Microevolution and Macroevolution
History of micro-macroevolution
Macroevolution was the type of evolution Darwin described in his time. Genetics and microevolution were not discovered until after Darwin died and Gregor Mendel published his pea plant experiments. Darwin proposed that species changed over time in morphology and anatomy.
Microevolution
A change in the
frequency of alleles in populations over time is called microevolution. Over longer timescales, microevolutionary processes
result in large-scale changes.
Large-scale changes that result in extinction and the formation of new species are called “Macroevolutionary”
The five "forces" that
can cause shifts in gene frequency (microevolution) are:
1. Natural Selection: when some genes are more
beneficial than others to survival and/or reproduction, those genes tend to
increase in frequency in the population over generations.
2. Gene Flow (Migration): when there is
mixing of genes from previously isolated populations that have diverged, this
can rapidly change gene frequencies in the newly merged population.
3. Mutation: when an advantageous mutation spontaneously arises in an organism, this mutated gene can increase in frequency over generations if it conveys an advantage over those who do not have it. If a neutral mutation (one that is neither beneficial nor harmful) arises in a population, it can increase in a population by genetic drift. If a deleterious mutation arises in an organism, it is likely to be selected against and will generally not increase in frequency. While recombination during meiosis can shuffle genes into new combinations, mutation is the only source of new genes.
4. Genetic Drift (Bottlenecks, Founder Effects): In a small population, gene frequencies can change rapidly due to random events. For example, in a population of only 10 individuals, each one carries 10% of the genes. In a population of 100 individuals, each one carries only 1% of the genes. In a population of 1000 individuals, each one carries only 0.1% of the genes. If an accident happens to one individual in each of these populations, it is more likely to significantly change gene frequencies in the small population.
5. Non-random Mating: With respect to the gene of interest, the population must be mating at random. If for example, white haired rabbits prefer to mate with other white haired rabbits, and brown haired rabbits prefer to mate with other brown haired rabbits, the population can fragment and gene frequencies within each sub-population would shift. Non-random mating is a special form of natural selection called sexual selection, which is performed by the members of a species on each other during mate choice. Some of the most extreme examples of sexual selection are in birds like the peafowl, where the females are drab and well camouflaged, but the males, who have to attract a female to mate with, put on elaborate displays and have incredibly colorful plumage. While this dramatic coloration may be good for their reproductive success, it can be a survival disadvantage. Such trade-offs can lead to interesting evolutionary compromises.
Macroevolution
Macroevolutionary changes are difficult to observe in progress because of the geological timescales that are usually involved. Evidence that macroevolution occurs, however, is compelling. This evidence is in the form of patterns of plant and animal distribution, fossils, biochemical molecules, anatomical structures, and developmental processes. Just like a burglar cannot carry out a crime without leaving some kind of evidence behind, organisms leave evidence of what they looked like and how they lived. Evolutionary investigators piece this evidence together and provide detailed accounts of the lives of extinct organisms and their relationships to modern forms. The sources of evidence for macroevolution are described in the next section.
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