John Tyler Bonner
- Published in print:
- 2013
- Published Online:
- October 2017
- ISBN:
- 9780691157016
- eISBN:
- 9781400846429
- Item type:
- chapter
- Publisher:
- Princeton University Press
- DOI:
- 10.23943/princeton/9780691157016.003.0005
- Subject:
- Biology, Evolutionary Biology / Genetics
This chapter discusses how the sexual cycle varies in a general way depending on the size and complexity of organisms. Natural selection has burnished the sexual system so that in each generation the ...
More
This chapter discusses how the sexual cycle varies in a general way depending on the size and complexity of organisms. Natural selection has burnished the sexual system so that in each generation the degree of variation in the offspring is optimal—not too little and not too much—which allows natural selection to take place and makes evolutionary progress possible. However, in many simpler, small organisms this sexual variation-control mechanism can be turned on and off: periods of sexual reproduction will be interspersed with periods of asexual reproduction. In larger forms (with some rare exceptions) only the sexual route is possible. And it is only in the sexual cycle through recombination that the all-important controlled variation that is so essential for evolutionary progress is produced.Less
This chapter discusses how the sexual cycle varies in a general way depending on the size and complexity of organisms. Natural selection has burnished the sexual system so that in each generation the degree of variation in the offspring is optimal—not too little and not too much—which allows natural selection to take place and makes evolutionary progress possible. However, in many simpler, small organisms this sexual variation-control mechanism can be turned on and off: periods of sexual reproduction will be interspersed with periods of asexual reproduction. In larger forms (with some rare exceptions) only the sexual route is possible. And it is only in the sexual cycle through recombination that the all-important controlled variation that is so essential for evolutionary progress is produced.
John Tyler Bonner
- Published in print:
- 2013
- Published Online:
- October 2017
- ISBN:
- 9780691157016
- eISBN:
- 9781400846429
- Item type:
- book
- Publisher:
- Princeton University Press
- DOI:
- 10.23943/princeton/9780691157016.001.0001
- Subject:
- Biology, Evolutionary Biology / Genetics
This book challenges a central tenet of evolutionary biology. The book makes the bold and provocative claim that some biological diversity may be explained by something other than natural selection. ...
More
This book challenges a central tenet of evolutionary biology. The book makes the bold and provocative claim that some biological diversity may be explained by something other than natural selection. The book makes an argument for the underappreciated role that randomness—or chance—plays in evolution. Due to the tremendous and enduring influence of Darwin's natural selection, the importance of randomness has been to some extent overshadowed. The book shows how the effects of randomness differ for organisms of different sizes, and how the smaller an organism is, the more likely it is that morphological differences will be random and selection may not be involved to any degree. The book then traces the increase in size and complexity of organisms over geological time, and looks at the varying significance of randomness at different size levels, from microorganisms to large mammals. The book also discusses how sexual cycles vary depending on size and complexity, and how the trend away from randomness in higher forms has even been reversed in some social organisms.Less
This book challenges a central tenet of evolutionary biology. The book makes the bold and provocative claim that some biological diversity may be explained by something other than natural selection. The book makes an argument for the underappreciated role that randomness—or chance—plays in evolution. Due to the tremendous and enduring influence of Darwin's natural selection, the importance of randomness has been to some extent overshadowed. The book shows how the effects of randomness differ for organisms of different sizes, and how the smaller an organism is, the more likely it is that morphological differences will be random and selection may not be involved to any degree. The book then traces the increase in size and complexity of organisms over geological time, and looks at the varying significance of randomness at different size levels, from microorganisms to large mammals. The book also discusses how sexual cycles vary depending on size and complexity, and how the trend away from randomness in higher forms has even been reversed in some social organisms.
Leo W. Beukeboom and Nicolas Perrin
- Published in print:
- 2014
- Published Online:
- August 2014
- ISBN:
- 9780199657148
- eISBN:
- 9780191748103
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199657148.003.0002
- Subject:
- Biology, Evolutionary Biology / Genetics
The sexual cycles of eukaryotes vary immensely in terms of the relative importance of the haploid and diploid phases, the differentiation between gametes, and the timing and mode of sex ...
More
The sexual cycles of eukaryotes vary immensely in terms of the relative importance of the haploid and diploid phases, the differentiation between gametes, and the timing and mode of sex determination. The chapter discusses the evolutionary advantages of haploid and diploid phases, the conditions for the maintenance of haplo-diplontic cycles, and the role of disruptive selection in the evolution from isogamy to anisogamy and oogamy. The chapter proposes a typology for sexual cycles based on the relative importance of haploid and diploid phase, whether sex is determined at the haploid or diploid stage, and whether the initial trigger is genetic or epigenetic. The chapter develops the concepts of heterothallism versus homothallism, haplo- versus diplo-genotypic sex determination, dioicy versus dioecy, monoicy versus monoecy, self-incompatibility systems and secondary mating types. The chapter considers the diversity of epigenetic sex-determination systems (mating-type switching, simultaneous and sequential hermaphroditism, as well as environmental, social, maternal, or parasite control of sex determination) and discusses the ultimate and proximate causes favouring their evolution, as well as their likely role in transitions from haplo- to diplo-genotypic sex determination.The electronic addendum of this chapter (Section 2.2) describes in more detail the diversity and phylogenetic distribution of sex-determination types among extant eukaryotes.Less
The sexual cycles of eukaryotes vary immensely in terms of the relative importance of the haploid and diploid phases, the differentiation between gametes, and the timing and mode of sex determination. The chapter discusses the evolutionary advantages of haploid and diploid phases, the conditions for the maintenance of haplo-diplontic cycles, and the role of disruptive selection in the evolution from isogamy to anisogamy and oogamy. The chapter proposes a typology for sexual cycles based on the relative importance of haploid and diploid phase, whether sex is determined at the haploid or diploid stage, and whether the initial trigger is genetic or epigenetic. The chapter develops the concepts of heterothallism versus homothallism, haplo- versus diplo-genotypic sex determination, dioicy versus dioecy, monoicy versus monoecy, self-incompatibility systems and secondary mating types. The chapter considers the diversity of epigenetic sex-determination systems (mating-type switching, simultaneous and sequential hermaphroditism, as well as environmental, social, maternal, or parasite control of sex determination) and discusses the ultimate and proximate causes favouring their evolution, as well as their likely role in transitions from haplo- to diplo-genotypic sex determination.The electronic addendum of this chapter (Section 2.2) describes in more detail the diversity and phylogenetic distribution of sex-determination types among extant eukaryotes.
