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BIOE 109 Summer 2009 Lecture 10- part I Mating systems. Types of Mating Systems. Mating Systems In Nature. Mating Systems In Nature. Monogamy. Monogamy. Monogamy. Polygyny. Polyandry. Polygyny. Mating Systems In Nature. ?. Mating Systems In Nature. Promiscuous.
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BIOE 109 Summer 2009 Lecture 10- part I Mating systems
Mating Systems In Nature Monogamy Monogamy Monogamy Polygyny Polyandry Polygyny
Mating Systems In Nature Promiscuous
Mating Systems In Nature Promiscuous
Hypothesis for the evolution of mating systems • Based on parental care and ecological constraints
Hypothesis for the evolution of mating systems • Based on parental care and ecological constraints Who can ditch first?
Hypothesis for the evolution of mating systems • Based on parental care and ecological constraints Who can ditch first? Is ditching worth it?
Sex allocation • the allocation of resources to male versus female production in sexual species (Charnov 1982). • Sex Ratio?
What is sex ratio? • sex ratio is defined as the proportion of males to females.
What is sex ratio? • sex ratio is defined as the proportion of males to females. • two distinct sex ratios exist:
What is sex ratio? • sex ratio is defined as the proportion of males to females. • two distinct sex ratios exist: 1. the population sex ratio i.e., the proportion of males to females in the population
What is sex ratio? • sex ratio is defined as the proportion of males to females. • two distinct sex ratios exist: 1. the population sex ratio i.e., the proportion of males to females in the population 2. the individual sex ratio
What is sex ratio? • sex ratio is defined as the proportion of males to females. • two distinct sex ratios exist: 1. the population sex ratio i.e., the proportion of males to females in the population 2. the individual sex ratio i.e., the sex ratio of progeny from a female
The evolution of sex ratio -In many species sex chromosomes cause 1:1 sex ratio
The evolution of sex ratio Mammals: females are homogametic (XX) males are heterogametic (XY) Birds: males are homogametic (ZZ) females are heterogametic (WZ)
Mammals: females are homogametic (XX) males are heterogametic (XY) Birds: males are homogametic (ZZ) females are heterogametic (WZ) Sex chromosomes do not guarantee a 1:1 sex ratio!
Why equal numbers of males and females? • R.A. Fisher (1930) provided a genetic explanation for the evolution of a stable sex ratio of 1:1.
Why equal numbers of males and females? • R.A. Fisher (1930) provided a genetic explanation for the evolution of a stable sex ratio of 1:1. • since every individual has one mother and one father, each sex contributes equally, on average, to subsequent generations.
Why equal numbers of males and females? • R.A. Fisher (1930) provided a genetic explanation for the evolution of a stable sex ratio of 1:1. • since every individual has one mother and one father, each sex contributes equally, on average, to subsequent generations. • therefore, males and females must have the same average fitness.
Suppose: • 25% male males will have high fitness • 75% female because they mate with multiple females • Suppose: • 75% male females will have high fitness • 25% female because they mate with multiple males • Members of the rarer sex will experience increased reproductive success relative to common sex • frequency-dependent selection results in stable equilibrium sex ratio of 1:1.
Exceptions to Fisher’s theory NOT ALWAYS 1:1
Exceptions to Fisher’s theory • Local mate competition (Hamilton 1967) • 2. Condition-dependent sex allocation (Trivers and Willard 1973)
Exceptions to Fisher’s theory 1. Local mate competition (Hamilton 1967) • proposed to account for female-biased sex ratios (e.g., parasitoid wasps).
Exceptions to Fisher’s theory 1. Local mate competition (Hamilton 1967) • proposed to account for female-biased sex ratios (e.g. parasitoid wasps). • here, a single foundress produces a small group of closely related individuals that mate among themselves.
Exceptions to Fisher’s theory 1. Local mate competition (Hamilton 1967) • proposed to account for female-biased sex ratios (e.g. parasitoid wasps). • here, a single foundress produces a small group of closely related individuals that mate among themselves. • females invest heavily in daughters and don’t “waste” effort in producing sons.
Exceptions to Fisher’s theory 1. Local mate competition (Hamilton 1967) Mother Male 1 son to 20 daughters Females Dust mites (Acarophenox)
Exceptions to Fisher’s theory 2. Condition-dependent sex allocation (Trivers and Willard 1973) Red deer, Cervus elaphus
Exceptions to Fisher’s theory 2. Condition-dependent sex allocation (Trivers and Willard 1973) • occurs in polygynous species when females invest heavily in producing and caring for their young.
Exceptions to Fisher’s theory 2. Condition-dependent sex allocation (Trivers and Willard 1973) • occurs in polygynous species when females invest heavily in producing and caring for their young. • a good mother can produce larger, or healthier, individuals when they mature.
Exceptions to Fisher’s theory 2. Condition-dependent sex allocation (Trivers and Willard 1973) • occurs in polygynous species when females invest heavily in producing and caring for their young. • a good mother can produce larger, or healthier, individuals when they mature. • theory predicts that females in extremely good condition should produce males.
Exceptions to Fisher’s theory 2. Condition-dependent sex allocation (Trivers and Willard 1973) • occurs in polygynous species when females invest heavily in producing and caring for their young. • a good mother can produce larger, or healthier, individuals when they mature. • theory predicts that females in extremely good condition should produce males. • Why?
Exceptions to Fisher’s theory 2. Condition-dependent sex allocation (Trivers and Willard 1973) • occurs in polygynous species when females invest heavily in producing and caring for their young. • a good mother can produce larger, or healthier, individuals when they mature. • theory predicts that females in extremely good condition should produce males. • Why? Because sexual selection (usually) occurs more strongly in males and condition matters!
How is sex ratio adjusted by mother? • Not known
Sex Allocation Recap • Sex ratio • Why we see an unbiased sex ratio • Sex chromosomes • Frequency dependent selection • Exceptions to sex ratio: • Local mate competition • Condition-dependent sex allocation
Sex in Plants ♂ ♀
Sex in Plants • Why and how do they outbreed? • Why do they inbreed?
The evolution of inbreeding and outbreeding • many plant species have evolved traits to avoid inbreeding.
The evolution of inbreeding and outbreeding • many plant species have evolved traits to avoid inbreeding. 1. Asynchronous male and female functions • pollen shed after or before plant’s stigmas are receptive.
The evolution of inbreeding and outbreeding • many plant species have evolved traits to avoid inbreeding. 1. Asynchronous male and female functions • pollen shed after or before plant’s stigmas are receptive.
The evolution of inbreeding and outbreeding • many plant species have evolved traits to avoid inbreeding. 1. Asynchronous male and female functions • pollen shed after or before plant’s stigmas are receptive. 2. Monoecy • male and female flowers separated on same plant.
The evolution of inbreeding and outbreeding 2. Monoecy • male and female flowers separated on same plant.
The evolution of inbreeding and outbreeding • many plant species have evolved traits to avoid inbreeding. 1. Asynchronous male and female functions • pollen shed after or before plant’s stigmas are receptive. 2. Monoecious • male and female flowers separated on same plant. 3. Dieocy • sexes are separated in different individuals.
The evolution of inbreeding and outbreeding 3. Dieocy • sexes are separated in different individuals.
The evolution of inbreeding and outbreeding 4. Self-incompatibility loci • prevent selfing or breeding with close relatives.
The evolution of inbreeding and outbreeding 4. Self-incompatibility loci • prevent selfing or breeding with close relatives. 5. Heterostyly • two (distyly) or three (tristyly) forms of flowers exist in a species (on different plants).