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What about the Population Bottleneck Scenario? • • Sequencing studies of MHC alleles in humans and chimpanzees show the existence of many ancient polymorphisms in the human species Because of balancing selection (selection favoring heterozygotes) at MHC loci, many different alleles are maintained in a population and may persist for millions of years; some alleles in chimpanzee are more similar to their homologs in humans than they are to other alleles in the chimpanzee Conclusion from MHC studies • Population bottlenecks would act to greatly reduce allelic diversity at the MHC loci. Evidence suggests that this is not the case. • Based on computer simulations, best estimate of the minimum human population size of about 10,000 (minimum estimate is 500). 1 New Topic: Animal Behavior • Definitions • Ethology – The study of animal behavior in relation to the natural environment; the ecology & evolution of animal behavior – Pioneering ethologists: Konrad Lorenz, Niko Tinbergen & Karl von Frisch: first and only animal behaviorists to win a Nobel prize (in physiology and medicine, 1973) • Psychology – study of how behavior is modified by experience through learning and memory Two types of questions about animal behavior • How Questions = Proximate Questions: – What are the mechanisms underlying behavior? – How is the nervous system wired? – What stimuli elicit a behavior? – How do hormonal levels influence the development and expression of the behavior? • Why Questions = Ultimate Questions = Evolutionary Questions: – What is the purpose or function of a behavior? – How does the behavior affect the probability of survival and/or reproduction of an individual – What was the original step in the historical process that led to the existence of a behavior – How has the behavior evolved and how has it changed over evolutionary time 2 Part 1: How Questions Part 2: Why Questions (Behavioral Ecology) Types of Behavior: Instinctive vs. Learned Behaviors • Instinctive Behaviors – Behaviors which appear in fully functional form the first time they are performed: The term “geneticallydetermined” is somewhat misleading – Called Fixed Action Patterns (FAP) by ethologists ß e.g., Web-building in spiders ß Egg-rolling in geese ß Pecking response in gulls • Learning: the durable modification of behavior in response to experience 3 Where does behavior come from? • In reality, most behaviors fall on a continuum between innate (hardwired) and learned • Even instinctive behaviors are influenced by the environment in the sense that their expression is a product of their genotype + development. Testosterone has two effects on male rat behavior • Primary sexdetermining signal stems from TDF on Y • Organizational effect on the development of the brain when the animal is very young; • Activational effect when the hormone triggers sexual behavior 4 Female Rat Sexual Behavior • In absence of TDF, ovaries develop and testis regress • Female fetus develops under the influence of estrogens (estradiol) • Brain develops estrogen receptors • Maturing brain develops mechanisms for mating & maternal behavior Fetal Environment Influences Male Aggressive Behavior 5 Testosterone’s effects not all bad ... How de we figure out whether behaviors are instinctive or learned? Deprivation experiments • Many studies in laboratory – Rear individuals in isolation to see if they still exhibit the behavior • Natural experiments – Galah parrot & pink cockatoo 6 Example of galah parrot and pink cockatoo • Both species nest in tree holes in Australia • Sometimes the larger cockatoos eject adult galahs from nest after egg laying has taken place • The cockatoos unwittingly rear galah chicks • Provides natural experiment to investigate instinctive versus learned behaviors • Galah chicks give alarm call of galah parents despite having been completely isolated from other galahs • By contrast, galah chicks learned contact call of cockatoos How de we figure out whether behaviors are instinctive or learned? Twin concordance studies: • Concordance: The presence of a given trait in both members of a pair of twins. • In twin studies, researchers assess both members of identical (monozygotic) and fraternal (dizygotic) twin pairs, who typically are exposed to common environmental influences. If genes influence the probability of exhibiting a particular trait, identical twin pairs, who share the same genes, will tend to be concordant-that is, both will exhibit or not exhibit the trait. Fraternal twin pairs, on the other hand, are no more similar genetically than non-twin siblings, and so will be less concordant - there will be more pairs in which one twin exhibits the trait and the other does not. By comparing the degree of concordance in identical and fraternal twins, researchers can estimate the extent to which genes influence trait expression. 7 How de we figure out whether behaviors are instinctive or learned? Reverse Genetics: The experimental procedure that begins with a cloned segment of DNA, or a protein sequence, and uses this knowledge to introduce programmed mutations (through directed mutagenesis) back into the genome in order to investigate gene and protein function. 8 fruitless Splicing Specifies Male Courtship Behavior in Drosophila • “Male courtship requires products of the fruitless (fru) gene, which is spliced differently in males and females. We have generated alleles of fru that are constitutively spliced in either the male or the female mode. We show that male splicing is essential for male courtship behavior and sexual orientation. More importantly, male splicing is also sufficient to generate male behavior in otherwise normal females. These females direct their courtship toward other females … The splicing of a single neuronal gene thus specifies essentially all aspects of a complex innate behavior.” (From Cell, Vol. 121, 785–794, June 3, 2005) What factors favor instinctive (innate) over learned behaviors? • Short generation time and absence of parental care, e.g., insects, spiders etc (no time for learning) • Expression of behavior is appropriate in almost every context: e.g., pecking response is "feed me" • Expression of behavior has to be right the first time, e.g., alarm calls; predator avoidance in moths and kangaroo rats 9 How are Fixed Action Patterns (FAPs) elicited? Releaser: the specific stimulus required to elicit a FAP • Red bill spot in gulls: only a red spot on an elongate rod is needed to elicit pecking response in chicks • Nut releases digging behavior in squirrels 10 Red underside releases male aggression in sticklebacks 11 Learned behaviors Learning: the durable modification of behavior in response to experience Types of learning I Classical conditioning: studied by Pavlov; involuntary activity becomes associated with a stimulus • Animal learns to associate involuntary activity with a stimulus • Repeated association of stimulus with reward or punishment causes stimulus alone to elicit response • Turn on light, give dog meat powder; dog salivates; eventually light alone causes dog to salivate • Occurs in organisms ranging from roundworms to humans 12 Types of learning II Operant Conditioning: animal learns to associate voluntary behavior with a stimulus • Trial and error learning • Skinner boxes 13 Skinner: Reductio ad absurdum • Thought much of human behavior could be reduced to operant conditioning • Built a Skinner Box for his daughter 14 Types of learning III: biased learning Behavioral Imprinting: form of learning in which individuals exposed to certain key stimuli early in development form a lifelong association with the object Konrad Lorenz's geese 15 Nick Smythe's agoutis and pacas (large, Neotropical rodents) By imprinting the paca on large social groups, Smythe was able to domesticate the paca in one generation. 16