Population Structure Elements of Population Evolution Reproductive isolation (homing to natal sites) Differential natural and sexual selection Heritable traits Results Genetically determined population specific variation Challenges To identify the ecological basis for the variation and the implications for conservation of the populations
Population structure and local adaptation Variation in genetic attributes that are neutral to selection are useful for determining current isolation or gene flow, and post-glacial history Variation in traits subject to selection are useful for studying evolution, though the environment also affects trait values
Divergence of allele frequencies over many generations 0.85 0.5
B. C. pink salmon protein polymorphisms Common Ancestor Odd year Even year South Coast Quinsam North Coast Babine Queen Charlotte Island North Coast Babine South Coast Quinsam Beacham et al. (1988)
Relationship between geographical and genetic distances 0.25 0.20 coastal cutthroat FST 0.15 chinook Atlantic 0.10 sockeye 0.05 steelhead chum pink 0 Hendry et al. 2004 0 1000 2000 3000 4000 5000 6000 7000 Maximum distance (km)
Variation in phenotypic traits We can observe differences among populations within species in virtually every phenotypic trait that can be measured, including but not limited to size, shape, color, timing of migration and breeding, egg size, etc.
Izaak Walton, The Compleat Angler, 1653, on population-level variation among salmonids: there are several kinds of [brown] Trouts; but these several kinds are not considered but by very few men, for they go under the general name of Trouts: just as Pigeons do in most places; though it is certain there are tame and wild Pigeons: and of the tame, there be Helmits and Runts, and Carriers and Cropers, and indeed too many to name. And t is so with many kinds of fish, and of Trouts especially, which differ in their bigness, and shape, and spots, and color. P. 70
Western Alaska rainbow trout
Cutthroat trout
Local adaptations These phenotypic traits that are under some degree of genetic control They are often assumed to result from local regimes of selection, and referred to as local adaptations. However, the are also affected by environmental conditions in many cases.
Criteria for proving local adaptation (Taylor 1991) 1. The trait being investigated must have a genetic basis 2. Different phenotypes must have different fitness 3. A mechanism of selection should be demonstrated
Relationship between spawning date and latitude: North American sockeye salmon Median spawning date 28-Dec 28-Nov 29-Oct 29-Sep 30-Aug 31-Jul 1-Jul 45 50 55 60 65 Hodgson and Quinn 2002 Latitude (N) of the spawning site
Temperature o C Spawning times and mean incubation temperatures of nine Fraser River sockeye salmon populations 9 8 7 6 5 4 3 2 1 0 Brannon 1987 Scotch Chilko Stellako Forfar Nadina Aug. Sept. Oct. Nov. Month Adams Cultus Weaver Harrison
Relationship between gravel size and egg size for sockeye salmon populations in the Iliamna and Wood River systems, Alaska Adjusted egg weight (mg) 130 120 110 100 90 80 70 1 10 100 Geometric mean particle size (mm) Quinn et al. (1995), and additional unpublished data
Genetic components of fry migration Incubation Area Chilko River Incubation Area Upper Pitt River Flow Fry distribution Flow Fry distribution Pitt Lake Chilko Lake Brannon 1972
Rheotaxis of fry tested in the dark Stock Upstream (%) Downstream (%) Neither (%) Chilko 87.4 6.7 5.9 7-Mile 6.3 90.7 3.0 Brannon 1972: IPSFC Bulletin 21
Disease resistance Summer steelhead and Ceratomyxa shasta Held for 30 days in Columbia River water and then 133 days in pathogen free water Region Population % mortality Oregon coast Siletz 100 Lower Columbia Skamania 9 Clearwater 9 Deschutes 2 Buchanan et al. 1983. TAFS 112: 541-543
Survival of coho salmon from the Kitimat and Big Qualicum (BQ) rivers and their hybrids and sockeye from the Fulton River, Weaver Creek and their hybrids after experimental exposure to Cryptobia salmositica Experimental populations Species vulnerable Hybrids resistant coho Kitimat x Kitimat BQ x Kitimat Kitimat x BQ BQ x BQ % survival 3 22 38 95 sockeye Fulton x Fulton Fulton x Weaver Weaver x Fulton Weaver x Weaver % survival 43 66 74 88 Bower et al. 1995
Genetic control of ocean distribution Columbia River coho salmon Recovery Locations Source WA OR CA CR Toutle (S) 26.1 43.4 8.5 21.0 Cowlitz (N) 50.6 21.0 0.6 27.8 Washington Department of Fish and Wildlife data
Relationship between the depth of the spawning site and the body depth of male sockeye salmon from Iliamna and Wood River Male body depth (mm) Quinn et al. (2001) 200 180 160 140 120 100 1 10 100 1000 Spawning site depth (cm)
Isolated populations can evolve as a result of various forms of selection, but how do we explain highly differentiated populations in sympatry? 1) Allopatric divergence, followed by contact through secondary colonization 2) Sympatric divergence 3) Or maybe both?
Wisconsin glacial period, about 50,000 to 10,000 years ago McPhail and Lindsey 1970
Colonization from the Pacific Refuge
Colonization from the Bering Refuge
Populations, long isolated in different refuges, may come into contact later, as their ranges expand. Is this why odd and even year pink salmon are so different? Even year pinks are smaller and spawn earlier than odd year pinks in the same river. Beacham and Murray 1985 Males POH length in mm Females River Even Odd Even Odd Glendale 363 403 390 407 Quinsam 389 428 387 403 Puntledge 375 392 377 402
Hypothesized evolution of sockeye, kokanee, and residuals Lakes were colonized after glacial retreat by anadromous sockeye salmon. In rare cases a male did not go to sea. These residuals mated and likely passed to their offspring the tendency not to migrate. Female residuals were not courted by anadromous males, only by residual males. Thus assortative mating began the process of sympatric divergence because the growing conditions (lake vs ocean) caused such differences in size. Later, this pre-zygotic isolation was later reinforced by postzygotic isolation as hybrids of the two forms were less fit than either parental form. Thus kokanee (freshwater progeny of freshwater parents) evolved from residuals (freshwater progeny of anadromous parents).
Lough Melvin, Ireland, displays both sympatric divergence and also allopatric origins for brown trout forms. Ferox trout, Lough Corrib (and Melvin, etc.) Sonaghan trout, Lough Melvin Gillaroo, Lough Melvin