Tale of Speciation on Daphne Major

Tale of Speciation on Daphne Major “Speciation, the process by which two species form from one, involves the development of reproductive isolation of two divergent lineages.” This is an interesting tale of a small population, an immigrant, incest, and isolation. Good thing these are birds. In 1981 a population of finches of the species Geospiza fortis lived on the Galapagos island of Daphne Major. An immigrant landed on the island. It was a hybrid of two finch species from another island. It’s genetic make up was 66% Geospiza fortis and 34% Geospiza scandens (another common Galapagos finch species). It came from an unknown island, was unusually large, had a huge beak, and sang a strange song. Over four generations this hybrid bred with other G. fortis finches on the island until a drought occurred, wiping out all but one brother and sister. The brother and sister had no choice but to breed with each other (scandal!). Their offspring were different from either of them or other finches on the island. The male’s song was strange because in learning the song from the father, it was copied incorrectly. But, females of this new population understood the song even though it was wrong. Members of the greater G. fortis population ignored the incorrect song and would not mate with the males who sang it. A barrier was in place that originally resulted from one immigrant being separated from its own population somewhere else, and its lineage ended up being behaviorally isolated by an incorrect learned behavior. 1. What kind of speciation do we normally associate with members from one population being geographically displaced to a new, smaller population? allopatric 2. What kind of speciation do we normally associate with members of a population that become ecologically or behaviorally distinct within that population such that they become reproductively isolated? sympatric 3. Examine the graphic and description. Propose a definition for “endogamous lineage”? Only breeding with their species 4. What forced brother and sister mating in the birds? Would you expect generations after this even to be more or less diverse? Explain your reasoning. Brother and sister mating could occur because there's not a lot of other birds to mate with, this could result in less diverse species depending on if they have the same genotypes. Song Differences There are two main reasons why Geospiza birds might not breed with each other: song and body shape. Bird songs in males are a crucial way to attract mates. Being able to recognize the shape of a mate is also very important. Bird songs are learned in a very short window of time in a sensitive period of a young male’s life. G. fortis only sings one song and although it varies a bit, it is distinct. Females do not sing, but they choose mates based on the song communication. As members of the interbreeding population were born and learned songs, they learned them incorrectly. Figure 2A below shows sonographs songs of the original immigrant (5110), a son (15830) and a fifth generation descendant (19668), compared with Figure 2B which show three Daphne G. fortis individuals that sang a standard form. Figure 2: Sonographs showing the immigrant interbred lineage (A) and the native G. fortis lineage (B). 5. In terms of the barriers in place that keep species separate, explain whether song recognition is a prezygotic barrier or a post-zygotic barrier. Song recognition is a pre-zygotic barrier because in order for birds to mate with each other, they have to recognize each other’s songs 6. Explain which type of isolating mechanism this would be once you determine the barrier type. This is behavioral isolation 7. Discuss why song would be a barrier to mating with another species and having the incorrect song might change an individual’s overall fitness. Songs would be a barrier to mating with another species because they would not recognize a different song from their own. Incorrect songs might change an individual’s overall fitness by prohibiting them to mate Beak Shape In terms of body shape the two populations appear quite different. In looking at this feature, there is a clear difference in beak size between the immigrant lineage and native G. fortis lineages. The Grants nicknamed the immigrant lineage the “big bird” lineage. Graph shows distribution of beak size for native G. fortis on Daphne Major. The bottom figure shows the beak size of the immigrant offspring. The original immigrant is designated by the asterisk. Right image: (A) shows a member of the fifth generation offspring of the immigrant “Big Bird”. (B) shows a native G. fortis resident. 8. The immigrant lineage has only a population of 20 while the native lineage 280 birds sampled. What event on the island reduced the overall diversity of this group? How might the size of the gene pool contribute to the low variety in beak size for the immigrants? Behavioral isolation reduced the overall diversity of this group since the birds do not mate with of=ther species, there’s not much diversity. The gene pool contributes to low variety in beak size by only having a small selection of birds to mate with. 9. Compare the photos of the birds. What differences do you note between the birds? How might these differences contribute to reproductive isolation? The birds look different so they might not be attracted to each other Isolation Image shows the nest sites of all members of the immigrant breeding population before the drought (1993) and later after interbreeding isolation (2007). The numbered dots show nesting sites and the large black areas show the volcanic craters on the island. 10. In the map above, brother and sister mated at nest site 16833. This type of mating is called endogamy. Surrounding nest 16833 are typical G. fortis nesting sites. Suggest a reason why two closely related individuals would mate with each other rather than other nearby birds. The nearby birds might have different song compared to the birds at 16833 11. Why do you think the immigrant lineage moved to the nesting sites (2007) away from the original sites? The immigrant lineage might not want to be around another species 12. How is this isolation an example of sympatric speciation? This is sympatric because the birds are in the same area but they don’t want to mix 13. After many generations in isolation, would you consider the Big Bird (immigrant) lineage a new species? Why or why not? Yes, the Big Bird lineage is a new species because it has developed into its own different traits Final Synthesis The Grants had these parting thoughts: “These observations provide important insight into the process of speciation at the beginning of the sympatric phase following divergence in allopatry. They also raise a question that is rarely if ever asked: How many generations of exclusively within-group mating are needed before the group is recognized as a separate species that deserves taxonomic status? There is no non-arbitrary answer. We treat the endogamous group as an incipient species because it has been reproductively isolated from sympatric G. fortis for three generations and possibly longer.” 14. Discuss how the finch species on Daphne Major illustrate both sympatry and allopatry. What did the Grant’s mean when they said they would treat the endogamous group as an incipient species? This illustrates allopatry because the finches were once separated and came to the island via immigration. Sympatry happens when the two types of finches don’t want to mate but live in the same area. The endogamous group is incipient because they developed completely different genes from the native species