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| ADAPTATIONS | Last Updated • May 8, 2022 | |||||||||||||||||||||||||||||||||||||||||||||||||
When studying the different organisms we encounter in the natural environment we notice that "each form functions in its own particular circumstances". The specialized features of animals and plants that perform one or more useful functions are known in evolutionary biology as adaptations. Each individual organism possesses many adaptations that function together to equip it for its particular way of life. These adaptations were not really recognized or well understood in the past, and it was assumed that all the details identified within the bodies of living organism, all features of species, were perfect mechanisms that had been specially "designed" to allow the species to function optimally within its ecological niche. Over time scientists came to realize that even specific details in organisms' bodies actually actually had a lot of flaws. This happens because every single part of a living organism forms as a consequence of changes that occurred on previously existing features, and were not "created" on purpose for a specific function. Adaptations always originate from modification of what is already existing, and not simply by making something up from scratch. That is, evolution can operate only by changing what is already present: remodeling, rather than new construction.
Common ground plans also suggest common origins: groups of species of the modern world share a common evolutionary heritage. The idea of evolutionary divergence of groups of organisms with common ground plans became accepted because of the powerful ideas of Charles Darwin.
| Charles Darwin's contribution
| Last Updated • May 8, 2022 |
Charles Darwin's contribution to science is of two kinds:
In 1831 Darwin embarked on a scientific cruise around the world on H.M.S. Beagle. He took careful notes about what he saw during the trip and eventually published his observations in a book called The Voyage of The Beagle.
Charles Darwin had left England to sail on board of the Beagle with a copy of Charles Lyell's book "Principles of Geology". In that book, Lyell had detailed his extreme view of uniformitarianism, based on James Hutton's ideas and observations. He was finally back home in London in 1836. During and after his trip, his observations of the natural world convinced him of the fact that life had evolved on Earth through natural selection. At the time, he did not feel that the world was ready for to accept these powerfully new ideas and decided to postpone any publication about evolution. At that time, he social and religious environment of Britain and the western world in general was very conformist and hostile, and when his work was finally finished, he locked it in a drawer, with instruction to his wife to publish it only after his death. But in 1858 a young scientist named Alfred R. Wallace wrote him a letter. In this letter Wallace had independently outlined concepts that were very similar to Darwin's own. Faced with this new reality, he changed his mind about publishing and the two scientists agreed to give a joint public presentation at the Linnean Society. None of the two was present at the meeting, and their work did not get much attention from fellow scientists, but the fact that somebody else had come to conclusions similar to his was the sparkle that convinced Darwin that times had finally changed and that he could finally go along and publish his book On the Origin of Species by Means of Natural Selection.
Darwin's idea can be summarized in the concept that "organisms that inherit favorable variations for their immediate environment will tend to survive more often than others". Darwin called this idea (evolution by) natural selection. In approaching Darwin's thought we have to keep in mind the general mentality of his epoch: the dominant school of thought in western society was that everything in nature was created by a divinity, in the exact same way as it was observed. Questioning that belief meant going against society, tradition, and religion, and become, in the best case, an outcast.
(Darwin's first great contribution to science) Evidence for evolution is expressed in two different ways, geographical and anatomical:
Humans have been able to breed, or select, races in nature for a long time (for instance, cats, dogs, cattle, etc.). So what would cause the same phenomenon, a selection of different species, in nature? Darwin proposed the mechanism of natural selection:
(Darwin's second great contribution to science). He put evolution in a perspective of either longevity or rate of reproduction: if certain individuals within a population lived longer, then they would also have a chance to produce more offsprings. But also, if certain individuals within a species reproduce faster and at a younger age then they would also have more offsprings.
| Genes, DNA and Chromosomes
| Regulatory Genes and Patterns of Development Populations, Species and Speciation Horizontal Gene Transfer Last Updated • May 8, 2022 |
I would strongly recommend that your read these four paragraphs in your textbook for completeness of information. Keep in mind that the vast majority of the concepts expressed in these paragraphs can be found in the pages of Cradle Of Life. In particular, there are two major points that you should know about:
| Rates of Origination
| Last Updated • May 8, 2022
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The study of fossils allowed scientists to assess rates of evolution and extinction: we can then tell how often new species appeared on Earth by studying the fossil record. Rapid evolutionary expansions of new genera or species from one or more phyla, classes, orders, or families have occurred many times in Earth's history, and are known as evolutionary radiations. Evolutionary radiations are recognized, for instance, after every major mass extinction, such as at the Permian/Triassic or at the Cretaceous/Paleogene boundaries. Evolutionary radiations occur very quickly geologically speaking (a few million years), because the modes of life of the newly originated groups often differ from those of the other groups from which they originated: for instance they might have occupied different ecological niches, where no competitors existed, or they may have thrived because upon entering a new area there were no effective predators around. Some evolutionary radiations may also have occurred because of the sudden disappearance of a certain group of predators. For instance, most present-day mammals orders, from bats to whales, came into existence as a consequence of an evolutionary radiation that started within 12 million years after the demise of dinosaurs and of other reptiles at the Cretaceous / Paleogene (K / Pg) boundary (about 65 my ago).
Radiations have often occurred through adaptive breakthroughs, or the appearance of key features in certain organisms. These changes can be physical (for instance, the advent of a porous skeleton in hexacorals) or behavioral (for instance, still in hexacorals, the rise of symbiosis). Other examples would include the development of toes that were used by certain fishes to wade and walk in shallow waters, and that were used later as legs when their descendants expanded on land, thus originating amphibians. Local radiations seem to be connected also with, again, physical isolation: consider for instance not only the previously seen case of Darwin's Galapagos finches but also the case of the fishes of Lake Victoria, in Africa, where, in a lake that formed only 13,000 years ago (and hence contained no previous aquatic life) we can find today about 500 different species of cichlid fishes, thus making for a spectacularly rapid radiation.
A similarly quick evolutionary radiation has occurred nearby Death Valley, California, when a population of fish became isolated and was separated in distinct groups because of the formation of different water bodies as a consequence of a drop in water level. That is, the fishes of Death Valley at first remained isolated from others in the area and then the same remnant lake in Death Valley was further split into a series of minor ones by a drop in lake water, and they had no more communication with each other. All of these lakes started with the same species of fish, and each lake has seen the evolution of a different species starting from the common ancestor. The persistent isolation of distinct groups of the same species eventually originated a new species in each of the new basins. | The Molecular Clock and the Times of Origination
| Last Updated • May 8, 2022 |
Fossils may be rare or unknown for some taxa (remember the discussion on preservation of organic matter in the chapter and notes about paleontology). Still, it is possible to get an idea of when a certain taxon originated in the past, even in absence of a fossil, by using what we call a molecular clock.
A molecular clock details the accumulation of mutations at a fixed rate. That is, it deals with the study of the rate at which mutations occur in a species, calculated by the study of the accumulation of genetic differences in living organisms. In order to be able to use a molecular clock it is then necessary to have at least one fossil and a living species related to it, and then perform a statistical analysis of the data. | Evolutionary Convergence
| Last Updated • May 8, 2022 |
As a group of organisms undergoes an evolutionary radiation, some of the taxa that arise may come to resemble taxa that evolved separately, during other radiations. The evolution of forms that are similar in two or more different biological groups is called evolutionary convergence. Evolutionary convergence offers evidence that the biological form is adaptive. This principle is particularly evident in the similarity between the marsupial mammals of Australia and the placental mammals of the rest of the world. Marsupials mammals diverged one from the other according to environmental factors, like placental mammals did, but converged, both in the way of life and in the form of the body, with one or more groups of placental mammals living at other locations.
Many marsupials match the placentals but, for instance, there are no placental mammals herbivore-equivalent in Australia: the early evolution of kangaroos there (that have no equivalent in other parts of the world) limited the possibilities for the development and the evolution of marsupial ungulates similar to our deer, cattle, antelopes, etc. | Extinction
| Last Updated • May 8, 2022 |
Fossils not only show evidence that life has changed substantially over time (evolution) but also that millions of species have disappeared as well (extinction).
Natural historians from ancient times realized that many fossils bore no resemblance at all to any of the currently living organism. Nonetheless, the idea that these "strange" fossils might represent animals that were now extinct was rejected by most. Scientists thought that certain species might simply have found refuge in yet unexplored areas. It was only in 1786 that Georges Cuvier simply pointed out that, for instance, there were no possible areas left on Earth where, say, mammoths could have possible retreated. Mammoths could not be seen anymore simply because they became extinct. Again Cuvier in 1796 finally proved extinction as a matter of fact by describing the differences that existed between the living Asian and African elephants and a mysterious elephant whose fossil bones had been found in North America. If a number between 5 billion and 50 billion (50,000,000,000) species (and very likely more) have lived on Earth since the beginning of life - as they were identified from the fossil record - but only 50 million (50,000, 000) are alive today, then about 99.9% of all species that have ever lived are now extinct. If you think of this from a statistical point of view, to a first approximation, all species are extinct. Extinction is a hard fact of nature, not a mark of obsolescence. Dinosaurs' extinction seems, to some scientists, the proof of their inadequacy for this planet but they dominated it for over 150 million years. We humans have been on this planet for only 100,000 years so we should think again about this concept. Most species in the fossil record last a few million years before becoming extinct. Extinction is not limited to the pure and simple disappearance of a group but it may also occur when a species evolves into another. In this case we should talk about a pseudoextinction: it is true that the original species does not exist anymore, but only because it changed into another, different species over time. The rates of extinction vary greatly: extinction rates have varied greatly within most large groups of plants and animals. Those groups of animals and plants that are well represented in the fossil record and that have experienced high extinction rates (they lasted only for a limited amount of time) tend to serve well as index, or guide fossils. A very good example is provided by the Ammonoids, which were swimming mollusks that became extinct at the K / Pg boundary. Before their extinction, they had a very high turnover rate all throughout the Mesozoic (particularly famous are the Jurassic Ammonoids of Great Britain). As a consequence, their fossil remains are very useful today as index fossils. Most of the stages of the Mesozoic are established on the basis of Ammonoides biozones.
 An Ammonite: an example of Ammonoids from the Lower Triassic of Southern California
By the 1830s, geologists were aware that there were big differences between fossils found in the time interval from the Cambrian to the Permian (Paleozoic) and those found in the interval from the Triassic to the Cretaceous (Mesozoic), as well as those found in the Paleogene and younger rocks (Cenozoic).
As you remember from Chapter 6, the boundaries between these great eras of geologic times were not picked randomly: they are marked by two of the biggest mass extinctions in Earth history, that is the Permo-Triassic (P / TR) extinction, that separates the Paleozoic from the Mesozoic, and the Cretaceous / Tertiary, or K / T (now more properly defined as Cretaceous / Paleogene, or K / Pg), that separates the Mesozoic from the Cenozoic. Mass extinctions clearly stand out from the background of normal patterns of extinction, and some are truly massive. Estimates for the Permo-Triassic event are for a 96% extinction for marine species while for the K / T event the estimate is up to 75% of marine species disappearance. Beside these two most important ones, other important mass extinctions have been identified during:
We have a mass extinction going on today, probably caused by humans through the destruction of the habitat of other species, and in particular the microhabitats of the tropical rain forest. New species, mostly opportunistic ones, might occupy the suddenly open niches caused by human-induced extinction, and impair the quality of human life itself.
In the picture above it is possible to see a small remnant of the tropical rain forest in a region of the Amazon basin, in Brasil, surrounded by vast areas of soybean fields. The destruction of rain forest in the Mato Grosso state of Brazil (but also in places like Central America, Africa, Malaysia and Indonesia) is a major factor in the present-day observed reduction of diversity of floras and faunas throughout the world. What seems a very small rate of extinction on a human time scale, becomes frightening when projected on a geological time scale. (Image source: © Time magazine, April 7, 2008). | Evolutionary Trends
| Last Updated • May 8, 2022 |
It is possible to observe general trends in evolution by examining the evolutionary history of any higher taxon that has left an extensive fossil record. What we can see in the fossil record is the following:
1. Animals tend to evolve toward larger body size (Cope's rule). There is a number of problems that may arise with big size (see the example on the book about the elephant head and its neck). Elephants and manatees possess some unusual features that makes it very unlikely that they will ever evolve into very different types of animals. Whales were able to grow in size once they moved from land to water thanks to the increasing support provided by the aquatic environment.
2. Evolutionary trends can be simple or complex We have recently realized that this is not true: some species seem to have lived without appreciable change for millions of years (for instance, stromatolite-building cyanobacteria). This seems to suggest that evolution works its ways very slowly. While most species seem to evolve relatively slowly and gradually; many other seem to have stepped out of rapid (geologically speaking) speciation. (See the example of the bat, which evolved from small rodent-like mammals in 12 million years when the average typical survival time for a mammal species is about 2 my). Also noteworthy is the case of the axolotl and the salamander: it is possible to artificially force an axolotl to become a salamander. (While both are existing today as two distinct species, the axolotl represents just the juvenile state of the salamander. That is, a salamander exists anyway as an axolotl during its early development). Apparently, a genetic mutation occurred in some salamanders in the past that prevented their thyroid gland from developing fully. So, the juvenile stage matured into an adult without turning into a salamander proper. Since that individual was still able to reproduce, the result was the axolotl: a new species was born. Such a rapid branching of new species from existing species has changed the point of view of many paleontologists about slow, gradual evolution, in favor of a model associated with speciation, that is with the rapid branching of new species from existing species. This view is known as the punctuated model of evolution (see figure below). 
(A) The classic notion (inherited from Darwin) was that evolution should show gradual transformations of species (shown by bell-shaped frequency distributions) through time, or phyletic gradualism.
From:
Eldredge, N., and Gould, S. J., 1972. Punctuated equilibria: an alternative to phyletic gradualism. Species that have not evolved in a very long period of time are called living fossils Examples from our labs include Lingula and Nautilus.
In other words, once a species has become extinct, or has turned into another species, it is gone forever. Materials on this chapter have been integrated with concepts from: Prothero, D. R., 2004. Bringing Fossils To Life: An Introduction to Paleobiology, © McGraw Hill, New York, NY 10020. © Alessandro Grippo, 1994-2016 | |||||||||||||||||||||||||||||
