Unmasking Natural Selection

BOOK REVIEWS || JOURNAL OF CREATION 32(2) 2018

Unmasking natural selection

40 Years of Evolution: Darwin’s finches on Daphne Major Island Peter and Rosemary Grant Princeton University Press, Princeton, NJ, 2014

Jean K. Lightner

o many people, the word evolution Tis equated to humans descending from ape-like creatures, or all life originating from simpler life forms. However, if one studies biology, it quickly becomes apparent that the word has a much broader range of meanings. The Grants are evolutionary biologists, and the evolution In fact, this review is being written described in this book refers to the after the third time I went through change in heritable characteristics the whole book. I think that speaks of a population over time. This book highly to the value of the book: the presents the results of their 40 years basic concepts can be understood by observing patterns of change in several reading the book once, but a deeper populations of finches on an island in understanding is engendered by the Galápagos. No, the finches didn’t reading it multiple times a few years turn into a different kind of bird; apart. instead, the Grants’ observations are a gold mine for creationists wishing to understand natural history in a biblical The draw of the finches worldview. on Daphne The book is very nicely illustrated, The preface and early chapters containing not only pictures but tables, charts, and boxes with more spend a lot of time providing the detailed information concerning context for their study, including what is discussed in the main text. It relevant information about the includes multiple appendices, a list of Galápagos in general, and Daphne abbreviations, a glossary, references, Major Island (hereafter, Daphne) in and a subject index. Despite their particular. As evolutionary biologists, best attempts to make the book as the Grants are interested in knowing lay-friendly as possible, some of how new species form—a topic of the chapters include rather technical great interest to creationary biologists population genetics concepts and as well. They discuss reasons for discussion of analyses. The summary choosing finches on an island at the end of each chapter is helpful (i.e. a small population of easily to gauge if the main concepts of the approachable birds that can be marked chapter were grasped. I found myself for individual identification and needing to reread portions of the book. tracked through time; figure 1). Their

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method of tracking finch populations of a missing competitor. They list Heritable traits and through time is what makes their five assumptions of the character natural selection work so valuable. It steps away from release hypothesis, and spend several For evolution (in the change over the armchair philosophy and just-so chapters discussing how they went time sense, that no one denies) to stories that predominate when looking about demonstrating the veracity of occur, heritable variation must exist in at the current state of a population and those assumptions in their study on the population. The Grants demonstrate attempting to infer how it reached its the Galápagos finches. It is when that beak length, width, and depth are present condition. they list the five assumptions that the all highly heritable (and variable) in As the bird species and the foods proverbial elephant enters the room. the medium ground finch (G. fortis). they eat are being presented, the The listed assumptions are reasonable They also show that reproductive Grants explain what brought them to enough, but the variability to expand output varies with the amount of rain Daphne in particular. Many islands into a new niche has to come from and available food, and thus has a contain both the medium and small somewhere. This is the primary blind heritability close to zero. The primary ground finches Geospiza( fortis and spot of most evolutionary explanations. factor that affected successfully raising G. fuliginosa, respectively). Daphne That is, there is a hidden assumption offspring that themselves reproduced only harboured the former granivorous that appropriate variability is just was related to longevity. And longevity finch, but it was smaller than on other sort of going to magically be there was related to survival during the dry islands in the Galápagos. Thus, on and natural selection gets the credit season. Daphne, the medium ground finch for any adaptation. Now, to be fair to Four years into their study, the not only occupies the niche it does the Grants, they do discuss sources Grants observed natural selection on other islands, but also that of the of variability in the book (standing in action. There was a drought that small ground finch. The Grants were variation, interspecific hybridization, resulted in selective mortality. Beak intrigued, and wanted to know why. and mutation). However, they do not depth was the strongest contributor, Drawing on previous work, deal in depth with how appropriate as a greater beak depth was necessary particularly that of David Lack, they variation shows up in appropriate to effectively crack and consume the describe the concept of character timeframes, especially as it might larger seeds that remained available release, or expansion into the niche relate to mutation. well into the drought. This, and other related observations over the study, make it clear that natural selection does not always act slowly and imperceptibly bringing gradual changes for the good of the organisms involved, as Darwin had argued.1 The Grants found that natural selection occurs most strongly when the environment changes. In the Grants’ study of the finches, this was during the droughts. Not only did natural selection occur most strongly then, but it was inconsistent in the direction it acted. The first drought they observed resulted in a selective mortality affecting smaller-beaked birds. However, due to exceptionally wet intervening years that resulted in explosive growth of plants bearing smaller seeds, another drought resulted in selective mortality in the larger-beaked birds. This oscillating Figure 1. An example of the variation in beak size and shape among finches in the Galápagos. The large ground finch (1) and medium ground finch (2) were observed on the island of Daphne Major. pattern of natural selection removes The small tree finch and green warbler-finch (3 and 4, respectively) are found on other islands. useful variation and can potentially

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put the finch population at risk of being The number of large ground breed with the medium ground finch extirpated from the island. finches increased slowly over the next (G. fortis). Hybrid offspring which As natural selection acted on decade, and then suddenly increased survived to reproduce would breed the medium ground finches during more dramatically. For many years with a medium ground finch, which the droughts, evolution resulted. there wasn’t much competition brought variation into the population. This simply means that in the next between them and the larger-beaked This was tremendously valuable, generation there was a shift in the medium ground finches (G. fortis). particularly after a drought which had average beak size. However, other That changed near the end of a two- selectively removed birds with smaller aspects of their study made it clear that year drought (2003–2005) when the beaks. Birds with smaller beaks are even in cases where natural selection large seeds they both consumed were much more efficient at consuming the appears to be acting, evolution does not depleted. The death toll was heavy for smaller seeds, and this hybridization always result. Instead, the net effect both species. In the medium ground allowed the resident medium ground of natural selection is influenced by finch G.( fortis), there was a size biased finch population to effectively exploit antagonistic selection, which can occur mortality (the larger-beaked birds were that resource again. on the same trait at different times, more severely affected), but in the Hybridization between these two or on different linked traits at the large ground finch (G. magnirostris) granivorous finches also explains mortality was not size biased. same time. Thus, it is incorrect to say the puzzle that brought the Grants The size biased mortality was an that natural selection is equivalent to to the island initially. The medium example of natural selection in action, evolution; it is merely one of several ground finch was relatively small on and evolution resulted. There was a possible causes. Daphne because it received genetic dramatic drop in the average beak material from the small ground finches size for the medium ground finch who occasionally stayed to breed. Character displacement population in the next generation. The Rather than establish a population, Grants point out that this example which could have eventually led to During the course of their study, of character displacement was not competition between the two species, the Grants also observed character from constant, ongoing competition. the small ground finch brought in displacement, the opposite of character Special conditions were required for useful variation. Since the medium release. In character displacement, it to occur. The reduction in average ground finch on Daphne never competition between species can beak size of the medium ground finch hybridized with the large ground finch result in divergence between the two. was dramatic, and this measurement (G. magnirostris), it had no way to The most striking example observed remained smaller throughout the rest regain the lost variation in the larger in their study is worth examining in of the study. size range after the drought depleted more detail. it in 2005. When their study began there A second factor that affects how were no large ground finches (G. Hybridization and genetic correlation a trait can change is how they are magnirostris) that bred on Daphne. correlated. Beaks can vary in three Occasional migrants would visit, It wasn’t just natural selection dimensions: length, width, and depth. but until a very favourable El Niño that caused changes in traits, such Often, two of the dimensions are year (1982) none stayed to breed. as average beak size. Hybridization correlated, and this has to do with the One important observation made between the medium ground finch underlying gene expression differences by the Grants is that of the migrants (G. fortis) and two other species (G. that control beak size and shape.2 who visited, those that stayed to fuliginosa and G. scandens) was While natural selection can easily shift breed were not a random sample important as well. The difference was the beak size up or down, changes in of the total visitors. They differed that hybridization was more persistent, beak shape are more complex. genetically (more heterozygous) and occurring at low levels throughout In addition to granivorous finches, phenotypically (larger beaks) from the study. This is in contrast to the Daphne is home to the cactus finch those that left. This is significant dramatic impact of natural selection (G. scandens). In the granivorous because evolutionists commonly operating during a drought. finches, beak length and depth increase assume that colonization is random There was no breeding population similarly with increases in size. In the for their models of gene flow and of small ground finches (G. fuliginosa) cactus finch, beak length increases statistical tests designed to detect on the island, but sometimes migrants considerably faster than beak depth natural selection. from this species would stay and as size increases. The longer beak

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facilitates feeding on the cactus plant (G. scandens). Then hybridization was having a long beak to feed on cactus, (Opuntia echios; figure 2). While observed between the two species. It the cactus finch beak became blunter natural selection is less effective in was never pervasive, as only a few from the incoming ground finch genes. transforming beak shape, hybridization individuals were involved in any While hybridization can cause two between the two species broke down particular breeding season. However, different species to coalesce, as was the correlation. it became persistent, happening in most beginning to happen in the above years. The hybrids would breed back example, it can also have a different to one of the parental species, bringing outcome. The Grants document this Coalescence or speciation in new variation. As they did so, a by tracing the offspring of a large juvenile that arrived on Daphne In the first few years of the study, dramatic shift in beak shape occurred. in 1981 and stayed to breed with there was a dramatic difference in beak This was most noticeable in the cactus the medium ground finches. In the finch population, as it was smaller. shape between the medium ground book it was suggested to be a cactus finch (G. fortis) and the cactus finch Despite the adaptive advantage of finch/medium ground finch hybrid, perhaps backcrossed to a medium ground finch (based on microsatellite comparisons). Subsequent genome sequencing indicates it was actually an Española cactus finch (G. conirostris), originating from over 100 km away.3 Despite being 70% larger and singing a different song, this large male immigrant bred with the medium ground finch. Some of his smaller offspring continued to do so, and became absorbed into the G. fortis population. However, part of his lineage quickly (within three generations) began interbreeding only among themselves, forming a distinct population. Initially the Grants had no idea what the outcome would be, so they called this population the Big Bird lineage and continued to track it. For over 30 years, the Big Bird lineage has remained separate, thus qualifying it as a separate species under standard rules of taxonomy. Its beak size is between that of the medium and large ground finch populations. In addition to consuming seeds in the expected size range, this new species can effectively exploit cactus as a food source.

Singing and hybridization In several different chapters, the Grants discuss the importance of song and morphology (i.e. size and shape) in choosing a mate. Young birds normally learn the song of their fathers, and the Figure 2. The Galápagos prickly-pear cactus (Opuntia echios) found on Daphne. Due to the greater males sing it as an adult. Females length in their beaks, cactus finches are skilled at biting open the fruits and buds on this plant, as recognize the song, and tend to mate well as probing the flowers for nectar. with a male like their fathers. There

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are a few circumstances that can claimed. The Grants’ observational However, in contrast to claims cause a breakdown in this pattern, data helps expose some of the poorer of ‘armchair’ evolutionists, natural including the death of the father during arguments that have been used to selection is not an explanation for any the formative years of the young. support natural selection as the adaptation because it does not explain Occasionally, a young bird might learn dominant explanation for the variety the origin of any trait. It was observed the song of a neighbour, which might of species we see today. Consider to be intermittent and oscillating, in not be of the same species. the following quote from the contrast to Darwin’s belief that it was In cases of hybridization between Understanding Evolution website (a continuous and always worked for the medium ground finch and the collaborative project of the University the benefit of the organisms involved. cactus finch, the offspring would of California Museum of Paleontology Natural selection is not the major breed back to the paternal species. and the National Center for Science factor involved in the diversification of Song and morphology appear to be the Education): the kinds God created at the beginning, reason why. In the case of the Big Bird “An adaptation is a feature that is but hybridization between species can lineage, these appear to be the factors common in a population because it now be seen as another important that prevent them from hybridizing provides some improved function. component of God’s providential care with other finches on the island. Adaptations are well fitted to their for his creatures.2 Thus, the main reason the species function and are produced by remain separate is behavioural, rather natural selection.”4 than the inability to produce viable, This is utter nonsense; no trait can References fit offspring. be produced by natural selection! The 1. Darwin, C., On the Origin of Species by Means of Natural Selection, or the Preservation of trait must already exist in the population Favoured Races in the Struggle for Life, 1st edn, (standing variation), or be brought in by p. 84, John Murray, London, 1859. Extrapolation some means (migration, hybridization, 2. Lightner, J.K., Finch beaks point to a Creator who provides, J. Creation 26(2):8–10, 2012. In the last few chapters, the Grants or mutation) within a timeframe where 3. Lamichhaney, S., Han, F., Webster, M.T., summarize their major findings, it can be helpful. Once the trait is in Andersson, L., Grant, B.R., and Grant, P.R., Rapid hybrid speciation in Darwin’s finches, including the long-term effects they the population, natural selection is Science 359(6372):224–228, 2018. observed from both natural selection one of several possible mechanisms 4. ‘Adaptation’, Understanding evolution, evolution. and hybridization. They point out that by which the trait can increase in berkeley.edu/evolibrary/article/evo_31, accessed 21 February 2018. there is an ebb and flow of various frequency, and thus become common. morphological traits over time, and Other mechanisms that can alter the that species can be ephemeral. They frequency of a trait include migration use what they have learned to make (in or out, based on suitability of traits inferences about the past (through for that environment), hybridization, a molecules-to-man evolutionary and meiotic drive (a well-known type lens) and the future. Except for the of non-Mendelian inheritance that needs timeframe and the assumption of to be considered in future studies). universal common ancestry, they It is realistic to view natural make a number of points that are selection as a shaping force. It can quite reasonable in a creationary result in divergence by eliminating worldview as well. What is missing overlapping phenotypes between two from the discussion is a sense of awe species. It may, at times, explain why for the Creator who provides for these certain variations are uncommon or birds even in a changing, sometimes absent. It is also one factor that may hostile, environment in our fallen contribute to the formation of a new world (Matthew 6:26; Luke 12:6–8). species, especially through allopatric speciation. While the latter wasn’t What does this mean for directly observed in the Grants’ study, they do provide examples of creationists? divergence between the same species One of the more obvious impli on different islands associated with cations of the Grants’ work is that different environmental conditions. If natural selection does not operate as this continues long enough, it could many evolutionists have traditionally result in speciation.