184 Scholarship Question: Stickleback Speciation

Speciation in three-spined sticklebacks

The three-spine stickleback (Gasterosteus aculeatus) are freshwater fish found above 30° north in marine and freshwater environments. It has been the subject of many scientific studies for many reasons including its breeding behaviour, varied morphology, and population genetics. The three spined-stickleback may be anadromous, migrating from the sea to lakes or they may be entirely restricted to fresh water.

In British Columbia (Canada) there are a five lakes that contain two forms of stickleback (known as stickleback species pairs). They remain in the lake throughout their life cycles. There is a limnetic form that lives in the shallows and feeds on free-swimming plankton and a benthic form that lives deeper in the lake and feeds on prey in the lake sediment.

Studies of these two forms from the five different British Columbia lakes have found the following:

` Mitochondrial DNA analysis shows that the benthic and limnetic forms in each lake are more closely related to each other than to sticklebacks in other lakes. ` The two forms never mate with each other. ` The benthic forms in one lake will spawn successfully with the benthic forms in another lake, and the limnetic forms in one lake will spawn successfully with the limnetic forms in another lake. ` A benthic form will display to benthic forms of stickleback from Japan. ` Analysis of nuclear DNA shows the limnetic forms in all 5 lakes are more related to each other (and marine lineages) than to benthic forms. ` Sampling of the lakes over three years produced frequencies of three forms of stickleback:

Year Total fish in sample Frequency of benthics Frequency of limnetics Frequency of F2 hybrids

1 1057 0.50

2 962 3 994 0.50 0.49 0.48 0.49 0.49 0.019 0.010 0.014

` Experiments into the selection factors in the three-spined stickleback bred limnetic forms with benthic forms by artificially fertilising eggs from one form with sperm from another (and vice-versa), producing a population of hybrid fish. The fish were introduced into an artificial pond and left to interbreed. Measurements and observation of the second generation of hybrids were then taken. It was found that the second generation (F2) hybrids were sorting into benthic and limnetic niches. Those fish that were at the extremes of this sorting (they spent all their time either feeding on free swimming prey or bottom dwelling prey) grew faster than those that ate a mixture of prey. 1. Analyse the information presented. Justify whether the benthic and limnetic forms are separate species and establish a justified speciation history of the three-spined stickle back. Your justification should include:

• Evidence for and against speciation • Factors that influence speciation • A justified history of the formation of the benthic and limnetic forms