Abstract
The life cycle is the standardized, cyclic sequence of physiological, genetic, and morphological events that mark the passage of one generation to the next.
The ‘survival machines’ of genes are the life cycles that link parental zygotes to progeny zygotes.
Harper and Bell, 1979
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Notes
- 1.
The origin of rusts is highly controversial and speculative. The so-called ‘ancient fern hypothesis’ advanced by several classical rust taxonomists evidently was based largely on the tenuous assumption that ancient hosts should harbor ancient parasites, in contrast to those found on evolutionarily younger hosts such as the gymnosperms and angiosperms. If correct, this fern association implies a vast geological time frame of coevolution between host and parasite. However, phylogenetic studies by Aime and colleagues (2006, 2014) show that the most ancient (basal) of the rusts in her phylogram of 46 species is Caeoma torreyae, originally found on California nutmeg (Bonar 1951), a member of the Taxaceae (i.e., the evolutionary much more recent yew family). Thus, the fern hypothesis has been substantially weakened but not yet entirely disproven by Aime’s work, as well as by other molecular phylogenetic studies (see Sjamsuridzal et al. 1999; Wingfield et al. 2004), which are generally consistent with the earlier cladistic analysis of Hart (1988) that challenged the dogma. Hart’s thorough work was based on an analysis on 28 morphological or character states. For further details, see footnote 2.
- 2.
Savile (e.g., 1953, 1955, 1971a, b, 1976) and Leppik (e.g., 1953, 1959, 1961), among others since the early 1900s, reconstruct the evolution of the rusts based on evidence derived from comparative morphology, host relationships, and biogeography. The synopsis in the following paragraphs is based mainly on their work together with the pioneering synthesis by Jackson (1931). A careful cladistics analysis by Hart (1988) reaches different conclusions as to the likely evolutionary history. As discussed in footnote 1, in the ensuing years the advent of molecular phylogenetics has refined the story (e.g., Maier et al. 2003, 2007; Aime 2006; Aime et al. 2006; van der Merwe 2007, 2008; Duplessis et al. 2011), in some cases (Sjamsuridzal et al. 1999; Wingfield et al. 2004; Aime 2006) undermining the early work and strengthening the arguments of Hart. Importantly, Hart’s work showed that the most primitive (basal) clade he studied was tropical short-cycle rusts on angiosperms. Extant rusts on conifers and ferns highlighted by the early workers were actually a nested terminal (i.e., most recent) clade. Thus, the larger picture evolutionary syntheses characteristic of Savile and Leppik, now considerably dated, are probably best regarded as hypotheses given the rudimentary fossil evidence and constraints in dating fungal divergences (e.g., Lücking et al. 2009; Berbee and Taylor 2010; see Thompson’s [1994] comments on difficulties of reconstructing phylogenetic events after the fact, pp. 60–63).
- 3.
To the extent to which increasing age-related mortality reflects primarily intrinsic physiological decline as opposed to increasing vulnerability to extrinsic mortality factors, the two sources have been controlled as an incidental effect of raising animals in zoos (though such ‘experiments’ have their own shortcomings; see discussion in Ricklefs 2008). These manipulations have shown reduced mortality for some protected species compared to their counterparts in the wild, suggesting that older individuals are indeed more vulnerable to extrinsic sources of mortality; there are also cases of similar patterns of mortality in zoos and in the wild, implicating primarily intrinsic factors for senescence (Ricklefs and Scheuerlein 2002; Ricklefs 2008).
- 4.
Here and elsewhere in our discussion of models in this text, the famous and succinct words of Box and Draper (1987, p. 424) should be kept in mind … “Essentially, all models are wrong, but some are useful”.
- 5.
The term ‘clone’ has been used variously by bacteriologists and, depending on the author, may or may not be equivalent to ‘strain’ (see e.g., Selander et al. 1987 and previous discussion of clones in Chaps. 2 and 5). ‘Strain’ normally refers to descendants of a given isolation that frequently but not necessarily arise from a single colony, which in turn is assumed to have arisen from a single cell.
- 6.
Buss (1987, pp. 142–144) extends Medawar’s logic to populations reproducing both sexually and asexually. Asexual individuals begin with a physiological age of zero but a genetic age equivalent to that of their parent, going back to the last zygote. Buss’s argument is consistent with conclusions of the theoretical models of cloning noted earlier: senescence, though much attenuated in its appearance, will nevertheless occur eventually in asexually reproducing species. Furthermore, a key evolutionary ontogenetic development was that “germ-line sequestration not only closed off asexuality as a developmental alternative, it limited organisms possessing it to only a brief span of life” (p. 144).
- 7.
Bell (1984, 1992), among others, emphasizes that the real criterion is whether there is a distinction between parent and offspring or, at the extreme, whether reproduction produces a symmetrical or asymmetrical result. Martínez and Levinton (1992; see also Roach 1993) argue that the evolution of somatic differentiation, and not germline sequestration per se, was the critical precursor to senescence.
Suggested Additional Reading
Baudisch, A. 2008. Inevitable Aging? Contributions to Evolutionary Demographic Theory. Springer-Verlag, Berlin. A refreshing perspective on senescence dogma, emphasizing organisms where senescence is delayed or absent.
Bonner, J.T. 1965. Size and Cycle: An Essay on the Structure of Biology. Princeton Univ. Press, Princeton, N.J. The case for considering the whole life cycle, not just the adult, as the organism.
Charlesworth, B. 1994. Evolution in Age-Structured Populations. Cambridge Univ. Press, Cambridge, U.K. Mathematical treatment of the evolution of life histories and senescence.
Finch, C.E. 1990. Longevity, Senescence, and the Genome. University of Chicago Press, Chicago. This comprehensive treatise, though now dated, remains a benchmark in the literature.
Rose, M.R. 1991. Evolutionary Biology of Aging. Oxford Univ. Press, NY. A concise, well-argued interpretation of senescence in an evolutionary context. This book is reviewed by Bell (1992). Evolution 46: 854-856.
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Andrews, J.H. (2017).
The Life Cycle.
In: Comparative Ecology of Microorganisms and Macroorganisms. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-6897-8_6
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