Darwin’s Finches: A Thirty Year Study.

I. Mit-DNA Based Phylogeny (Fig-ure 1). 1. All Darwin’s finches descended

from South American grassquit (small finch) ancestor circa 3 Mya.

2. Galapagos colonized 1st

3. Extant Cocos Island species de-rived from Galapagos species.

II. Recall Allopatric Model of Speci-ation. 1. Division of originally panmictic

population into isolates.

2. Morphological divergence in re-sponse to local selection.

3. Post-divergence sympatry & se-

lection for reproductive isolation.

Figure 1. Mt-DNA based phylogeny. P. inornata is the Cocos Island finch.

Figure 2. Wallace model of Geospizid evolution.

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III. Contrast with A. R. Wallace Peripatric Model (Figure 2). 1. ARW imagined successive inva-

sion and ecological divergence produced existing species.

2. Implicitly assumed all islands and niches present ab initio.

IV. Subsequent Realization: Finches

and archipelago co-evolved – Wallace-Grant model. 1. Number of islands increased

due to crustal spreading.

2. Climatic conditions changed with recurrent fluctuations (El Niño) superimposed on overall trend of increasing aridity.

a. An initially warmer, moister climate may have favored

long, narrow beaks for exploiting nectar and insects.

b. Subsequent aridity may have favored alternative feeding strategies (seeds) and morphologies (deep beaks).

Figure 3. Coevolution of Galapagos Islands and Geospizid finches.

Figure 4. Changing bill shape (length/depth) ra-tio shape and body size.

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V. Bill Size Selection (Figure 5). 1. G & G confirmed famous British ornithologist David Lack’s

earlier (1947) conjecture that larger bills facilitate con-sumption of larger (harder) seeds.

2. G & G observed two episodes of selection on bill size – both associated with droughts due to El Niño weakening.

3. 1977. Selection for large beaks in G. fortis.

a. Population declined with small-beaked birds disappear-ing to a greater extent than large-beaked conspecifics.

b. Not enough small seeds – birds forced to eat medium and large seeds.

4. 1987. Selection for small beaks. a. This time plants producing large seeds were hit hardest.

b. Small-beaked individuals favored over large.

5. In both drought years, there was less mortality in the cac-

tus finch (G. scandens), which is less dependent on seeds.

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Figure 5. (Left) G. fortis abundance (top) and seed abundance (bottom) from 1975 to 1978. Right. Selection for large beaks in 1977. Beak size (depth) distribution in 1976 and 1978. Birds with small beaks experienced greater mortality than their large-beaked conspecifics.

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VI. Adaptive Landscapes (Figure 5). 1. Utilization and seed abun-

dance used to construct maximum bird density as function of beak size.

2. No more than one species per peak.

3. Close association of opti-

mal and observed beak siz-es (Figure 6).

VI. Hybridization.

1. Species within Geospizid genera can hybridize.

2. Hybrid individuals have in-

termediate beaks that may or may not reduce fitness as estimated in Figure 6.

3. Principal barriers to hybridization in Geospizidae are visual

and acoustical – i.e., how the birds look and sound.

Figure 6. Estimated maxi-mum densities vs. beak size for Geospiza spp. on 15 dif-ferent islands. Circles, etc. indicate mean beak sizes of resident finch populations.

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VII. Reproductive Isolation. 1. Mechanism is mate choice by ♀♀.

2. If female preference based solely on

appearance, ecological divergence would => reproductive isolation.

3. But song is also involved. Song evo-lution consequent to

a. Passive changes in resulting from

changing morphology (body size, syrinx volume, beak size).

b. What females consider to be a “good” song1.

VIII. River of Life – Think River Delta.

1. Tree of life branches coalesce – result of hybridization.

2. At its tips, the “tree” of life becomes a “web”.

1 Females appear to evaluate the songs of would-be mates vis-a-vis vocali-zations that they heard when they were young, i.e., their father’s.

Figure 7. Air sacs and syrinx in birds. Air sacs function in respiration; the syr-inx, in vocalization.