Introduction to Natural and Sexual Selection
Behavior is shaped by natural selection. Example: Young male zebra finches first listen to the songs of nearby males of their species, particularly their fathers. Anole lizards that spend more time on the ground, for example, need But can different behaviors be favored by natural selection in different environments? traits that occurred in parallel to natural selection for behavior. Consider the familiar example of runaway the association between behaviors and From such information we can determine whether behavioral trait arose.
Notice, that this relationship is much weaker than W1. Moreover, the regression line is not signficant, which is to say that we would might expect to see a relationship between body size of males and fecundity of females as strong as that seen here by chance alone. The final component was probability of offspring survival or success with which eggs hatched into froglets W3.
Bull frog dads may have good or bad territories from the point of view of hatching success and this might vary as a function of size. Index Stabilizing Selection Stabilizing selection tends to reduce the amount of variation in the phenotype distribution over an episode of selection.
Individuals in the center of the phenotype distribution tend to be favored and have higher fitness than individuals in the "tails" of the distribution. A simple form of disruptive selection would act on single locus with two alleles where overdominant heterozygous individuals are at an advantage relative to the two homozygous classes see Side Box 2.
If the heterozygotes had higher fitness, then selection would tend to remove the extreme homozygous classes. The phenotype is also stabilized by selection. In contrast, to the response to selection seen in the case of directional selection, pure stabilizing selection does not change the mean of the phenotype distribution.
Stabilizing selection reduces variation and favors individuals with an average phenotype over the extremes. This mode of selection is often referred to as optimizing selection. The examples of stabilizing selection that we will explore involve fundamental issues of the life history as it relates to parental investment.
We will consider stabilizing selection on offspring size and offspring number in a vertebrate without parental care a lizardand two vertebrates with extended parental humans and birds. In this exploration of selection and the life history, we will explore the concept of trade-offs.
These life history concepts are central to parental investment models that we explore in subsequent chapters. As we shall see in the chapter on mating systems and parental careoffspring number is a central trait which influences not only fitness of the parent e. I have also chosen these examples to illustrate how counterbalancing selection can constrain reproductive traits and result in the process of adaptation.
Classic paradigms developed by Lack ; and Williams provide the selective context for understanding the evolution of parental investment. David Lack is best known for his hypothesis that avian clutch size was limited by the amount of care that parents could provide to offspring. The selective premium accrued by investing in additional offspring would be counter-balanced by the decreased fledging success in large clutches owing to limitations on parental effort per offspring. The most successful nests would be those that produced an intermediate clutch size.
Clutch size in birds should be under stabilizing or optimizing selection. This process reflects adaptation of the life history, and any behavioral traits that are associated with the life history.
Similarly, in organisms without parental care, Lack further suggested that production of large numbers of small offspring was balanced by the high survival of a few large offspring Sinervo et al, Likewise, a second paradigm of life history biology, costs of reproductive allocation, provides the selective context for understanding selection during adult phases of the life history.
Williams refined Lack's ideas and considered the parental costs of reproduction that would arise from further investment in energy per offspring to offset deficiencies in offspring survival.
The parents might have to forage further, or for longer periosds of time during the day in order to satisfy the demands of the growing brood. Investment in current reproduction is expected to result in a cost to future reproductive success that should either lower survival of the parent, lower fecundity, or reduce growth rate and consequently affect body size and fecundity.
3. Adaptation and Selection
Lack developed classic manipulation of clutch size to test for selection on parental effort and care. Index Stabilizing Selection on Clutch Size in Birds For over a decade, Lars Gustaffson and his colleagues have been studying Lack's and William's hypotheses in the collared flycatcher, Ficedula albicollis, on the island of Gotland which is south of the Swedish mainland.
Each spring they monitor a vast array of nest boxes. They tag each and every male and female and record the number of offspring that the female lays. Results from their studies support both Lack's and Williams hypotheses. Gustaffson and Sutherland found that the number of recruits produced by unmanipulated nests was higher than the number of recruits with eggs that were either removed from the nest, or eggs that were added to the nest.
Parents that were induced to rear enlarged clutches produced lower quality offspring that had lower survival to maturity. In contrast, parents that were induced to produce smaller clutches could have handled more offspring but the number of recruits that they produced was reduced because they started out with a smaller clutch size.
Finally, unmanipulated clutches had the optimum number of offspring -- not too many or too few, but just right. In addition to observing stabilizing selection on the number of fledglings produced, they also found that the quality of the offspring at maturity was also affected by the clutch size manipulation. When the female birds matured the next season, progeny that came from nests with one egg removed produced more fledglings in their own nests when they matured.
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If the offspring came from enlarged or reduced nests they produced fewer offspring. Lack's hypothesis concerning an optimal clutch size apparently holds for the collared flycatcher. Finally, they also found that female parent's with manipulated clutch size produced fewer eggs the next season if they had eggs added to their nest and produced more eggs the next season if they had eggs removed their nest.
Thus, investment in a large clutch in one season results in a cost of reproduction that reduces the clutch size that the female parent produces the next season. X The operation of Lack's tradeoff between offspring number and quality and William's tradeoff between current and future reproduction in collared flycatchers.
The effect of clutch size manipulation in collared flycatchers, Ficedula albicollis on a fecundity of the female parent the next season, b number of recruits in a nest that return to the breeding grounds the next spring, c fecundity of the recruits when they return to breed.
While selection on costs of reproduction that the female experiences in terms of reduced fecundity tends to favor the production of small clutches, the selection on offspring survival to recruitment and the number of offspring that offspring produce at maturity are both under significant stabilizing selection. Optimal Brood Size in Cichlids Index Stabilizing Selection on Maternal Investment Our discussion of parental investment began with a birds, a group with what we think of as fairly advanced parental care.
In a simple phylogenetic sense, we consider parental care of eggs or juveniles a more derived condition, and no parental care to be the ancestral condition.
Species with parental care evolved from an ancestor that presumably did not have advanced parental care. By examining organisms that typify the ancestral and derived states for parental care, we can see if there are common features of life history that are subject to the action of stabilizing natural selection. These aspects of life history involve the fundamental selective constraints originally considered by Lack and Williams.
Many egg-laying reptiles have little or no parental care after laying their eggs. The side-blotched lizard is a useful system for investigating how parental investment influences offspring survival.
Quantifying parental investment in egg-laying animals without parental care entails a measurement of energy content of egg production. Because the amount or mass of yolk in an egg reflects most of direct energy invested by the female in her young, we can measure survival of the adult female and survival of her offspring as a function of egg mass to determine whether there is a net stabilizing selection on offspring size.
In conjunction with a number of colleagues, I have collected data on the survival of female lizards as a function of the quantity of yolk that female parents put into the egg. I measured the survival of these female parents to the production of a second clutch.
One of the factors thought to influence patterns of adult mortality is related to costs of reproduction. If reproduction is costly, then heavy investment in current reproduction might be expected to lower survival or future reproductive sucess.
From the fitness surface which describes female survival as a function of her investment in individual offspring it is clear that the distribution of survivors is much narrower than the distribution of females that died. Females that laid extremely large eggs had relatively low survival and selection was stabilizing. It appears that production of very large eggs and very small eggs is costly in terms of survival.
Why should the production of small eggs be a liability? In these lizards females that produce small eggs also tend to lay many eggs or have large fecundity.Animal Behavior - CrashCourse Biology #25
In a separate series of experiments Sinervo and DeNardo enhanced the survival of females that laid large clutches of small eggs by surgically ablating follicles. Females with experimentally reduced clutch size had greatly enhanced survival.
Why should the production of large eggs be a liability? Using the same technique of experimental clutch-size reduction, Sinervo and Licht reduced clutch size down to the smallest possible, a single egg. Females with experimentally-reduced clutches experienced a slightly different problem in that they became egg bound at high frequency, and required a ceasarian section to remove the eggs that were far too large to lay.
Congdon and Gibbons have suggested that similar constraints limit adaptive evolution of clutch size and offspring size of turtles. All vertebrates must pass These studies illustrate two points. First, there is an optimal offspring size from the point of view of the female parent's survival. Second, experimentally altering the phenotype allows for the revelation of the causes of natural selection.
In this case the optimizing selection on a female parent's survival results from two separate causes: Survival of adult females represents a single episode of selection, Sinervo et al. In this case, the optimal offspring size arises from a classic life history trade-off: Stabilizing selection on offspring size occurs during a number of separate life history episodes. Selection on maternal investment should have a genetic basis if the trait is to respond to natural selection and indeed egg size of the mother is positively correlated with egg size of daughter's.
Given the results on heritability from this two year study we have also used selection estimates measured across additional years to correctly predict the response to selection on egg size. Survival of adult female lizards as a function of investment egg size of individual offspring.
The fitness of a female was either 0 she died or 1 she survived.
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The histograms at the top of the graph gives the distribution of egg sizes laid by female parents that survived, relative to the histogram at the bottom that gives the distribution of those females that died.
The curve describes the fitness surface for the probability of adult female survival as a function of the egg size that she laid. Natural selection favored females that laid intermediate-sized eggs.
Index Selection on Human Birth Weight. In light of the constraints on maximum offspring size that arise from the pelvic girdle of lizards, do such constraints operate on other vertebrates? Lets look at a problem more close to home. Humans invest an inordinate amount of energy into their young before they are born and indeed in terms of extended parental care after they are born. The reader is well aware of the duration of parental care in humans after birth which can be more than two decades.
Little data is available in any animal on how parental investment leads to offspring success. However, data is available on offspring size and the problems of birth. The parturition problems experienced by lizards are a very general problem for all vertebrates that lay relatively large and costly offspring.
The problem with offspring size in humans is exacerbated by the large size of the cranium of the newborn relative to the size of the birth canal. Karn and Penrose collected data from hospitals on the survival probability of offspring as a function of neonate size, and gestation duration. Length of gestation could confound neonate size as premature babies are usually much smaller.
Thus, neonate size is expressed as the difference in size from the mean of the population after removal of factors such as length of gestation. Two patterns emerge from the analysis of selection on human birth weight. First, there is significant stabilizing selection on neonate size.
Small infants and large infants die during child birth at a higher rate than intermediate-sized infants. Second, there is also a directional component to selection. Notice that the optimal infant size is one-half of a pound higher than the average infant size in the population. The pattern of low survival of large offspring undoubtedly has different causes than the probability of low survival of small offspring.
For example, small offspring may have had high mortality because of inadequate nutrition during gestation. Conversely, large offspring may have died because of the large diameter of the cranium relative to the pelvic girdle and its effects on duration and difficulty of labor.
The data that Karn and Penrose collected back in X took place before the advent of modern techniques for the care of neonates. It would interesting to know if the widespread use of caesarian sections and other medical techniques have altered the selection on neonate size.
Additional data on the efficacy of techniques of intervention would provide experiments of a sort that could be used to pinpoint the causes of stabilizing selection, much like the techniques used in female lizards. In the case of humans experiments per se could not be used, but reference or control populations could be compared to the results from experimental populations that received different kinds of post-parturition treatment. Probability of human infant survival after birth as a function of neonate size.
Original data is from Karn and Penrose and reanalyzed by Schluter Data are standardized relative to average infant size at birth which is located at 0. Index The evolution of litter size in primates We have seen that the human species is subject to stabilizing selection for offspring size.
In addition, the simple interpretation of the difference in optimum offspring size relative to the average offspring size 0. Has a functional ceiling in offspring size been reached in humans? Does the pelvic girdle limit the size and number of offspring in other primates? Humans and primates have a shared evolutionary history and limitations on parental investment might have constrained other primates during the evolutionary history of the group.
This issue briefly touche upon the kind of historical and design constraints that Gould and Lewontin suggested might be important to the process of evolution in addition to the process of selection.
Karn and Penrose's data set reflects over 35, individual births. Is such data available on other primates? It would be incredibly difficult to collect this kind of data of on free-ranging chimpanzees, because we would have to census all existing chimpanzees found in nature. The data on mountain gorillas would be even more difficult to collect because this species has been dwindling in population size in recent years owing to habitat fragmentation.
Is there any other kind of data available on offspring size or litter size of primates that we could use instead of actual data on natural selection?
There is a wealth of comparative data on neonate size, body size and litter size of primates that have been used to construct hypotheses on adaptation and constraint during the evolution of primate reproduction. The comparative tradition has a rich history in evolutionary and behavioral studies Pagel and Harvey. In the comparative tradition, data is gathered on a large number of related species, the patterns for the species are graphed, and inferences are made regarding the process of adaptation.
While not as powerful as direct assessment of the process of natural selection, such data provides us with raw insight into past processes that might have limited evolutionary change. In addition, it is impossible for us to observe natural and sexual selection on past events. We must rely on such inferences if we are to understand the evolution of behavior. Luetnegger used the comparative approach to investigate whether the tendency for the evolution of large cranial size in primates may have constrained their reproductive biology.
Many small species of primates tend to give birth to two offspring whereas all large species of primates give birth to a single progeny. This pattern presents us with a minor paradox. If fitness is increased by the number of progeny, and if a larger animal should be able to produce more offspring, then why don't large primates produce more than a single offspring?
The answer to this problem is certainly more complicated than fecundity. Perhaps it is important for larger primates to invest more in a single offspring given their ecology. I am not thinking in terms of these ecological constraints, I am thinking of just the size constraints on primates as it affects reproduction.
The answer to this question lies in the field of scaling and allometry.
In the field of allometry, the relative size of various structures compared to overall body size is the pattern of central importance. In terms of primates, the evolution of large brain size has been an evolutionary trend that distinguishes homo sapiens from other primates, and all of primates from other terrestrial vertebrates many marine mammals or cetacea have evolved relatively large brains that rival the human brain.
Addressing the constrains on reproduction addresses the constraints that are imposed on evolution of large brain size in humans. You may have noticed that human infants have relatively large heads relative to adult humans. When plotting the neonate size against maternal body size, Luetenegger observed that neonate head size was relatively speaking much larger for small-bodied primates compared to large-bodied primates.
If the pelvic girdle was relatively fixed in size across these groups, then it is the small-bodied primates that might experience more difficulties during birth than large-bodied primates.
Small-bodied primates show a much higher incidence of twinning and this is hypothesized to result from selection for more and smaller offspring. These small primates could not necessarily produce a single large offspring. While such hypotheses are difficult to prove without direct evidence on the fitness of a small primate that produced one versus two young, the example serves to illustrates Gould and Lewontin's point on alternative causes for evolutionary patterns.
An adapationists might speculate that small-bodied primates are selected to produce lots of small offspring because the optimal litter size is two young whereas a large primate is selected to produce a single large offspring. The argument based on architectural grounds contends that small primates are physically incapable of producing anything but two small young.
These are two drastically different explanations of the same pattern -- one based on adaptational causes, the other based on constraints on organismal design.
Index Our consideration of animals with a vastly different evolutionary history fish, amphibians, reptiles, birds, and mammals leads to a robust conclusion that can be made with regards to natural selection and the life history trade-offs that involve, offspring size or offspring number. While the parental investment trade-off operates on slightly different aspects of the reproductive biology of each group, there is a common mode of selection that operates on the reproductive traits -- stabilizing or optimizing selection.
Indeed many researchers have identified similar patterns in a wider array of taxa. These trade-offs can either be left unresolved and identified as simple stabilizing selection on the variance in offspring size or offspring number in the population, or the stabilizing selection can be further analyzed in terms of counterbalancing components of directional selection that act on different life history episodes.
Chapter 51 - Behavioral Ecology | CourseNotes
Regardless of whether or not the causes of stabilizing selection are elucidated, their impact on species is fairly clear. Stabilizing selection would act to maintain the constancy of a species over a long time frame. Moreover, if species had different optima for traits then stabilizing selection would tend to keep species differentiated. Finally, additional design constraints can limit the process of adaptation.
Elucidating these design constraints, requires a deeper understanding of the proximate mechanisms of behavior, which we will uncover in future chapters. Disruptive selection is perhaps the most elusive mode of selection. Despite the paucity of actual examples of disruptive selection, the process is thought to play a major role in the process of speciation or the origin of new species Templeton, X. The action of disruptive selection is much more complicated than the action of directional selection in which a single agent of selection shapes a trait.
Furthermore the action of disruptive selection is likely to be more complicated than stabilizing selection which in many instances is composed of counterbalancing trade-offs. Disruptive selection acts against the individuals in the middle of the range of phenotypes and tends to favor individuals in the extremes. A simple form of disruptive selection on a single locus with two alleles where the heterozygous individuals are at a disadvantage relative to the two homozygous classes see Side Box 2.
In the case of such underdominance in fitness, selection favors the more extreme homozygous classes. The action of disruptive selection on the phenotype distribution.
In many cases, behaviors have both an innate component and a learned component. Behavior is shaped by natural selection. Many behaviors directly increase an organism's fitness, that is, they help it survive and reproduce. Introduction Do the squirrels in your neighborhood bury acorns underground? Does your cat start meowing around the time you usually feed her? If you've noticed any of these things, congratulations—you've made your first observations in behavioral biology!
These are all examples of animal behaviors. Yep, you and I count as animals too. In fact, these behaviors are just a tiny sampling of the amazing and diverse behaviors we can see in nature.
We could ask what behavior is used for, but it might be better to ask, what isn't it used for? Animals have behaviors for almost every imaginable aspect of life, from finding food to wooing mates, from fighting off rivals to raising offspring. Some of these behaviors are innate, or hardwired, in an organism's genes. For instance, this is true of the squirrel and its acorn.
Broadly speaking, animal behavior includes all the ways animals interact with other members of their species, with organisms of other species, and with their environment. Behavior can also be defined more narrowly as a change in the activity of an organism in response to a stimulus, an external or internal cue or combination of cues. For example, your dog might start drooling—a change in activity—in response to the sight of food—a stimulus.
Modern behavioral biology draws on work from the related but distinct disciplines of ethology and comparative psychology. Ethology is a field of basic biology, like ecology or genetics. It focuses on the behaviors of diverse organisms in their natural environment. Comparative psychology is an extension of work done in human psychology. It focuses largely on a few species studied in a lab setting. Behavioral biology also draws on many related areas of biology, including genetics, anatomy, physiology, evolutionary biology, and, of course, neurobiology—which traces the neural circuits that underlie animal behavior.
Four questions to understand a behavior Nikolaas Niko Tinbergen was a Dutch ornithologist, or bird biologist, who studied behavior and is now considered one of the founders of the field of ethology.
Based on his own research, Tinbergen proposed four basic questions helpful in understanding any animal behavior. Let's look at these questions, using the production of song by the zebra finch—a common songbird—as an example. What triggers the behavior, and what body parts, functions, and molecules are involved in carrying it out?
Singing is triggered in zebra finches by social cues, such as the proximity of a potential mate, as well as the appropriate hormonal state. The ability to produce songs is influenced by male hormones and occurs mainly in male birds. Songs are produced when air flows from air sacs in the bronchii through an organ called the syrinx.