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1. Angiosperms, commonly known as the flowering plants, are the most abundant group of plants that exist on earth today. Hailing from the phylum Anthophyta (Raven 1999), the evolution of Angiosperms began around 110 Ma (Freeman 2004) while the domination of Angiosperms over the plant kingdom began between 80-90 Ma (Raven 1999). This newly evolved form was the second radiation of plant development (Freeman 2004) and changed the face of plant kingdom from what could have been one of a world solely of evergreens to one infused with exotic flowers.
The development of the daisy can attribute its beautiful, simple, and efficient flower back to a group called Progymnosperms, the spore-bearing plants. Seed bearing plants arose from the Progymnosperms eventually giving rise to the Gymnosperms. The Angiosperms evolved from Gymnosperms (Raven 1999), representing the last step in the stages of plant developments that led to the type of plant life that we see today (Gould 2001).
The basic concept of the seed allows for greater protection of offspring during reproduction and for greater longevity and survival during harsh climactic conditions. The Gymnosperms, although varied in size, shape, color, etc., all exhibit an “…ovule, which becomes a seed, rest[ing] exposed on a scale [which] is not completely enclosed by sporophyte tissues at the time of pollination” (Raven 1999).
The evolution of the Angiosperm from the Gymnosperms went one step further in the effort of increasing fitness by creating a vessel in which they carry out their reproduction, thereby even further protecting the development of the seed (Raven 1999). The Gymnosperms may have come first in the history of the earth, but simply because you arrive first does not mean you are the best. Most evolution occurs towards higher, more complex, more organized pathways, and plant life is no exception. The early success of Angiosperms during their development has led to there presently being 240,000 known species in the one phylum of Angiosperms while there are only 721 species in the combined four remaining phyla of Gymnosperms which include Coniferophyta, Cycadophyta, Gnetophyta and Ginkgophyta (Raven 1999).
Progymnosperms a Gymnosperms a Angiosperms
(Spore) a ( Seed (Exposed Seeds) (Protected Seeds)
Without definitive answers as to what caused the actual success of the Angiosperms, there are some very clear indicators that either alone, or in conjunction with others, helped to make Angiosperms the most successful plant group on earth. These differences include the flower along with its color, odor and secondary chemicals, the development of fruit, the co-evolution of insects, the developments of surface waxes and hairs on the leaves of Angiosperms along with “…their stomatal mechanisms, their specialized conducting tissues, and their different modes of photosynthesis” (Raven 1999).
Flowers: Color, Odorand Secondary Chemicals
To start, the most obvious difference is the development of the flower, whose petals are really just modified leaves (Raven 1999). The flower is crucial to reproduction for many Angiosperms (Gould 2001). The success that the flowers bring to Angiosperms over Gymnosperms is the attention that the bright color, shape and smell of the flowers can bring to the plant (Gould 2001) which aid in the distribution of pollen from anthers to stigmas (Raven 1999).
One might not think that drawing attention to itself would be an evolutionary choice that would be made for survival, but the use of animals and insects not only distributes pollen in the same plant or plants in the immediate surrounding area, but plants that are also far distances away. Cross-pollination over large distances helps to ensure outcrossing and prevents inbreeding depression (Raven 1999).The odor associated with flowers would also have of played a role in the success of Angiosperms. Some odors are attractants, some are replants, and some lack odor all together. Those Angiosperms that use odor to its advantage enjoy all the same benefits as discussed above for the use of color. Others use odor or secondary chemical compounds as a defense mechanism to ward off would be predators. Other chemicals called secondary chemical compounds also occur widely in Angiosperms and may have been
another one of the large, although not visually apparent, reasons for their successes. Secondary chemicals can act as insect and herbivore repellant warding off herbivore predators (Raven 1999).
Energy and Density
The use of attractants is also an energy saver in pollen production. Gymnosperms must produce great quantities of pollen as they passively shed it from one tree to another. Although Angiosperms spend more energy on attractants, they spend less energy on pollen; due to their attractants, their pollen can reach greater distances and as an effect their plants need not be located so closed together, therefore their individual survival rates might increase as their densities decrease, lessening demand and competition for available resources in a given area (Raven 1999).
In addition to the morphological changes in Angiosperms in regards to the carpel and the protective surroundings of reproduction against fungus and dehydration (Gould 2001), the ovary can also develop into a fruit, which can further its chances of reproductive success. In the discussion of plant ‘fruits' one must turn the eyes away from the customary oranges and apples in the grocery isle. The fruits of plants can be fleshy, hardened, hooked, burred, light and fluffy or thorny, tasty, bitter, poisonous, or medicinal. Fruits are eaten, carried, buried, drifted away, washed up, digested, flown, clung, wafted away, kicked, and excreted. The point being that the main effort of the fruit is to aid in the dispersal of the seed (Raven 1999).
The Co-evolution of Insects and the Presence of other Organisms
As the Angiosperms begin to rise in the fossil record, dating back 80-90 Ma, so do the insects that we know to be associated with pollinating Angiosperms (Raven 1999). This suggests that the Angiosperms and those Insecta co-evolved, furthering each other's existence and evolution with their own. The pollinating insects that would have been present would have been the pollinating beetles, flies, butterflies, moths, ants, bees and wasps (Gould 2001).
Other groups also contributed to the success of Angiosperms by developing and evolving alongside them, although they may not have had such an impact as the co-evolution as the Insecta groups. Besides the classical reptiles and amphibian dinosaurs that continued to evolve in the Cretaceous period, the Mammalia kingdom was also starting to rise; representatives would have been seen from the Multituberculata, the Monotremata, the Edentata, the Proteutheria and the Condylarthra (Gould 2001).
Very distinct morphological changes also took place between the Gymnosperms and Angiosperms that let the Angiosperms thrive on the inner continent of Gondwanaland. Gondwanaland at that time would have exhibited arid to semi-arid conditions, and the Angiosperms would have had to contend with those conditions while evolving. The Gymnosperms, or at least the pines, were adapted more for colder climates and combating water loss during frozen months. The Gymnosperms have tough, needle like leaves, with thick cuticles and recessed stomata that reside in clumps of two to five. The typical leaf of the Angiosperm is flattened and green with a transparent epidermis containing a waxy cuticle with glands and trichomes. Generally, the stomata are located on the underside of the leaf and are used for water and temperature regulation. Angiosperms, unlike Gymnosperms, also may have hairs on the leaves that help to create a barrier of air to further regulate temperature and water loss. A further difference is leaf abscission that may be present in Angiosperms. Similar morphological changes could be further elaborated on that exemplify differences in the wood and bark (when applicable) of the two type of plants (Raven 1999)
The evolutionary success of Angiosperms is apparent in the fact that today their species number over 330 times as many as existing Gymnosperms. The protection that the evolutionary and morphological differences bring their seeds have obviously led to success for Angiosperms. The question remains as to whether this is actually the last evolutionary change that will be seen for the plant kingdom. In what direction can evolution take the organization, morphology, and ecological symbiosis for plant seed protection, pollinator attraction and seed dispersal?
2. Throughout time, change takes place; whether it occurs over the course of a century, millennia, or a million years it is the one thing that can be guaranteed to occur. A major change that can be seen thought history over and over again is the occurrences of mass extinctions. Distinct and drastic changes within the geological evidence point to huge changes in physical, atmospheric and paleontological remains. This evidence suggests that disastrous mass extinctions took place that changed the flora and fauna of the earth time and time again (Gould 2001).
The exact cause of mass extinctions continues to be debated but it is known that physical change to the earth of some kind is involved. It has been suggested that asteroids could have caused dust or chemical clouds, changing the chemical composition of the atmosphere of the given time causing mass extinction. While it has been suggested that the Cretaceous-Tertiary Extinction, or Alvarez event, can be correlated with the crater located on the tip of the Yucatan Peninsula (Mayr 2001), others argue that there is no compelling evidence to support this theory as the crater age and size do not correlate to produce the needed effects in order to produce a mass extinction of that magnitude (Gould 2001).
Other explanations for mass extinctions turn towards the environmental changes associated with geological changes such as continental drift, the formation of Pangaea, associated glacial episodes, Noah's Biblical Flood, deposits of salt or at least decreased salinity, global cooling/warming or even a series of catastrophes that only together would have brought together the mass extinctions. This last theory is also known as ‘catastrophism'. The prevailing theory is that most extinctions must be caused by a combination of continental drift and interaction in combination with the climate changes that continental interaction can cause (Gould 2001). Even though all the explanations for mass extinction are very different, they have one very similar factor: they are all chance occurrences (Mayr 2001).
Combining the knowledge that the mass extinction events are chance occurrences along with the fact that not one of the causes of any of the great extinctions can be proven does not leave any room for calculating when the next mass extinction may occur, or does it? There are certain people who believe that a pattern can be calculated to the mass extinctions that show 26 Ma between mass extinction events. There is much controversy over these calculations, to include the fact that there is an element of error inherent in our capability to date the events that occurred millions of years ago (Gould 2001). As technology increases and better and larger amounts of samples from those time eras are analyzed we may be better able to pinpoint closer data ranges and re-analyze the calculations. A better way to think about an organized system of mass extinctions may be an alternative theory posed by some scientists called ‘chaos theory', in which underlying patters exist but never quite work the same way twice (Gould 2001).
There have been five major mass extinctions, four being category twos and one, occurring during the Permian period, being a category one. During the Permian extinction event only five percent of the species present survived (Mayr 2001). See Table one for a comparable view of five major extinctions.
In addition to the five main events, it can be argued that up to 20 mass extinction events have occurred throughout time, but most have had much less impact than these listed (Gould 2001).
Despite the mass extinction events that have occurred, it seems that organisms continue to move from an organizational standpoint of simplicity to more complex. At least, that is what one would think from the change of primordial slug to aeronautical engineering man. However Mayr, 2001, argues that “…among the prokaryotes there is no indication of ever increasing complexity in the long period of their existence. Nor does one find any evidence for such a trend among the eukaryotes”. Mayr, 2001, suggests that there are just as many lineage lines that move towards simplicity as there are that move towards complexity.
Another factor that must be considered is that although a maojority of the organisms may perish during a mass extinction, not all creatures perish. After every major mass extinction, the numbers of species rebounded, they just happened to be different species than were previously there (Gould 2001); this indicates that there would be no need to regress to simpler organisms or even to re-evolve from the primordial elements. All that would be needed by the surviving organisms would be time to recover, and an appropriate environment to do it in.
While evolutionary movement towards greater complexity has been discussed, what about movement towards greater, or larger, size? During some evolutionary time periods, organisms reached massive sizes as compared to today. After a mass extinction event, the remaining organisms were able to grow to sizes they were previously unable to due to a lesser degree of competition now present. When in competition with other species and families, some organisms evolve using miniaturization as a way to increase maneuverability, conserve energy, specialize and utilize niches (Gould 2001). Without the selective pressures of competition, the remaining organisms are able to evolve to larger sizes.
After each extinction drastic changes were seen in that previously non-dominant creatures quickly rose to utilize niches that were no longer filled by those that they competed against. This type of niche filling and explosive speciation by one group of organisms so quickly after the loss of another group of organisms may be an indicator that higher taxa competition was present all along. This pattern was exhibited by the mammals after the dinosaurs went extinct during the Cretaceous-Tertiary Boundary mass extinctions (Mayr 2001).
Larger sizes allows for greater range of habitat variation as the organisms are able to cover more territory. A larger organism also has a larger bulk, which can be a positive thing when discussing heat loss in the winter, but a negative thing when discussing heat loss in the summer. Size, for a reptile, reduces the cost associated with the need to keep warm. Another factor that may have selected for large size is the environment itself. A warm environment would allow larger organisms to evolve due to reduced requirements for maintaining body temperatures. The extremely large organisms, like those that were seen in the Jurassic, were organisms that developed in conditions like these (Gould 2001). The changes in continent formations, global climate patterns and the like do not provide for the same conditions today. That may be why we do not have organisms of the Jurassic size in today's time period.
The physical changes brought about by the chance occurrence of mass extinctions have caused untold changes time and time again to the face of this planet. Although we can only theorize the exact causes, there are many people who are sure that the next mass extinction will be our own doing. It has been suggested that we are in the beginning stages of the next mass extinction, which will be the direct results of the actions of one of its own inhabitants. The habitat destruction and pollution that the human population creates on a daily basis not only causes other species to go extinct (Mayr 2001) but may ultimately result in their own extinction.
3. It is in the Devonian period that we see the first amphibian crawl from the water and onto the shore (Gould 2001). Although the land that this tetrapod crawled onto was teaming with plant life, this amphibian was a carnivore. The amphibian was not the quick moving amphibian we know today, but a slow one that ate worms and insects (Gould 2001). Why would this creature not been one ready to tackle a world teaming with a utilizable resource?
In order to discuss why amphibians were not herbivorous we must first discuss why they evolved onto land in the first place. Alfred Sherwood is the author of one of the classical views of why fish moved onto land at all. His theory, developed in the 1950s, “…was that fishes moved on to land in order to escape from drying pools” (Gould 2001). The ability to move on land would either allow them to find new homes while theirs dried up, or to find a suitable place or means, such as a mud burial, to wait until the pools returned (Gould 2001). The end result that only predatory fossils survived in conjunction with this theory may have been because all of the herbivorous fish that made the transition were eaten while the pools were drying, and too few remained to be retained as fossils.
Other theories suggest that the move to land was made to utilize the resources available there, to include the plants and oxygen (Gould 2001). This theory would not necessarily select for carnivores, but again, if the to groups made the transition around the same time, it may be that the prey simply were not adapted to escape from their predators on land yet and were therefore unable to survive into the fossil record.
The evidence shown in the fossil record that the first vertebral organisms were predators is not reflection of environment that they were evolving into, but a reflection of the environment that they were evolving out of. Life had evolved in water. Although there were carnivorous, herbivorous, and omnivorous organisms that occupied the waters, it seems obvious that the more aggressive of the feeders would be the predatory carnivores. Being more aggressive, it is plausible that they would be the first to move onto land. Herbivores which are usually more docile, and who would already have plant matter to eat in the water, would have less pressure to evolve onto land, save of that to escape from the predators.
It is generally agreed upon that the first vertebrates, the amphibians, had descended from the lobed finned fishes (Raven 1999, Gould 2001) and DNA evidence shows that it is the lungfish that bears the most similar DNA to amphibians (Raven 1999). The lungfish had many characteristics that allowed it to evolve onto land that include the most essential items, the paired lungs and a pattern in the lobe-fins that resembles the legs of the early amphibians (Raven 1999). It may be that these evolutionary characteristics that allowed the move brought whatever else was attached to the amphibian with it, to include its digestive tract.
Some of the first vertebrates to make the transition include Ichthyostega, Acanthostega, Metaxygnathus and Tulerpeton (Gould 2001). The diet of these creatures, as was discussed in The Book of Life, 2001, reflected the fact that they were slow moving having freshly evolved out of the water. The Ichthyostega is supposed to have thrived off of worms and arthropods when not eating fish from the water.
It has also been suggested that the coal deposits we find today are another marker that the evolution of the time had been able to evolve plants, but not a way to utilize them. The utilization of cellulose and lignin that are found in plan materials requires special equipment in the digestive tracts of animals. Beyond this, it is suggested that not even the bacteria or fungi were developed at this point to help decay plant materials and that is why the plant material survived breakdown and eventually became coal (Gould 2001).
The amphibians, as suggested by Gould, 2001, gained the ability to break down plant matter and use it as a nutritive substance in the later Paleozoic as their morphological changes in the fossil record may suggest. The flattened teeth, larger bodies which suggest longer digest tracts and possible new intestinal flora along with the cleidoic egg also moved amphibians into the new category of reptiles (Gould 2001). Whether the evolution of herbivory came first or the cleidoic egg, it does not matter. There are no herbivorous amphibians so both changes must have occurred in those organisms evolving into the eventual line of the reptiles. The reptiles, although continued in their success, also gave lineage to the dinosaurs.
During the time of dinosaurs, the herbivores were grouped as the prosauropods and are found in late Triassic paleontological data. The eating of plant materials during their eras was hard work, using gizzard stones to aid in the decimation of plant foods that could not be pulverized by the simple cutting motion of their teeth (Gould 2001).
In order for nutrition to be gained from plant materials, an appropriate digestive tract already given, the plant material needs to be broken down as much as possible. Tooth-to-tooth contact, along with a rotatable jaw, as seen with mammalian teeth, allows the crushing of plant food. “Reptilian herbivores generally have teeth that cut past each other like a pair of scissors but not occlude, which limits their ability to crush plant food” (Gould 2001). This may be one of the reasons that there are few herbivorous reptiles today. They evolved the ability to utilize a new source of food, but did not evolve the necessary morphological tools, or evolved and then lost the tools, to maximize the food usage.
The digestive system of the prosauropods was good enough, however, to give rise to the Brontosaurus in the Jurassic period (Gould 2001), a dinosaur so common it needs no introduction. If a dinosaur this large was able to utilize the same system, then something else must have gone wrong to explain why there simply are not many remaining herbivorous reptiles.
Perhaps it is as simple of an explanation that the end of the Permian mass extinction, which took out ninety-five percent of all species on earth and almost all of what we consider to be classical dinosaurs, took enough of the herbivorous reptiles with it that it could not recover. Perhaps, now being a minority, it could not compete with all the new organisms that occurred during the species explosions and is currently under higher taxa competition for resources.