Stabilizing selection Fig. 16.9 Human birth weight, an example of stabilizing selection.

Evolution:Microevolution

Chapter 16

I. Microevolution

A. evolutionary changes below species level

• refers to change within a species

B. changes in gene frequency due to natural selection

C. driving force of evol.

II. Population Genetics

A. study of microevolutionary change

B. evol. takes place as changes occur in a gene pool

• all of the genes present within a pop.

C. gene frequencies change as organisms adapt to a changing env.

1. alleles for more adaptive phenotypes increase in freq.

2. alleles for less adaptive phenotypes decrease in freq.

D. if gene frequencies change, then evolution occurs

III.Hardy-Weinberg (HW) Principle

A. math. model used to predict behavior of allele freq. in an ideal pop.

B. allele freq. remain constant from generation to generation, unless outside forces change them

1. HW equilibrium

2. changes occur through natural selection and other means

C. p2 + 2pq + q2 = 1

D. if a population is not in HW equilibrium, then evol. is occurring

• gene freq. are changing

IV. Outcomes of Natural Selection

A. most traits within a pop. form a normal distribution

B. stabilizing selection

1. intermediate (avg.) phenotype is most adaptive and selected for

2. alleles for average individuals increase in number

Stabilizing selection

Fig. 16.9 Human birth weight, an example of stabilizing selection

C. directional selection

1. one extreme phenotype is most adaptive and selected for

2. a new “average” is established

Directional selection

Fig. 16.10 An example of directional selection. Guppies become more colorful (blue and yellow lines on graph) in the absence of predation, and less colorful (red graph line) when in the presence of predation.

D. disruptive selection1. both extreme phenotypes are adaptive and

selected for2. two “averages” are formed

Disruptive selection

Fig. 16.8 Three different kinds of selection: (a) stabilizing, (b) directional, and (c) disruptive

Another real example of the three different kinds of selection

V. Other Ways Evolution Can Occur in Addition to Natural Selection

A. genetic drift

1. change in allele freq. due simply to random chance

2. more pronounced effect in small populations

3. can lead to the complete loss or fixation of certain alleles in a pop.

An example of genetic drift

Fig. 16.6 Genetic drift

B. population bottleneck

1. pop. is greatly reduced in a small amount of time

• usually due to a natural disaster or major disease of some kind

2. gene freq. change according to those possessed by the survivors

Schematic view of a population bottleneck

C. founder effect

1. small group of individuals leave main pop. and establish a new colony

2. gene freq. change according to those possessed by these “founders”

D. gene flow

• exchange of genes between pops. of the same species

Schematic view of founder effect