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Species and Communities BIOLOGICAL SPECIES CONCEPT (MAYR 1970, POPULATIONS SPECIES AND EVOLUTION) Species What is a species? Population structure Evolution of geographic variation Gene flow and clines Assortative mating Local variation Geographic isolation Secondary contact and hybridization Ecology of speciation Geographic replacement Behavior and speciation Communities Factors shaping patterns of species diversity and abundance “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” PHYLOGENETIC SPECIES CONCEPT (CRACRAFT 1989, SPECIATION AND ITS ONTOLOGY) A species is the smallest aggregation of populations diagnosable by a unique combination of character states in comparable individuals. The goal of the phylogenetic species concept is to define indivisible taxa that can be used in cladistic analysis. Generalized model of speciation Speciation is driven by changes in the spatial structure of populations • Dispersal Genetic Divergence A’ A Isolation – Where young birds settle to reproduce B Isolating Mechanisms Colonization Reproductive Fragmentation Ecological • Philopatry – Tendency of birds to return to their birth place Behavioral Hybrids 1 Dispersal processes determine effective population size Geographic variation drives speciation Deme - a reproductively cohesive population Effective Population Size – number of individuals in a deme The more individuals interbreeding the higher the effective population size • Geographic variation in color and size is characteristic of about 1/3 of American birds Small demes evolve faster Effective population size decreases with dispersal distance Other factors that decrease effective population size Fragmentation of populations into small isolated populations with limited dispersal Founding of new populations by a small number of colonists Existence of non-monogamous breeding systems Opposing forces explain geographic differences • – Changes in gene frequencies caused by differential fecundity and survival • Assortative mating Natural Selection • Assortative mating produces geographic variation Gene Flow – Preferred pairing of like types – Genetic blending caused by interbreeding Proportion of red-phase Screech Owls 2 Geographic isolation is an important component of speciation Secondary contact tests species • Large Scale (allopatric) – Colonization of islands • Galapagos finches • Hawaiian Honeycreepers – Habitat fragmentation • Toucans • Small Scale (sympatric) • Mascarene White-eye Hybrids occur when ecological isolation breaks down • Reproductive isolation limits effects of hybrids • Stable hybrid zones can persist for centuries • Ecological isolation is less resistant to hybridization Oriole hybrids • When ecologically similar species become sympatric one will usually replace the other 3 Blue-winged, Golden-winged Warblers Behavioral isolating mechanisms promote speciation • Speciation promoted by Range expansion of Blue-winged Warblers Lawrence’s – Complex behaviors – Enhanced brains – Cultural transmission of information Brewster’s What is a species? • Exploding Species (TREE 11:314-315, 11:509) – Typical estimate for birds is 10,000 species – Molecular techniques and better field studies may increase this to 20,000 – Authors argue that biological species concept needs revision “Failure to observe inbreeding does not mean lineages are completely independent; neither does interbreeding necessarily prohibit the continued divergence of two lineages in contact” Communities • Factors shaping patterns of bird species diversity and abundance are: – Time – Energy – Space Robert H. MacArthur. 1958. Population ecology of some warblers of northeastern coniferous forests. Ecology 39:599-619. 4 Bird diversity reflects habitat and prey diversity • Vegetation structure correlated with bird species diversity (Karr and Roth 1971) • Prey size diversity associated with bird diversity (Schoener 1971) Tropical Temperate Other evidence of competition • • • Ecological displacement in Caribbean hummingbirds Altitudinal displacement in Appalachian thrushes Ecological equivalents in European and North American tits Character displacement indicates competition • Character displacement refers to morphological or behavioral adaptations caused by competition. • Gant’s (1986) study of Galapagos Finches is a classic example of character displacement driven by competitive exclusion. – When three species occur on one island (Santa Cruz) they have distinct bill sizes. Single species on islands (Daphne Major and Los Hermanos) have intermediate bill sizes. Patterns of species diversity • Temperate – tropical gradients in diversity reflect: – More food – Benign climate – Stable environment (MacArthur 1969) Number of landbird species 5 Island biogeography Evidence of equilibrium dynamics 80 Number of species 70 60 50 40 30 20 10 0 10 100 1000 10000 100000 Island Area (km 2) • The number of species on an island reflects a balance between rates of immigration (colonization) and extinction. Extinction rates increase and immigration rate decrease as the number of species present on an island increases. The intersection of the two curves for any particular island size defines the expected equilibrium number of species (S). • • – Immigration rates on islands that are distant from source areas are lower than rates on islands close to source areas – Extinction rates on large islands are lower than rates on small islands (MacArthur and Wilson 1967 Sources and sinks • • • • Habitat patches on a landscape function as islands of varying habitat suitability Source habitats generate surplus productivity (R0>1), sink habitats cannot sustain stable populations (R0<1) Populations in source habitats show less variability than populations in sink habitats Landscapes with larger and less isolated patches show less variability than landscapes with smaller or more isolated patches Pulliam, H.R. 1988. Sources, sinks, and population regulation. The American Naturalist 132: 652-661. 6