Natural Selection, Adaptations, and Patterns of Evolution

Types of Natural Selection

Natural selection can change how a trait appears in a species in three different ways. Roll your cursor over each of the images below to learn more about the different types of natural selection, including convergent evolution, divergent evolution, and coevolution.

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Examples of Natural Selection Types

• Directional Selection: Giraffes with longer necks are able to reach higher leaves. Directional selection favors giraffes with longer necks.
• Stabilizing Selection: The birth weight of babies in many species is an example of stabilizing selection. If a baby is too small, it has a smaller chance of survival due to heat loss. If a baby is too big, it can hurt the mother during childbirth. Therefore, natural selection will favor a birth weight that is more average in a population.
• Disruptive Selection: The peppered moth is an example of disruptive selection. Those populations of moths that lived in areas with high levels of particulates in the air selected for dark colored wings. Those that lived in areas in which there was little pollution selected for light colored wings.

Factors Impacting Natural Selection

As you know, the organisms in a population compete for resources, like food, water, shelter and mates. These different factors of competition are called selective pressures. The severity of these selective pressures, coupled with the rate of reproduction, and the complexity of an adaptation impact how quickly a species evolves. Some structural adaptations take millions of years to develop. Sometimes, intense selective pressures can cause populations to experience rapid changes and adapt very quickly to their environment. In this topic's Warm-Up, you examined how peppered moths adapted rapidly to the substances in the air during the Industrial Revolution of the 1800s.

Isolation of Species and Speciation

When populations of species become isolated and there is no gene flow between them, they continue to adapt based on the selective pressure present in their individual environments. Factors, like geography, differences in mating behaviors, and differences in the time of mating, can isolate populations from one another. If populations continue to adapt to a point where they are no longer able to reproduce together, they are said to be reproductively isolated. This reproductive isolation is the final step to these populations becoming different species from one species, which is know as speciation.

Patterns of Evolution

Evolution does not happen in the same pattern over and over again. Instead, it occurs based on environmental pressures faced by different populations. In this interactivity, learn more about the different patterns of evolution, including convergent evolution, divergent evolution, and coevolution. Click the player button to begin.

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Punctuated Equilibrium and Adaptive Radiation

Charles Darwin believed that evolution by natural selection happened continuously and gradually over a long period of time. In 1972, scientists Niles Eldredge and Stephen Jay Gould theorized that evolution of species actually occurs in short bursts, which are preceded and followed by long periods of stability with very little change. When many species arise from one ancestral species in a short period of time, this is known as adaptive radiation. You already know that the finches studied by Darwin rose from a common ancestor through divergent evolution. Because of their rapid diversification based on food resources, they are also an example of adaptive radiation. These very similar descendant species inhabit the same environment with very little competition for resources.

 

Natural Selection, Adaptations, and Patterns of Evolution Review

Now that you have learned about natural selection, adaptations, and patterns of evolution, review your knowledge in this non-graded activity. Read each statement and indicate whether it is “True” or “False,” and then click SUBMIT to check your response. Click the player button to get started.