A4.1.5 Convergent evolution as the origin of analogous structures Notes

Same Function, Different Structure

1. The wings of birds and insects perform the same function but differ fundamentally in structure and origin.
2. These similarities, arising from independent evolutionary paths, are a result of convergent evolution, which produces analogous structures.

Analogous Structures = Similar Function

1. They are similar in function, performing the same role (e.g., flight, swimming).
2. But arise from unrelated ancestral traits,
3. Adapting and evolving separately due to similar environmental pressures.

Characteristics of Analogous Structures

1. Analogous structures arise not from a shared ancestor, but from similar selection pressures acting on unrelated organisms.
2. They usually appear in organisms that:
   1. Occupy similar habitats (e.g., aquatic environments).
   2. Face similar survival challenges (e.g., need for flight, defense, or water conservation).
   3. Develop functionally equivalent solutions despite unrelated evolutionary pathways.

How Convergent Evolution Works

1. Convergent evolution occurs when unrelated species face similar selection pressures which are environmental challenges that favor specific traits.
2. Over time, these pressures lead to the development of comparable adaptations, even in species with distinct evolutionary histories.

Steps in Convergent Evolution

1. Similar Environmental Challenges: Aerial navigation, aquatic locomotion, or acute vision.
2. Independent Adaptations: Species evolve traits suited to their environment, but starting from different ancestral features.
3. Functional Similarity: Analogous structures emerge, performing similar tasks despite differing origins.

Examples of Analogous Structures

1. Wings of Birds and Insects

1. Birds and insects both use wings to fly, but their structures and origins are vastly different:
   1. Bird Wings: Modified forelimbs with bones, muscles, and feathers.
   2. Insect Wings: Thin, membranous structures supported by veins, without skeletal components.
2. Despite these differences, both types of wings evolved to solve the same problem: navigating through the air.

2. Eyes of Humans and Octopuses

1. The eyes of humans and octopuses are remarkably similar in function, despite their distinct evolutionary paths:
   1. Human Eye: Nerve fibers in front of the retina create a blind spot.
   2. Octopus Eye: Nerve fibers are located behind the retina, eliminating the blind spot.
2. These similarities arose independently as both species adapted to environments where acute vision was advantageous.

3. Fins of Dolphins and Sharks

1. Both dolphins and sharks have streamlined bodies with fins for efficient swimming. However:
   1. Dolphin Fins: Made of bone and muscle, covered by skin.
   2. Shark Fins: Composed of cartilage, with a distinct internal structure.
2. These adaptations evolved independently as solutions to the challenges of aquatic locomotion.

Distinguishing Analogous from Homologous Structures

1. Homologous Structures: Share a common ancestry but may serve different functions (e.g., bat wings and human hands).
2. Analogous Structures: Evolve independently but serve similar functions (e.g., bird wings and insect wings).

Cladistics and the Problem of Analogy

1. Morphological similarity can sometimes mislead classification.
2. For example, dolphins and sharks were historically grouped together due to body shape.
3. Cladistics, based on DNA or amino acid sequences, helps distinguish homology (true ancestry) from analogy (convergence).