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:
GroupABCDE
ZonosemataZonosemataZonosematahouseflyhousefly
Treatmentuntreatedown wingswith houseflywith Zon.untreated
Regluedwingswings
Effect testedwings markssurgerywingwingno wing marks
+ wavingswavingmarkingsor wavings
______
Hypothesis PredatorPredicted outcome (X = attack and/or killed)
______
H0 j. spiderXXXXX
otherXXXXX
(no mimicry)
H1 j. spiderXXXXX
otherleft aloneleft aloneXXX
(mimicry - deters other predators)
H2 j. spiderleft aloneleft aloneXXX
otherXXXXX
(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.
ABCDE
No. of spiders retreating:1515220
No. of spiders attacking/killing:55181820
- 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.