Taxonomic systems used by biologists are hierarchical, that is,

Options

A. taxonomic groups reflect shared characters, not evolutionary relationships.
B. each higher taxonomic group contains all the groups below it.
C. taxonomic groups reflect common habitats.
D. a hierarchy of traits is used to establish classifications.

Answer

To answer the question regarding the hierarchical nature of taxonomic systems used by biologists, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Understanding the Hierarchical Nature of Taxonomy**:
- Taxonomy is the science of classification of living organisms. It organizes biological diversity into a structured system. This system is hierarchical, meaning it is arranged in levels from broad to specific categories.

2. **Evaluating Option 1**:
- Option 1 states that "taxonomic group reflects shared character, not evolutionary relationship."
- This statement is **incorrect** because taxonomic groups do reflect both shared characteristics and evolutionary relationships. Taxonomy often uses phylogenetic relationships to classify organisms.

3. **Evaluating Option 2**:
- Option 2 states that "higher taxonomic groups contain all the groups below it."
- This statement is **correct**. In a hierarchical system, each higher taxonomic category (like kingdom or phylum) encompasses all the lower categories (like species or genus) within it. For example, the genus Solanum includes multiple species such as potato and tomato.

4. **Evaluating Option 3**:
- Option 3 states that "taxonomic groups reflect common habitats."
- This statement is **incorrect**. Taxonomic classification is based on shared characteristics and evolutionary relationships rather than common habitats. Different species can occupy similar habitats but belong to different taxonomic groups.

5. **Evaluating Option 4**:
- Option 4 states that "a hierarchy of traits is used to establish classification."
- This statement is **correct**. Taxonomists use various traits and characteristics to classify organisms, and these traits are organized in a hierarchical manner to establish relationships among different groups.

### Conclusion:
Based on the evaluation of the options:
- The correct statements regarding the hierarchical nature of taxonomic systems are:
- Option 2: Higher taxonomic groups contain all the groups below it.
- Option 4: A hierarchy of traits is used to establish classification.

### Final Answer:
The correct options are **Option 2 and Option 4**.

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