complete bio practical #1

What are descriptive statistics?

Statistics used to summarize or describe a dataset, such as the mean, median, standard deviation, and range.

What are inferential statistics?

Statistics used to test hypotheses and make inferences about populations using sample data.

What is a population?

All members of the group being studied.

What is a sample?

A subset of a population used to learn about the population.

Why do scientists usually study samples instead of entire populations?

Measuring every member of a population is often impossible or impractical.

What is a parameter?

A numerical value describing a population.

What is a sample statistic?

A numerical value calculated from a sample that estimates a population parameter.

What is sampling error?

The difference between a sample statistic and the true population parameter caused by random sampling.

Does sampling error necessarily mean someone made a mistake?

No. Randomly selected samples naturally differ from their populations.

What generally happens to sampling error as sample size increases?

It generally decreases.

Why does increasing sample size generally improve an estimate of a population mean?

Larger samples are less strongly affected by random sampling variation.

What is a categorical variable?

A variable that places observations into groups or categories rather than measuring a numerical quantity.

Give an example of a categorical variable from the turtle experiment.

Shell color/group, such as blue versus green turtles.

What is a numerical variable?

A variable whose values represent measured or counted quantities.

What is an explanatory (independent) variable?

The variable hypothesized to influence or explain another variable.

What is a response (dependent) variable?

The measured outcome that may change in response to the explanatory variable.

What is the mean?

The sum of all observations divided by the number of observations.

What is the formula for a sample mean?

x̄ = ΣXi / n.

What does Σ mean?

Sum all the indicated values.

In x̄ = ΣXi/n, what does n represent?

Sample size, or the number of observations.

What is the median?

The middle value after observations are arranged from smallest to largest.

How is the median found when there is an even number of observations?

Average the two middle observations.

What is the range?

Maximum value minus minimum value.

What do measures of dispersion describe?

How spread out or variable the observations are.

What is standard deviation?

A measure of how spread out observations are around their mean.

What does a small standard deviation indicate?

The observations are relatively clustered around the mean.

What does a large standard deviation indicate?

The observations are more spread out around the mean.

What is variance?

Standard deviation squared.

Approximately what proportion of observations in a roughly normal distribution fall within one SD of the mean?

About two-thirds.

What is the interquartile range (IQR)?

The range containing the middle 50% of observations, from Q1 to Q3.

What percentile is Q1?

25th percentile.

What percentile is Q3?

75th percentile.

If two datasets have the same mean but one has a larger SD, what differs?

Their variability; the dataset with the larger SD is more spread out.

What is accuracy?

How close an estimate is to the true value.

What is precision?

How consistent or tightly clustered repeated estimates are.

Can a measurement be precise but inaccurate?

Yes. Repeated measurements can be tightly clustered around the wrong value.

What happens to the precision of sample means as sample size increases?

Precision generally increases; sample means become more tightly clustered.

Why do small samples show greater variation in sample means?

Random selection of a few individuals has a larger influence on the calculated mean.

What is the major lesson of the turtle sample-size activity?

Larger samples generally provide more precise estimates of population characteristics.

Does a difference between two sample means automatically prove the populations differ?

No. The difference could result from sampling error.

What is a null hypothesis?

A hypothesis stating that there is no real effect, association, or difference.

What is an alternative hypothesis?

A hypothesis stating that a real effect, association, or difference exists.

For a t-test comparing two population means, what is the null hypothesis?

The two population means are equal.

For a t-test comparing two population means, what is the alternative hypothesis?

The population means differ.

What is a P-value?

Assuming the null hypothesis is true, the probability of obtaining the observed result or one more extreme.

Does a P-value tell you the probability that the null hypothesis is true?

No.

What does a small P-value indicate?

The observed result would be relatively unlikely if the null hypothesis were true.

What significance level is commonly used in this lab?

α = 0.05.

What do you generally conclude when P < 0.05?

Reject the null hypothesis; the result is statistically significant.

What do you generally conclude when P > 0.05?

Fail to reject the null hypothesis.

Why should you not say 'accept the null hypothesis' when P > 0.05?

A nonsignificant result may reflect insufficient evidence or statistical power; it does not prove the null is true.

Does statistically significant necessarily mean biologically important?

No.

P = 0.03. Is it significant at α = 0.05?

Yes; reject the null hypothesis.

P = 0.23. Is it significant at α = 0.05?

No; fail to reject the null hypothesis.

P = 0.0015. Is it significant at α = 0.05?

Yes; reject the null hypothesis.

P = 0.16. Is it significant at α = 0.05?

No; fail to reject the null hypothesis.

What does a t-test compare?

The means of two groups while accounting for variability and sample size.

What happens to the magnitude of t when the difference between group means increases, all else equal?

The magnitude of t generally increases.

What happens to t when variability increases while the mean difference stays the same?

The magnitude of t generally decreases.

How does increasing sample size affect the ability of a t-test to detect a real difference?

It generally increases statistical power by reducing sampling uncertainty.

For two equal-sized samples of n observations each, what are the degrees of freedom in the lab's t-test example?

df = 2n - 2.

If each group has n = 20, what are the degrees of freedom using df = 2n - 2?

38.

What graph did the lab use to visualize distributions and compare groups?

A box-and-whisker plot.

What does the box of a box-and-whisker plot represent?

The interquartile range, from Q1 to Q3.

What does the line inside a box in a box-and-whisker plot normally represent?

The median.

What should a scientific graph include?

An informative title or figure caption, clearly labeled axes, units where appropriate, and a legend when multiple groups need identification.

If two boxplots overlap, does that alone prove there is no significant difference?

No. Statistical significance should be determined using the appropriate statistical test.

What is taxonomy?

The science of naming and classifying organisms.

What is hierarchical classification?

A classification system in which organisms are placed into nested groups from broad to specific.

What are the major traditional taxonomic ranks from broad to specific?

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

What is binomial nomenclature?

The two-part scientific naming system using genus and specific epithet.

How should a scientific species name be formatted?

Genus capitalized, specific epithet lowercase, and both italicized or underlined when handwritten.

What is the scientific name of humans?

Homo sapiens.

After a genus has already been written out, how can it be abbreviated?

Use the first letter of the genus followed by the specific epithet, such as H. sapiens.

What is systematics?

The study of biological diversity and evolutionary relationships among organisms.

What is phylogenetics?

The study and reconstruction of evolutionary relationships.

What is a phylogenetic tree?

A branching diagram representing a hypothesis about evolutionary relationships.

What does a branch on a phylogenetic tree represent?

An evolutionary lineage.

What does a node represent?

A common ancestor at which lineages diverged.

What are sister taxa?

Two lineages that share an immediate common ancestor.

What is a clade?

A common ancestor and all of its descendants.

What is another term for a clade?

A monophyletic group.

What is a monophyletic group?

A group containing a common ancestor and all of its descendants.

What is a synapomorphy?

A shared derived character inherited from the common ancestor of a clade.

What is an outgroup?

A taxon outside the group of primary interest that helps determine ancestral versus derived character states.

What is the ingroup?

The taxa whose evolutionary relationships are the main focus of the analysis.

Why is an outgroup useful when building a phylogenetic tree?

It helps identify which character states are ancestral and which are derived.

How do you determine which two taxa are most closely related on a phylogenetic tree?

Find the pair that shares the most recent common ancestor.

Does physical closeness of two names at the tips necessarily mean they are closely related?

No. Relatedness is determined by branching pattern and common ancestry.

Can branches around a node be rotated without changing evolutionary relationships?

Yes.

Does the order of taxa across the tips of a tree determine relatedness?

No. The branching pattern determines relatedness.

What does a character matrix contain?

Taxa and their character states, often coded numerically such as 0 and 1.

What can 0 and 1 represent in a morphological character matrix?

Typically absence and presence, or ancestral and derived states, depending on how the character is defined.

When constructing a phylogeny from characters, why group organisms sharing derived traits?

Shared derived traits can indicate common ancestry.

What is convergent evolution?

Independent evolution of similar traits in different lineages.

Why can convergent evolution complicate phylogenetic reconstruction?

Similar traits can evolve independently and falsely appear to indicate close common ancestry.

What is a homoplasy?

A similar character state not inherited from the most recent common ancestor, often caused by convergence or reversal.

Why might wings in two distantly related groups not prove close ancestry?

Wings can evolve independently through convergent evolution.

What is a phylogenetic hypothesis?

A proposed explanation of evolutionary relationships that can be tested with additional evidence.

Why can molecular data improve a phylogenetic hypothesis based only on morphology?

Molecular sequences provide many additional characters that can independently test proposed relationships.