Evolutionary Background Key Concepts Adaptation Descent with modification (Evolution as history) Natural Selection (Evolution as process) Some Special Problems What do we mean by adaptation? Altruism Group vs Individual Selection Kin Selection Sexual Selection ADAPTATION = solution to a specific problem that contributes to survival and/or reproductive success. improbably useful i.e., too precisely functional to have arisen by chance alone Mimicry as a classic example DESCENT WITH MODIFICATION Any 2 species can be traced back ultimately to a common ancestor. The differences between them are the result of the history of the adaptations of their respective lineages since they diverged from their common ancestor. DESCENT WITH MODIFICATION Any 2 species can be traced back ultimately to a common ancestor. The differences between them are the result of the history of the adaptations of their respective lineages since they diverged from their common ancestor. A primate phylogeny: Based on combination of data from dozens of proteins used as molecular clocks. [Freeman & Herron, Fig 19-6] 1
NATURAL SELECTION 1. The Struggle for Existence Example: Giraffe ecological niche = a high browser 2. The Mechanism: Natural Selection individuals vary within a species variation is in part heritable variation is related to adaptation Thus, if individuals vary re some trait related to survival and reproductive success, the better adapted individuals will leave more offspring and the next generation will on average be different re that trait. Natural selection occurring over a long time will lead to dramatic changes in a species - to evolution. Example: Giraffe ecological niche = a high browser Natural selection = Differential reproductive success (RS) of individuals within a population due to genetic differences among them. Some important implications and qualifications 1. If no genetic basis, then no natural selection. 2. Reproductive success (not adaptation, not survival) is bottom line for natural selection. Alleles A1 A2 A3 # offspring 3 2 1 Alleles = alternative forms of a gene. Here A alleles contribute to neck length. 3. Selection acts on individuals (not species, not groups, not populations). 4. Note: adaptation not mentioned in definition 2
Special problem with behavior: Not all behaviors appear truly adaptive GROUP SELECTION Selection acts on individuals or genes (not species, not groups, not populations). Langur Modern study of animal behavior coincides with the realization that selection acts at the level of the individual (or the gene actually) and rarely at level of larger groups (known as group selection). Two problems with group selection thinking: 1. Logical problem Lion Infanticide: Maladaptive or Adaptive? 2. Generally fails to predict correctly The logical problem with group selection GROUP SELECTION & ALTRUISM But some behaviors do appear to benefit the larger group at a cost to the individual Altruism: behavior that benefits another individual at a cost to the altruist's personal fitness (ability to produce offspring). - C + B Altruist Recipient Belding s ground squirrel 3
ALTRUISM & KIN SELECTION Kin Selection: selection of a trait through helping relatives, either 1. descendant kin (offspring): direct selection -OR- 2. non-descendant (collateral) kin: indirect selection inclusive fitness = direct + indirect fitness ALTRUISM & KIN SELECTION Three factors are important in the spread and maintenance of an altruism gene by kin selection: 1. benefit to recipient, B 2. cost to altruist, C 3. degree of relatedness between altruist and recipient, r Coefficient of Relatedness, r = 1. probability that a rare gene is shared by two individuals, OR 2. the proportion of genes Identical By Descent (IBD) shared by two individuals. ALTRUISM & KIN SELECTION Three factors are important in the spread and maintenance of an altruism gene by kin selection: 1. benefit to recipient, B 2. cost to altruist, C 3. degree of relatedness between altruist and recipient, r Hamilton s Rule states the conditions under which altruism will spread. In its simplest form it is: rb > C When should you be altruistic? When B > C/r Recipient identical twin 1 parent full sib half sib niece/nephew* uncle/aunt ** 1 st cousin ** 1/2 1/2 1/8 B>C/r B > C B > 2C B > 2C B > 8C * assumes your sib was full sib ** assumes your parent s sib was full r 1 4
Alarm-calling in Belding s ground squirrel: an example of altruistic behavior better explained by kin selection than group selection Individual and group selection arguments make different predictions as to whether these classes of animals should alarm call: Males Females w/o pups Females with pups Pups Group Selection: Behaviors that maximize survival of group/species are selected Individual (Kin) Selection: Behaviors that maximize the propagation of individual s genes are selected Predictions: Who alarm calls and who doesn t? Class Group Indiv. Males Females w/o pups Females with pups Pups? * depends on whether other relatives are present /* Outcome Need to know: males disperse from natal group, females stay; males also move during breeding season Sexual Selection: selection for traits that increase mating success (usually at the expense of survival or parental care). Instrasexual Natural Selection Sexual Selection Survival Mating Success Parental Care Intrasexual Competition Mate Choice (Epigamic) Intersexual or Epigamic 5