Note for BSC 220 at UA-Chapter 12 Life in Groups An Introduction to Biological Evolution
Note for BSC 220 at UA-Chapter 12 Life in Groups An Introduction to Biological Evolution
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Date Created: 02/06/15
Chapter 12 An Introduction to Biological Evolution Class February 21 2012 Life in Groups Some evolutionary biologists make the case that cooperative life in groups with competition also drives evolution Alarm Calls When danger threatens sometimes one or more members of the animals group may give of a call usually a high pitched yell or distinctive screech and nearby members of the group scatter for cover or take wing in a burst of ight These vocal calls under stress are alarm calls that alert others to danger Dangers emerge from many directions Vervet monkeys of Africa produce alarm calls when approached by threatening predaceous snakes mammals or large birds Each type of predator poses a different threat so different alarm calls are emitted 0 When a snake is encountered the alarm call is lowamplitude alerting other monkeys nearby They respond by looking at the ground 0 When a stalking mammal leopard is discovered the alarm call is a very loud low pitched series of chirps and the monkeys scatter for a secure sanctuary in the trees 0 When a large bird eagle approaches from the air the alarm call is short loud staccato grunts signaling the approach of a menacing bird The monkey look up or immediately retreat into dense covering vegetation o The calls alone elicit the retreat response Alarm calls are common features in mammals and birds especially if they are the subject of predator interest Alarm calls are widespread and at first seem to be disadvantageous to the individual signaling danger 0 One suggestion is that in fact individuals that give alarm calls benefit directly from the advantages of such a vocal warning For example alarm calls alert the predator to the fact that it has been spotted 0 Or the alarm call sets colleagues all around into chaotic pandemonium confusing the predator de ecting it from the caller 0 Or perhaps the caller gains by simply causing all neighbors to scatter safely denying the predator a meal making the thwarted predator less likely to return in the future to this unproductive location Some biologists propose that in such circumstances the called is acting unselfishly Such trait or behavior is termed altruism wherein an individual s trait or behavior here emitting alarm calls reduces its own relative chances of successful reproduction it more often gets caught and eaten but this same trait or behavior enhances the relative chances of others in its group to survive and reproduce successfully Individual Selection and Group Selection The idea of altruism gained followers as other examples of apparent unselfish behavior accumulated o In response to threats male baboons step forward to intercept an approaching predator thereby protecting females and young but increasing their own chances of meeting an untimely end 0 Group bene ts outweigh individual advantages 0 Biologists saw selection acting at two levels one was individual selection acting on the particular phenotype of one organism the other group selection acting on favorable trait held in common by the group 0 But argued that we do see animals engaged in behaviors that limit overall group size 0 Many animals have a social hierarchy so that top males breed lowerranking males don t and this limits overall population size 0 Group values altruism prevail or individuals bene ts sel sh trump them 0 Altruistic behavior biologically speaking is characterized by loss of tness by the giver to the bene t of neighbors Sel sh behavior is the opposite gain or the giver at the expense of neighbors Kin Selection 0 An individual bene ts directly by helping its own offspring and the individual bene ts indirectly by helping relatives that carry proportionately some of the same genes such as nieces nephews aunts and uncles Natural selection that favors actions bene ting offspring and relatives is kin selection 0 It is a kind of individual selection because individuals bene tior really their particular genotype benefitsithrough kin bene ts 0 Together the success directly and indirectly in promoting one s own genotype is inclusive tness 0 Genetically speaking parental care is very sel sh Parents may exhaust themselves in rearing young and expose themselves to threats when defending their young offspring o The best outcome genetically would be for both female and all offspring to survive Coef cient of Relationship 0 The female enhances her tness by advancing the success of her offspring o The amount of effort and risk she spends on others will depend on the amount of genetic relationship she shares with them 0 It is highly advantageous to promote one s own genes and disadvantageous to spend time and energy on another s with whom one has no shared genetic investment This idea is expressed in the term coef cient of relationship 0 It expresses the degree or fraction of shared identical genes between two individuals 0 In animals parental care is not a conscious decision but like anatomical traits is genetically based 0 Some animals exploit this innate parental behavior of others 0 When a nest with new eggs is temporarily left unattended the female of another bird species may slip in and lay her eggs unknown to the host leaving the host parent with the expensive task of raising the foster young Such birds are brood parasites cuckoos nches cowbirds etc o Cuckoo chicks often hatch rst grow rapidly and if any host nestlings still remain the cuckoo chicks may muscle them out of the nest sending them to certain death on the forest oor below Levels of Selection 0 Dawkins argues that selection acts directly on DNA the genetic material itself o In the group the individual tness comes from the favorable action of natural selection upon individuals engaged in bene cial behaviors 0 Individuals engaged in behaviors producing successful propagation of individual genotypes into future generations have higher fitness than those not practicing such advantageous behaviors o This is kin selection a kind of individual selection based on inclusive fitness 0 Problem with claims of group selection 0 There is no mechanism to selectively eliminate contending groups 0 Group selection does not address how to contend with cheaters iindividuals enjoying benefits at the expense of the good group 0 If members of a community are not closely relatedia low coefficient of relationshipithen kin selection does not occur 0 There are no clearcut examples of group selection have been found in nature Microevolution and Macroevolution o Microevolution is the evolutionary event seen close up and zoomed in Concerned with patterns of change within a population or species It is a short term perspective 0 Clinical variation ring species and changes in allele frequencies are examples 0 Microevolution focuses on populations 0 At the microevolutionary level natural selection acting on variation within a population may be gradual and continuous adding one character to another but something big seems nessesary to drive big changes and something sudden to produce sudden changes 0 Macroevolution an evolutionary event seen zoomed back the overall pattern The origin of species and higherlevel taxa such as families or classes are the concerns of macroevolution It is a long term view 0 Phenomena as the rise diversification and demise of dinosaurs the origin of birds the origin of mammals o Largely been the province of paleontologists who take a longer view of events unfolding through geologic time 0 Big changes characterize macroevolution 0 Species appeared suddenly in geological time persisted and then suddenly disappeared Such a sudden appearance often of major new groups is called quantum evolution 0 They coined the term punctuated equilibrium to describe both a pattern and eventually a process Long periods of little change equilibrium interrupted punctuated by sudden change The punctuated moment is marked by speciation thereby producing new lineages or clades this pattern is known as cladogenesis o The gaps in the fossil record did not result from discontinuities of preservation but instead rapid real biological events marked by rapid evolutionary change 0 A species persisted more or less unchanged for long periods of time which then were suddenly in geological terms punctuated by rapid change 0 Driven by species selection 0 This is in contrast to phyletic evolution anagenesis wherein a species undergoes transformation through time eventually become quite distinct but without benefit of frequent speciation along the way 0 Species selection o More often claimed than con rmed 0 There are no clear cut examples in nature because it would be a process operating over many thousands of years 0 It is hard to imagine what the selective agent might be acting on the species as an individual rather than collectively through individual outcomes Rapid Evolution 0 Descent with modi cation by means of natural selection produces continuous change On The Edge 0 When a barrier forms to divide a population it often produces many fragmented and isolated groups out of the original single population 0 Because isolated groups are smaller favorable variation may collect there without being swamped by the overwhelming variety in the larger population and new favorable characteristics come quickly to predominate 0 These isolated populations may be fragments on the margins of the major species rangei peripheral isolates 0 Here each small population lives in a slightly different environment than that of its neighboring populations and hence experiences different selective pressures and different microevolutionary changes 0 Within peripheral isolates evolution may speed up Genetic Drift 0 Here at the extremes of a specie s range individuals meet extreme conditions relative to those of the major populations in the middle 0 Such random uctuations of an allele resulting from chance alone is genetic drift 0 Chance not selection rules 0 Rare alleles may arrive with the few colonists and thereby become common in undiluted isolation This is the founder e ectia type of genetic drift in which rare pioneering alleles by chance become common in new populations they establish 0 Bottleneck e ect wherein only a few out of the whole population breed restricting genetic variability to just those individuals randomly enjoying reproductive success Small Isolated Populations 0 Isolated habitats can be a kind of genetic sanctuary where random events and extreme conditions can shelter new but adaptive features that prosper and build to significant numbers Macro Changes at Micro Levels Preadaptation o Often old traits serve new functions Feathers in birds evolved initially as insulation to conserve body heat and ight came later 0 Preadaptation meaning that a structure or behavior possesses the necessary form and function before being remodeled into a new role it later serves 0 A preadadapted part can do the job before the job arrives Embryonic Changes 0 Another way to produce rapid changes in through major adjustments during embryonic development based on genetic mutations that affect embryology Hox Genes o A more direct way to produce rapid major changes in morphology is be gene action especially by master control genes called H ox genes 0 Hox genes regulate the appearance of major body parts such as body regions legs antennae and wings o A simple change in one of these master control Hox genes can produce a major change in the body design Evolutionary Signi cance 0 Big changes in morphology can be initiated by the relatively few but important master control genes leading to rapid and big evolutionary changes
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