Ecological Speciation a sticklebacks study Andrew Yul Leung

Ecological Speciation: a sticklebacks study Andrew Yul Leung Biol 402

Three-spined Stickleback Gasterosteus aculeatus • • Kingdom: Animalia Phylum: Chordata Class: Actinopterygii Family: Gasterosteidae

Three-spined Stickleback Gasterosteus aculeatus - Related to pipefish and seahorses - Lack scales - Some have bony armor plates - Distinguished by the presence of strong isolated spines in the dorsal fin

Three-spined Stickleback Gasterosteus aculeatus has both Anadromous (migrating from salt water to fresh water to spawn) and Stream-resident (fresh water) ecotypes. Ecotype = a subspecies that is adapted to a particular set of environmental conditions

Three-spined Stickleback Gasterosteus aculeatus - Widely distributed and is seen in waters in Europe, North America, United Kingdom and Japan - Anadromous and Stream-resident stickleback breed sympatrically, therefore showing little introgression exhibit pre-mating isolation

Three-spined Stickleback Gasterosteus aculeatus - Generalist: seen in a variety of lake depths - Benthic: inhabits the bottom, and specializes on littoral invertebrate prey - Limnetic: inhabits the open water, focusing on pelagic zooplankton

Three-spined Stickleback Gasterosteus aculeatus Generalist Benthic Limnetic - Evolved multiple times via parallel diversification - Reproductively isolated (low levels of gene flow)

Diversification • • Effects ecosystems by: - changing prey community structure - changing total primary production - changing the nature of dissolved organic materials which regulate spectral properties of light transmission

Scientific Papers Evidence for ecology’s role in speciation (Mc. Kinnon, J. S. et al. 2004) Evolutionary diversification in stickleback affects ecosystem functioning (Harmon, L. J. et al. 2009)

Evidence for ecology’s role in speciation Darwin’s Finches: an example of adaptive radiation/speciation

Evidence for ecology’s role in speciation Q: What is the role of natural selection in isolation leading to speciation? Q: What phenotypic traits mediate isolation? • Selective environment (ecology) leads to isolation via parallel speciation or assortive mating

Evidence for ecology’s role in speciation Anadromous vs. Stream-resident Sticklebacks - Large size seen in ancestral anadromous ecotype may be an adaptation for relatively long migrations - Stream-resident ecotypes have undergone evolution repeatedly - The difference in size between the two ecotypes is substantially heritable

Evidence for ecology’s role in speciation • Methods: samples taken from Alaska, British Columbia, Iceland, Scotland, Norway & Japan - Close relationship among geographically adjacent populations , rather than those of the same ecotype - Origins of freshwater sticklebacks originated from anadromous ancestors in similar geographic regions - Stream-resident stickleback ecotypes have evolved repeatedly

Evidence for ecology’s role in speciation 80 70 60 50 mean length 40 (mm) 30 20 10 0 Anadromous Female Stream Female Stickleback ecotype - Anadromous sticklebacks in study have a larger average body size than stream-resident sticklebacks (2 -tailed t-test, n = 10, P = 0. 001*)

Evidence for ecology’s role in speciation Mating compatibility: Grey = different ecotype combinations Black = same ecotype combinations

Evidence for ecology’s role in speciation - Isolation (low mating compatibility) in nearby streams and corresponding anadromous pairs - Reproductive isolation from individuals from different regions - Female stream and anadromous sticklebacks preferred males of their own ecotype (strong signature of divergent selection) - Reproductive isolation is brought about by adaptation to different environments

Evidence for ecology’s role in speciation Regression lines for mating compatibility - Decline of mating compatibility with increasing difference in body size (P < 0. 001*)

Evidence for ecology’s role in speciation - Levels of reproductive isolation are due to body size - Probability of smaller (stream sticklebacks) mating with larger (anadromous sticklebacks) is negatively related to size difference - Comparing Body size and Ecotype match, only size difference is significant in reproductive isolation (but ecotype may make a 2 ndary contribution)

Evidence for ecology’s role in speciation Methods: - raised large and small females of each ecotype and tested mating with B. C. males Found that different ecotype combinations were compatible if size difference was small - small preference for males of their own ecotype

Evidence for ecology’s role in speciation To contrast body size, divergence in red coloration was not significant to reproductive isolation (P = 0. 76)

Evolutionary diversification in stickleback affects ecosystem functioning Q: What is the effect of evolutionary diversification on the ecosystem? Q: Does evolutionary diversification of stickleback populations affect ecosystem function? (previous studies show sticklebacks can cause trophic cascades in pond ecosystems) - Predictors of ecosystem level processes are determined by the rate of productivity, biomass sequestration and decomposition of a number of species

Evolutionary diversification in stickleback affects ecosystem functioning Ecological factors influence adaptive radiation, which has a profound effect on ecosytems (Evidence for ecology’s role in speciation Mc. Kinnon, J. S. et al. ) Adaptive radiation is the diversification of ancestral groups into a variety of related forms Specialized to fit different habitats Generalist Benthic Limnetic

Evolutionary diversification in stickleback affects ecosystem functioning Ecological theory: - Diversification affects ecosystem aspects (prey diversity, energy flow, food chain length…) Diversification leads to greater predator diversity increasing top-down control over a larger range of species This increases cascading affects of predators on lower trophic levels, thus modifying environmental conditions in ecosystems and shaping selective pressures in other species

Evolutionary diversification in stickleback affects ecosystem functioning Methods: - 4 mesocosm treatments: Generalist (G), Limnetic (L), Benthic (B) and Species pair (L, B) - G & L, B tests overall evolutionary diversification - L, B & either B or L tests effect of speciation

Evolutionary diversification in stickleback affects ecosystem functioning Differences in community structure and productivity among treatments a) Non-metric Multi. Dimensional Scaling for pelagic vs. invertebrate biomass b) Non-metric Multi. Dimensional Scaling for benthic vs. invertebrate biomass Hypothesis: 1) Trophic cascade theory: Strongest cascades are seen in preferred foraging habitats, Increasing primary productivity with increased predatory specialization 2) Distinct types of sticklebacks differ in their engineering effects on ecosystems

Evolutionary diversification in stickleback affects ecosystem functioning - Stickleback diversification affects community composition of organisms at lower trophic levels - G vs L, B Species pair showed strong effects of sticklebacks on pelagic prey communities (P = 0. 02*) as generalists transitioned to limnetic and benthic forms - Pelagic prey communities with 2 specialists were significantly different compared with only 1 specialist, Benthic (P = 0. 02*)

Evolutionary diversification in stickleback affects ecosystem functioning Amount of chlorophyll: c) Phytoplankton in pelagic d) Periphyton in benthic Treatments differed in gross primary productivity, net primary productivity, respiration and algal biomass Generalist treatment had most algae which implies that neither specialization or diversification lead to stronger trophic cascades

Evolutionary diversification in stickleback affects ecosystem functioning Sticklebacks strongly affect the composition of DOC pool and thus affect the physical light environment Variation in DOC composition was significant (DOC produced by algae vs. leaf litter), although treatments had similar total DOC Generalist treatment: DOC was primarily from algae Benthic treatment: DOC was primarily from leaf litter

Conclusion Speciation may arise from ecological difference and divergent selection on a small number of phenotypic traits Ecology can create mating incompatibilities between stickleback ecotypes based on one trait, ie. Body size Both specialization, from generalist to limnetic and benthic species, and speciation of predators can affect ecosystem functions such as DOC composition (physical light) Short periods of adaptive radiation can have the potential to transform structure and function of an ecosystem

~ Fin