Untitled

• Lecture 6 is about Sympatric Speciation
• Readings are from Chapter 4

Lecture Outline

• Sympatric Speciation will be covered
• Theory surrounding sympatric speciation will be discussed
• Experimental and natural evidence will be covered

Sympatric Speciation

• Sympatric speciation refers to the evolution of an isolating mechanism within an interbreeding population
• Speciation occurs within the average dispersal distance, or "cruising range," of a single individual
• If this wasn't the case it could be microallopatric speciation

Controversial Aspect

• Sympatric speciation is a controversial aspect of speciation
• Darwin proposed that species could form in sympatry to fill empty niches, using a morphological species concept
• Without genetics, it was impossible to know that interbreeding could prevent new taxa

Influence of Mayr

• The view that sympatric speciation was possible prevailed for decades
• Mayr (1963) reviewed and critiqued sympatric speciation utilizing arguments from natural history and genetics
• Mayr's work had a huge impact on the field causing sympatric speciation to no longer be considered the norm

Theory: Problems

• All models of sympatric speciation face two main theoretical problems
• The first problem is the antagonism between disruptive selection and recombination
• Interbreeding continually breaks up beneficial gene complexes
• Interbreeding can break up genes for habitat fitness and habitat preference
• The second problem is the need for coexistence of new species.
• Populations must develop sufficient ecological differences to coexist
• Selection partially drives the ecological divergence
• Models based on disruptive sexual selection do not easily solve the problem of coexistence

Theory: Models

• There are two main types of models for sympatric speciation
• Disruptive sexual selection
• Disruptive natural selection

Disruptive Sexual Selection

• In many adaptive radiations, species differ more strongly in sexual traits than ecological adaptations.
• African lake cichlids are an example of this
• These observations have inspired models where sympatric speciation is driven by sexual selection
• Disruptive sexual selection models show some success, but often under unrealistic or restrictive assumptions
• Most provide no mechanism for the coexistence of new species in the same habitat
• Some more recent models have added ecological divergence in addition to sexual selection with better success

Disruptive Natural Selection

• There are two types of natural disruptive selection models:
• Discrete-habitat models
• Continuous-resource models

Discrete-Habitat Models

• Sympatric species seek out different niches and preferentially mate with members of their own niche
• An example would be insects that feed and mate on a specific host plant
• Models of discrete-habitat focus on three traits:
• Niche preference
• Niche adaptation
• Assortative mating
• Most early models only combine two of these three traits
• The model that explores all three traits simultaneously (Johnson et al. 1996) is the most realistic and successful

Continuous-Resource Models

• Discrete-habitat models might not be appropriate for many species
• Sympatric African cichlids don't show dramatic differences in habitat
• Continuous-Resource models are based on co-evolution of:
• Ecological traits
• Assortative mating
• Continuous-Resource models splits population into two sympatric groups using a different part of the resource distribution

Continuous-Resource: Dieckmann and Doebeli

• The most famous continuous resource model is that of Dieckmann and Doebeli (1999)
• This model is based on an important resource (e.g. seed size) unimodally distributed
• An ecologically-relevant trait (e.g. beak size) is unimodally distributed
• As population grows, frequency- and density based selection favors individuals with extreme values (larger and smaller beaks)
• Splitting occurs if individuals with extreme traits prefer individuals with similar traits

Continuous-Resource: Problems

• The Dieckmann and Doebeli model works best when individuals have innate preference for mating with individuals having traits similar to their own
• If mate preferences are not very strong, speciation takes so long that initial conditions probably change
• For example, speciation might take up to ~1,000,000 generations
• As the ends of a distribution become occupied there is less competition for the central resource which favors intermediate individuals.
• Can result in a continuum of interbreeding forms rather than discrete groups.

Model Conclusions

• Some models show reproductive isolation in the face of gene flow, meaning that sympatric speciation is theoretically possible
• Conditions for sympatric speciation are more stringent than allopatric speciation
• Allopatric speciation should be regarded as the null hypothesis
• Some models may have unrealistic assumptions
• Few models have varied their parameters to examine effects on probability of speciation
• It is more prudent to judge the likelihood of sympatric speciation based on laboratory experiments and evidence from nature

Experimental Evidence

• Some lab studies have demonstrated reproductive isolation between selected lines that interbreed
• Many are "kill the hybrid” experiments where hybrids are removed in each generation
• Leads to successful isolation but not that realistic

Geotaxis: Hurd and Eisenberg

• House fly geotaxis studies (Hurd and Eisenberg 1975)
• Strong disruptive selection applied to flies moving up or down from a central chamber
• The flies were reared apart but allowed to interbreed
• Flies diverged in geotaxis after many generations, and mating tests showed strong sexual isolation

Drosophila: Thoday and Gibson Studies

• Bristle number in Drosophila (Thoday and Gibson 1962, 1970)
• 12 generations of disruptive selection for bristle number
• Flies showed complete assortative mating by bristle number
• Results could not be replicated by 19 future studies
• Other studies found moderate assortative mating

Mating Maze: Rice and Salt

• Rice and Salt's (1990) "no gene" assortative mating maze was the most successful experiment
• Individuals mate only with others who choose the same habitat
• Flies could choose habitats differing in:
• Phototaxis(light or dark)
• Geotaxis (up or down)
• Chemotaxis (ethatnol vs. acetylaldehyde)
• Newly hatched flies were released into a central chamber
• Sorted themselves into 1 of 8 habitat vials
• Those choosing extreme habitats were then allowed to breed in order to simulate extreme habitat selection
• The habitats were: Up/dark/acetylaldehyde (A) and Down/light/ethanol (B)
• Additional selection on development time
• Early eclosing flies were from habitat A, and Late eclosing flies were from habitat B
• Mutations and chemical markers were used to identify a fly's parental habitat
• Complete habitat isolation occurred in 30 generations, as all flies who chose each extreme environment were offspring of parents from that environment
• There was no evolution of complete habitat preference, meaning offspring flies from extreme habitats continued to choose intermediate habitats
• These offspring were considered lethal and were discarded
• Flies did not mate assortatively when in a common environment i.e. assortative mating was a product of habitat isolation

Rice and Salt Conclusion

• Rice and Salt (1990) study shows that habitat isolation can evolve if selection is strong and assortative mating is a byproduct of habitat choice
• Experiments are helpful, yet their biological realism is questionable
• Strong disruptive selection seems necessary but isn't sufficient for sympatric speciation

Evidence from Nature

• Allopatric speciation is viewed as the null hypothesis
• Allopatric speciation is considered easier than sympatric speciation
• Selection or drift acting on geographically isolated populations will eventually produce isolating barriers
• Strong and pervasive evidence for allopatric speciation is present
• Many opportunities for geographic isolation have been generated in the last 2 million years due to ~20 major glacial advances and more frequent cycles of temperature change

Sympatric Speciation Criteria

• Sympatric speciation should meet four criteria:
• Species must be largely or completely sympatric
• Must have reproductive isolation, preferably of genetic origin
• Must be sister groups
• Biogeography and evolutionary history must make allopatry very unlikely
• There is:
• Evidence from habitat "islands"
• Host races
• Host-specific species
• Allochronic (temporal) isolation
• Comparative studies

Species on Islands

• A small monophyletic group confined to a small isolated habitat would be convincing in terms of evidence
• Several endemic Coleoptera and Orthoptera on St. Helena were thought to have speciated in sympatry
• However, the species are all flightless and could have speciated microallopatrically
• Flightless weevils on the island of Rapa
• Could have speciated microallopatrically or on small islets that were connected to the mainland during the Pleistocene
• Lepidoptera on the island of Rapa
• Possibility of multiple colonizations
• Negative correlation between endemism and mobility indicates allopatry rather than sympatry
• Surveys of endemic birds are not sister species on isolated islands, which fails to provide evidence of sympatric speciation on islands

Postglacial Lakes

• Many lakes in North America and Eurasia were formed after the last glaciation (only 15,000 years ago)
• Endemic fish in these lakes likely evolved since that time
• Some lakes contain closely related fish differing in morphology, behaviour, habitat, or life history, thus they're often considered an example of sympatric speciation
• Connection to rivers and the sea can allow for multiple invasions of allopatric taxa
• Gene exchange after secondary contact (hybridization) can yield misleading conclusions that the fish are sister taxa
• Mitochondrial DNA especially can be especially misleading and nuclear DNA provides a clearer picture

Lake Galtabol, Iceland

• Strong evidence comes from Arctic charr in Lake Galtabol, Iceland
• Limnetic and benthic morphs more closely related to each other than taxa in four nearby lakes
• Genetic similarity does not reflect current hybridization
• They share no alleles at one of six nuclear loci and are strongly differentiated at other loci

Cichilds

• Cichild fish have a dramatic adaptive radiation consisting of over 1500 species
• Consists of:
• Victoria, Malawi, and Tanganyika lakes
• Lake Nabugabo is a satellite lake separated from Lake Victoria by a sandbar 4000 years ago
• Includes small crater lakes such as Barombi Mbo and Bermin

Support for Sympatric Speciation

• Hard to envision geographic barriers allowing formation of hundreds of species in only 2 million years in the cichilds
• Speciation has also occurred in limnetic and deep water forms less likely to encounter geographic barriers
• Movement between habitats may prevent allopatric speciation
• Closely related species differ more in colour than morphology and habitat use
• Some pelagic species show no genetic structuring across Lake Malawi indicating allopatry is not possible as the fish are so mobile
• Monophyletic groups of pelagic species show that speciation occurred in pelagic ancestors

Support for Allopatric Speciation

• Levels of all three major lakes have risen and fallen repeatedly, creating isolated small lakes
• Many species have highly localized ranges and shorelines have diverse habitats (rocky, sandy, swamps, bays, rivers)
• Habitat specificity can reduce gene flow
• Genetic patterns sometimes mirror fragmentation of lakes
• Littoral fish have limited migration but occasionally colonize new habitats, promoting allopatric or parapatric speciation
• Some pelagic species return to littoral zone for spawning which could cause allopatric speciation by habitat segregation
• Some allopatric populations evolved behavioural isolation during periods when there were barriers to dispersal

Conclusion - Cichilds