SEX AND REPRODUCTIVE SUCCESS

Transcription

1 Chapter 15 SEX AND REPRODUCTIVE SUCCESS Genetic system refers to the way a species reproduces itself: sexually or asexually, self-fertilize or outcross, hermaphroditic or have separate sexes. THE EVOLUTION OF MUTATION RATES One hypothesis proposes that the optimal mutation rate could be favored because populations, or lineages within populations, that experience beneficial mutations and thus adapt faster to changing environments might persist longer than populations or lineages that do not experience mutations. It can be inferred from this hypothesis that the faster the environment changes the faster the mutation rate must be in order to avoid extinction. According to the hypothesis of the minimal mutation rate, mutation exists only because the repair system is as efficient as it can be, or because selection is not strong enough to favor investment of energy in a more efficient repair system. The process of mutation is not an adaptation. Deleterious mutations occur in many loci. Mutator genes will usually be associated sooner or later with a deleterious mutation they have caused and will decline in frequency. Therefore, natural selection in sexual populations will tend to eliminate any allele that increases mutation rates, and mutation rates should evolve toward the minimal achievable level. Mutation rates are expected to evolve to the optimal achievable level because selection is not strong enough to favor investment in a more efficient system. Evolutionary biologist believe that the existence of mutation is not an adaptation in most organisms but rather is a by-product of imperfect DNA replication that either cannot be improved or can be improved only at too great a cost in fitness. Alleles that increase mutation rates are generally selected against because they are associated with the deleterious mutations they cause. It should be expected that mutation rates will evolve to the minimal achievable level, even if this should reduce genetic variation and increase the possibility of a species extinction. SEXUAL AND ASEXUAL REPRODUCTION Sex usually refers to the union (syngamy) of two genomes. Gametes are produced by the process of meiosis and gametogenesis, and usually carry the genomes of their originators. Sex usually involves the outcrossing between two individuals, but it can occur by self-fertilization in some organisms. Sex normally includes segregation of alleles and recombination among loci, although the extent of recombination varies greatly.

2 Anisogamy: difference in the size or in the size and form of the gametes. The smaller gamete is considered to be the male, and the larger, the female. Isogamy: the gametes look alike. Dioecious or gonochoristic: individuals are either male or female. Hermaphroditic or cosexual: individuals can produce both kinds of gametes. Parthenogenesis: the offspring develops from a single cell, e.g. unfertilized egg. Apomixis: meiosis is suppressed and the offspring develops from an unfertilized egg. It is a type of parthenogenesis. THE PARADOX OF SEX PARTHENOGENESIS VERSUS THE COST OF SEX The traditional explanation of the existence of recombination and sex is that they increase the rate of adaptive evolution of a species, either in a constant or a changing environment, and thereby reduce the risk of extinction. There is evidence that most asexual lineages of eukaryotes have arisen from sexual ancestors. Asexual lineages are recent. Disadvantages of sex: 1. In general, asexual reproduction preserves adaptive combination of genes, whereas sexual reproduction breaks them down and reduces linkage disequilibrium between them. Recombination also destroys adaptive combination of genes, a disadvantage. 2. Cost of sex: about only half of the offspring of sexual females contribute to the growth of the next generation, whereas all of the (all-female) offspring of asexuals do so. Given equal fecundity, sexually reproducing females will produce only half the number of female offspring of asexually reproducing females. HYPOTHESES FOR THE ADVANTAGE OF SEX AND RECOMBINATION PREVENTIGN MUTATIONAL DETERIORATION 1. In asexual populations, fitness declines because genotypes with few deleterious mutations, if lost by genetic drift, cannot be reconstituted, as they are in populations with recombinations: Muller s ratchet. 2. Deleterious mutations can be more effectively purged by natural selection in sexual that in asexual populations, thus maintaining higher mean fitness. Sexual reproduction reduces the chances of producing a defective offspring.

3 ADAPTATION TO FLUCTUATING ENVIRONMENT 3. Recombination enables the mean of a polygenic character to evolve to new, changing optima in a fluctuating environment. ENHANCED ADAPTATION UNDER DIRECTIONAL SELECTION 4. The rate of adaptation, by fixing combinations of advantageous mutations, may be higher in sexual than asexual populations, if the populations are large. Slower adaptation in asexual populations is likely to be a major reason for their high rate of extinction. SEX RATIOS AND SEX ALLOCATION The theory of sex allocation attempts to explain why some species are hermaphroditic and other are dioecious, and what account for variation in sex ration among dioecious species. Sex ratio is the proportion of males. Population sex ratio and individual sex ratio refer to the proportion males in the population or in the progeny of the individual female. A genotype with an individual sex ratio of 0.5 is an ESS, evolutionarily stable strategy, because it has, per capita, the greatest number of grandchildren. In some species mating occurs not randomly among member of the large population, but within small local groups descended from one or a few founders. After one or a few generations, progeny emerge into the population at large, the colonize patches of habitat and repeat the cycle. This populations are female biased. Genotypes whose individuals sex ration is biases in favor of females contribute more individuals and genes to the population as a whole than groups founded by unbiased genotypes. See THE LOCAL MATE COMPETITION explanation on page 394. The theory of sex allocation says that an even sex ratio, (0.5), is the result of an even fitness in male and in female functions. The evolution of separate sexes versus hermaphroditism depends on how reproductive success via female or male function is related to the allocation of an individual s energy or resources. In dioecious species, individuals allocate all of their reproductive energy to one sexual function or the other. The reproductive success that individuals achieve might be a linear function of their proportional allocation of energy or resources to male function or to female function. If most structures associated with reproduction enhance both male and female function (e.g. petals), hermaphroditism may be favored, because with relatively little extra expenditure the individual can produce offspring via both sexual functions.

4 If male and female functions require different structures, a pure male or female incurs the cost of developing one set of structures, whereas the hermaphrodite incurs both costs and thus is likely have lower fitness. INBREEDING AND OUTCROSSING Outcrossing can be advantageous because it prevents inbreeding depression in the next generation. Dioecy, asynchronous male and female functions in the same organism, and several kinds of self-incompatibility promote outcrossing. If inbreeding depression is caused by homozygous recessive alleles, selection is expected to eliminate those deleterious alleles, decrease variation and increase fitness. Advantages of self-fertilization: Two obstacles must be overcome by obligate self-fertilizing species: inbreeding depression and loss of reproductive success through outcrossing. 1. Selfers save some energy by producing small flowers and little pollen. 2. Occasionally they may produce a highly fit homozygous genotype that sweeps to fixation, carrying with it the alleles that increase the selfing rate. 3. Outcrossing depression may occur when distant population cross-pollinate. Different coadapted gene combinations, perhaps adapted to different environment, may cause this. Selfing will prevent outcrossing depression. Self-fertilization may evolve if fewer resources need to be expended on reproduction, if an allele for selfing becomes associated with advantageous homozygous genotype or if selfing ensures reproduction despite low density or absence of pollinators. THE CONCEPT OF SEXUAL SELECTION Darwin introduced the concept of sexual selection to describe the difference between individuals of the same sex in producing offspring by the number of mates they obtain. Variation in the reproductive success of males depends on the number of their mates, the fecundity of their mates, and the proportion of female s eggs they fertilize. Both mating partners have a genetic interest in the production and welfare of their offspring. If the mates are unrelated, neither has any genetic interest in the other s survival or reproductive success with other partners. Mating success is about the same in males and in females. Forms of sexual selection proposed by Darwin:

5 Male to male combat: it is usually males who fight. Mate choice: usually female choice of male mates. Other forms recognized today by biologists see list on page 397, Table Sexual selection exits because females produce relatively few, large gametes, and males produce many small gametes. The difference creates an automatic conflict between the reproductive strategies of the sexes. A male can mate with many females and suffers little or no reduction in fitness if he mates with an inappropriate female. A single male can potentially fertilize all of a female s eggs, and her fitness can be significantly lowered by inappropriate matings. Females are a limiting resource for males, which compete for mates; but males are not a limiting resource for females. The operational sex ratio, the relative numbers of males and females in the mating pool at any time, is often male-biased because males mate more frequently and because the cost of breeding is greater for females. Females are a limiting resource for males. Variation in mating success is greater among males than among females. There are cases of sex role reversal in which females compete for males. Red phalaropes and sea horses where the males care for the young and tend fewer offspring than a female can produce. CONTEST BETWEEN MALES AND BETWEEN SPERM Sexual selection can create two kinds of traits - weapons and ornaments. Weapons are used in male-to-male combat, that is, competition for mating opportunities. Plumage patterns and songs are used to establish dominance by many birds. Females often prefer to mate with males with external ornaments - exaggerated features of morphology. Sexual selection by male contest, directional selection for greater size, weaponry or display features can cause and arms race that results in evolution of ever more extreme traits. This escalation becomes limited by opposing ecological selection if the cost of larger size or weaponry becomes sufficiently great.

6 As Darwin noted, the duller coloration and lack of exaggerated display features in females and non-breeding males of many species imply that these features of breeding males are ecologically disadvantageous. Males reduce the opportunities for many matings with other males: Protecting a territory. The animal that claims the best territory usually has the best reproductive success. Clasping and guarding the female as long she produces fertile eggs. The seminal fluid of the male reduces the attractiveness of the female. Production of the seminal plug in the vagina. Sperm competition is another way for males to compete. Greater production of sperm increases the chance of fertilization by that male. Larger testes will produce more sperm: polygamous species of primates have larger testes than monogamous species. In many damselflies, the male penis is adapted to remove sperms of previous mating. Sperm precedence: the degree of sperm precedence is affected not by the order of mating but by genetic variation among females and males in the ability of their sperm to displace other males sperms and to resist displacement. Additional reading: Experimental evolution of sperm competitiveness in a mammal. Renée C Firman and Leigh W Simmons. Centre for Evolutionary Biology (M092), University of Western Australia, 35 Stirling Hwy, Nedlands, 6009, Australia. BMC Evolutionary Biology 2011, 11:19. See electronic version of this article in: A contrasting hypothesis in The American Naturalist, Vol. 138, No. 1, Jul., 1991 SEXUAL SELECTION BY MATE CHOICE In many species of animals, individuals of one sex, usually the male, compete to be chosen by the other. Females of many species of animals mate preferentially with males that have larger, more intense, or more exaggerated characters. Color patterns, vocalization, ornaments or display behavior. The preferred male character is often disadvantageous. The time females spend searching for acceptable males have been shown to reduce reproductive output in several species. Hypotheses proposed to explain the female preference for arbitrary traits: 1. Direct benefits 2. Indirect benefits: a) Runaway sexual selection; b) Good genes model 3. Sensory bias 4. Antagonistic coevolution

7 1. DIRECT BENEFITS OF MATE CHOICE According to this hypothesis, the female prefers the male that provides a direct benefit to the female or her offspring, such as nutrition, a superior territory with resources for rearing offspring, or parental care. The selection pressure is to recognize males that are superior providers by some feature that is correlated with their ability to provide. Once this capacity has evolved in females, their preference selects for males with the distinctive, correlated character. 2. INDIRECT BENEFITS OF MATE CHOICE Runaway sexual selection: the sons of females that choose a male trait have improved mating success because they inherit the trait that made their fathers appealing to their mothers. Females will choose a male among many competitors that has the greatest chance of producing male offspring ( sons ) with the best chance of reproductive success. This theory was proposed by Ronald Fisher in 1930 and has been called the Sexy Son Hypothesis. Linkage disequilibrium will develop if the gene that makes the female prefer a male with an exaggerated trait becomes associated with the gene that makes the male have the exaggerated trait because the female always chooses the male with that trait. The frequency of the male trait is accompanied by an increase in the frequency of the female preference through hitchhiking. This is a case of non-random mating in the population. The process is termed "runaway" because over time, it would facilitate the development of greater preference and more pronounced traits, until the costs of producing the trait balance the reproductive benefit of possessing it. (RA Fisher, 1958): According to Fisher, if a majority of females prefer a particular kind of male, other females would be favored if they mate with the same kind of males because their sons will be attractive to many females. Every individual will tend to inherit its mother s genes for preferring its father, and its father s genes for the qualities preferred. These two (groups of) genes will then segregate together and under certain circumstances, due to positive feedback, may lead to runaway selection of more and more exaggeration of the quality preferred. This would continue until the disadvantage, in terms of male survival, exceeds the reproductive advantage for males. Eventually, all the males in the population will end up with a tail length at the optimum point. When all males come to have the same trait, there would be no genetic advantage to a female in choosing one rather than the other. Because of this major problem with this theory, more elaborate forms of it have been developed. M.Tevfik Dorak,

8 If females have genetically variable responses to each of several or many male traits, different traits or combinations of traits may evolve, depending on the initial genetic conditions. The run-away selection can follow different paths in different populations, so that populations may diverge in mate choice and become reproductively isolated. Sexual selection is therefore a powerful potential cause of speciation. Indicators of genetic quality This hypothesis is called either the Good Genes hypothesis or the Handicap hypothesis. According to the handicap hypothesis, males develop secondary sexual characteristics (e. g. bright plumage) that could reduce the male s survival. Females invest a lot in reproduction. It is to their advantage to be selective in choosing a mate, one who will pass on the best genes to the next generation. Females will select males that will pass on good genes that will increase the quality of her offspring. Females suffer substantial losses of fitness if their offspring do not survive or reproduce, therefore, females should select males with high genetic quality, so that their offspring will inherit good genes and so have a superior prospect of survival and reproduction. Any male trait that is correlated with genetic quality (indicator of good genes) could be used by females as a guide to advantageous matings. This theory presumes the evolution of three traits: a male handicap, a female attraction for the handicap, a viability trait. Males who have survived in spite of these handicaps are the stronger Female choice may also carry an ecological cost; rejecting a male in searching for a better one will delay reproduction or entail other risks. Natural selection should reduce variance in fitness. How can genetic variance be maintained and provide at the same time for female preference? It has been proposed that only males resistant to parasites would be able to display conspicuous features -not necessarily costly to them- to attract females. Thus, the ornaments (such as long tails, inflated throat poaches or bright plumage) simply reveal the state of health without damaging it so they constitute revealing handicaps. Female choice for males with the bestdeveloped sexual characters would result in offspring that are likely to inherit geneticallydetermined resistance to parasites from their father. M.Tevfik Dorak, Ph.D. See examples on page 404, Evidence for the Role of Indirect Effects. 3. Sensory bias Phylogenetic studies have shown that female preferences sometimes evolve before the preferred male trait.

9 Certain traits may be intrinsically stimulating and evoke a greater response simply because of the organization of the sensory system. Animals frequently show greater responses to supernormal stimuli that are outside the usual range of stimulus intensity. See the example of the fish Xiphophorus and Priapella of page Antagonistic coevolution Conflict between the sexes is also pervasive in nature. Unless a species is strictly monogamous, a male profits if he can cause a female to produce as many of his offspring as possible, even if this reduces her subsequent ability to survive and reproduce. Genes that govern male versus female characters may conflict, resulting in antagonistic coevolution. See examples, abalone and Drosophila, on page 406. An arms race for more enhanced characters may result. Females evolve a resistance to mate, and their resistance selects for features that enable males to overcome the females reluctance. This is called chase-away sexual selection. Under chase-away selection, males may evolve increasingly strong stimuli, such as brighter colors or more elaborate song, to induce reluctant females to mate. More elaborate traits evolve not because females prefer them but because females resist them. ALTERNATIVE MATING STRATEGIES Males also incur in a cost for reproduction. Delayed maturation may evolve if larger males are more successful in attracting or competing for mates. Alternative mating strategies also called polymorphic mating phenotypes have evolved especially in males of many species. The sneaker male who lurks around the harem of females defended by a strong male waiting for the opportunity to sneak in and mate a few times and then run away unscathed. Males that mimic females like the Coho salmon whereas small jack males are smaller, resemble females, do not fight, and breed when they are only about a third as old as the hooknose dominant males.

10 Species that grow throughout their reproductive life, a sex change can be advantageous if reproductive success increases with size to a greater extent in one sex than in the other. o Protandry and protogyny. Warner suggests that selection for protandry may occur in populations where female fecundity is augmented with age and individuals mate randomly. Selection for protogyny may occur where there are traits in the population that depress male fecundity at early ages (territoriality, mate selection or inexperience) and when female fecundity is decreased with age, the latter seems to be rare in the field.[1] An example of territoriality favoring protogyny occurs when there is a need to protect their habitat and being a large male is advantageous for this purpose. In the mating aspect, a large male has a higher chance of mating, while this has no effect on the female mating fitness.[18] Thus, he suggests that female fecundity has more impact on sequential hermaphroditism than the age structures of the population.[1] Warner, R.R (1975). "The Adaptive Significance of Sequential Hermaphroditism in Animals.". The American Naturalist 109: 61 82; Kazancioğlu, E; SH Alonzo (2010). "A comparative analysis of sex change in Labridae supports the size advantage hypothesis". Evolution; international journal of organic evolution 64 (8): Proximate causes of sex change: Kroon, FJ; Munday, P. L., Westcott, D. a, Hobbs, J.-P. a, & Liley, N. R. (2005). Aromatase pathway mediates sex change in each direction. Proceedings. Biological sciences / The Royal Society 272 (1570): Read this interesting article: Adult male phenotypes may be determined from birth (mites, Radwan 1995; isopods, Shuster & Sassaman 1997) or they may be directed by a range of environmental influences (many species; Oliveira et al. 2008). Well-fed males in some species may delay maturation and become territorial (dung beetles, Moczek 1998), whereas rapid growth in other species leads males to mature early, live fast, and die young (salmon, Hutchings & Myers 1994). Stephen Shuster. Westneat_Chap-25%20-%20sms.pdf The evolution of features of genetic systems, such as rates of mutation and recombination, sexual versus asexual reproduction, and rates o f inbreeding, can usually be understood best as consequences of selection at the level of genes and individual organisms, rather than group selection.