Chapter 8: Sex and Evolution

CHAPTER 8: Sex and Evolution Sexual selection based on eye span – stalked-eyed flies.  Terms: Gametes – Eggs or sperm. Zygote – Single celled embryo. Gonads – Sexual organs. Meiosis – Cell division that forms the gametes. Haploid – One chromosome of the diploid...

CHAPTER 8: Sex and Evolution Sexual selection based on eye span – stalked-eyed flies.  Terms: Gametes – Eggs or sperm. Zygote – Single celled embryo. Gonads – Sexual organs. Meiosis – Cell division that forms the gametes. Haploid – One chromosome of the diploid pair. Diploid – Two homologous chromosomes. Sources of genetic variation following reproduction: 1. Independent assortment. 2. Recombination (crossing over).  Asexual reproduction: Offspring are usually identical to one another and their parent. Most common in plants, vegetative reproduction. Clone – Refers to offspring of asexual reproduction that lack variation. Parthenogenesis – Asexual reproduction by producing diploid eggs. - Have some genetic variation as a result of independent assortment and crossing over. - Observed in insects, fish, lizards, turkeys Selfing – Organisms that have both male and female gametes fertilize themselves, common in plants.  Costs of sexual reproduction: Maintenance of gonads Mate attraction Two fold cost of meiosis -Only ½ the parents genes are passed on versus 100% in asexual reproduction. Asexual Organism Family Trees https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcTrpKu8rYijXwXjGoFD5lT7xt2C95R6_SYvyhlW1ZSaYe1fpYeD Advantages of sexual reproduction: Genetic variation - Provides the potential for offspring to adapt to a changing environment. - Do our best ecological models indicate that it sufficiently offsets the costs? - What is more prevalent among multicellular organisms, sexual or asexual? WHY?? Genetic variation allows organisms to cope with changes in the biological environment. Red Queen Hypothesis – Organisms must constantly alter their genotype in order to prevent being overcome by pathogens, present a “moving target.”  Hermaphrodism Male and female function combined in the same individual - May be simultaneous or sequential. In plants: -Monoecius = Male and female flower parts together in the same plant. - Dioecious = Separate sexes. Many plants prevent selfing through self-incompatibility (SI) genes. Dioecious Monoecious -Perfect flower – More than 2/3 of all plant species.  Sex Determination https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQzxAfAp9o9626pJQbg4O-GnrAT0ksftyWnxaB7UncMiksJZXT7 XY sex determination -Factors that may alter sex ratios - Competition between X and Y sperm. - Selective abortion. - Segregation of certain chromosomes into certain eggs - Practiced by many bird species (XY are females). Temperature dependent sex determination. - Alligators, turtles, and some lizards. Socially dependent sex determination. - Blue-headed wrasse = sequential hermaphrodite. 1. In isolation all females. 2. Small groups some are males. 3. Females may become males later in life. Alteration of California sheephead life history resulting from fishing pressure.  Evolution of sex ratio The rare sex advantage leads to a 1:1 sex ratio. Frequency dependent selection. Local mate competition (competition among brothers) favors the production of daughters - Haplodiploidy – Fertilized egg = female Unfertilized egg = male  Mating Systems Promiscuity – By far the most common in animals, the only system seen in outcrossing plants. Polygamy – Long term bonds formed with more than one member of the opposite sex. - Polygny – One male, multiple females. - Polyandry – One female, multiple males. Monogamy – Pair bond during offspring raising, possibly beyond. -Common when both parents can equally contribute to care of young. -Extra-pair copulations may occur even within monogamy - Up to 1/3 of offspring in monogamous bird species - EPCs have led to the selection of nest guarding Mating Systems – Bluegill The Polygyny Threshold Model  Sexual Selection Selection by one sex of specific characteristics in the opposite sex Leads to sexual dimorphism, difference in outward appearance of sexes. - Secondary sexual characteristics Arises in three ways: 1. Dissimilar sexual function - Female fecundity depends on size 2. Male combat 3. Mate choice Long tailed widow birds – demonstration of female choice  Explaining sexual selection Handicap principle – Demonstrate strength by enduring handicap. Parasite mediated sexual selection – Presence of parasites prevents the display of elaborate plumage or other features.