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Evolution and Biodiversity - Classification and cladistics (HL)

Grade 11IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The binomial system of nomenclature, developed by Carl Linnaeus, uses a two-part name: the Genus (capitalized) and the species (lowercase), e.g., HomoHomo sapienssapiens.

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Taxonomists classify species using a hierarchy of taxa: Domain →\rightarrow Kingdom →\rightarrow Phylum →\rightarrow Class →\rightarrow Order →\rightarrow Family →\rightarrow Genus →\rightarrow Species.

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The three domains of life are ArchaeaArchaea (extremophiles), BacteriaBacteria (true bacteria), and EukaryotaEukaryota (plants, animals, fungi, protists).

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Natural classification involves grouping organisms together based on their most recent common ancestor, rather than purely physical similarities.

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A clade is a group of organisms that have evolved from a common ancestor, including the ancestor and all its descendants.

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Evidence for clades is primarily derived from DNA base sequences or amino acid sequences of proteins. The number of differences is proportional to the time since divergence.

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Molecular Clock: Mutations in DNA sequences occur at a relatively constant rate. The time since two species diverged can be estimated using the formula: T=DrT = \frac{D}{r} where DD is genetic distance and rr is the mutation rate.

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Analogous traits (convergent evolution) are similar because of similar selection pressures (e.g., wings in birds and butterflies), while homologous traits (divergent evolution) are similar due to common ancestry (e.g., the pentadactyl limb).

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Cladistics has led to the reclassification of many groups, most notably the Scrophulariaceae (figwort family), because DNA evidence showed they did not share a unique common ancestor.

📐Formulae

T=DrT = \frac{D}{r}

Mutation Rate(r)=Number of Sequence DifferencesTime since Divergence (years)\text{Mutation Rate} (r) = \frac{\text{Number of Sequence Differences}}{\text{Time since Divergence (years)}}

Sequence Similarity%=(Identical BasesTotal Bases)×100\text{Sequence Similarity} \% = \left( \frac{\text{Identical Bases}}{\text{Total Bases}} \right) \times 100

💡Examples

Problem 1:

Two species of primates have 1212 differences in a specific 500500 base pair gene sequence. If the known mutation rate for this gene is 22 mutations per million years, estimate the time TT since these two species shared a common ancestor.

Solution:

Using the molecular clock formula T=DrT = \frac{D}{r}, where D=12D = 12 mutations and r=2r = 2 mutations per million years: T=122=6 million yearsT = \frac{12}{2} = 6 \text{ million years}

Explanation:

The accumulation of mutations over time acts as a clock. Since 1212 mutations have occurred and the rate is 22 per million years, 66 million years must have elapsed since the lineages diverged.

Problem 2:

Identify the plant phylum based on these characteristics: no vascular tissue, no true roots, and reproduces via spores released from a capsule.

Solution:

The phylum is BryophytaBryophyta.

Explanation:

Bryophytes (mosses, liverworts) lack xylem and phloem (vascular tissue) and rely on diffusion. They produce spores in capsules at the end of stalks.

Problem 3:

In a cladogram, Species A and B share a more recent node than Species A and C. Which pair is more closely related?

Solution:

Species A and B are more closely related.

Explanation:

On a cladogram, the proximity of the most recent common ancestor (node) determines the degree of relatedness. A more recent node indicates fewer genetic differences and a shorter time since divergence.