Transcript Chapter 10
What is adaptation? Adaptation Comes from ad (to or toward) and aptus (a fit) Evolutionary context – ◦ genetically-based change in response to a problem ◦ achieved through the process of natural selection. ◦ A period of time passes before adjustment occurs Adaptation is both a process and a state of being (a phenotypic trait or character) We identify traits as adaptations only when they are evolved for the solution of a specific problem (selected function or purpose). Adaptation - a working definition: a genetically based trait or integrated suite of traits that has evolved in response to selection for the function that it currently performs and that increases the fitness of its possessor Just because a trait has a specific effect does not mean that the trait is adaptive for that purpose. Evolution is blind! Selection acts only on what is before it and not with an end goal in mind. Selection may lead to adaptations that … are either a further modification of traits already present (by changing their use) or… selection may act to eliminate traits All of the trends in selection that cause traits to change over time are adaptive but the changes are not done with some predetermined end goal in mind Identifying adaptations can be difficult and care must be taken Many traits evolved under one selective regime but are now being used under a very different selective regime. The current function may not reflect the context in which the trait was originally evolved Need to distinguish current utility from historical origin What is the significance of these slides as they relate to adaptation? Giraffes What kind of story do the slides tell us What was the original explanation for the adaptive value of the Giraffe neck? What other possibilities have now been investigated? Does the giraffe neck give any feeding advantage to those who possess it? If the giraffe neck was actually selected for as a means of defeating other males in a battle over females, then the neck is now coopted for use in feeding higher in the trees than other organisms can. Sometimes the term pre-adaptation is used for such co-opted traits But preadaptation is a bad label to use here. The term exaptation has been suggested by Stephen Gould refer to situations in which traits perform a certain function now but either arose originally for some other function or had no function at all originally Gould has also proposed the use of the term spandrel for an adaptation that originally arose with no adaptive purpose at all but now has adaptive value In male giraffes if long necks were originally adapted for fighting then their current advantage for feeding would be an exaptation (originally arose for a different adaptive reason) but in females long necks would be a spandrel since they originally arose with no adaptive value for females but now may impart a feeding advantage. Examples of exaptations: bone tissue skull sutures Adaptationist program Seeks to find adaptive explanation for every trait in organisms Much difference of opinion on this approach Everything is not adaptive. Some things are actually maladaptive or non-adaptive. Some traits or variations in traits are actually selectively neutral Adaptations are not always perfect Need to keep in mind that Adaptation is also constrained by compromises, trade-offs and correlations with other traits We will discuss each of these ideas later Various types of studies are involved in the attempt to explain what is happening in an evolutionary context To determine that a trait is actually an adaptation we need to…. determine what the trait is for and then …. show that individuals that have that trait actually contribute more genes to the next generation than the individuals that don’t have the trait The obvious answer is not always the right one Giraffe story in the text What are the two current hypotheses? The case of the giraffe demonstrates the importance of being able to look at things with fresh eyes and come up with alternative hypotheses. Examine the picture to the left does it suggest a third possible hypothesis for long necks? Three major approaches to determining adaptive significance of traits Experiments Observational studies Comparative studies Experimentation Experimental example Zonosemata (snowberry) flies and jumping spiders What is being investigated? Zebra Jumping spiders stalk their prey. Warn others of their species off with leg waving behavior A prey of the jumping spider, the snowberry fly, exhibits a curious behavior that resembles the leg-waving of the jumping spider. QUESTION: Why do the flies wave their striped wings? Experimental example Zonosemata flies What are 3 hypotheses that might explain this behavior? Experimental example Zonosemata flies •What was the experimental set-up What were some of the controls used in the experiment and why was each important? What predictions were made? Experimental example Zonosemata flies What were the experimental results? Experimental example Zonosemata flies RESULTS What important points of experimental design are demonstrated in the Snowberry Fly example? What were the control groups in this experiment? Why were they important? Allow us to work towards eliminating competing hyotheses. Effective Controls What were some of the things considered here? ◦ ◦ ◦ ◦ Same arena Method of presentation of flies Timing of presentation Others? Handling all treatments exactly alike What needed to be randomized and why? ◦ Order the flies were presented in ◦ Others? Radomization Sample size needs to be large enough for statistical analyses. Replicated experiments reduce the amount of distortion because sample size is larger which lowers the amount of bias. Allow the measurement of variation in data points. Repetition Observational Studies With Garter Snakes When are these type of studies done? In this method we need to show two things: 1. Occurrence of trait is non random in the population 2. The observed trait is adaptive Example – Garter Snake study How did this study show that snake movements is an adaptation to control body temperature? Observational Studies Show they are choosing a particular temperature more often than would happen by random movements Watched snakes, where they spent their time and what their body temps were Found that they maintain their body temperatures between 28 and 32 degrees Celsius. Discovered options for thermoregulation sun/shade, under rocks (thin, medium, thick), or moving up or down in burrows. Found that of the 3, all could be used to effectively maintain desired daytime temps but only rocks could provide enough warmth at night Studied thin, medium and thick rocks. Predicted only medium rocks work for the right temps both night and day. Most snakes found under rocks. Now have to show that being under medium rocks is not random behavior Compared availability of thin, medium and thick rocks in the habitat to the frequency that each was used by garter snakes All rocks are equally represented in the habitat so if random events, the snakes should be found equally under each type of rock. Results …. Table 10.1 Comparative Studies Tests for patterns across species Proper application of comparative methods requires knowledge of the evolutionary relationships among the species under study. Example bat studies Comparative Studies Bats: Is the larger size of testes in some bats adaptive due to sperm competition? Do bats from larger social groups have larger testes because there is more competition for passing on their genes? Example of comparative study Showed initial correlation between social group size and testes size. But the data could be skewed by evolutionary relationships. The testes size could still be related to who evolved from whom. Perhaps the larger testes groups are simply from one common ancestor and the smaller from another. Need to do a correction for this If we replace the individual points for A, B and C and for D,E and F with a single point representing their most recent common ancestor we get…. But two data points is not very reliable for making extended conclusions. Felsenstein’s method of phylogenetically independent contrasts. Plot sister species independently When species diverge from a common ancestor does the species that evolves larger group sizes also evolve larger testes? Drag point closest to the vertical axis to the origin Erase lines Bat results: show that when a bat species evolved larger group sizes than its sister species, it also tended to evolve larger testes for its body size. Phenotypic Plasticity De Meester’s first study with Daphnia De Meester’s second experiment No stocking Heavy Stocking Reduced stocking Every Adaptive Trait Evolves from something else In order to show that one thing has evolved from similar structures in ancestors must be able to … 1. Establish the ancestral condition 2. Understand the transformational sequence, how and why the characters changed through time Video demonstrating transitional adaptations Whale evolution You may review this video at http://www.pbs.org/wgbh/evolution/library/03/4/quicktime/l_03 4_05.html Adaptations work with what is available Many structures are far from optimally designed The fact that everything evolves from something else is just one reason why an organism’s traits, even when clearly adaptive, are often imperfect Contrivances - less than optimally designed structures (if interested visit http://www.talkorigins.org/faqs/jury-rigged.html) The Panda’s thumb Eye Development see the link at http://www.pbs.org/wgbh/evolution/library /01/1/quicktime/l_011_01.html Tradeoffs Trade-offs Two evolutionary forces may work on the same part from different directions and the resources devoted to one body part or function may be stolen resources from another part or function Giraffe’s long neck may allow to fight off competition but it sure makes getting a drink inconvenient, difficult and maybe even dangerous. A trade-off example The Begonia In Begonias there is a trade-off between the size of female flowers and the size of the inflorescence. Even though larger female flowers attract more pollinators, the female flowers remain smaller than optimal for pollination because bees also visit larger inflorescences and larger inflorescences can not contain individual flowers as large as the optimal flower size alone would dictate trade-off between the number of female flowers and individual flower size may be dictated by two things. 1) more flowers, more seeds and 2) perhaps more bees will be attracted to larger inflorescences Constraints Two types of constraints Developmental – based on how an organism develops in embryo or how an organism’s structure is related to function. Phylogenetic – based on inheriting the needed genetic variation from its ancestors Constraints (developmental example) A behavior or trait that would be adaptive is physiologically or mechanically impossible. Why does the Fuchsia retain its flowers and turn them red for 5 days AFTER pollination, when pollinators are no longer visiting, the flowers are of no more use but are still tapping needed resources. Investigations showed that it was not a cue for pollinators telling them which flowers to visit. Turns out that pollen tubes need to grow through the area of the abscission layer for the petals and if petals are dropped too soon the pollen tube never makes it to the ovules. So the petals need to remain for at least 4 days after pollination for fertilization to take place. Constraint - phylogentic Pigs can’t fly Animals can’t do photosynthesis Beetle example on page 388-389 ◦ Need to be able to detoxify the chemicals in individual host plants they may prey upon. ◦ Showed genetic variation which allows beetles to attack different host plants, and detoxify the new hosts unique chemical defenses depends on which phylogenetic group the beetles belong to. Constraints in Arthropods Arthropods inherited both an exoskeleton and jointed legs. These traits have opened up many opportunities in arthropod evolution, but they have also blocked other possibilities. Can you think of any constraints on the size that arthropods will reach? Molting: Molting is more hazardous for larger animals. Exoskeleton strength: The exoskeleton may not be strong enough to support larger animals. Respiration: Many arthropods can only get enough oxygen to support small bodies. Sickle cell and Malaria Adaptive Compromise End Day One Adaptation Definitions Some Definitions Preadaptation(exaptation): A character that was adaptive under a prior set of conditions and later provides the initial stage for evolution of a new adaptation under a different set of conditions. Examples: 1. A bird’s flight feathers (from feathery scales on certain dinosaurs, where they served the function of insulation); 2. The vertebrate eye (from a series of lightsensitive organs). Definition Vestigial structure: an anatomical structure found in all or most normal individuals of an extant species; typically very small in size, and with apparently little or no important function now. Such parts typically would be found in ancestors of this species, but as larger and clearly functional structures. An example would be the human earwagging muscles. Definition Atavism (Atavistic structure): a vestigial structure found in only a small fraction of the normal members of an extant species Example The rudimentary thigh bone found in about 5% of individual whales. Definition Contrivance: A structure modified and used for a function which is different from the original (or previous) function for that structure in an ancestor. Example The vertebrate eye The design of the vertebrate retina is “insideout.” The retina is behind the nerves that form the optic nerve. Where the optic nerve leaves the eye, there is a hole, which results in a blind spot. Definition Imperfection: A “contrivance” which still retains some of the features of its ancestral source structure, to a greater or lesser degree; clearly not fully or perfectly “adapted” to its new function, but serving adequately. Example The panda’s “thumb” is an example here, as are the many contrivances found in orchid flowers. This term could also be applied to vestigial or atavistic structures. Exercise on examples of some adaptations and imperfections The End