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== Developmental Reprogramming == |
== Developmental Reprogramming == |
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Evolution is often portrayed as an interplay between mutation and selection, with the former providing a supply of variation, and the latter acting as fittness-based. But this picture has a major flaw, because [[mutation]] provides new [[Gene|genes]], whereas[[Natural selection|selection]] acts not on genes but on [[Phenotype|phenotypes]]. The missing link is how we get from altered gene to new phenotype. This is the process of developmental reprogramming that is, a mutationally driven change in something that is itself a state of change. If a particular ontogeny is represented by a trajectory through multi-dimensional phenotypic space, then after reprogramming we have a different trajectory. Developmental reprogramming is a mutation-based, and thus inherited change in the overall genetic/epigenetic/ecological program through which we get from genome to phenome. Reprogramming becomes controversial if it is proposed that it can be systematically biased, in that mutation more readily produces changes in certain directions than others, including the extreme case of some directions being prohibited. |
Evolution is often portrayed as an interplay between mutation and selection, with the former providing a supply of variation, and the latter acting as fittness-based. But this picture has a major flaw, because [[mutation]] provides new [[Gene|genes]], whereas[[Natural selection|selection]] acts not on genes but on [[Phenotype|phenotypes]]. The missing link is how we get from altered gene to new phenotype. This is the process of developmental reprogramming that is, a mutationally driven change in something that is itself a state of change. If a particular ontogeny is represented by a trajectory through multi-dimensional phenotypic space, then after reprogramming we have a different trajectory. |
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'''Developmental reprogramming''' is a mutation-based, and thus inherited change in the overall genetic/epigenetic/ecological program through which we get from genome to phenome. Reprogramming becomes controversial if it is proposed that it can be systematically biased, in that mutation more readily produces changes in certain directions than others, including the extreme case of some directions being prohibited. |
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== Developmental Bias: Constraint vs. Drive == |
== Developmental Bias: Constraint vs. Drive == |
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A bias in developmental reprogramming is reffered to as a mutation bias or a developmental bias (evolution biased by development). The term developmental bias is used for both positive and negative effects. Developmental |
A bias in developmental reprogramming is reffered to as a mutation bias or a developmental bias (evolution biased by development). The term developmental bias is used for both positive and negative effects. Developmental is a term used solely for '''negative biases''', whereas '''positive biases''' have recently been termed developmental drive. Evidence for a directional evolutionary role for development bias is limited and difficult to aquire. |
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== Developmental Bias: Absolute vs. Relative == |
== Developmental Bias: Absolute vs. Relative == |
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If a developmental system is incapable of producing a certain variant ontogeny, it is an absolute constraint. If the particular ontogeny x is difficult to produce, in the sense that a small proportion of mutations lead in that direction while many lead elsewhere, it is a relative constraint. The same distinction can be applied to developmental drive (positive biases). |
If a developmental system is incapable of producing a certain variant [[ontogeny]], it is an '''absolute constraint'''. If the particular ontogeny x is difficult to produce, in the sense that a small proportion of mutations lead in that direction while many lead elsewhere, it is a '''relative constraint'''. The same distinction can be applied to developmental drive (positive biases). |
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== Developmental Drive in the Real World == |
== Developmental Drive in the Real World == |
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To show the involvement of developmental drive, it is often necessary to compare existent and nonexistent morphologies, which is a much more difficult. This causes a problem because there must be a decision made about which nonexistent morphologies to compare with the existent. However, there is a broad scope for uninformative comparisons to be made so it must be done with careful consideration. |
To show the involvement of developmental drive, it is often necessary to compare existent and nonexistent [[Morphology (biology)|morphologies]], which is a much more difficult. This causes a problem because there must be a decision made about which nonexistent morphologies to compare with the existent. However, there is a broad scope for uninformative comparisons to be made so it must be done with careful consideration. |
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== Developmental Drive: Centipede Segments == |
== Developmental Drive: Centipede Segments == |
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There exists at least 3000 species of centipede with trunk segment numbers that range from 15 to 191. |
There exists at least 3000 species of [[centipede]] with trunk segment numbers that range from 15 to 191. There are two orders of centipedes in which developmental drive is evident. Lithobiomorpha and Geophilomorpha are characterized '''by very different developmental systems but exhibit the same absolute constraint regarding even-segment-number phenotypes.''' |
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There are two orders of centipedes in which developmental drive is evident. Lithobiomorpha and Geophilomorpha are characterized by very different developmental systems but exhibit the same absolute constraint regarding even-segment-number phenotypes. |
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The order [[Lithobiomorpha]] consist of about 1100 species. All have 15 trunk segments at their adult stage. However, these centipedes hatch from the egg with fewer than 15 segments and the segments are added with age through a series of moults through which the hatchling grows towards adulthood. Lithobiomorph centipedes with even numbers of trunk segments exist, but only as juveniles. The strange thing about this is why, given the all-pervasive role of heterochrony in evolution, there have been no shifts of relative timing so that reproductive maturity and a cessation of segment addition have occurred in (say) a 12-trunk-segment juvenile in at least one of the many lithobiomorph species. So the argument for constraint is persuasive but not conclusive. It becomes even more persuasive when we consider the geophilomorph situation. |
The order [[Lithobiomorpha]] consist of about 1100 species. All have 15 trunk segments at their adult stage. However, these centipedes hatch from the egg with fewer than 15 segments and the segments are added with age through a series of moults through which the hatchling grows towards adulthood. Lithobiomorph centipedes with even numbers of trunk segments exist, but only as juveniles. The strange thing about this is why, given the all-pervasive role of heterochrony in evolution, there have been no shifts of relative timing so that reproductive maturity and a cessation of segment addition have occurred in (say) a 12-trunk-segment juvenile in at least one of the many lithobiomorph species. So the argument for constraint is persuasive but not conclusive. It becomes even more persuasive when we consider the geophilomorph situation. |
Revision as of 00:46, 27 November 2017
Developmental drive is characterized by positive biases toward other trajectories or phenotypes. Developmental drive is a type of developmental reprogramming of ontogenetic trajectory within a lineage in favor of certain changes. Developmental drive therefore has an influence on the direction of evolutionary change.
History
Over the last three decades, there has been rapid growth of a new approach to comprehending the evolution of organisms and the effect that development has on the direction of evolution. Evolutionary developmental biology or "evo-devo", is focused on the developmental genetic machinery that lies behind embryological phenotypes, which were all that could be studied in the past, stemming from the work of Von Baer and Ernst Haeckel.
Present day evo-devo erupted out of the discovery of the homeobox in the early 1980s. Proposals that biases (positive and negative) can potentially lead to the direction of evolutionary change being determined by developmental dynamics as well as by population dynamics are in contrast with the historical thrust of Darwinism and Neo-Darwinism, that the direction of change is determined exclusively by selection. Development has long been the missing link of evolutionary theory.
Developmental Reprogramming
Evolution is often portrayed as an interplay between mutation and selection, with the former providing a supply of variation, and the latter acting as fittness-based. But this picture has a major flaw, because mutation provides new genes, whereasselection acts not on genes but on phenotypes. The missing link is how we get from altered gene to new phenotype. This is the process of developmental reprogramming that is, a mutationally driven change in something that is itself a state of change. If a particular ontogeny is represented by a trajectory through multi-dimensional phenotypic space, then after reprogramming we have a different trajectory.
Developmental reprogramming is a mutation-based, and thus inherited change in the overall genetic/epigenetic/ecological program through which we get from genome to phenome. Reprogramming becomes controversial if it is proposed that it can be systematically biased, in that mutation more readily produces changes in certain directions than others, including the extreme case of some directions being prohibited.
Developmental Bias: Constraint vs. Drive
A bias in developmental reprogramming is reffered to as a mutation bias or a developmental bias (evolution biased by development). The term developmental bias is used for both positive and negative effects. Developmental is a term used solely for negative biases, whereas positive biases have recently been termed developmental drive. Evidence for a directional evolutionary role for development bias is limited and difficult to aquire.
Developmental Bias: Absolute vs. Relative
If a developmental system is incapable of producing a certain variant ontogeny, it is an absolute constraint. If the particular ontogeny x is difficult to produce, in the sense that a small proportion of mutations lead in that direction while many lead elsewhere, it is a relative constraint. The same distinction can be applied to developmental drive (positive biases).
Developmental Drive in the Real World
To show the involvement of developmental drive, it is often necessary to compare existent and nonexistent morphologies, which is a much more difficult. This causes a problem because there must be a decision made about which nonexistent morphologies to compare with the existent. However, there is a broad scope for uninformative comparisons to be made so it must be done with careful consideration.
Developmental Drive: Centipede Segments
There exists at least 3000 species of centipede with trunk segment numbers that range from 15 to 191. There are two orders of centipedes in which developmental drive is evident. Lithobiomorpha and Geophilomorpha are characterized by very different developmental systems but exhibit the same absolute constraint regarding even-segment-number phenotypes.
The order Lithobiomorpha consist of about 1100 species. All have 15 trunk segments at their adult stage. However, these centipedes hatch from the egg with fewer than 15 segments and the segments are added with age through a series of moults through which the hatchling grows towards adulthood. Lithobiomorph centipedes with even numbers of trunk segments exist, but only as juveniles. The strange thing about this is why, given the all-pervasive role of heterochrony in evolution, there have been no shifts of relative timing so that reproductive maturity and a cessation of segment addition have occurred in (say) a 12-trunk-segment juvenile in at least one of the many lithobiomorph species. So the argument for constraint is persuasive but not conclusive. It becomes even more persuasive when we consider the geophilomorph situation.
The order Geophilomorpha consists of about 1000 species with variable trunk segment numbers. The overall range of segment number for the order is from 27 to 191. Despite the larger number of segments in geophilomorphs, all are formed during embryonic development. The tiny hatchling has its full adult complement of segments. Although postembryonic growth involves a series of moults, and some morphological changes accompany these, there are no segments added at any moult in any geophilomorph species.
As if in response to Raff’s (1996) statement that the dis- covery of shared regulatory genes ‘has already begun the process of bringing such nonstandard animals as crayfishes and centipedes into the experimental mainstream’, there have been several recent studies of the developmental genetics of centipedes. However, others focused on the segmentation gene engrailed and so are relevant to the formation of segments and the determination of segment number. The study by Kettle et al (2002) is particularly relevant because it goes beyond gene sequence data to visualization of the pattern of expression of engrailed in geophilomorph embryos. It is clear from this study that segmental stripes of engrailed expression form in strict anteroposterior sequence. Moreover, the stripes appear one at a time, not in pairs. Eventually, this head- to-tail process concludes when the posteriormost stripe of expression occurs just in front of the telson.
It would seem reasonable to expect that such a one-at- a-time process could be stopped at any point – that is, after the formation of any given number of segmental expression stripes. Even if there is some reason why it cannot, the final rudimentary segment could be deleted by apoptosis, in the same way that occurs in the interdigital regions of tissue in tetrapods. Yet clearly, neither stopping the process at any even-number stage nor later reducing any odd number to a lower even number ever occurs.
Within this range almost all odd numbers are represented whereas all even numbers are absent. This complete absence of even-number trunk segments contrasts not just with the wide range of odd numbers that is found, but with the density with which this range of odd numbers is populated. This is an example of constraint or, another way of putting it, the universality of odd numbers is an example of drive. A dominant feature of the overall morphological pattern found in nature is probably caused by drive unless there is some unknown form of selection against any even-numbered variants, which seems highly implausible. All variant centipede ontogenies, from whatever starting point, are driven into odd-segment-number character states. So there is absolute drive in this direction, and equivalently absolute constraint regarding even numbers of segments.
Further Reading
Biologie évolutive du développement