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As we have touched on in other sections, species evolve from a common ancestor. In eukaryotes, this is evident even at the cellular level due to the presence of membrane-bound organelles, linear chromosomes, and genes that contain introns. All of which we discussed in Unit 2. If all organisms evolved from something else, it is also reasonable to say that evolution is still happening. There are several patterns in which this continued evolution follows, and which past evolution followed.
Patterns of Evolution
Evolution over time can follow several different patterns. Factors such as environment and predation pressures can have different effects on the ways in which species exposed to them evolve. Figure 7.05 shows the three main types of evolution: divergent, convergent, and parallel evolution.
Figure 7.05: Types of evolution; a)divergent, b)convergent, and c)parallel.
Divergent Evolution
When people hear the word "evolution," they most commonly think of divergent evolution, the evolutionary pattern in which two species gradually become increasingly different. This type of evolution often occurs when closely related species diversify to new habitats. On a large scale, divergent evolution is responsible for the creation of the current diversity of life on Earth from the first living cells. On a smaller scale, it is responsible for the evolution of humans and apes from a common primate ancestor.
Convergent Evolution
Convergent evolution causes difficulties in fields of study such as comparative anatomy. Convergent evolution takes place when species of different ancestries begin to share analogous traits because of a shared environment or other selection pressure. For example, whales and fish have some similar characteristics since both had to evolve methods of moving through the same medium: water.
Parallel Evolution
Parallel evolution occurs when two species evolve independently of each other, maintaining the same level of similarity. Parallel evolution usually occurs between unrelated species that do not occupy the same or similar niches in a given habitat.
Continuing Evolution
If you are questioning whether evolution is still happening today, there are several examples of how we see evolution in the present. One of these is the evolution of resistance to chemicals meant to kill organisms. Humans often try to control their environment by using antibiotics, pesticides, herbicides, or other chemicals. In all of these situations, the organisms that the chemicals were designed to kill, gradually develop resistance to the chemicals. Bacteria can survive antibiotics, insects can survive pesticides, and weeds can survive herbicides. These changes happen through the reproduction of the small number of organisms that do survive. In other words, the population evolves.
