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FIGURE 6.3 Continental landmasses during the Mesozoic showing how dinosaur populations became increasingly isolated through time. (A) Late Jurassic (about 140 Ma). (B) Late Cretaceous (about 80 Ma). From Cowen (1995), History of Life, 2e, Blackwell Science, Inc., Malden, MA, p. 82, figs. 5.13 and 5.14.

FIGURE 6.3 Continental landmasses during the Mesozoic showing how dinosaur populations became increasingly isolated through time. (A) Late Jurassic (about 140 Ma). (B) Late Cretaceous (about 80 Ma). From Cowen (1995), History of Life, 2e, Blackwell Science, Inc., Malden, MA, p. 82, figs. 5.13 and 5.14.

during the Late Triassic Period, when evidence for the first dinosaurs is recorded. Dinosaurs became widespread soon afterward, inhabiting every continent, except Antarctica, by the Early Jurassic (which also reflects their rapid migration rates) before significant splitting up of Pangea. However, as the continents split farther apart by seafloor spreading during the Jurassic and Cretaceous Periods, increased diversification of dinosaurs took place. Some similarities endured within species on still-connected continents, but noticeable differences appeared in those on separate continents. Thus, the most prominent barriers to gene flow and subsequent causes of reproductive isolation and allopatric speciation over time were the oceanic expanses. For terrestrially-bound dinosaurs, this circumstance meant that any of them inhabiting landmasses that later separated from Pangea then formed populations that became distinct from their ancestral populations through time. Additionally, linear mountain systems and inland seas (caused by global sea-level highs) also resulted in geographic barriers that could have been a mechanism for dinosaur spe-ciation (Chapter 13).

However, allopatric speciation through geographic isolation is not the only hypothesis proposed for how species originated in the fossil record. Indeed, reproductively-isolated species with recent common ancestors can have overlapping geographic ranges. Those closely-related species that occur in the same region have sympatry, and the origin of new species from populations within these regions is possibly through sympatric speciation. Sympatric speciation is regarded as the result of intraspecific factors, rather than environmental factors such as climate changes or predation by other species. Sexual selection (through competition for mates) is an example of an intraspecific factor that could cause natural selection and subsequent changes in genotype frequencies in a population. This was illustrated through the hypothetical example of the less-endowed Centrosaurus earlier. As these differences within a species occur in the same geographic area through time, the increased genetic distance between their inheritable traits is termed character displacement. The role of character displacement in dinosaur evolution is poorly understood, but is hypothesized through synapomorphies (connected by cladograms) and speculations about character traits that would relate to this proposed mechanism for speciation. Examples of such characters include horns, head frills, and feathers, which might have served as sexual displays in dinosaurs or were otherwise used for intraspecific competition (Chapters 9, 11, and 13).

Natural selection and the subsequent co-evolution of two or more species that occurs as a result of their interactions are summarized by the Red Queen hypothesis. The Red Queen is a character in Through the Looking Glass, by writer and mathematician Charles Lutwidge Dodgson (1832-98; more popularly known by his pseudonym of Lewis Carroll). In the book, Alice meets the Red Queen chess piece, who appears to run across the chess board at high speed, yet never leaves her square: "Now, here, you see, it takes all the running you can do, to keep in the same place." This serves a metaphor for a co-evolutionary process in which two species of organisms continuously match one another's defenses only to maintain the status quo. For example, plants may evolve chemical defenses against insect herbivores, which in turn evolve resistance to the plant's chemicals, and so on. This type of equilibrium state should cause regular extinctions through time of species with two or more lineages, so the Red Queen hypothesis is scientifically testable. This hypothesis has been proposed to explain some changes in character traits of dinosaurs through time, such as in Cope's Rule, whereby prey and predatory dinosaurs became progressively larger as a result of their "arms race" interactions (Chapters 9 and 10). Additionally, increased amounts of dermal armor in ankylosaurs and apparent defensive weaponry in stegosaurs comprise other presumed evolutionary responses to pressures from theropod predation (Chapter 12). Although the preceding is a simplistic analogy with regard to modern predators and prey, this hypothesis has also been applied to changes in herbivorous dinosaur dentition and digestive systems in response to changes in vegetation types throughout the Mesozoic Era.

Finally, an important point to keep in mind with natural selection is that some species may have inheritable variations that are "pre-adapted" for a change in either the magnitude or rate of an environmental factor unprecedented in the history of a species. For instance, a large-scale volcanic eruption that deposits ash in only a few weeks over a large area of a forest may favor the reproductive survival of taller adult plants of a species, as the taller plants can still disperse their seeds above the ash layer. The shorter adult plants of the same species, completely covered by the ash, may not survive to reproduce. This chance possession of inheritable traits, favorably adapted for a selective pressure before it happened, is called exaptation. Exaptations also are hypothesized as features that had a neutral (non-harmful and non-beneficial) effect on an organism's adaptation that in later generations become advantageous for survival. This hypothesis for natural selection is especially applicable to explaining the survival of certain lineages of organisms after mass extinctions recorded by the geologic record. The lack of some currently undefined exaptations in dinosaurs at the end of the Cretaceous may have resulted in their demise in the face of a global catastrophe (Chapter 16).

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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