74. Mechanisms of evolution. Anagenesis..

Mechanisms of Evolution: Anagenesis (Phyletic Evolution)

1. Definition and Overview

• Anagenesis, also referred to as phyletic evolution, is one of the two major evolutionary processes, the other being cladogenesis or speciation.
• It involves the slow, continuous transformation of a single species over a long period of time.
• Unlike cladogenesis, anagenesis does not involve the splitting of one species into multiple distinct species. Instead, it is a straight-line evolution within a single lineage.
• Over time, the accumulated changes in this evolving population may become so significant that it is recognized as a new species. The original form is then considered extinct, even though its descendants continue to exist.

2. Evolutionary Mechanism

• The changes that drive anagenesis are often shaped by natural selection, especially when they offer advantages in survival or reproduction.
• Some changes may be neutral, meaning they do not significantly affect the organism's fitness and therefore are not selected for or against.
• This process is most commonly observed in species that are well adapted to their environments and do not face extreme selection pressures to change rapidly or split.

3. Role in Macroevolution

• Anagenesis is a part of macroevolution, which includes large-scale evolutionary changes over extended periods.
• Macroevolution encompasses both anagenesis and cladogenesis, offering a comprehensive picture of how species evolve and diversify through time. 4.

Examples of Anagenesis in Nature

a. Eye Evolution in Molluscs

• Although the fossil record lacks detailed intermediate stages of eye development in vertebrates, molluscs show a complete range of eye complexity among existing species.
• These variations allow scientists to observe each stage of gradual eye evolution, demonstrating how the organ has incrementally improved over time.

b. Core Histones as Conserved Structures

• Some biological structures show very little evolutionary change, which is also a form of anagenesis referred to as evolutionary stasis.
• Core histones, which help package DNA in cells, are an example. They are so efficient at their function that further changes are unnecessary.
• Histone H4, for instance, differs by only two amino acids when comparing cows and peas, despite the vast evolutionary distance between them.

c. Nephron Adaptation in Vertebrates

• The kidneys of different vertebrate classes show gradual adaptations to their respective environments.
• Marine fish and reptiles have reduced or no glomeruli to conserve water in salty environments.
• Freshwater fish and amphibians, on the other hand, have large glomeruli to help expel excess water. These adaptations reflect continuous evolutionary adjustments within lineages.

d. Lung Evolution in Bony Fish