Three subspecies are currently recognized by the IUCN:
G. a. aculeatus is found in most of the species range, and is the subspecies most strictly termed the three-spined stickleback; its common name in Britain is the tiddler, although "tittlebat" is also sometimes used.

G. a. williamsoni, the unarmored threespine stickleback, is found only in North America; its recognised range is southern California, though isolated reports have been made of it occurring in British Columbia and Mexico;
G. a. santaeannae, the Santa Ana stickleback, is also restricted to North America.

These subspecies actually represent three examples from the enormous range of morphological variation present within three-spined sticklebacks.
These fall into two rough categories, the anadromous and the freshwater forms.

The anadromous form spends most of its adult life eating plankton and fish in the sea, and returns to freshwater to breed.
The adult fish are typically between 6 and 10 cm long, and have 30 to 40 lateral armour plates along their sides.
They also have long dorsal and pelvic spines.
The anadromous form is morphologically similar all around the Northern Hemisphere, such that anadromous fish from the Baltic, the Atlantic and the Pacific all resemble each other quite closely.

Three-spined stickleback populations are also found in freshwater lakes and streams.
These populations were probably formed when anadromous fish started spending their entire lifecycle in fresh water, and thus evolved to live there all year round.
Freshwater populations are extremely morphologically diverse, to the extent that many observers (and some taxonomists) would describe a new subspecies of three-spined stickleback in almost every lake in the Northern Hemisphere.
One consistent difference between freshwater populations and their anadromous ancestors is the amount of body armour, as the majority of freshwater fish only have between none and 12 lateral armour plates, and shorter dorsal and pelvic spines.
However, also large morphological differences occur between lakes.
One major axis of variation is between populations found in deep, steep-sided lakes and those in small, shallow lakes.
The fish in the deep lakes typically feed in the surface waters on plankton, and often have large eyes, with short, slim bodies and upturned jaws.
Some researchers refer to this as the limnetic form.
Fish from shallow lakes feed mainly on the lake bed, and are often long and heavy bodied with relatively horizontal jaws and small eyes.
These populations are referred to as the benthic form.

Since each watershed was probably colonised separately by anadromous sticklebacks, morphologically similar populations in different watersheds or on different continents are widely believed to have evolved independently.
A unique population is found in the meromictic Pink Lake in Gatineau Park, Quebec.

One aspect of this morphological variation is that a number of lakes contain both a limnetic and a benthic type, and these do not interbreed with each other.
Evolutionary biologists often define species as populations that do not interbreed with each other (the biological species concept), thus the benthics and limnetics within each lake would constitute separate species.
These species pairs are an excellent example of how adaptation to different environments (in this case feeding in the surface waters or on the lake bed) can generate new species.
This process has come to be termed ecological speciation.
This type of species pair is found in British Columbia.
The lakes themselves only contain three-spined sticklebacks and cutthroat trout, and all are on islands.
Tragically, the pair in Hadley Lake on Lasqueti Island was destroyed in the mid-1980s by the introduction of a predatory catfish, and the pair in Enos Lake on Vancouver Island has started to interbreed and are no longer two distinct species.
The two remaining pairs are on Texada Island, in Paxton Lake and Priest Lake, and they are listed as Endangered in the Canadian Species at Risk Act.

Other species pairs which consist of a well-armored mar
Three subspecies are currently recognized by the IUCN:
G. a. aculeatus is found in most of the species range, and is the subspecies most strictly termed the three-spined stickleback; its common name in Britain is the tiddler, although "tittlebat" is also sometimes used.
G. a. williamsoni, the unarmored threespine stickleback, is found only in North America; its recognised range is southern California, though isolated reports have been made of it occurring in British Columbia and Mexico;
G. a. santaeannae, the Santa Ana stickleback, is also restricted to North America.
These subspecies actually represent three examples from the enormous range of morphological variation present within three-spined sticklebacks. These fall into two rough categories, the anadromous and the freshwater forms.
The anadromous form spends most of its adult life eating plankton and fish in the sea, and returns to freshwater to breed. The adult fish are typically between 6 and 10 cm long, and have 30 to 40 lateral armour plates along their sides. They also have long dorsal and pelvic spines. The anadromous form is morphologically similar all around the Northern Hemisphere, such that anadromous fish from the Baltic, the Atlantic and the Pacific all resemble each other quite closely.
Three-spined stickleback populations are also found in freshwater lakes and streams. These populations were probably formed when anadromous fish started spending their entire lifecycle in fresh water, and thus evolved to live there all year round. Freshwater populations are extremely morphologically diverse, to the extent that many observers (and some taxonomists) would describe a new subspecies of three-spined stickleback in almost every lake in the Northern Hemisphere. One consistent difference between freshwater populations and their anadromous ancestors is the amount of body armour, as the majority of freshwater fish only have between none and 12 lateral armour plates, and shorter dorsal and pelvic spines. However, also large morphological differences occur between lakes. One major axis of variation is between populations found in deep, steep-sided lakes and those in small, shallow lakes. The fish in the deep lakes typically feed in the surface waters on plankton, and often have large eyes, with short, slim bodies and upturned jaws. Some researchers refer to this as the limnetic form. Fish from shallow lakes feed mainly on the lake bed, and are often long and heavy bodied with relatively horizontal jaws and small eyes. These populations are referred to as the benthic form.
Since each watershed was probably colonised separately by anadromous sticklebacks, morphologically similar populations in different watersheds or on different continents are widely believed to have evolved independently. A unique population is found in the meromictic Pink Lake in Gatineau Park, Quebec.
One aspect of this morphological variation is that a number of lakes contain both a limnetic and a benthic type, and these do not interbreed with each other. Evolutionary biologists often define species as populations that do not interbreed with each other (the biological species concept), thus the benthics and limnetics within each lake would constitute separate species. These species pairs are an excellent example of how adaptation to different environments (in this case feeding in the surface waters or on the lake bed) can generate new species. This process has come to be termed ecological speciation. This type of species pair is found in British Columbia. The lakes themselves only contain three-spined sticklebacks and cutthroat trout, and all are on islands. Tragically, the pair in Hadley Lake on Lasqueti Island was destroyed in the mid-1980s by the introduction of a predatory catfish, and the pair in Enos Lake on Vancouver Island has started to interbreed and are no longer two distinct species. The two remaining pairs are on Texada Island, in Paxton Lake and Priest Lake, and they are listed as Endangered in the Canadian Species at Risk Act.
Other species pairs which consist of a well-armored mar
Three-spined stickleback