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What is Anagenesis? - Definition & Example

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  • 0:02 Evolutionary Mechanisms
  • 1:46 Anagenesis
  • 3:04 Punctuated Equilibrium
  • 4:33 Lesson Summary
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Lesson Transcript
Instructor: Joseph Said
This lesson reviews anagenesis, which is a mechanism of biological evolution. We'll also discuss the main alternatives to anagenesis, punctuated equilibrium.

Evolutionary Mechanisms

Why are there so many plant and animal species? Why do they look so different from each other yet have a lot of similarities? These are the questions biologists ask, and the answer to all of them is evolution, the scientific theory that states that species change over time, which explains the diversity of life on Earth. Okay, but how does evolution do what it does? How do different species form? There is some disagreement, so let's talk about two of the big ideas: anagensis and punctuation equilibrium.

Anagenesis, also known as phyletic transformation or gradualism, is a mechanism in which one species evolves into another by evolutionary changes within a lineage. The new species completely overwrites the ancestral species causing the ancestral one to be extinct, as there are no members of the ancestral species remaining.

Punctuated equilibrium, alternatively proposes that species undergo little to no change for perhaps hundreds of thousands if not millions of years, called stasis. Speciation only occurs when a geographical event causes the population to split into two. Then the two populations quickly diverge from one another forming two distinct groups we call clades. For this reason, punctuated equilibrium is sometimes called cladogenesis.

So in anagenesis, one species evolves into another within the same lineage. In punctuated equilibrium, one species breaks into two populations, then evolve separately and form two distinct groups. Both have merit as viable mechanisms of evolution.

Anagenesis

Let's take a look at our ancient ancestors and the path they took to evolve into you and I. We know from the fossil record that our ancestors from Australopithecus africanus, and Homo habilis (among others) slowly evolved into Homo sapien (you and I). But what was the mechanism?

Scientists that support anagenesis would argue that Australopithecus africanus slowly acquired traits within the lineage until they eventually 'became' Homo habilis. This transformation would be so complete that there would be no residual traces of Australopithecus africanus, as all of them had completely evolved into Homo habilis, thus making Australopithecus africanus extinct. With anagenesis, each of our ancestral forms would simply be phased out by the next succeeding form.

A documented example of anagenesis is the peppered moth. In Europe, peppered moths were mostly white and blended in with the lightly colored trees. The industrial revolution produced so much pollution that the trees became covered in black soot; as a result, the white moth's coloration made them quite visible to predators and pushed the moths to adapt towards a darker coloration, phasing out the white one.

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