WEIT 2012 Chap1
WHAT IS EVOLUTION
What is the definition of Evolution?
Evolution is a gradual process in which the biological populations change into a different and usually more complex or better form. Life on Earth began with a universal common ancestor. It changed gradually, taking several millions of years. This universal common ancestor, named LUCA (Last Universal Common Ancestor), branched out over time, throwing off many new and diverse species. This mechanism of evolutionary change is called natural selection. When you break that statement down, you find that it really consists of six components.
What are the six components of the statement of evolution?
The six components are
1. Evolution
2. Gradualism
3. Speciation
4. Common ancestry
5. Natural selection
6. Nonselective mechanisms of evolutionary change
Let see each of these components closer.
What is Evolution?
Evolution is the fact that a population of organisms undergoes genetic change over time. Individual organisms do not evolve. The differences are based on the changes into the DNA, which originate as mutations. All the populations evolve but not at the same rate. The theory of evolution doesn't predict that population will constantly be evolving, or how fast they will change when they do. It depends on the evolutionary pressures they experience.
- See Appendix 1
What is Gradualism?
Gradualism is the fact that it takes many generations to produce a substantial evolutionary change. The evolution of new features can take over hundreds or thousands (even millions) of generations. A species evolve faster or slower as evolutionary pressures wax and wane. Also, when the natural selection is strong, evolutionary change can be fast. But once a species becomes well adapted to a stable habitat, evolution often slows down. Many living species share fundamental traits such as biochemical pathways that we use to produce energy, our standard four letters DNA code, and how that code is read and translated into proteins. This tells us that every species goes back to a single common ancestor (who had those traits and passed them on to its descendants).
Gradualism being evolutionary changes over time, it's often represented as a tree whose roots would be the common ancestor of all the species it represents. The difference between a gradualism and a punctuated equilibrium representation is that a punctuated equilibrium representation illustrates the evolutionary changes at given time and that gradualism one tries to show all the phases a species went through. In Appendix 2, there's a comparison of the two representations.
- See Appendix 2
What is Speciation?
Speciation, also called splitting, is a very slow process which explains how a single ancestral species can split in several descendant species. More precisely, it's the appearance of slight differences between two different populations of the same species and we consider that the splitting really occurred when the differences become so important that the two populations can no longer interbreed. Species themselves cannot split and don't have to. This process doesn't happen quite often, but enough to explain us the great diversity of the terrestrial flora and fauna. Since it doubles the number of opportunities for future speciation for a species, speciation might increase exponentially the number of species, but more than 99 percent of the species did actually extinct without leaving any descendant.
One of the most interesting examples brings us millions years ago around the Triassic period and explains us the link between birds and reptiles or how a bipedal dinosaur, let’s call him ancestor X, split in order to give two new kinds of dinosaurs, one having actual reptiles’ features and producing bipedal, carnivorous dinosaurs, like the T.rex, and the other producing all the birds. This splitting would at first only be noticed by slight differences, like two populations of the same species. We’ll have to wait thousands generations to see the dinosaur kind producing bird start flying.
This process is studied in a more detailed way in ch.7: "The Origin of Species".
What is Common ancestry?
Common ancestry is the flip side of speciation. We can always look back in time, using either DNA sequences or fossils, and find descendants joining at their ancestors. Every pair of species shares a common ancestor sometimes in the past. To express the evolutionary relatedness among species through time, we use The Tree of Life. This Tree illustration has been first proposed by Darwin in the 19th century. This Tree of Life could be done by looking at physical traits. Now we have the ability to make a Tree of Life even more precise with the knowing of DNA.
Appendix 3, Picture A, represents Charles Darwin’s first diagram of an evolutionary tree from his “First Notebook on Transmutation of Species” (1837) is on view at the American Museum of Natural History in New York City. It stands to reason that if the history of life forms a tree, with all species originating from a single trunk, then one can find a common origin for every existing species by tracing each twig (=existing species) back through its branches until they intersect at the branch they have in common. This node as we have seen is their common ancestor. On Appendix 3, Picture B, we can see an example of an evolutionary Tree on which we can clearly see that each branch intercept a single common ancestor. Finally, Appendix 3, Picture C, represents an evolutionary tree of vertebrates, showing how evolution produces a hierarchical grouping of features, and thus of species containing those features. The dots indicate where on the tree each trait arose.
- See Appendix 3
What is Natural selection?
Natural selection is a process which requires that individuals of a species vary genetically in their ability to survive and reproduce in their environment. In a more detailed way, if individuals within a species differ genetically from one another, and some of those differences affect the abilities of an individual to survive and reproduce in its environment, then in the next generation the “good” genes that lead to a higher survival and reproduction level will have relatively more copies than the “not so good” genes. Over time, the population will gradually become more and more suited to its environment as helpful mutations arise and spread through the population, while deleterious ones are weeded out. Ultimately, this process produces organisms that are well adapted to their habitats and way of life.
Let illustrate this process by a simple example. The woolly mammoth inhabited the northern parts Eurasia and North America, places were the weather can be especially cold. He was adapted to those extreme temperatures by bearing a thick coat of hair. It probably descended from mammoth ancestors that had little hair. Mutations in the ancestral species led to some individuals being hairier than others. When the climate became cold the shaggy individuals were better able to tolerate their frigid surroundings and thus left more offspring than their balder fellows. By the way, the next generations will have a hairier average mammoth. Let this process continue over some thousands of generations and the smooth mammoth will be replaced by a shaggy one. In Appendix 5, Picture A, there is an illustration comparing a woolly mammoth and a modern elephant. A clear difference can be observed concerning the thickness of the hair of the mammoth especially on his ventral side.
Another example: Lamarck believed that giraffe's were no bigger than horses, but as there was little food around other than that on high up branches, a giraffe managed to stretch its neck and make it longer to reach the branches. When it mated, its offspring also had long necks. Ultimately, this process will continue through generations until our modern giraffe and by the way the giraffes with shorts necks will disappear. Illustration of that theory, Appendix 4, Picture B.
- see Appendix 4
What are the Nonselective mechanisms of evolutionary change?
These mechanisms, also called genetic drift, have nothing to do with adaptation; it is more about the random changes in the proportion of alleles due to the sampling effects of reproduction. Because natural selection is the only process that generates adaptation, the influence of the nonselective mechanisms of evolutionary change is less. This also helps us explain some neutral (nor useful or injurious) and non-adaptive features, like some DNA variations, even if we cannot prove that these features have absolutely no selective advantages. It can only play an evolutionary role on small population, and, because it has nothing to do with adaptation, it might sometimes just preserve bad alleles instead of destroying them, for example the high rate of genetically based diseases in small, isolated human communities, like Gaucher’s disease in northern Swedes.
Can we consider Evolution as a theory?
First of all, we have to make clear what a theory is: "A theory is a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena." So in science, a theory is much more than only a supposition about how things are: it is a well-thought-out group of propositions meant to explain facts about the real world. It is something conveying far more assurance and rigor than the notion of a simple guess. For a theory to be considered scientific, it must be "testable" and "make verifiable predicions". A good theory makes predictions about what we should find if we look more closely in nature. And if those predictions are met, it gives us more confidence that the theory is true.
For example, "Athomic theory" isn't just the statement that "atom exist"; it's a statement about how atoms interact with one another, form compounds, and behave chemically. Similarly, the theory of evolution is more than just the statement that "evolution happened": it is an extensivly documented set of principles (those we described just above) that explain how and why evolution happens.
- see Appendix 5
References
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