SEL - Natural Selection [LESSON]

Natural Selection

Charles Darwin

Darwin's theory of evolution was strongly influenced by the contributions of Lyell, Hutton, and Malthus. His observations of fossils, living organisms, characteristics that enhance an organism's chance of survival, and the environment became the foundation of Darwin's theory of evolution.

Take a look at the Evolutionary Scientist Timeline and click through the timeline to review the material.

During his travels, Darwin made several concluding observations:

  1. Members of a population vary in the traits they exhibit.
  2. Traits are inherited from parents to offspring.
  3. All species are capable of producing more offspring than the environment can support.
  4. Due to competition for resources (food, space, mates, etc) not all offspring of a population will survive to reproduce.

Darwin observed that beak shape varies among finch species.   This illustration shows the beak shapes for four species of ground finch

Image of Darwin seatedDarwin studied all types of living things during his work as a naturalist. One of his most famous examples is the Galapagos finches. Scientists have observed that in dry years when those seeds are in short supply, these finches resort to eating larger seeds that are harder to crush. Variation of beak shape and size occurs even within a single population. During long droughts, the medium ground finches with larger beaks are able to crush those large seeds better than those with smaller beaks. This means that the finches with larger beaks are more “fit” for the dry conditions, so they survive and reproduce more often than those with small beaks during a drought. In years with very high amounts of rain, more birds with the smaller beaks survive to produce offspring.

Based on his observations, Darwin made two specific inferences:

  1. Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment will leave more offspring with those traits than other individuals.
  2. The unequal ability of individuals to survive and reproduce leads to the accumulation of specific traits in a given population within a specific environment, over the course of several generations.

During his travels, Darwin found evidence that supported the ideas of previous scholars and collected his findings into two major ideas: descent with modification to explain the unity and diversity of organisms; and natural selection comparing the adaptations of organisms to their environment.

Natural Selection

Watch the Evolution by Natural Selection video below to learn more about natural selection.

We generally describe the process of natural selection according to the four “steps” below:

  1. Selective pressure – something is acting on the population causing it to be difficult for organisms to survive.
  2. Genetic variation – there is a genetic difference between members of the population.
  3. Selection – some members have a genetic difference that is selected for, meaning they survive longer and reproduce more offspring with that beneficial trait.
  4. Selection over generations – after repeated selection, the trait that is selected for becomes more common in the population.
Click here to reveal a MAJOR misconception when it comes to natural selection.

Remember that individual organisms do not evolve; only populations evolve. The traits that make them more or less successful in given environmental conditions are expressions of their genetic makeup, so they cannot change those traits. Some traits can be changed or enhanced through environmental factors and behavior, such as diet or exercise, but those traits typically cannot be passed on to offspring.

A great example of natural selection (but not so great for us!) is the increase in resistance of the bedbug to pesticides. Fifty years ago, the bed bug population was nearly eradicated in the United States, thanks in part to the use of pesticides like DDT. Today, the population is growing in number and even harder to exterminate! Hotels, homes, movie theaters, and even the Empire State Building have documented cases of bed bug infestation in recent years. In 2010, the Centers for Disease Control and Prevention (CDC) and the Environmental Protection Agency (EPA) issued a joint statement on the resurgence of bed bugs throughout the United States. So what happened?

In the past, spraying with pesticides used to quickly eliminate these blood-sucking pests. However, mutations to just two amino acids in a certain bed bug protein can make the insect 250 times more resistant to the commonly used pesticide. This means that a few bed bugs with this mutation can survive a potential extermination and then reproduce, passing on the pesticide resistance to the next generation. As this process continued through several generations, the population evolved such that every individual carries the resistance mutations. This evolution was beneficial to the bed bug population but not so much to the humans sharing their beds! 

Selective pressure – the pesticide is killing the bedbugs.

Genetic variation – some bedbugs have the resistance to the pesticide and some don’t.

Selection – those that have the resistance survive longer and reproduce more offspring, passing down the mutation.

Selection over generations – after repeated selection, most of the population is resistant to pesticides.

Complete the Natural Selection activity below by clicking the cards to reveal more information, then click the arrow.

Types of Natural Selection

Natural selection can alter the frequency distribution of heritable traits in three distinct ways depending on the influence of environmental factors.

  • Stabilizing selection favors the intermediate phenotypes. For example, if bees preferred pink petunias over red or white, then the pink ones would be more fit and its population would increase while the red and white ones would decrease.
  • Directional selection favors one extreme or the other. For example, it seems that cacti with more spines would be less frequently eaten and the population of spiny cacti would increase.
  • Disruptive selection favors the extreme phenotypes. In the case of the petunias, if bees preferred either the red or white, but not pink, then the pink ones would not be pollinated and would decline in population. This type is more rare in nature but leads to greater differences between subsets of a population.

types_selection

Can you identify each type of natural selection from the image above?

1 – disruptive, 2 – stabilizing, 3 - directional

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