Adaptation • What is an adaptation? • It is a genetically based trait, or integrated suite of traits, that increases the fitness of its possessor.
What is adaptation. • Adaptations come about in response to a problem. That is, it is achieved through the process of natural selection. • It is through the adaptations brought about by natural selection that organisms appear to be well designed.
Identifying Adaptive Traits • Not all traits are adaptive • Learned traits • Traits that were originally adaptive for one problem, now used to fix another. • Traits that have no current benefit toward fitness i.e. molecular genetic drift • Traits must be rigorously tested to determine whether they are adaptive
All hypothesis must be tested • Hypothesis are tested by • using the hypothesis to make predictions • Next, the predictions are tested by either - experimentation, - observation, - Or comparative studies • Then the test are analyzed to determine if the predictions are correct.
The Awkwardness of Giraffes • Why are Giraffes so tall? • Is it an adaptation? • What is it an adaptation for?
Giraffe’s Neck Reconsidered • Simmons and Scheepers conducted studies to observe whether the giraffe’s long neck was used to gain an advantage in forging. • They found that giraffes most often feed on foliage that is at shoulder height. • In times of drought when foliage is scarce they feed on low brush
The giraffe neck was actually adapted as a means of defeating other males in a battle over females. • Bull giraffes employ their heads and necks as clubs and occasionally even kill each other.
If the giraffe neck was actually selected for as a means of defeating other males in a battle over females. The neck is now coopted for use in feeding higher in the trees than other organisms can. • Is this adaptation? • Adaptation: a 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. (fighting) • The giraffe’s neck is an exaptation when it is used to eat leaves off tall trees.
Exaptation • The term exaptation refers to situations in which traits perform a certain function now but either arose for some other function or originally had no function at all. • A type of exaptation in which there was no original function is called spandrel
Example • In male giraffes long necks were originally adapted for fighting then their current advantage for feeding would be an exaptation (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.
Points to Consider • Should differences among populations or species always be considered adaptive? • Exaptive? • Spot patterns on giraffes Reticulated giraffe Masai giraffe
How do We determine whether a trait is an adaptation? • Three major approaches to determining adaptive significance of traits • Experiments • Observational studies • Comparative studies
Experimental exampleZonosemata (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 exampleZonosemata flies • What are 3 hypotheses that might explain this behavior?
Experimental exampleZonosemata flies • What was the experimental set-up • What were some of the controls used in the experiment and why was each important?
Experimental exampleZonosemata flies • What predictions were made?
Experimental exampleZonosemata flies • What were the experimental results?
RESULTS Retreat Stalk and attack Kill
Observational Studies • When are these type of studies done? • When experiments are impractical or inappropriate, observations can yield sufficient information to evaluate a hypothesis • Experimental study: Why giraffes have long necks?
Observational Studies • Observational studies must employ the following criteria: 1. Hypothesis must lead to observed predictions. 2. Observed occurrence of the trait must be shown to be non random in the population. 3. The observed trait is adaptive Example – Garter Snake study
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) 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 mediumrocks 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 ….
Comparative Studies • Compares traits across different species lines • Proper application of comparative methods requires knowledge of the evolutionary relationships among the species under study.
Example of comparative studyBats • Question • Why do some bats have bigger testes than others? • Hypothesis: • David Hosken hypothesized that large testes are an adaptation for sperm competition
Prediction • If you compared different species of bats, those that form larger social groups will have larger testes because there is more competition for passing on their genes?
Studies initial showed correlation between social group size and testes size.
However a closer look indicates that the data could be skewed by evolutionary relationships. • Perhaps the larger testes groups are simply from one common ancestor and the smaller from another. • Example 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….
Plot sister species independently Felsenstein method A better question: when species diverge form a common ancestor does the species that evolves larger group sizes also evolve larger testes? Erase lines Drag point closest to the vertical axis to the origin
Felsenstein’s method • Each data point represents the divergence that arose between a pair of sister species as they evolved away from their common ancestor.
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 Section 9.5
Phenotypic Variation • Phenotypes are influenced by the environment • Total variation is called the phenotypic variation and it has two components: VG and VE. • contributions come from both the environment and the genetic makeup • Phenotypes may respond in different degrees to different environmental situations
Phenotypic Plasticity • When an individuals phenotype is influence by its environment it is said to be plastic. • When phenotypes are plastic, individuals with identical genotypes may have different phenotypes, provided they live in different environments.
Can plasticity be adaptive? • Show that the plastic trait is positively selected for • show that individuals that are plastic for the trait are more fit than those who are not plastic
Daphnia magna: The water flea • Tiny crustacean that lives in freshwater lakes. • Daphnia reproduce asexually (clones). • Thus researchers are able to grow genetically identical fleas in different environments
The Water flea Experiment • Researchers studied 10 clones (genotypes) from each of three different lakes. • They were grown in 2 different environments • One where fish predators had lived and one where fish had not lived. • checked phototactic (to light) response in the two different environments
Water Flea Results • The Daphnia from Blandaart lake, where fish are present, were much more plastic in their ability to respond to light than Daphnia from lakes that had no fish • Showed that selection for phenotypic plasticity has been selected for in a lake where fish are present.
Phenotypic Plasticity • The Phenotypic plasticity is the result of differences in phenotypic expression (for a given genotype) based on the interaction of specific individuals with the environment in which each lives. • Phenotypic plasticity may evolve. • Phenotypic plasticity may or may not be adaptive.
Tradeoffs and constraints Factors that limit adaptive evolution
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