2 Natural Selection through Sexual Reproduction From so simple a beginning, endless forms….
3 Phenotype: observable characteristics, e.g., beak size Genotype: genes consisting of pairs of alleles that code for the phenotype, e.g., B codes for big beak size. b codes for small beak size.
4 Good weather, nuts are soft, B = 40% Dry weather, nuts are hard, B = 70%
5 Many more generations later with the dry weather persisting:
6 Industrial pollution in peppered moths in 1800 England and Bernard Kettlewell’s experiments of 1950
7 A summary of the principles of natural selection Principle of Variation: Individuals within a species show variation in their physical and behavioural traits. Principle of Inheritance: Some of this variation is heritable. Principle of Adaptation: Individuals are in competition with one another for scarce resources and some inherited variations will have survival advantages. Principle of Selection: as a consequence of being better adapted to an environment, some individuals will produce more offspring, who will inherit the same advantages. Fitness is thus defined as reproductive success (RS) or the number of surviving offspring produced. Evolution results in differential rate of reproduction and survival of different genotypes in a population
8 1.Variation of a trait 2.Differential reproduction of the trait 3.Heritability of the trait 4.End result: brown is more fit than green
9 Where does variation come from? Mutation or copying errors. Fitness = The relative number of surviving offspring, more specifically: the extent to which copies of an individual’s genotype are present in succeeding generations, relative to other genotypes; in other words, reproductive success (RS) Spencer’s quote, “survival of the fittest,” can be misleading. Fitness is not related to survival except for survival to the reproductive age. Fitness does not refer to physical well-being. A wise man lived a long life alone. Wise but not fitness. A dumb head had children and died. Dumb but fitness.
10 Evolutionary Language A trait or characteristic is selected for, is adaptive, is an adaptation, is a fitness. A phenotype is selected against because it is not adaptive to the ecology. Evolution is blind, random, unconscious, unintentional, non-directional and non-progressive. But just as a speaking habit, we often talk about evolution as if it is a conscious process by referring to phenotypes as “designs,” “design features,” “strategies,” “solutions” or as something that is “useful,” “functional,” or “successful.”
11 Inclusive fitness: Extending Darwin’s natural selection through sexual reproduction Hamilton (1964) reformulated evolutionary theory by showing that the measure of an individual’s direct reproductive success (RS) was too narrow a concept. He introduced the term inclusive fitness to refers to the fitness or RS of the individual plus the effects of a particular behavior or trait of the individual on RS of the individual's relatives. C < B*r Inclusive fitness = Direct fitness + Indirect fitness = RS of Individual + RS of Relatives Altruism: Decrease in direct fitness < than increase in indirect fitness Altruism: Selfish restraint, e.g., not being too mean to relatives.
13 Reciprocal Altruism (Trivers, Axelrod & Hamilton) Why do organism help genetically unrelated? C < B*r, r = reciprocating What facilitate r or the probability of meeting each other again? Stable residence Small community characteristic of ancestral past Long term memory Still true today: crimes higher in large cities than small towns crimes more likely when identity is hidden than shown we treat people well in part because we run into them again We feel bad when we fail to return a favor from others when others fail to return our favor
14 Prisoner’s Dilemma B A Cooperate Defect CooperateDefect B:2.50 A:2.50 A:5 B:0 A:0 B:5 A:1 B:1
15 Sexual Selection Whereas natural selection increases the frequency of a trait because its bearers are favored by nature, sexual selection theory states that the frequency of a trait increases because its bearers are favored by the opposite sex.
17 Who Chooses Whom? Baterman’s Principle The sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete. For humans and many other animals, females are the limiting sex because there are far fewer eggs than there are sperms, the eggs carry far more nutritional investment for the next generation than sperms, and internal gestation limits reproductive capacity of females compared to males.
18 Sexual selection can take two forms Intrasexual selection Also known as Intrasex Competition and Male-to-Male-Combat, in which members of the less limited sex (typically males) compete aggressively among themselves for access to the limiting sex. Intersexual selection Also known as Intersex Competition or Mate Choice or Female Choice, in which males compete with each other to be chosen by females.
19 Weapons Intersex selection selects traits that give males the physical advantage to fight against, to injur, and to kill other males. These include size, strength, aggression, and violence and other related traits that serve as weapons in the male to male combat. Ornaments Intrasex selection selects traits which serve as brags to attract females. The best known example is the peacock tail. These male ornaments originally fascinated Charles Dawin and motivated his theory of sexual selection. Zahavi’s Handicap Principle The elaborated ornaments that take much energy to sustain and cut into one’s fitness serve as indicators for the overall wellbeing and genetic quality of the individual carrying the extra cost of fitness. The showy males are telling their females that they must have good genes because they can afford the competition with a handicap. Sex Dimorphism Intersex and intrasex selection lead to sex differences in these “weapon” and “ornament” traits so that males are bigger, stronger, and more aggressive and have brighter color, deeper voice or are more oratory or showy or bragging than females.
20 Parental Investment Theory: Extending Darwin’s sexual selection theory Robert Trivers (1972) argued that a driving force behind sexual selection is the degree of parental investment each sex devotes to their offspring. That is, parental investment differences between the two sexes are the engine driving sexual selection and the evolution of all related sex differences.
21 Why do females choose? High parental investment InitialPostnatal Why do males compete? Female Low parental investment WeaponsOrnaments Males Excessive cheap spermsLimited valuable eggs Maternity Certainty Paternity Uncertainty What do females choose? Good genesGood providersGood fathers
22 The sex that invests most heavily in an offspring will be more choosy about who they mate with (i.e females). The sex that invests least in an offspring should compete more vigorously for access to the higher-investing sex (i.e. males). Male competition leads to physical dimorphism (weapons and ornaments), which correlates with male parental investment and male reproductive success variation. Variations open ways for other reproductive strategies: –Increased male parental investment –Reduced sex dimorphism –Reduced male reproductive variation
23 Where male parental investment approaches that of females, competition will be reduced as will sexual dimorphism (e.g. swans). Where male parental investment is higher, females will possess typically male characteristics (e.g. seahorses, jacana).
24 Mating Systems Monogamy: pair bond for the breeding season or for life; parental investment from both parents; low sex dimorphism. Polygyny: males have multiple female partners; parental investment from mothers but not fathers; high dimorphism Resources based: males defend territories that sustain females. Non-resources based: males gather at lek to compete for females. Polyandry: females have multiple male partners; parental investment from fathers but not mothers.