Speciation

1. Speciation

2. We can separate speciation into a three-step process: • An initial step that isolates populations; • A second step that results in the divergence of characteristics between populations; • A final step that results in reproductive isolation (re-enforcement)

3. Mechanisms of Isolation • Speciation process often begins when populations become isolated and gene flow is reduced • Allopatric speciation is the formation of a new species in isolated geographical areas • Populations often become isolated through dispersal and colonization • Populations can also become geographically isolated as the result of vicarianceevents (=splitting of a population’s former range into 2 or more isolated patches),including formation of mountains ranges, river formation, drying and the subsequent fragmentation of forests, lava flows, etc.

4. Geographic Isolation via Dispersal and Colonization • This often occurs as the result of founder events • One consequence of a founder event is that colonizing individuals are isolated from the original ancestral population • The founding population experiences drift and differences in mutation and selection as the result of being in different environment • In the process of adapting to a different environment, the population undergoes different genetic substitutions and gradually becomes genetically differentiated • As the result of accumulated genetic differences, distinguishable morphological and physiological differences may arise • When populations have changed enough to be recognizably different, but not to be reproductively isolated, they are usually referred to as racesor varieties • The first recognizable step in allopatric speciation is when populations have become genetically differentiated in the process of adapting to different environments

5. Dispersal and Colonization: Example involvingHawaiian Drosophila • Hawaii contains a diverse assemblage of drosophilids • Many of the Hawaiian Drosophila are endemic to particular islands of the archipelago; leading hypothesis for their diversification is founder events • Based on the manner in which the islands were formed, the founder event hypothesis makes two predictions: 1) closely related species should be found on adjacent islands; 2) phylogenetic relations among the flies should correspond to the manner in which the islands were formed

6. Results • Mitochondrial DNA sequence data for 4 species of flies supports these predictions • The most recent species are found on the youngest islands and the sequence of branches for the phylogenetic hypothesis of flies is consistent with the order of island formation

7. Geographic Isolation through Vicariance • An example of a vicariance event separating populations relates to the isthmus of Panama – the land bridge that developed between North and South America some 3 million years ago • A comparison of mitochondrial DNA sequences of snapping shrimp on either side of the isthmus (e.g., 7 pairs of closely related morphospecies, with one member of each pair found on either side of the isthmus) indicated that species pairs from either side of the land bridge are each other’s closest relatives

8. Changes in Chromosomes as a Barrier to Gene Flow Chromosomal mutations that lead to polyploidization can result in reproductive isolation between populations because if the incompatibilities between gametes with different number of chromosomes

9. Mechanisms of Divergence • The Role of Genetic Drift • The impact of drift on small populations is not always through a loss of overall genetic variation • When a population is reduced to a small size, only rare alleles are lost due to drift; for genetic diversity to be dramatically reduced the population would have to be extremely small • Most models indicate that genetic variation in the population can actually increase as a result of bottlenecking • This can still results in a rapid genetic divergence of the isolated population due to epistatic component of genetic variance being converted to additive variance

10. Mechanisms of Divergence cont. • The Role of Natural Selection • Example: The apple maggot fly, Rhagoletis pomonella • This fly is common throughout the midwest and northeastern US • Traditionally associated hawthorne fruit trees • Adults reproduce on the fruit, and females lay eggs in the fruit • When the fruit ripens and falls off the tree, the larvae burrow in the ground and emerge next spring and summer to repeat the life cycle • Interestingly, these flies have more recently become associated with apple trees; probably when they were introduced from Europe some 300 years ago Hawthorne tree Apple fruit

11. Rhagoletis pomonella study cont. • When given a choice flies show a strong preference for their original host • Because mating occurs on the plant, host preference results in strong nonrandom mating • Were flies associated with different hosts distinct races/species? Can natural selection based on host preference for different food create different races/species of flies? • Interestingly, host-associated populations of flies are not physically isolated from one another • Electrophoretic analysis indicated that the flies are genetically differentiated; exhibited significant allele frequency differences at 6 enzyme loci • It appears that rather than being isolated by dispersal or vicariance, the flies are isolated on different host species

12. Rhagoletis pomonella study cont. • Results imply that selection has favored distinct habitat preferences in apple and hawthorn flies. • There appears to be an increase in fitness from switching to apple trees: • Escape from parasitoids: The average level of wasp parasitism is 70% less on apple than on hawthorne • Escape from intraspecific competition due to the large size of apple fruit • Escape from interspecific competition

13. Mechanisms of Divergence cont. • The Role of Mutation • For speciation, the primary role of point mutations, gene duplications, and chromosomal inversions is to provide the raw materials for drift and selection once gene flow is reduced

14. Sympatric and Parapatric Speciation • A big debate in evolutionary biology has been whether physical isolation between populations is necessary prerequisite for populations to diverge or whether selection for divergence can overwhelm gene flow thereby triggering speciation • Sympatric speciation is the formation of new species in sympatry; reproductive isolation without geographic isolation • Thus, the difference between allopatry and sympatry is a distance factor • Genetic modeling suggests that populations can diverge with low to moderate gene flow provided that 2 conditions are satisfied: • 1) selection for divergence is strong; • 2) mate choice must be correlated with the factor that is promoting divergence

15. Parapatric speciation • The establishment of reproductive isolation between adjacent populations; formation of contiguous races and species • 1. Strong selection for divergence is thought to cause gene frequencies in the continuous population to diverge along the gradient • 2. The formation of parapatrically distributed races could be accomplished by diversifying selection • 3. If the dissimilar habitats are sharply delineated, there could be strong selection against the transmission of genes responsible for the local adaptations across such a boundary • 4. Hybrid individuals or their offspring would be ill adapted in either environment; pre-zygotic isolation would be favored, resulting in speciation

16. Reinforcement (reproductive isolation) • The third stage in speciation may occur if diverged populations come back into contact and have an opportunity to interbreed • There are a number of possible fates of the hybrids: 1) they may survive and interbreed with the parental populations and eliminate the divergence; 2) may have new characteristics and become a distinct population • Dobzhansky indicated that if populations diverged sufficiently while allopatric, they may no longer be compatible - post-zygotic isolating mechanisms - isolating mechanisms that prevent hybrid zygotes from developing into viable, fertile adults • Selection should favor assortative mating, and that this reinforcement would finalize the speciation process • Selection would favor mutations in populations that would prevent mating from occurring – prezygotic isolating mechanisms

17. Post-zygotic Isolating Mechanisms 1. Hybrid Mortality (Inviability) Incompatibility in parental genes results in the breakdown of genetic and cellular mechanisms that regulate development Cessation of development and death before the individual reaches adulthood 2. Hybrid Sterility Reproductive isolation in which hybrid zygotes are sterile Cannot reproduce and their genes can’t flow between species Probable causes: a) abnormal development of the gonads; b) failure of parental genes to produce gametes with correct numbers of homologous chromosomes (=polyploidy) 3. Hybrid (F2) Breakdown F1 hybrids are normal, vigorous, and fertile, but the F2 contain inviable or sterile individuals

18. Pre-zygotic Isolating Mechanisms • Under the circumstances of post-zygotic isolation, selection would favor assortative mating • It would favor some mechanism that would decrease the possibility of the 2 populations mating in the first place - pre-zygotic isolating mechanisms (reinforcement) • There are 5 main types: habitat isolation, temporal isolation, ethological isolation, mechanical isolation, gametic isolation

19. 1. Habitat Isolation Two species may never come into contact with one another because they occur in different habitats in the same geographical area 2. Temporal (Seasonal) Isolation Potential mates do not meet because mating or cross fertilization takes place at different times of the year or different times of the day 3. Ethological Isolation Populations are isolated by different and incompatible behavior before mating e.g., incompatible special signals or courtship displays that attract mates Also, species may come into contact with one another, but they never mate because there is no sexual attraction between males and females

20. 4. Mechanical Isolation Closely related species may attempt to mate, but fail to consummate the act because they are anatomically incompatible Common in plants - physical differences in the flowering parts Sometimes observed in animals, whereby differences in the genital structure of animals can also lead to unsuccessful mating 5. Gametic Isolation In organisms with external fertilization, male and female gametes may not be attracted to one another In organisms with internal fertilization, although mating occurs and the gametes may meet, they do not form a zygotedue to gametic mortality

21. Hybridization and Hybrid Zones • Hybridization • Reinforcement should occur when hybrids have reduced fitness, but what if they have increased fitness?

22. Hybrid Zones • A hybrid zone is a region where interbreeding between divergent populations occurs and hybrids are frequent • Three possible outcomes dictate their size, shape and longevity

23. Hybrid Zones cont. • 1. Under some circumstances, there may be no measurable differences in the fitness of hybrids and purebred lines • When this is the case, the hybrid zone is usually wide, with the hybrids having their highest frequency at the center of the zone and decreasing frequencies with increasing distance • The width of the zone is a function of: how far individuals from each population disperse and how long the zone has existed • Zones are wider if individuals disperse far and the populations are in contact for long periods

24. Hybridization and Hybrid Zones cont. • 2. Under some circumstances, the resulting hybrids have lower fitness that the parental offspring • The fate of the hybrid zone under this scenario depends on the strength of selection against them • If selection is strong and reinforcement occurs then the hybrid zone is narrow and short lived • If selection is weak, then the zone is wider and longer-lived

25. Hybridization and Hybrid Zones cont. • 3. Under other circumstances, the resulting hybrids can have a higher fitness than the purebred offspring in newly colonized environments or certain restricted habitats • When the hybrids are more fit, the fate of the hybrid zone depends on the extent of the environments in which hybrids have an advantage • If hybrids have higher fitness in environments outside the range of the parental species, then a new species may form • If hybrids have an advantage at the boundary of each parental species range, then a stable hybrid zone may form • The idea being that hybrid individuals have intermediate characteristics and have a fitness advantage in the transitional habitats

26. Rates of Speciation • The Rift Lakes of East Africa (Lakes Victoria, Tanganyika, and Malawi) contain s diverse assemblage of freshwater cichlid fishes (e.g., 1000 of the 1300 species that are known worldwide) • Many have very specific feeding strategies and habitat preferences • Many of elaborate colors and behavioral studies suggest that sexual selection is intense • For a long time, biologists have suggested that the cichlid faunas of these lakes are the result of rapid speciation

27. Cichlids cont. • For a long time, biologists have suggested that the cichlid faunas of these lakes are the result of rapid speciation

28. Cichlids cont. • The longstanding idea was that radiation that occurred in each of the lakes was the result of many different founder populations • However, phylogeny estimates from DNA sequence data suggest that the cichlid fauna of each lake may have descended from a common ancestor (descended from a single population) • The phylogenetic hypotheses that have been generated to date clearly separate the species found in each lake • Thus far, no lineage seems to have descendents in more than one lake

29. Cichlids cont. • If the phylogenetic data are correct then speciation rates for these lakes has been faster than previously envisioned • There is evidence to suggest that Lake Victoria almost completely dried up some 10-12,000 years ago • This would suggest that the approximately 300 species of this Lake descended from a common ancestor roughly 10,000 years ago • What is responsible for these rapid rates of evolution? • The original colonization event was apparently from a single founder species • After the original colonization, periods of drought produced repeated vicariance events that isolated populations in lakes • In addition, the ecology and mating systems of these fishes provided numerous opportunities for divergent selection on mate choice characteristics, habitat choice, and feeding strategies