Speciation and hybridization Where do species come from and where are they going? Darwin’s “Origin of Species” said little about how new species arise –We now know that physical isolation is an important factor (allopatric speciation) –Can new species arise in sympatry? If so, how? Why are the Oscine Passerines so diverse? Species concepts –Biological Species Concept –Morphological Species Concept –Evolutionarily Significant Units and/or Phylogenetic species concept

Geographic Variation in Song Sparrows: Incipient Speciation? For a conservationist or evolutionary biologist, what is the most important thing to know about the origin of this variation? Why care what the origin is?

e.g. How do we know that eastern song sparrows aren’t lighter than northwest coast song sparrows because the intense sun east of the Cascades is just bleaching their feathers? Why is it important to know this? And how do you figure it out? Natural selection acts on variation in the population, but evolution can only occur if the variation is heritable. Therefore, we have to know if the traits are heritable if we are interested in evolution and ultimately speciation. If we want to conserve a subspecies we again should know that we are conserving something evolutionarily significant and not something that just has developed different colors because its in the sun or not. Neutral genetic markers suggest reduced geneflow among some song sparrow subspecies, but populations across the Cascades are not isolated. Most likely song sparrow subspecies characteristics are genetically determined and may represent important local adaptation, but no one has actually done the appropriate experiments to know for sure. Until then, it might be meaningless to talk about subspecies or hypothesize that speciation is in progress.

Populations diverge all the time, but what prevents merging and collapsing back into one? At least 10% of the “good species” listed in your field guide can and do hybridize occasionally with other species. –Are these really “good species”? What are the isolating mechanisms that keep sympatric or parapatric species from interbreeding?

Hybrid zones: Windows into the speciation process What happens when two recently diverged geographically isolated populations come into secondary contact? X time reinforcement of mating signals Full reproductive isolation (Prezygotic isolation) (Postzygotic isolation) If hybrids are selected against—suggests that ecological specialization has begun.

If hybrids are less fit than parentals, then you can get selection for reinforcement. Example: African Tinkerbirds—one species occupies savannah and the other forest. At the forest/savannah interface there is selection against hybrids and hence greater ability for females to distinguish males of each population. Divergence in sexual and social signals leads to prezygotic isolation (in animals) Kirschel et al. PNAS 2009 Frequency (Hz) Time (sec) AllopatrySympatryAllopatrySympatry reinforcment

What happens if two populations randomly diverge in mating signals by cultural processes? For example, white crowned sparrow dialect zones within a subspecies…is this enough for genetic isolation and speciation? MacDougall-Shackleton EA & SA MacDougall-Shackleton Cultural and genetic evolution in mountain white-crowned sparrows: song dialects are associated with population structure. Evolution 55: White-crowned sparrow dialect boundaries are not strict barriers to geneflow. No known ecological differentiation, and no known penalty to mating across dialect boundaries. Young males can disperse into new dialect zones, learn that dialect and produce ecologically competitive offspring although they may prefer to stay within dialect zone.

White crowned sparrow dialect zones on west slope of Cascades 2 dialects from Oregon (4 individuals each) Baptista 1976 Dialect 1 Dialect 2 Seattle Dialect Zone

Species collapse in secondary contact. Rohwer et al These two species likely separated during the last ice age. Two populations migrated to separate summer habitat in the Rockies and along the coast. Note that the habitat in the Rockies was smaller. The population there evolved to be more aggressive because territories were in short supply. Now the more aggressive species has expanded its range and Is starting to overtake the range of the other species hybridizing as it goes. Currently, the hybrid zone is in western Washington. Hermit Warbler Townsend’s Warbler

Species collapse in secondary contact. Rohwer et al These two species likely separated during the last ice age. Two populations migrated to separate summer habitat in the Rockies and along the coast. Note that the habitat in the Rockies was smaller. The population there evolved to be more aggressive because territories were in short supply. Now the more aggressive species has expanded its range and Is starting to overtake the range of the other species hybridizing as it goes. Currently, the hybrid zone is in western Washington. Hermit Warbler Townsend’s Warbler

Hybrid zone

Note that Northwards along the coast, all the birds bear mixed mitochondrial DNA Even though they look like Townsend’s warblers. This is testament to the fact that male Townsend’s Warblers have been moving south along the coast hybridizing with female hermit warblers as they go. The only pure hermit warblers left are in Oregon and California. Hermit warblers may be completely extinct in another 1000 years or less (but their mtDNA may live on). Similar scenarios could happen in other warblers “species” pairs too. Pies represent the proportions of hermit mtDNA haplotypes to Townsend’s haplotypes. Townsend’s males are more aggressive than Hermit males. Kick out the Hermit males and Hermit females then mate with Townsend’s males.

Transects across the hybrid zone. One in the southern WA Cascades and one in Olympic Mtns.

Speciation by isolation on islands in Galapagos finches. What happens when finch populations occasionally fly back to an island that already has other finches on it?

Adaptive radiation Darwin’s finches: –Divergence in bill characters in isolation on separate islands. –Further divergence through competition in secondary contact (when they fly to an island already inhabited) –Song differences related to bill differences (males learn song from father, and bill size/shape may determine how fast trill notes can be repeated) –Song may reduce geneflow enough to maintain morphological differences (females prefer bill shape and song type of father), and to allow divergent populations to function as separate species (even if they can and do interbreed) –But, no penalty to hybridization…it does occur. Hybrids survive as well as parentals in some cases. –So how “good” are these species? Will they collapse back into 1? Depends on whether in long run birds with divergent characters survive and reproduce better than hybrids with intermediate phenotypes. –If resources specialized upon are separated in space or time on an island this can help along the process of divergence by semi-isolation. Grant and Grant 2003

Conclusions No debate that birds can diverge in allopatry, by adaptive or random processes. Long enough time in allopatry will lead to divergence and reproductive isolation. For sympatric or parapatric speciation to occur, the importance of mating signals relative to ecological divergence is debatable Generally agreed that both are necessary Which comes first is an open question, although generally assumed that ecological divergence comes first (hence adaptive radiation), often in allopatry or parapatry (Note that allopatry and parapatry because of their spatial differentiation both reduce geneflow between populations) Divergent characters will be maintained if there is a fitness cost to hybrids and/or if geneflow between populations is reduced via some other means. Fitness costs to hybridization often lead to evolution of divergence in mating signals and functional isolation by mating signals.