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Chapter 9. Milosz Szymoniak Arelis Diaz Tiffany Lavallee. Dr. B’s Question #7.

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chapter 9

Chapter 9

Milosz Szymoniak

Arelis Diaz

Tiffany Lavallee

dr b s question 7
Dr. B’s Question #7

Natural Selection may take one of three forms: directional (also called purifying), stabilizing (also called balancing), and disruptive (also called diversifying) selection. Distinguish between these types and give real-life examples of each.

natural selection
Natural Selection
  • The differential survival and reproduction of individuals in a population, brought about the evolutionary change.
  • It is the process by which populations adapt to their changing environment.
  • The Galápagos finches is a great example of this process. The birds with beaks better suited for eating cactus got more food. Because they could get more food they were in better condition to mate. 
  • Those with beak shapes that were better suited to getting nectar from flowers or eating hard seeds in other environments were at an advantage in the Galapagos. 
  • Nature selected the best adapted varieties to survive and to reproduce.

Belk and Borden

the process
The Process

finches from the gal pagos islands
Finches from the Galápagos Islands

directional selection
Directional Selection
  • Directional selection occurs when natural selection favors a single phenotype.
  • The distribution of phenotypes in a population sometimes changes systematically in a particular direction.
  • The allele frequency continuously shifts in one direction.
  • Genetic changes occur as a consequence, because the genotypic fitnesses may shift so that differences are favored.
  • Organisms colonize new environments where the conditions are different from those of their original habitat.
  • The appearance of a new favorable allele or a new genetic combination may prompt directional changes as the new genetic constitution replaces the preexisting one.
  • Directional (purifying) selection removes deleterious mutations from a population.
  • It is in favor of the advantageous heterozygote. sets of alleles evolution


stabilizing selection
Stabilizing Selection
  • Stabilizing selectionrefers to genetic diversitywhich decreases as the populationstabilizes on a particular trait value or phenotype.
  • Extreme values of the character are selected against. This is probably the most common mechanism of action for natural selection.
  • This type of selection acts to prevent divergence of form and function. In this way, the anatomy of some organisms, such as sharks and ferns, has remained largely unchanged for millions of years.
  • Stabilizing selection can sometimes be detected by measuring the fitness of the range of different phenotypesby various direct measures.


~In stabilizing selection, individuals with extreme characteristics die off or fail to reproduce resulting in populations of individuals with intermediate characteristics.

~It favors the norm, the common, the average traits in a population.

~Stabilizing selection is most common in unchanging environments.


She’s all muscle and is the sled puller

This is My Xena,an Alaskan Malamute

She weighs 30 more lbs

This is a Siberian Husky

Look at the Siberian Husky, a dog bred for working in the snow. The Siberian Husky is a medium dog, males weighing 16-27kg (35-60lbs). These dogs have strong pectoral and leg muscles, allowing it to move through dense snow. The Siberian Husky is well designed for working in the snow. If the Siberian Husky had heavier muscles, it would sink deeper into the snow, so they would move slower or would sink and get stuck in the snow. Yet if the Siberian Husky had lighter muscles, it would not be strong enough to pull sleds and equipment, so the dog would have little value as a working dog. So stabilizing selection has chosen a norm for the size of the Siberian Husky.

disruptive selection
Disruptive Selection
  • Disruptive selection, also called diversifying selection, is a descriptive term used to describe changes in population genetics that simultaneously favor individuals at both extremes of the distribution.
  • Individuals at the extremes contribute more offspring than those in the center, producing two peaks in the distribution of a particular trait.
  • Its favoring the two extremes rather than the intermediate