1. Lack of a Clear Defining Characteristic:
* Animals: Animals share a common ancestor and exhibit clear defining characteristics like mobility, heterotrophic nutrition (consuming other organisms), and a complex nervous system.
* Plants: Plants are incredibly diverse and don't share a single, universally defining characteristic. They exhibit a wide range of adaptations, including different photosynthetic mechanisms, reproductive strategies, and growth forms.
2. Convergent Evolution:
* Animals: Animal evolution often involves diverging into distinct groups with unique adaptations.
* Plants: Plants often evolve similar features independently due to similar environmental pressures. This phenomenon, called convergent evolution, makes it challenging to distinguish between closely related groups based on appearance alone. For example, cacti and euphorbs share spiny features and succulent stems, but they're not closely related.
3. Extensive Hybridization:
* Animals: While hybridization occurs in animals, it's less common and usually results in distinct species with unique characteristics.
* Plants: Hybridization is widespread in plants, creating a continuum of variation and blurring the lines between species. This makes it difficult to define species boundaries based on morphology alone.
4. Importance of Molecular Data:
* Animals: Traditional morphological classification is often sufficient for animals due to their clear characteristics and distinct evolutionary lineages.
* Plants: Molecular data (DNA analysis) is increasingly crucial for understanding plant relationships due to the prevalence of convergent evolution and hybridization.
5. Historical Factors:
* Animals: Animal classification has a longer history and a more established framework due to their greater visibility and more apparent differences.
* Plants: Plant classification has faced more challenges due to their diversity, cryptic nature, and historical reliance on solely morphological features.
Overall, plants are a more challenging group to classify due to their complex evolutionary history, widespread convergence, hybridization, and the need for molecular data to resolve relationships. This complexity highlights the ongoing work and importance of plant taxonomy in understanding biodiversity and evolutionary processes.