Basic Concepts in Evolution
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Evolution is 'blind to the future'. It is "blind" in that evolution does not foresee what might occur sometime in the future, but on what is occuring in the here and now. It is imperfect in that there is always room for improvement (i.e. one characteristic may be beneficial in a particular environment, but may be deleterious in another) towards more optimal traits. No single trait is good in all environments -- instead, it must be a collective effect of all traits in an individual for survival and reproduction. The Forces of Evolution There are four known forces that are involved in evolution: 1) mutation, 2) genetic drift, 3) gene flow or migration, 4) selection. Mutation: Mutation is the ultimate source of variation in a population. Without variation, there can be no evolution. However, taken alone, mutation is considered the weakest force of evolution. This means that if mutation is the only force presenting itself, it would take many (e.g. 1000s, even 10s of 1000s) of generations to significantly alter the allele frequencies. This can be shown mathematically, but I'll not go into that. There are three kinds of mutation: (1) micromutation, which refers to DNA level changes such as nucleotide substitutions and frame shifts; (2) macromutation, which refers to the chromosomal level, such as gene duplications, sequence inversions, and chromosomal exchanges; and (3) megamutation, which involves foriegn genomes (symbiosis). The frequency of each type of decreases from micro- to macro- to mega- mutations. This is due to the fact that small changes (i.e. micromutations) are less likely to be as harmful as large mutations (i.e. megamutations). However, when megamutations (or symbiosis) do occur, they are biologically significant, since a new species is immediately formed. There are many examples of symbiosis, but I will deal with them in a later section. Drift: Drift is the process of random change in the allele frequencies. Here are its characteristics: 1) allele frequencies change over time due to chance, 2) loss of variation due to chance fixation of an allele, 3) leads to genetic divergence between populations, 4) drift occurs more rapidly in small populations. The "Founder effect" and "genetic bottlenecks" are examples of drift. Gene Flow: Gene flow or migration is the movement/exchange of genes (via individuals) from one population into another. The effects of gene flow are: 1) allele frequencies change over time, 2) gene flow tends to homogenize allele frequencies between two or more populations (i.e. the exact opposite of drift). Selection:
Selection is considered by many to be the strongest of evolutionary forces. It is responsible for the adaptations of species (such as mimicry). Selection can work on different levels: 1) level of the gene, 2) level of the individual, and 3) level of the kin group. Selection has been argued to not work at the level of groups (other than kin groups), such as populations, species, and higher taxa, since there is no compelling evidence to support the group selectionist view. Read The Selfish Gene by Richard Dawkins for an argument against the good of the group view. There are three modes of selection: 1) stabilizing (the mean of the frequencies does not shift with selection against the two extremes in a population), 2) disruptive (the middle portion of the population is selected against causing a bi-modal effect), and 3) directional (the mean frequencies shift higher or lower with selection against one extreme in the population). Thus, selection can change allele frequencies or maintain them. There are essentially two kinds of selection: natural selection, and artificial selection. Natural selection is the theory proposed by Darwin to account for biological adaptions. Some selective force in the environment selects against individuals that cannot effectively survive and reproduce, compared to other individuals. Artificial selection is what humans do whenever they breed/cross a particular species of animal/plant. Some trait is selected for, such as increased seed yield, milk yield, or simply for some aesthetic trait. In other words, artificial selection has a conscious element driving the evolution of a species for some purpose. Natural selection is blind to the future: there is no consiousness or "purpose" behind it.
Phylogenetics deals with the reconstruction of evolutionary relationships or the 'pattern' of evolution. Modern phylogenetics follow two basic principles: 1) monophyly, and 2) synapomorphy. A monophyletic group is a group of related species on a phylogenetic tree that contains the ancestor and all of its descendents. For example, gorillas, chimps and humans are a monophletic group, while gorillas and chimps alone, are not (and would instead be a "paraphyletic" group since it lacks one or more descendents). Pleisiomorphies are "ancestral character states" (or characteristics that have been around since the first ancestor). Sympleisiomorphies are "shared ancestral character states" (or ancestral characteristics that are shared by more than one species/taxon) Apomorphies are the "derived character states" (or characteristics that have changed from, or are not found in, the ancestral character states). Synapomorphies are defined as the shared derived character states between species, and autapomorphies are derived character states in only one species. Thus, a phylogenetic tree should be based on the shared derived character states between related species, the resulting groups should be named according to monophyly. Most phylogenetic trees are (or should be) dichotomously branching. Anagenesis is evolutionary change within a lineage (or along a line). A node (the point of origin of branching, which also represents a theoretical ancester) indicates a speciation event. This is known as cladogenesis -- the divergence of a lineage. As the number of species increases, the number of possible trees, from every possible arrangement of those species, increases exponentially (which is why computers are now used for this). Parsimony is the principle used in deciding which tree is the 'correct' one -- i.e. the tree with the fewest evolutionary or speciation events. You may associate "parsimony" with "Occam's Razor" -- i.e. use the solution that contains the fewest assumptions or variables. This is only an asssumption, but it is a reasonable one since it is more probable that fewer evolutionary events occured for any particular species. However, it is possible that the most parsimonious tree is not the correct one ('correct' meaning the actual historical events that occurred throughout evolutionary time). Thus, parsimony is simply used as a matter of convenience (since history is history -- we cannot simply go back in time to see what actually happened). The pattern of evolution is inferred from various data. As more and more data is accumulated, the more our phylogenies will approach historical reality.
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