The Scalesia Genus As an in Situ Model for Evolutionary Theory

The Scalesia Genus as an In Situ Model for Evolutionary Theory

Title Image. A young Scalesia pedunculata plant at the Darwin Research Station on Santa Cruz (Birnbaum, 2018).

Foster Birnbaum Evolution and Conservation in the Galapagos Professor Bill Durham 10/12/2018 Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018

Abstract

The towering Scalesia plants once formed the Galapagos’ most verdant environments: on the tops of otherwise arid islands rose lush Scalesia forests. These forests were even more unique given that Scalesia trees are, in fact, daisies. In total, fifteen Scalesia daisies grow throughout the archipelago, four of which are highland trees; the rest are arid- or transitional-zone shrubs.

Recognizing the Scalesia genus’s high species count, researchers consider it a paragon of adaptive radiation (Stöcklin 2009: pp.33-48). This paper further uses the Scalesia genus to explore tenets of evolution. To do so, this paper examines three hypotheses: (1) geographic distance affects the distribution of variation in the bush Scalesia affinis; (2) different quantities of pollinators explain why only some Scalesia species have ray florets; and (3) severe El Niño events exert a selective pressure on the tree Scalesia pedunculata to grow and die in generational cohorts. Genetic data on seven S. affinis populations support the first hypothesis; experimental data on how ray florets affect reproductive success support the second hypothesis; and observational data on S. pedunculata groves demonstrate the insufficiency of the third hypothesis. In addition, this paper discusses how S. pedunculata’s generational lifecycle affects efforts to conserve the species.

Introduction

The Galapagos are most well-known as the birthplace of not a living organism but an idea: Darwin’s theory of speciation by natural selection, also known as Darwinian evolution.

Recognizing the ongoing potential for the flora and fauna of the Galapagos to advance evolutionary principles, this paper proposes the Scalesia genus, an endemic genus in the

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Asteraceae (daisy) family, as a model for Darwinian conceptions of variation and natural selection. Genetic analysis of chloroplast DNA restriction sites suggests that Scalesia’s closest relative is the Pappobolus genus of the Andes Mountains, and the number of differences in the sites suggests that the genera diverged around 2 million to 6 million years ago (Schilling et al.

2004: pp.248-254). Since its arrival in the Galapagos, Scalesia has grown to include fifteen species. Eleven are shrubs that survive in the arid zone, and four are trees—two of which

(S. pedunculata and S. cordata) stand over 10 meters—that grow in higher-elevation, moist environments. Regarding the distribution of the fifteen species, older, larger islands harbor more species (e.g., Santa Cruz and San Cristóbal have six and four, respectively, each including S. pedunculata) than do younger, smaller islands (e.g., Wolf and Pinta have one species of shrub each). Further, “all the species are nearly completely allopatric in distribution, with a wide distance between their individual ranges in the archipelago,” and few hybrid forms have been observed (Itow 1995: pp.17-30). In addition to differing in size, Scalesia species display marked differences in leaf appearance, as shown in Figure 1. These differences are at least somewhat related to habitat conditions (Stöcklin 2009: pp.33-48). The distribution of and variation in

Scalesia species make the genus an excellent candidate for evolutionary study.

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A: Scalesia cordata; B,C: Scalesia microcephala var. cordifolia; D: Scalesia microcephala var. microcephala; E: Scalesia pedunculata; F–H: Scalesia aspera; I: Scalesia villosa; J: Scalesia stewartii; K–M: Scalesia atractyloides var. atractyloides; N–P: Scalesia divisa; R: Scalesia divisa; S,T,V: Scalesia baurii ssp. hopkinsii; U: Scalesia baurii ssp. baurii; W: Scalesia affinis ssp. gummifera; X: intermediate between Scalesia baurii and Scalesia crockeri; Y: Scalesia crockeri; Z: Scalesia affinis ssp. brachyloba; AA–CC: Scalesia helleri; Figure 1. Leaf variations among Scalesia species DD: Scalesia retroflexa. (Stöcklin 2009: pp.33-48).

Accordingly, this paper will analyze two Scalesia species, S. affinis and S. pedunculata.

Table 1 summarizes several characteristics of each species.

Characteristic S. affinis S. pedunculata Height shrub – grows up to 3 m tall tree – grows about 12 m tall (Figure 2B) (Figure 2C) Zone(s) of Residence arid and transitional highland – forms dense forests Islands of Residence Fernandina, Isabela, Santa San Cristóbal, Santiago, Santa Cruz, and Floreana Cruz, and Floreana (Figure 2A) (Figure 2A) Leaf Type pubescent (i.e., hairy) and pubescent and alternate alternate Ray Florets1 present (Figure 2D) absent Seed Dispersal Mechanism possibly land iguanas probably tree finches2

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Lifecycle seeds germination after about germination and growth occurs 20 days; subsequent growth quickly: plants grow 7 m, half occurs quickly their adult height, in their first two years Table 1. Characteristics of S. affinis and S. pedunculata. (Blaschke and Sanders 2009: pp.177-191; Durham 2016; Itow 1995: pp.17-30; Nielsen et al. 2002: pp.139-153; Traveset et al. 2016: pp.207-213)

Figure 2. A. Range of S. affinis (green) and of S. penduculata (blue) (Blaschke and Sanders 2009: pp.177-191). B. A juvenile S. affinis plant. C. A juvenile S. pedunculata plant. D. The head of an S. affinis flower with white ray florets. (Birnbaum, 2018)

In explaining how these species support Darwinian evolution, this paper attempts the following:

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1. to describe the variation in S. affinis populations—the only species for which data are

available—on different islands;

2. to investigate why S. affinis flowers have ray florets while those of S. pedunculata do

not; and

3. to explain the evolutionary history of the S. pedunculata lifecycle, characterized by

the growth and eventual mass die-off of one generational cohort and the resulting

growth of a new cohort.

Hypotheses

1. The variation in S. affinis is based on geography (i.e., organisms that live closer

together are more similar than those that live farther apart);

2. S. affinis developed ray florets while S. pedunculata did not because fewer pollinators

are present in the arid and transitional zones than in the highlands; and

3. S. pedunculata’s generational lifecycle resulted from periodic, major climate changes

(i.e., El Niño Southern Oscillation events).

In addition, this paper addresses how the S. pedunculata lifecycle affects efforts to conserve the species, ranked vulnerable by the International Union for Conservation of Nature

(Tye and Loving 1998). Thus, this paper will encourage further exploration of the Scalesia genus as a model for in situ evolutionary study and will provide advice on how to best protect

S. pedunculata, a critical member of many island ecosystems.

Part 1: Intraspecies Variation in S. affinis

Variation forms the backbone of Darwin’s theory of evolution in On the Origin of

Species. While the Galapagos mockingbirds catalyzed his doubts concerning the immutability of

Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018 species, his observations of domesticated animals—especially pigeons—led to his conclusion that variation exists not only between species but also within species. This concept, in turn, helped him show the arbitrariness of labelling any group of organisms that share similar features as an absolute, distinct species; further, he advanced the idea that any organisms considered as separate species are merely morphologically distinct variations of a common ancestor. In this way, one “species” with two isolated sub-populations can, given enough time, become two

“species”: i.e., a “species” can originate.

As L.R. Nielsen’s 2004 genetic research demonstrates, S. affinis displays the intra-species variation that Darwin discusses in On the Origin of Species. Nielsen (2004: pp.434-442) analyzed the DNA of seven S. affinis populations—four from Isabella, two from Floreana, and one from Santa Cruz—using biparental and maternal markers. (Biparental markers are located in autosomal chromosomes, meaning they are affected by both parents’ genetic makeup; maternal markers are located in maternally inherited mitochondrial DNA.) The biparental data show the overall variation in S. affinis. Of the 286 biparental markers, only 129 (45%) were monomorphic

(i.e., their DNA sequence was identical in greater than 98% of all individuals) (Nielsen 2004: pp.434-442). By contrast, a similar biparental analysis of Helianthus annuus, the common sunflower and another Asteraceae species, found 72% monomorphism (Gedil et al. 2000: pp.213-221). (The increased variation in S. affinis compared to its more widespread relative may be due to the Galapagos’ remoteness: evidence suggests that isolation increases the survival of mutant forms of an organism (Nielsen 2004: pp.434-442).) Genetic analysis of S. affinis thus reveals significant intraspecies variation.

Further, the biparental markers reveal that the variation in S. affinis populations corresponds to their geographic proximity to each other, supporting the paper’s first hypothesis

Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018 that variation and geography are linked. For example, individuals from Isabela and Floreana differ in sequence in 30 of the polymorphic markers. Analyzing the differences between each of the 21 possible pairs of the seven populations, Nielson (2004: pp.434-442) concludes that “the pairwise differences among populations within the same island were clearly lower than among populations from different islands”; he summarizes this in Figure 3. Indeed, the genetic analysis reveals differences at population-level resolution. Using the polymorphic markers to group the individuals by genetic similarity results in a perfect reconstitution of four of the populations: one of the four from Isabella, the two from Floreana, and the one from Santa Cruz. Using at least 140 markers to form the groups results in a miss-assignment rate of under ten percent. The two maternal markers also support the connection between geographic separation and genetic variation. One allele of one of the markers is present only in the four Isabella populations; among these four, the three southern populations have a different allele of the other marker than does the northern population. The relationship between distance and genetic variation holds even within a single population, at least at certain distances. Individuals 240 meters to 360 meters apart are significantly more genetically dissimilar than are those closer together. At further distances, however, the statistical significance disappears. Thus, on inter-island, inter-population, and even intra-population levels, geography governs the distribution of the variation in S. affinis: proximity positively correlates with genetic similarity. (Nielsen 2004: pp.434-442)

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Figure 3. A tree representing the relative differences between the seven populations (Nielsen 2004: pp.434-442).

In this way, S. affinis is a model for Darwin’s conception of variation. Substantial heritable variation exists within a supposedly uniform species in an endemic Galapagos genus; investigating the distribution of that variation reveals several sub-populations with distinct genetic markers. In Darwin’s terms, S. affinis is thus comprised of multiple variants, or “incipient species”: over time, the current, relatively small differences might grow into deviations large enough to warrant the creation of a new set of species labels (Darwin 1964: p.52). How, though, would this growth occur? Darwin asked and answered this question, too. His solution involves another central evolutionary tenet—natural selection.

Part 2: Natural Selection Acting on Ray Florets

After demonstrating the remarkable diversity within species, Darwin presents a chain of reasoning to reach natural selection. He starts by remarking on what he terms the “struggle for existence”—i.e., each organism constantly faces death and must compete for the resources to

Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018 survive and procreate (Darwin 1964: p.60). Then, he states that some adaptations increase an organism’s chance of survival and reproduction. Combining this idea with the concept of omnipresent variation, he concludes that organisms change over time because the prevalence of mutations that make an individual more fit should increase in each generation. Examining why S. affinis has ray florets while S. pedunculata does not demonstrates this process, which Darwin coined as “natural selection” (Darwin 1964: p.6).

Two possible explanations exist for this difference between S. affinis and S. pedunculata.

First, as stated in the paper’s second hypothesis, perhaps the relative lack of pollinators in

S. affinis’s environment prompted the ancestors of S. affinis either to retain or to develop ray florets to attract pollinators. (Some evidence suggests that the common ancestor of the Scalesia genus had ray florets (Nielsen et al. 2002: pp.139-153).) Second, perhaps the difference in stature of the two species explains the presence or absence of ray florets—i.e., S. pedunculata’s unusual height for an Asteraceae species may make ray florets less effective. In either case, the morphological variation between the two species resulted from differences in selective pressure: according to both explanations, selective pressure to develop ray florets exists only in S. affinis because only in that species do ray florets provide a reproductive advantage. Experimental data support the first hypothesis; observational data undermine the second.

In 2002, Nielsen et al. tested the importance of the presence or absence of ray florets: they identified pairs of capitula (i.e., flower heads) in S. affinis and S. penduculata plants; in each

S. affinis pair, they removed the ray florets from one of the capitulum, and in each

S. pedunculata pair, they attached ray florets to one of the capitulum. The unchanged capitulum of each pair acted as a control, as it was the same age and exposed to the same conditions as its neighboring, altered capitulum. Nielsen et al. (2002: pp.139-153) analyzed the fruit produced by

Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018 each capitulum to determine the effect of ray florets on reproductive success. In addition, for

S. affinis capitula, they counted how many pollinators visited each capitulum over fifteen hours, and they measured pollen deposition for each capitulum over eight hours.

They found that the precense of ray florets significantly increased reproductive success for S. affinis but not for S. pedunculata. Regarding S. affinis, capitula with ray florets received twice as many pollinator visits as did capitula without them. Further, the most important

Galapagos pollinator—the carpenter bee Xylocopa darwini—showed a preference for capitula with ray florets: X. darwini were nearly twice as likely to move to a rayed capitulum than to a rayless capitulum. The greater number of pollinator visits to rayed capitula translated into more pollen deposition. Figure 4 summarizes these data. In addition, rayed S. affinis capitula reproduced more successfully; fruit from rayed capitula had significantly more achenes (i.e., viable seeds) with embryos than did fruit from rayless capitula. Meanwhile, S. pedunculata plants showed no difference in reproductive success between rayed and rayless capitula; the percent of achenes with embryos was about two thirds regardless of the presence or absence of ray florets. Thus, S. affinis plants rely on ray florets to ensure reproductive success, and

S. pedunculata plants do not jeopardize their reproductive success by lacking ray florets.

(Nielsen et al. 2002: pp.139-153)

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Figure 4. A. Number of pollinator visits to S. affinis capitula with and without ray florets. B. Number of X. darwini movements to and from S. affinis capitula with and without ray florets. C. Difference in amount of pollen grains deposited on S. affinis capitula with and without ray florets; error bars show ± SEM. (Nielsen et al. 2002: pp.139-153) D. A female X. darwini bee on an S. affinis flower (Birnbaum, 2018).

This experiment supports the first hypothesis because it demonstrates how selective pressure might have resulted in S. affinis producing ray florets and S. pedunculata lacking them.

S. affinis grows in a region with fewer pollinators than the lush, highland environment in which

S. pedunculata resides. (For comparison, Nielsen et al. (2002: pp.139-153) observed that

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S. pedunculata received nearly three times more pollinator visits per hour than did S. affinis.)

Accordingly, S. affinis plants with ray florets have a significant reproductive advantage over S. affinis plants without them, especially regarding attracting X. darwini, the most valuable pollinator. (This difference is even more important because S. affinis exists on the edge of X. darwini’s range.3) This selective pressure explanation suggests that natural selection resulted in

S. affinis plants universally having ray florets. Meanwhile, as ray florets provide very little advantage to S. pedunculata, natural selection did not favor ray floret development in them.

Indeed, some selective pressure might have even incentivized a lack of ray florets, as growing them squanders resources. In this way, substantial evidence exists that selective pressure based on the availability of pollinators contributed to this morphological difference between S. affinis and S. pedunculata flowers. (Nielsen et al. 2002: pp.139-153)

As previously mentioned, another possible explanation exists for this difference: perhaps the selective pressure resulted from the difference in height between the two species. If this was the case, though, this pattern (i.e., short plants with and tall plants without ray florets) should hold for other species. This is not the case. In the Scalesia genus alone, nine of the eleven shrub species never grow florets, indicating that a small stature does not guarantee that ray florets present a selective advantage (Nielsen et al. 2002: pp.139-153) In addition, there exist tall trees with ray florets—albeit not in the Galapagos. For example, the Lepidaploa polypleura, the

Montanoa hexagona, and the Montanoa revealii trees, which reside in Southern Mexico, can grow up to about twenty meters and have ray florets (Pruski 2017: pp.1-39; Grantham and Tan

1999: pp.50-56). (The comparison of S. pedunculata to these three species assumes a similar pollinator condition exists in Southern Mexico and in the Galapagos.) Because there are several

Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018 cases of short plants without and tall ones with ray florets, the hypothesis that the variation in height between S. affinis and S. pedunculata explains the ray floret difference seems unlikely.

Although the available evidence supports only the first hypothesis in this section (i.e., the pollinator hypothesis), both hypotheses show the potency of using the divergent morphology of

S. affinis and S. pedunculata to demonstrate natural selection. As only a single gene seems to control ray floret development, ray florets are prime examples of the common, heritable variation that Darwin discussed in On the Origin of Species (Nielsen et al. 2002: pp.139-153). When ray florets convey a selective advantage—as they do in S. affinis, probably because of the dearth of pollinators in its environment—plants with the ray floret gene prosper, passing the gene to the next generation; eventually, the entire population will have ray florets, as is the case for

S. affinis. Thus, the Scalesia genus demonstrates not only Darwin’s conception of variation but also his idea of natural selection.

Part 3: Investigating S. pedunculata’s Lifecycle

Natural selection helps explain S. pedunculata’s lifecycle—characterized by generational growth and die-off. At first, this lifecycle seems counterintuitive. Why does S. pedunculata, which so dominates its environment that the region is named for it, have a similar reproductive strategy to mayflies? And what makes the S. pedunculata forests in the Galapagos different from other Asteraceae tree forests, which have normal, asynchronous lifecycles? Perhaps the answer is the Galapagos itself: one hypothesis for the evolution of S. pedunculata’s lifecycle is that the extremely harsh conditions during the periodic El Niño Southern Oscillation events exert a selective pressure against long life; as a result, all of the trees in an S. pedunculata stand tend to die at the same time, meaning there is only one time when new trees can grow, especially as

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S. pedunculata cannot grow in shade. Data on the ages of S. pedunculata stands over the past century support this hypothesis.

Syuzu Itow preformed an observational study of an S. pedunculata stand—established on an abandoned field—on Santa Cruz from 1978 to 1991. In 1978, the stand was four years old, and the average tree stem circumference was 20 cm. By 1981, the average circumference increased to around 35 cm, reflecting the aging of the trees from 1978 and the lack of new growth. By 1987, after a severe El Niño event in 1982/83, the distribution of tree stem circumferences had become bimodal: the majority of trees had small stem circumferences; a few had an even larger average stem circumference than did the trees in 1981. As Itow (1995: pp.17-30) speculates, this suggests that the majority of the 1978/81 cohort died during the El

Niño event, leading to a population comprised mostly of a younger generation of trees with some survivors from the older generation. By 1991, only one (now very large) original tree remained, and the aging younger generation comprised the rest of the stand. Figure 5 summarizes these populational data. (Itow 1995: pp.17-30) This study demonstrates that El Niño events (which generally stress Galapagos ecosystems) can cause mass die-offs among S. pedunculata populations, meaning plants that do not reproduce before these events do not contribute to the genetic composition of the next generation. These periodic climate events thus place selective pressure on S. pedunculata plants to prioritize (1) shorter lifespans and (2) new growth just after a mass die-off, when the most sunlight is available. (Severe El Niño events occur about once every two decades, not accounting for the effects of anthropogenic climate change (Cai et al.

2014: pp.111-116).) Assuming that severe El Niño events cause the periodic S. pedunculata die- offs, they likely have significantly contributed to the evolution of the generational S. pedunculata lifecycle.

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Figure 5. A. Quantitative data of the circumference at breast height for the Santa Cruz S. pedunculata stand from 1978 to 1991. 1978a and 1978b are subpopulations of slightly different ages. B. Images of the stand in 1978 (left), 1981 (middle), and 1987 (right). The red lines represent the 1982/83 El Niño event. (Itow 1995: pp.17-30)

However, two later studies challenge this assumption. First, Itow and Dieter Mueller-

Dombo (1998: pp.333-339) analyzed S. pedunculata stand data from as far back as the 1930s.

They found that the die-off that occurred in the late 1930s was not connected to a major El Niño event. Similarly, James R. and William A. Runkle (2005: pp.12-15), who followed Itow’s stand on Santa Cruz, found that the plants were unaffected by the powerful 1997/98 El Niño event. The

Runkles (2005: pp.12-15) surmised that Itow’s stand might have survived due to its youth; nonetheless, these two studies question whether El Niño events played the primary role in shaping the S. pedunculata lifecycle. Indeed, Itow and Mueller-Dombo (1998: pp.333-339) suggest that another factor was at least as important: the low diversity of canopy species and 15

Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018 resulting simple forest structure. This factor makes S. pedunculata stands susceptible not only to severe climate changes but also to such unpredictable conditions as an abnormally high tree density—caused by low mortality among young trees—and a suboptimal stand location. (Itow and Mueller-Dombo 1998: pp.333-339; J.R. and W.A. Runkle 2005: pp.12-15)

The Runkles’ (2005: pp.12-15) and Itow and Mueller-Dombo’s (1998: pp.333-339) studies thereby demonstrate the insufficiency of the initial hypothesis: El Niño events do not fully explain the evolution of the S. pedunculata lifecycle. These articles, though, do support the theory that the periodic El Niño climate perturbations combine with the simple forest structure to create the selective pressures described above. Several ways to further test this theory exist. First, growing individual S. pedunculata plants in a controlled environment would enable an evaluation of the lifespan and health (as assayed by, for example, trunk diameter and root depth) of the species in the absence of the variable Galapagos climate. Such measurements would help determine how much of the S. pedunculata lifecycle to attribute to genetics as opposed to the extreme Galapagos climate and poor forest structure. That is, perhaps S. pedunculata die-offs have no relationship to the plant’s genetics, and instead, the lifecycle directly results from suboptimal environmental conditions. Second, studying the lifecycle of S. cordata—one of the other two tall Scalesia trees—could provide further verification: finding that S. cordata also follows a generational lifecycle, in which severe El Niño events often separate the generations, would support the hypothesis. Indeed, one study provides preliminary support for such a lifecycle (Lawesson 1998: pp.87-93), while another finds the opposite (Hamann 2000: pp.223-250). Even in the absence of such research (which would take decades), the revised hypothesis demonstrates the potency of Darwin’s concepts of selective pressure and natural selection to generate reasonable explanations for biological phenomena.

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Part 4: Conservation of S. pedunculata

S. pedunculata—and the other giant daisies of the Galapagos—have faced substantial threats to their survival since the advent of the Anthropocene epoch in the archipelago. From

1915, only 1.1% of the S. pedunculata forest on Santa Cruz, less than 0.1% of the Santiago forest

(which exists only in artificial enclosures), and none of the San Cristóbal forest remain; regarding S. cordata, one of the other Scalesia trees, only 0.1% of the 1915 population on

Southern Isabella still grows (Mauchamp and Atkinson 2009: pp.108-112).4 Myriad reasons have contributed to the near extinction of S. pedunculata. Mauchamp and Atkinson (2009: pp.108-112) cite logging and agriculture as the causes of Scalesia decline during the early twentieth century. The introduction of ungulates, such as goats, pigs, and donkeys, also contributed to the destruction of the Scalesia forests. On Santiago, goats alone cleared the forests.

Conservation efforts, especially since the turn of the twenty-first century, have focused preservation efforts on addressing these problems. For example, as part of the Isabella Project

(an international multimillion-dollar conservation program), conservationists eradicated the goats on Santiago. Unfortunately, these efforts have not resulted in substantial Scalesia growth.

Despite the Isabella Project, on Santiago, there has been no S. pedunculata growth outside of the enclosures, and Scalesia trees have not reclaimed abandoned farms on inhabited islands. As

Mauchamp and Atkinson (2009: pp.108-112) report, invasive plant species form the primary obstacle to Scalesia regrowth.

One reason for this is S. pedunculata’s generational lifecycle, meaning that—in addition to providing fertile ground for exploring evolutionary hypotheses—the lifecycle demonstrates

Foster Birnbaum Evolution and Conservation in the Galapagos 10/12/2018 how Galapagos organisms’ adaptations to their environment now jeopardize their survival: invasive plants prevent shade-intolerant S. pedunculata shoots from growing after mass die-offs.

In 2006, Jorge L. Rentería and Chris Buddenhagen conducted an observational study of invasive species in an S. pedunculata forest on twenty-five hectares around the Los Gamelos volcano on

Santa Cruz. After recording every tree along sixty-nine parallel lines fifteen meters apart, they found significant numbers of ten invasive species; the two most threatening were Cedrela odorata, a semi-deciduous tropical tree that can reach thirty meters, and Rubus niveus, the blackberry bush (Figure 6). Rentería and Buddenhagen (2006: pp.31-35) surmise that many of the larger invasive trees (including C. odorata) gained a foothold during the 1982/83

S. pedunculata die-off, and they note that the dense R. niveus understory prevents new

S. pedunculata growth. While Rentería and Buddenhagen (2006: pp.31-35) recognize the need to continuously remove invasive species in S. pedunculata forests, they stress the acute need for such activity during and directly after mass die-offs.

Figure 6. A. A C. odorata tree. B. A R. niveus bush. (Birnbaum, 2018)

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Conclusion

The analyses here support the first two of the paper’s hypotheses:

1. genetic S. affinis data support the existence of a relationship between variation and

geography; and

2. experimental data on the reproductive advantage of ray florets support the pollinator

explanation for the presence of ray florets in only some Scalesia species.

The analyses also demonstrate the inadequacy of the third hypothesis:

3. observational data on S. pedunculata groves demonstrate that severe El Niño events

do not alone account for S. pedunculata’s lifecycle and suggest that the low forest

stability also contributes to the die-offs.

Thus, in identifying the heritable variation in the Scalesia genus and in using natural selection to understand adaptations of the genus, this paper demonstrates how the Galapagos daisies, like other endemic Galapagos organisms, are in situ models for evolutionary theory. The slight variation in S. affinis populations across different islands might, for instance, provide insights on how geographic distance contributes to differences between populations; the divergence between S. affinis and S. pedunculata with respect to ray florets and S. pedunculata’s generational lifecycle help to explain how the environment creates selective pressures for and against certain characteristics. However, for the Scalesia genus to provide such research benefits, the genus must exist; without conservation efforts that consider the adaptations of Scalesia organisms to the Galapagos, their existence is far from guaranteed. Future research can further clarify how the Scalesia genus illuminates tenets of evolutionary theory and how it contributes to the intelligent design of Galapagos preservation efforts.

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End-Notes 1. Ray florets are looped petals on the head of the flower that play a role in pollinator attraction.

2. Evidence—including the feeding habits of tree finches—suggests a co-evolutionary history

between Scalesia trees and tree finches (Durham 2018).

3. Some evidence suggests that Scalesia species on the edge or outside of X. darwini’s range

have ray florets while species in the center of the range lack them: S. affinis and S. baurri

subsp. hopkinsii fall in the former category, and S. pedunculata and S. divisi fall in the later

(Nielsen et al. 2002: 139-153).

4. Indeed, in September 2018, I spent eight days in the Galapagos, including substantial time on

Santa Cruz and San Cristóbal, and saw no S. pedunculata forests and very few

S. pedunculata trees.

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