The Role of Analogy in Adaptive Explanation

<p>Cases of 'convergence' (traits which have independently evolved in two or more lineages) could play an important role in the construction and corroboration of adaptive hypotheses. In particular, they could inform us about the evolutionary histories of novel traits. However, there is a problem of causal depth in the use of analogies. Natural Selection's affect on phenotype is constrained by phylogenetic history to a degree that we are unfounded in projecting adaptive stories from one lineage to another. I will argue for two approaches to resolve this issue. First, by constraining our catchment area to closely related lineages we can control for developmental noise. Second, by integrating analogies into explanations which incorporate other streams of evidence or bootstrapping an analogous model across many instantiations, we can overcome the problem of causal depth.</p>

The Role of Analogy in Adaptive Explanation

<p>Cases of 'convergence' (traits which have independently evolved in two or more lineages) could play an important role in the construction and corroboration of adaptive hypotheses. In particular, they could inform us about the evolutionary histories of novel traits. However, there is a problem of causal depth in the use of analogies. Natural Selection's affect on phenotype is constrained by phylogenetic history to a degree that we are unfounded in projecting adaptive stories from one lineage to another. I will argue for two approaches to resolve this issue. First, by constraining our catchment area to closely related lineages we can control for developmental noise. Second, by integrating analogies into explanations which incorporate other streams of evidence or bootstrapping an analogous model across many instantiations, we can overcome the problem of causal depth.</p>

The rise and fall of the ancient northern pike master sex-determining gene

The understanding of the evolution of variable sex determination mechanisms across taxa requires comparative studies among closely related species. Following the fate of a known master sex-determining gene, we traced the evolution of sex determination in an entire teleost order (Esociformes). We discovered that the northern pike (Esox lucius) master sex-determining gene originated from a 65 to 90 million-year-old gene duplication event and that it remained sex linked on undifferentiated sex chromosomes for at least 56 million years in multiple species. We identified several independent species- or population-specific sex determination transitions, including a recent loss of a Y chromosome. These findings highlight the diversity of evolutionary fates of master sex-determining genes and the importance of population demographic history in sex determination studies. We hypothesize that occasional sex reversals and genetic bottlenecks provide a non-adaptive explanation for sex determination transitions.

The Attraction of Synchrony: A Hip-Hop Dance Study

This study investigated an evolutionary-adaptive explanation for the cultural ubiquity of choreographed synchronous dance: that it evolved to increase interpersonal aesthetic appreciation and/or attractiveness. In turn, it is assumed that this may have facilitated social bonding and therefore procreation between individuals within larger groups. In this dual-dancer study, individuals performed fast or slow hip-hop choreography to fast-, medium-, or slow-tempo music; when paired laterally, this gave rise to split-screen video stimuli in which there were four basic categories of dancer and music synchrony: (1) synchronous dancers, synchronous music; (2) synchronous dancers, asynchronous music; (3) asynchronous dancers, one dancer synchronous with music; and (4) asynchronous dancers, asynchronous music. Participants’ pupil dilations and aesthetic appreciation of the dancing were recorded for each video, with the expectation that these measures would covary with levels of synchronization. While results were largely consistent with the hypothesis, the findings also indicated that interpersonal aesthetic appreciation was driven by a hierarchy of synchrony between the dancers: stimuli in which only one dancer was synchronous with the music were rated lower than stimuli in which the dancers were asynchronous with each other and with the music; i.e., stimuli in which the dancers were unequal were judged less favorably than those in which the dancers were equal, albeit asynchronously. Stimuli in which all elements were synchronous, dancers and music, were rated highest and, in general, elicited greater pupil dilations.

The rise and fall of the ancient northern pike master sex determining gene

Sexual reproduction is a ubiquitous basic feature of life and genetic sex determination is thus widespread, at least among eukaryotes. Understanding the remarkable diversity of sex determination mechanisms, however, is limited by the paucity of empirical studies. Here, we traced back the evolution of sex determination in an entire clade of vertebrates and uncovered that the northern pike (Esox lucius) master sex-determining gene initiated from a 65 to 90 million-year-old gene duplication and remained sex-linked on undifferentiated sex chromosomes for at least 56 million years. Contrasting with its ancient origin, we identified several independent species- or population-specific transitions of sex determination mechanisms in this lineage, including an unexpected complete and recent Y-chromosome loss in some North American northern pike populations. These findings highlight the diversity of the evolutionary fates of master sex-determining genes and raise the importance of careful considerations of population demographic history in sex determination studies. Our study also puts forward the hypothesis that occasional sex reversals and genetic bottlenecks provide a non-adaptive explanation for sex determination transitions.

Histological evidence of reproductive senescence in Atlantic herring (Clupea harengus)

Senescence is the age-specific decline in fitness of adult organisms principally associated with a decline in survival rate (actuarial senescence) and fecundity (reproductive senescence). Although common in natural populations of many taxa, there are few examples in fishes. A recent study found age-specific increases in the relative frequency of macroscopically non-reproductive Atlantic herring (Clupea harengus Linnaeus, 1758), consistent with reproductive senescence in a number of Canadian populations. However, a non-adaptive explanation for these patterns, unrelated to senescence, could not be definitively ruled out. Here, we present the results of histological examination of herring gonads undertaken to examine this and another hypothesis. The sample size of macroscopically senescent gonads was small (n = 4 females), a function of the low abundance of large, older, purportedly senescent, herring in these populations subjected to high mortality rates, and constraints on obtaining fresh samples. The results indicate that these fish were senescent and not merely skip spawning, providing further evidence of senescence and the occurrence of a post-reproductive period in herring in Atlantic Canada. Based on existing theory for the evolution of senescence, observations of actuarial senescence in Norwegian spring-spawning herring suggest that this population may also experience reproductive senescence and that the phenomenon may occur broadly in the species.

Repeats Of Unusual Size in Plant Mitochondrial Genomes: Identification, Incidence and Evolution

Plant mitochondrial genomes have excessive size relative to coding capacity, a low mutation rate in genes and a high rearrangement rate. They also have non-tandem repeats in two size groups: a few large repeats which cause isomerization of the genome by recombination, and numerous repeats longer than 50bp, often found in exactly two copies per genome. It appears that repeats in the size range from several hundred to a few thousand base pair are underrepresented. The repeats are not well-conserved between species, and are infrequently annotated in mitochondrial sequence assemblies. Because they are much larger than expected by chance we call them Repeats Of Unusual Size (ROUS). The repeats consist of two functional classes, those that are involved in genome isomerization through frequent crossing over, and those for which crossovers are rare unless there are mutations in DNA repair genes, or the rate of double-strand breakage is increased. We systematically described and compared these repeats, which are important clues to mechanisms of DNA maintenance in mitochondria. We developed a tool to find non-tandem repeats and analyzed the complete mitochondrial sequences from 135 plant species. We observed an interesting difference between taxa: the repeats are larger and more frequent in the vascular plants. Analysis of closely related species also shows that plant mitochondrial genomes evolve in dramatic bursts of breakage and rejoining, complete with DNA sequence gain and loss, and the repeats are included in these events. We suggest an adaptive explanation for the existence of the repeats and their evolution.

The naked ape as an evolutionary model, 50 years later

Evolution acts through a combination of four different drivers: (1) mutation, (2) selection, (3) genetic drift, and (4) developmental constraints. There is a tendency among some biologists to frame evolution as the sole result of natural selection, and this tendency is reinforced by many popular texts. “The Naked Ape” by Desmond Morris, published 50 years ago, is no exception. In this paper I argue that evolutionary biology is much richer than natural selection alone. I illustrate this by reconstructing the evolutionary history of five different organs of the human body: foot, pelvis, scrotum, hand and brain. Factors like developmental tinkering, by-product evolution, exaptation and heterochrony are powerful forces for body-plan innovations and the appearance of such innovations in human ancestors does not always require an adaptive explanation. While Morris explained the lack of body hair in the human species by sexual selection, I argue that molecular tinkering of regulatory genes expressed in the brain, followed by positive selection for neotenic features, may have been the driving factor, with loss of body hair as a secondary consequence.

Pollination unpredictability and ovule number in a South-Andean Proteaceae along a rainfall gradient

Pollen limitation occurs frequently in plant populations and, as result, many ovules are wasted. One possible adaptive explanation posits that ovule overproduction represents a ‘bet-hedging’ strategy against pollination inefficiency. This hypothesis is supported by comparative evidence showing that unpredictability in pollen receipt is positively associated with an increasing number of ovules per flower across species. Yet, this proposition has not been tested at the intraspecific level, where natural selection operates. Here, we evaluated the relationship between pollination unpredictability, considering both pollination quantity and quality, and number of ovules per flower, across 16 populations of the south-Andean generalist treelet Embothrium coccineum J.R.Forster and G.Forst from north-western Patagonia, which occurs along a west–east gradient of decreasing rainfall. Despite sizable variation in mean number of ovules per flower, we found no increase in ovule production with increasing pollination unpredictability across populations. Instead, we found that mean number of ovules per flower decreased with decreasing rainfall. Therefore, in this species, there was no support for the proposal that ovule overproduction represents a bet-hedging strategy against unpredictable pollen receipt. Rather, the number of ovules per flower seems to be conditioned primarily by resource availability.