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February 8th, 2010 Bioe 109 Winter 2010 Lecture 14 The adaptationist program What is adaptation? - the term “adaptation” has different meanings in different fields of biology. 1. Acclimatization. - acclimatization refers to the physiological adjustment of individual organisms to different conditions (e.g., temperature, photoperiod). - this definition clearly entails no genetic change. - to evolutionary biologists, adaptation is used in two very different contexts. 1. The process of becoming adapted. - this definition is synonymous with the process of natural selection. - for the process of adaptation to occur there must exist genetic variation among individuals in the population. - differences in fitness must also exist among the different genotypes to drive the selective process. 2. The state of being adapted. - the second definition of adaptation is perhaps more widely used. - it refers to the end-point of this process, i.e., the state of being adapted. - individual characters of organisms are viewed as adaptations. - all adaptations are assumed to have evolved by the process of natural selection. - unlike the previous definition, however, we can not this process directly but must infer its past action. - this is where difficulties arise - inferring the selective history of a trait can be difficult. How do we study adaptations? - there are two basic approaches: 1. The experimental approach - hypotheses for the adaptive origins of traits are tested by experiments. 2. The comparative approach - hypotheses for the adaptive origins or traits are tested by: (a) performing comparisons among species (b) making observations within species 1. Experimental approaches - let’s use an example from the textbook to illustrate the power of experiments because it is a classic example. - this is the study done by Erik Greene (now at the University of Montana) and his colleagues on the small tephretid fly, Zonosemata vittigera. - the study involved testing a hypothesis for the origin of two rather peculiar features of the fly: 1. distinctive dark wing bands 2. wing-waving behavior (when threatened, the behavior of holding the wings perpendicular to the body and moving them up and down). - this behavior was noted by entomologists to be similar to the territorial threat display of jumping spiders. - why would this fly apparently mimic this threat display? - the explanation initially given by entomologists was that the fly was using this display to avoid predation by a variety of different predators. - since jumping spiders are extremely quick and have a rather nasty bite, predators would avoid tephretid flies performing this display. - therefore, it may have evolved as an adaptation to deter predators that hunt tephretid flies. - Greene et al. had an alternative explanation - that it evolved to intimidate the jumping spiders themselves. - this idea that mimicry of a predator behavior by a prey had not been identified previously. - to test this possibility a series of experiments were performed. - the first step in doing any experiment is to formulate the question and generate a specific null hypothesis to test. - in the example outlined here, the question tested was: “Do the wing markings and behaviors used by the tephretid fly actually mimic the threat display of the jumping spider thus allowing them to escape predation by the spider?” - Greene et al. considered three hypotheses that they were able to test by their experimental design: H0: Does not mimic jumping spiders (null hypothesis). H1: Mimics jumping spiders to avoid other predators. H2: Mimics jumping spiders to avoid predation by jumping spiders. - Greene et al. then set up five experimental groups to test these hypotheses. - to test whether wing markings and wing wavings were involved in the mimicry they used normal houseflies which 1) do not markings and 2) do not wave their wings. - they did an ingenious series of manipulations of the wings of these two flies to discriminate among the 3 alternative hypotheses. - they cut and reglued the wings of Zonosemata vittigera and houseflies. - in some treatments they replaced the tephretid wings back on the same individuals, in other treatments they swapped them with the wings of houseflies. - five experimental groups were set up: Group A B C Treatment Zonosemata untreated Zonosemata own wings Reglued Zonosemata housefly with housefly with Zon. wings wings housefly untreated wing waving no wing marks or wavings Effect tested wings marks surgery + wavings D E wing markings ____________________________________________________________________________ Hypothesis Predator Predicted outcome (X = attack and/or killed) ____________________________________________________________________________ H0 j. spider other X X X X X X X X X X X left alone X left alone X X X X X X left alone X X X X X X X (no mimicry) H1 j. spider other (mimicry - deters other predators) H2 j. spider other left alone X (mimicry - deters j. spiders) ____________________________________________________________________________ - the experiment was performed in a test arena in which they introduced the experimental groups in random order to jumping spiders belonging to 11 different species that had been starved for 2 days. - they also exposed experimental groups A, C, and E to an assortment of different predators (lizards, mantises, assassin bugs) and recorded who was captured and eaten. - the results are unequivocal in their support of H2. No. of spiders retreating: No. of spiders attacking/killing: A B C D E 15 5 15 5 2 18 2 18 0 20 - this study exemplifies the power of experiments. - in setting up the experimental groups properly, Greene et al. were able to test and discriminate among alternative hypotheses. - there is still a drawback with this approach that relates to the criticism I raised at the start of class. - what is it? - well, we still have no insights into whether the markings and wavings evolved explicitly for mimicking jumping spiders, or whether both traits evolved for a different reason and then were “coopted” for this new function. - for example, what if some wing patterning and waving displays initially evolved as a courtship display by male Zonosemata. - what if by chance, these courtship displays also happened to deter predation by jumping spiders on male flies. - if a mutation occurred that now had female Zonosemata performing these behaviors they would also gain a significant advantage over females that did not. - the precise pattern of wing markings and displays may have been “fine-tuned” over evolutionary time periods to confer an ever improving mimicry. - how can we examine whether this scenario is true? - one way would be to examine wing patterns, wing waving displays and courtship behavior in Zonosemata with that of related species of tephretid flies. 2. The comparative method - how do we go about proving that a trait is an adaptation? - in asking this question, we are really interested in obtaining evidence that the trait under consideration has evolved by natural selection and not some alternative. - there are three steps in carrying out the so-called “adaptationist program”: 1. Observe or describe some organism trait. 2. Formulate an adaptive hypothesis for the evolution of that trait. 3. Test hypothesis by experiment or by collecting additional data. - one must be careful in carrying out these steps. A. Comparisons among different species – the evolution of testes size in fruit bats - the comparative method commonly involves comparisons among different species to test hypotheses of adaptation. - up until very recently, it was undertaken without a proper appreciation of the phylogeny of the group under study. - now, one cannot undertake comparisons among species and publish it in a reputable scientific journal without using a test that corrects for the lack of independence. - let’s go over the example in the book involving testes size in bats. - bats are like many mammals in showing a substantial variation in testes size among different species. - one of the most popular explanations for this variation is that it reflects the outcome of sperm competition. - this is a form of sexual selection that occurs among males. - when females of a species mate with multiple males in a single breeding period there is competition among the male’s sperm over who fertilizes the female’s egg(s). - one strategy to increase male reproductive success is to increase ejaculate size since this can either physically displace, or simply substantially increase, a male’s probability of successfully fertilizing eggs. - in bats, variation also exists among species in the typical roost size. - in species that form larger roosts, the opportunity for sperm competition may be greater. - thus we might expect greater male testes size to be correlated with mean colony size. - in this example, a strong positive relationship exists between mean testes size and mean group size that is consistent with the prediction made by the sperm competition hypothesis. - there is a problem here - namely that our comparisons may have been strongly affected by the phylogeny of the group. - suppose we compared six bat species. - suppose that the phylogeny of the group showed two groups of three closely-related species. - both groups may have inherited their large or small testes sizes from common ancestors (not independently). - the evidence is now compromised - the species can no longer be treated as independent data points because they may have inherited testes size and group size from common ancestors. - to undertake statistical comparisons it is necessary to correct for this lack of independence. - the textbook outlines how one of these methods works - that of Felsenstein’s method of independent contrasts. B. Comparisons among individuals of the same species – the case of the polar bear - polar bears are white. - because they are unique among bears in being white, we can reasonably assume that this is derived trait. - in other words, polar bears evolved from a brown ancestor. - since polar bears live in the arctic, where they spend much of their time silhouetted against a background of snow, we might venture an hypothesis that their white pelt is an adaptation to life in the arctic. - in formulating this hypothesis, we are postulating an adaptive explanation for the evolution of the white pelt of polar bears. - what could have been the selective advantage of a white coat? - one reasonable guess would be that it represents an adaptation to hunting. - polar bears hunt seals, and being white against a white background facilitates the hunting of seals in this environment. - this may be termed the camouflage hypothesis. - we can test the camouflage hypothesis by testing one, or more, predictions that it makes. - one simple prediction is that polar bears should hunt seals in a manner that should take advantage of their camouflaged pelt. - sometimes they do. - for example, a paper by Sterling (1974) described the hunting strategies of 288 polar bears. - here’s the breakdown: 1 “sneak and pounce” 54 “jump and crush” 233 “sit and wait” - the “sneak and pounce” strategy is consistent with the camouflage hypothesis. - in the “jump and crush” strategy, the bear would use its keen sense of smell to detect the presence of a seal in an under snow lair. - the bear would then rush from 50-100 meters downwind, leap into the air and crush the seal to death by landing on it with its great weight. - in 233 of the 288 bears observed by Sterling, the animal simply waited motionless by a breathing hole and waited for a seal to surface. - the “sit and wait” and “jump and crush” strategies seem to depend little on the camouflage strategy. - is there another adaptive hypothesis for why polar bears should be white? - one clue when polar bears are photographed under UV light they turn out to be black. - in other words, polar bear coats absorb UV light. - examined very closely, the individual hairs on the polar bear pelt are not white at all but are clear. - polar bear hairs simply lack the pigment found in the core of the hair in most other mammals. - a study was conducted by R.E. Trojan and colleagues on the optical properties of polar bear fur. - they found that the hairs function to trap incident light and reflect it back towards the animals skin. - in other words, the white pelt appears to function as a “solar heat collector”. - therefore, we have another working hypothesis which can be called the “solar heat collector” hypothesis. - it is not particularly effective, however, only trapping about 16% of the light back towards the skin. - the rest of the light is scattered and reflected and this makes the bear look white. - we now have a second alternative adaptive explanation for the whiteness of polar bears. - this hypothesis has not been definitely tested. - the example of the polar bear points out the obvious fact that we must be careful in carrying out the adaptations program. - it is not simply enough to uncritically accept a hypothesis because it is plausible. - it is necessary to subject these hypotheses to further tests and consider the likelihood of alternative explanations.