Abstract:The evolutionary history of vascular plants is reviewed by extrapolation back through time from a wide range of data recently derived from the present flora, using as the central theme evolutionary inferences gained from phylogenies reconstructed as cladograms. Any region of the genome can be used to infer relationships, but only a combination of knowledge of morphology and the developmental genes that underpin morphology can allow evolutionary interpretation of macroevolutionary transitions; this in turn is n… Show more
“…However, these changes do not involve major body-plan evolutionary innovations. Evolutionary innovation that results in major new body plans appears to be confined to those times and places where extrinsic drivers either create new habitats or cause extinctions, thus opening major resource spaces to colonization (Valentine 1980;Bateman and DiMichele 2003) and enhancing the likelihood of pulses of niche construction (Erwin 2008). These events are generally rare, and conform to periods of 'escalation' (Vermeij 1987).…”
Complex plant ecosystems first experienced the effects of major glaciation during the Late Paleozoic Ice Age. The general response of Carboniferous tropical vegetation to these climatic fluctuations, especially the transitions from greenhouse to icehouse conditions (ice age sensu lato) and a return to warm times, now can be characterized based on large paleobotanical data sets originally collected to solve stratigraphic and paleoecologic questions. These data come primarily from North America and central Europe, which at the time were part of a single continental mass situated in the tropics. At the onset of icehouse conditions, innovation (speciation leading to novel forms and ecologies) occurred in environments subjected to perhumid (everwet) climates, while floras in better drained, drier, and more seasonal conditions remained dominated by holdovers/survivors from older biomes. This pattern is termed the 'Havlena effect'. During the height of the ice age, glacial-interglacial cycles produced large sea-level fluctuations and concomitant climatic changes, such that significant areas of continents in the tropics were alternately covered by shallow seas or densely vegetated terrestrial coastal plains. In spite of the repeated destruction of wet lowland habitats during each marine transgression and their further fragmentation that accompanied a climate change from humid to sub-humid, seasonally dry conditions, most of the species and the basic configuration of the plant communities in the wetland biome remained stable. This resilience demonstrates that glacial-interglacial cycles by themselves are not responsible for either extirpation or extinction of these biomes. At the transition from icehouse-to-greenhouse conditions, dry-biome forms, which had been evolving outside of the taphonomic preservational window, became dominant across basinal landscapes while wet landscapes retained their 'conservative'species composition. This pattern is termed the 'Elias effect'. Thus, environmental threshold-crossing marked both the beginning and end of this cold interval, with the loci of response in different environmental settings. In contrast, the minor systematic changes that occurred during glacial-interglacial cycles did not influence the composition or structure of tropical lowland vegetation substantially.
“…However, these changes do not involve major body-plan evolutionary innovations. Evolutionary innovation that results in major new body plans appears to be confined to those times and places where extrinsic drivers either create new habitats or cause extinctions, thus opening major resource spaces to colonization (Valentine 1980;Bateman and DiMichele 2003) and enhancing the likelihood of pulses of niche construction (Erwin 2008). These events are generally rare, and conform to periods of 'escalation' (Vermeij 1987).…”
Complex plant ecosystems first experienced the effects of major glaciation during the Late Paleozoic Ice Age. The general response of Carboniferous tropical vegetation to these climatic fluctuations, especially the transitions from greenhouse to icehouse conditions (ice age sensu lato) and a return to warm times, now can be characterized based on large paleobotanical data sets originally collected to solve stratigraphic and paleoecologic questions. These data come primarily from North America and central Europe, which at the time were part of a single continental mass situated in the tropics. At the onset of icehouse conditions, innovation (speciation leading to novel forms and ecologies) occurred in environments subjected to perhumid (everwet) climates, while floras in better drained, drier, and more seasonal conditions remained dominated by holdovers/survivors from older biomes. This pattern is termed the 'Havlena effect'. During the height of the ice age, glacial-interglacial cycles produced large sea-level fluctuations and concomitant climatic changes, such that significant areas of continents in the tropics were alternately covered by shallow seas or densely vegetated terrestrial coastal plains. In spite of the repeated destruction of wet lowland habitats during each marine transgression and their further fragmentation that accompanied a climate change from humid to sub-humid, seasonally dry conditions, most of the species and the basic configuration of the plant communities in the wetland biome remained stable. This resilience demonstrates that glacial-interglacial cycles by themselves are not responsible for either extirpation or extinction of these biomes. At the transition from icehouse-to-greenhouse conditions, dry-biome forms, which had been evolving outside of the taphonomic preservational window, became dominant across basinal landscapes while wet landscapes retained their 'conservative'species composition. This pattern is termed the 'Elias effect'. Thus, environmental threshold-crossing marked both the beginning and end of this cold interval, with the loci of response in different environmental settings. In contrast, the minor systematic changes that occurred during glacial-interglacial cycles did not influence the composition or structure of tropical lowland vegetation substantially.
“…Los cambios biológicos que se han dado a través del tiempo, se pueden comprobar de manera diversa; por ejemplo por medio de los múltiples estudios numéricos que hoy son ampliamente utilizados y que han incentivado el entendimiento de muchos aspectos de la vida y sus procesos. Sin embargo, entre las observaciones históricas siguen siendo aquellas de las Ciencias de la Tierra, incluyendo a la paleontología y paleobiología, las únicas fuentes directas a través de la cual se pueden comprobar las hipótesis que se generan a través de estas investigaciones (e.g., Nixon, 1996;Bateman y DiMichele, 2003;Bradley et al, 2003, Crane et al, 2004Lewis, 2006;Hilton y Bateman, 2006 ). Lo anterior lo podemos ejemplificar con datos que requirieron de muchos años de observación y experimentación.…”
El origen de la vegetación actual de México y su diversidad tiene larga historia. Posiblemente es la extensión de esta historia el punto en que discrepan las propuestas, una planteando que inicia en el Cretácico (ca. 132 ma) y otras haciendo énfasis en procesos restringidos al Plio-Pleistoceno (5.3 ma), sobre todo si se refieren al origen de la vegetación actual. El aumento del conocimiento sobre la evolución geológica de México, y del constante cambio en su fisiografía, así como del estudio de las angiospermas fósiles de la región, genera un concepto más claro de cómo y cuándo las formas de vida fueron llegando y asociándose. Se presenta una hipótesis en la que se combinan procesos geológicos y cambios fisiográficos, con la presencia de plantas y vegetación en las partes emergidas que se van desarrollando. Se propone que la biodiversidad actual efectivamente inicia hace ca. 132 millones de años, aunque linajes que hoy viven en México se pueden reconocer desde este tiempo, es complicado ubicarlos en familias, pues posiblemente representen miembros del grupo troncal. En el Paleógeno (65-32 ma) las familias, y aun géneros, que continúan viviendo en el país son más fácilmente reconocidos, pero grupos extintos o que hoy crecen en otras regiones siguen siendo comunes. Es en el Neógeno (32-1.8 ma) que desde un punto de vista de la morfológico/anatómico las plantas fósiles se parecen más a las que viven de forma natural actualmente en el país, pero muestran diferencias que en general permiten proponer nuevas especies. Si las plantas fósiles y actuales de México se relacionan morfo/anatómicamente más solo en tiempos relativamente recientes, es de esperar que con los tipos de vegetación suceda algo similar. El registro fósil sugiere que a partir de comunidades que se desarrollaron bajo condiciones cálido-húmedas en el Cretácico, divergieron tipos de vegetación con capacidades diferentes ante el estrés hídrico, y comunidades que se favorecieron de condiciones templadas a frías. Esto sucede aparentemente en dos momentos distintos en dos regiones diferentes; durante el Paleógeno se afecta al norte y en al Neógeno al centro y sur del país. Trabajo geológico y paleobotánico conjunto y comparativo permitirá refinar esta propuesta que sugiere que los cambios que activan o restringen respuestas biológicas forman parte de otros componentes del Sistema Tierra.
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