Lecture 2.3. Answers.

Transcription

1 Lecture 2.3. Answers. 1. What is the adaptive significance of Allen s rule illustrated in the picture below? Allen's Rule in North American hares (Lepus spp.), From the northern arctic hare (L. arcticus) through the more southerly desert jackrabbit (L. alleni), members of the genus Lepus show progressively longer extremities (legs and ears) and leaner bodies. In this case, the cline is composed of species as opposed to geographic races. Longer legs / ears increase surface area to volume ratio, thereby promoting dissipation of excess heat in hot climates. The ears especially act as radiators. Another good example is the enormous ear African of elephants. 1

2 2. The picture at the right indicates an inverse relationship between temperature (50 + year averages) and body mass in contemporary populations of woodrats. Discuss. Absent changes in proportion (as in previous question), Increasing body size reduces surface area to volume ratio and promotes heat retention. The hares in the previous question violate Bergman s rule increasing body size in mammals with increasing latitude), but get away with it by becoming long and thin. 2

3 3. On Daphne Island in the Galapagos Islands, Peter and Rosemary Grant have identified the formation of what appears a new species of Darwin s finch. The birds are descended from a single individual now known to have been a hybrid from another island. They differ from the locals morphologically and with regard to song. Importantly, they mate solely among themselves. For additional details, go here and here. A new Darwin s finch. The bird s ancestor, # 5110 (almost all the birds are tagged) had a larger bill than the indigenous medium ground finches and a different song. Drought reduced its descendants to a single brothersister pair whose offspring have since mated only with each other. Discuss this event with regard to the modes of speciation given above and also with regard to pre- and postzygotic isolation. Note: (1) young birds learn their songs (how to sing and what to respond to) from Dad; (2) both song and bill morphology are important cues for recognizing conspecifics. Speciation is peripatric with regard to #5110 s ancestors, but sympatric in the sense that reproduction isolation arose in the midst of a population (the birds on Daphne Is) of conspecifics. In other words, 3

4 none of the four definitions fits exactly. Reproductive isolation is pre-zygotic mate choice by females based on song and possibly also bill morphology. Hybridization involved if #5110 in fact a hybrid. Produced the variability from which reproductive isolation would eventually evolve. However, reproductive isolation did not develop until #5110 s descendants had been reduced to a single brother-sister pair. So hybridization by itself insufficient. Song novelty presumably the result of imperfect copying of local songs by the immigrant. 4. (10 pts) Most hummingbirdpollinated flowers are red and have long tubes. How have evolutionary biologists accounted for this? (Requires outside reading). The traditional answer is that insects are optimized for UV vision and can t see red, or at least not well. But this has been called into question. An alternative explanation is that hummingbird flowers present a suite of characters making them attractive to hummingbirds and unattractive to insects such as bees. These characters include the long tube that often (but not always) keeps large insects out and nectar that, while relatively abundant, is less concentrated than that of most 4

5 bee flowers. According to this view, the red color is a form of advertising, a sign to hummingbirds that this is a good flower to visit because insects will not previously have removed the nectar. The two hypotheses are not mutually exclusive. 5. (8 pts) Where / what is Lake Nyasa (Malawi)? Why would the cichlid fishes of this lake have undergone extensive adaptive radiation? Lake Nyasa is the second largest freshwater lake in Africa. To a great extent, it is isolated from other freshwater environments, in effect being an island. In situ, speciation (adaptive radiation) made possible by a lack of colonizing species. Satellite view of Lake Nyasa. 5