimes closely related species. Different fish may also prefer different sub-habitats within that hydrological system.

In an aquarium that may not assume the same significance where fish are "stuck" with one another. Sooner or later a persistent male may succeed where he wouldn't in the wild.

Another real determining factor is whether a specific species' gonopodium (the modified anal fin) will even fit the vent of the female of another species. That is a key to defining whether a Poeciliid is a separate a species or not. If you Google gonopodium in an image search you will get a few close up photos of the fertilization organ of different Poeciliids. There indeed are differences. John Dawes' book also bears that out.

Without having made a study of this myself, it is still reasonable to assume that hobby hybrid types in the Xiphophorus group (the hobby swordtail/variatus/platys group) and mollies probably have gonopodia, which are somewhat intermediate between the original wild species.

It may also be that there would be a little local variation between populations of a widely distributed species. (Part of the to-do in the guppy/Endler's discussion, as to whether they are the same species or not, is how very similar the male organs are between those two groups of livebearers.)

Furthermore there may be some genetic drift within a group of fish kept in the hobby. In 1989, Noboru Iwasaki in his "Guppies: Fancy Strains and How to Produce Them" noted that there were differences in the gonodpodia of Japanese raised guppies and "imported guppies". I'm assuming the imported guppies he referred to were imports from Singapore, Hong Kong and Europe - especially Germany.

It sounds funny referring to the shape of a caudal (tail) or dorsal (upper) fin as a superficial difference between fishes. But those fins, other than impressing a female in a prenuptual dance, don't have much of a role in procreation.

They will usually breed quite successfully with one another. You didn't mention colors and I  couldn't off hand predict what would appear if this color was bred to that color. I'm sure there are aquarists who can. People who have been in the Florida fish farming industry like author Ed Taylor certainly would be pretty good at that.

The Sailfin is a contribution of a couple of wild species - P. latipinna from the southern U.S. and perhaps Mexico's P. velifera. The lyretail pattern is a hobby-generated sport which has been around at least 45-50 years. Black mollies were generated by breeders somewhere around 80 years ago. (Black mollies are found in a number of places around the world. Those are descendents of hobby fish which have been released though. Remarkably they have survived predation.)

I think one species has been left out of the following list, but ancestors of our hobby mollies would include Poecillia velifera, P. latipinna, P. sphenops and P. mexicana - the last two of whom have evidentially been confused over the years. Messing around with them is ok, so long as their offspring are not released into the wild.

There is more info to be had if you were to Google {Guppylog mollies} {Guppylog molly species} and the various species names mentioned above or hobby names such as you mentioned or color forms - black molly, dalmatian molly, silver molly, golden sailfin molly, chocolate molly... Searching for balloon belly mollies will plug you in with an oriental tendency to develop egg-shaped fishes and sometimes spirited controversy among aquarists over when it is ok to reproduce "sports" of the original species with skeletal alterations.

The various national livebearer groups, their web sites and their e-mailing lists will also have some fascinating info from time to time. Sometimes a little link hopping is needed. Perhaps start at the links list at http://livebearers.org/ALAPublic/Default.htm

Here are a couple of Guppylog threads which may be of interest too.

Brood Records:
http://www.guppylog.com/story/2005/1/3/2168/70736

A Little on Crosses:
http://www.guppylog.com/story/2003/10/15/81621/667

Browsing the Guppylog Immediate Help Links may be useful too.

Hope this is of use to you. Please let us know what transpires if you cross those molly strains.

Thanks and all the best!
uncle scott

Most hobby mollies are a jumble of heritages. It (none / 0) (#1)
by unclescott (unclescott at prodigy.net) on Mon Aug 8th, 2005 at 13:40:49 CST
(User Info) Poster's IP: 68.253.196.52 [Edit User]

really is incorrect to speak of a specific hobby molly as a particular species.

They will usually breed quite successfully with one another. You didn't mention colors and I  couldn't off hand predict what would appear if this color was bred to that color. I'm sure there are aquarists who can. People who have been in the Florida fish farming industry like author Ed Taylor certainly would be pretty good at that.

The Sailfin is a contribution of a couple of wild species - P. latipinna from the southern U.S. and perhaps Mexico's P. velifera. The lyretail pattern is a hobby-generated sport which has been around at least 45-50 years. Black mollies were generated by breeders somewhere around 80 years ago. (Black mollies are found in a number of places around the world. Those are descendents of hobby fish which have been released though.)

I think one species has been left out of the following list, but ancestors of our hobby mollies would include Poecillia velifera, P. latipinna, P. sphenops and P. mexicana - the last two of whom have evidentially been confused over the years. Messing around with them is ok, so long as their offspring are not released into the wild.

There is more info to be had if you were to Google {Guppylog mollies} {Guppylog molly species} and the various species names mentioned above or hobby names such as you mentioned or color forms - black molly, dalmatian molly, silver molly, golden sailfin molly, chocolate molly... Searching for balloon belly mollies will plug you in with an oriental tendency to develop egg-shaped fishes and sometimes spirited controversy among aquarists over when it is ok to reproduce "sports" of the original species with skeletal alterations.

The various national livebearer groups, their web sites and their e-mailing lists will also have some fascinating info from time to time. Sometimes a little link hopping is needed. Perhaps start at the links list at http://livebearers.org/ALAPublic/Default.htm

Here are a couple of Guppylog threads which may be of interest too.

Brood Records:
http://www.guppylog.com/story/2005/1/3/2168/70736

A Little on Crosses:
http://www.guppylog.com/story/2003/10/15/81621/667

Browsing the Guppylog Immediate Help Links may be useful too.

Hope this is of use to you. Please let us know what transpires if you cross those molly strains.