Milosz SzymoniakArelis Diaz

Tiffany Lavallee

Chapter 9

Dr. B’s Question #7Natural 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 SelectionThe 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.

http://anthro.palomar.edu/evolve/evolve_2.htm Belk and Borden

The Process

http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html

Finches from the Galápagos Islands

http://anthro.palomar.edu/evolve/evolve_2.htm

Directional SelectionDirectional 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. http://www.britannica.com/eb/article-49874/eerent sets of alleles evolution

http://trc.ucdavis.edu/biosci10v/bis10v/week6/directional.gif

Stabilizing SelectionStabilizing selection refers to genetic diversity

which decreases as the population stabilizes 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 phenotypes by various direct measures.

http://en.wikipedia.org/wiki/Stabilizing_selection

~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.

http://www.gwu.edu/~darwin/BiSc150/PopGen/stabilizing.GIF

This is My

Xena,an Alaskan Malamut

e

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.

She weighs

30 more lbs

She’s all

muscle and is

the sled puller

This is a Siberian Husky

Disruptive SelectionDisruptive 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

http://en.wikipedia.org/wiki/Disruptive_selection http://medical-dictionary.thefreedictionary.com/Disruptive+selection

http://trc.ucdavis.edu/biosci10v/bis10v/week6/disruptive.gif

Selection in A beetle Population