Pre-class Videos 1-3 9/14

Making Statistical Inferences, The Normal Distribution, Natural Selection of Quantitative Traits

A Statistical Population is…

a large body of data that contains all the data points of interest

What is a Statistical Sample and why/how is it used?

a smaller sample of data taken from the larger population

Populations are often too large to gather all the data points, so frequencies and relative frequencies calculated from the statistical sample are used to predict frequencies of the population.

Compare Frequency vs Relative Frequency

Frequency is the exact count of a value

Relative frequency expresses frequency as a decimal or percent

How do you use Statistical Tests and what are they used for?

involve probability calculations and are used to determine how reliable predictions based on statistical samples are

Standard Deviation is…

It is denoted…

a measure of the average deviation of data points from the sample mean x̄

denoted by the letter ‘s’

Normal Distribution (what is it, how can it be used)

a type of symmetrical bell-shaped distribution that models an approximation of many frequency distributions in biology

The sample mean x̄ is the center of the normal distribution bell curve.

It is very useful for making approximations about frequencies. For any data set that can be described by normal distribution, 68% of the data points can be found in ±1 s and 95% can be found in ±2 s. Almost all values lie in the range ±3 s.

The area under the curve can be treated as probabilities. The probability of finding a value that falls under the area of the curve between x̄ + 2s and x̄ - 2s is 0.95.

Quantitative traits are…

Traits that are encoded by multiple genes and show a continuous distribution of phenotypes rather than a discrete distribution of phenotypes

List the three major ways in which natural selection act on quantitative traits

Stabilizing Selection

Directional Selection

Disruptive Selection

Describe Stabilizing Selection

favors average phenotypes, decreases variation in phenotypes over generations

Describe Directional Selection

favors phenotypes that vary in one direction from the mean

Describe Disruptive Selection

favors phenotypes that vary in both directions from the mean, resulting in multiple peaks (can be more than 2)