Question

Which of the following is an example of homology (similarity due to common ancestry)? a. suspension feeding in sponges and clams b. ectoparasite lifestyle in aphids and ticks c. cnidocytes (stinging cells) in jellyfish and sea anemones d. radial symmetry in ctenophores and echinoderms

Short answer

The correct answer is (c) Cnidocytes (stinging cells) in jellyfish and sea anemones, as both organisms belong to the phylum Cnidaria and share a common ancestor. The presence of cnidocytes in both species is a result of their shared evolutionary history, making it an example of homology.

Step-by-step solution

Understanding Key Terms

Before answering the question, it is important to clarify the concept of homology. Homology is the similarity between species due to a shared common ancestor. These similarities can be found in morphological, physiological, or molecular features that were present in the ancestor and have been passed down through generations.

Examining the Options

Now, let's examine each of the given options: a. Suspension feeding in sponges and clams: Suspension feeding is a feeding method used by various aquatic organisms, including sponges and clams. While it may be tempting to assume that this similarity is due to a common ancestor, these organisms belong to different phyla (Porifera and Mollusca) and have evolved suspension feeding independently as an adaptation to their environments. This is an example of convergent evolution, not homology. b. Ectoparasite lifestyle in aphids and ticks: Ectoparasites are organisms that live on the external surface of their host organism, feeding on the host's blood or fluids. While aphids (insects) and ticks (arachnids) share this lifestyle, they belong to separate arthropod groups. The ectoparasitic lifestyle has evolved separately in each of these groups due to shared environmental pressures, which is another example of convergent evolution. This option does not represent homology. c. Cnidocytes (stinging cells) in jellyfish and sea anemones: Cnidocytes are specialized cells that are unique to the phylum Cnidaria. Both jellyfish and sea anemones belong to this phylum, indicating that they share a common ancestor. Since cnidocytes are present in both of these organisms and they have a shared ancestry, this option represents an example of homology. d. Radial symmetry in ctenophores (comb jellies) and echinoderms: Radial symmetry is an organismal body plan where the body parts are arranged symmetrically around a central axis. Ctenophores and echinoderms both display radial symmetry, but they belong to different phyla (Ctenophora and Echinodermata, respectively). This means that the radial symmetry in these groups has evolved independently, which is another example of convergent evolution and not homology.

Identifying the Correct Answer

Based on the analysis of each option, the correct answer is: c. Cnidocytes (stinging cells) in jellyfish and sea anemones This option is an example of homology because both jellyfish and sea anemones belong to the same phylum (Cnidaria) and share a common ancestor. Their shared cnidocytes (stinging cells) are the result of this evolutionary connection.

Key concepts

Convergent Evolution

Convergent evolution is a fascinating concept in evolutionary biology where different species evolve similar traits independently. This happens not due to a shared ancestry but rather as a result of similar environmental pressures or ecological niches. Imagine living in a hot desert. Over time, you might start evolving traits to help you survive better, just like another species far across the world that's unrelated to you might do the same. While the adaptations may look alike, they're like twins with different parents – they’re similar but originated separately.

Examples of convergent evolution include:

• The development of wings in bats and birds. Despite their similar structures, bats and birds come from entirely different evolutionary lineages.
• The evolution of streamlined bodies in sharks (fish) and dolphins (mammals), adapted for fast swimming in the marine environment.

This idea of unrelated organisms developing similar traits reinforces how powerful environmental factors are in shaping life on Earth.

Cnidaria

Cnidaria is a diverse phylum of animals known for their beautiful, often translucent forms. This group includes organisms like jellyfish, sea anemones, and corals. They are primarily aquatic, living in both the ocean and some freshwater habitats. They are most famous for their radial symmetry, meaning their body parts are arranged around a central axis, giving them a symmetrical appearance from above.

Key characteristics of Cnidaria:

• Cnidarians have two main body forms: the medusa, which is typically free-floating like the jellyfish, and the polyp, which is usually stationary like sea anemones.
• They exhibit simple tissue organization with two layers: the ectoderm and the endoderm, sandwiching a jelly-like substance known as mesoglea.
• Cnidarians are predators, using specialized cells called cnidocytes for capturing prey and for defense.

Understanding Cnidaria helps highlight the diversity and complexity of life in aquatic ecosystems.

Cnidocytes

Cnidocytes are specialized cells unique to the phylum Cnidaria. These cells are like tiny grenades with a hair-trigger mechanism. They are essential for capturing prey and providing a defense mechanism against predators. When triggered, they eject a structure known as a nematocyst, which can inject toxins into the prey, stunning or killing it instantly.

To understand cnidocytes, consider these points:

• They are a hallmark feature of Cnidaria, distinguishing them from other phyla.
• The structure of cnidocytes is complex, containing a nematocyst loaded with venom.
• These cells are typically located in the tentacles of jellyfish and sea anemones, from where they can efficiently capture food or deter threats.

Cnidocytes showcase the evolutionary innovations that can emerge in specific lineages, providing robust survival strategies in the natural world.

Evolutionary Biology

Evolutionary biology is the study of the processes that have led to the diversity of life on Earth. It helps us understand how species evolve through mechanisms such as natural selection, genetic drift, and mutation. By tracing lineages and mapping out relationships between species, evolutionary biology allows us to piece together the history of life.

Some fundamental aspects of evolutionary biology include:

• Natural selection, a process where organisms better adapted to their environment tend to survive and produce more offspring.
• Genetic drift, which involves random fluctuations in allele frequencies within a population, often having a larger impact in small populations.
• Understanding common ancestry, which explains homology as shared traits inherited from a common ancestor, contrasting with traits developed through convergent evolution.

Evolutionary biology links many scientific disciplines and reinforces our understanding of life’s complexity, helping us appreciate the intricate tapestry of biological adaptation over time.