Eighth Edition
by Harold L. Levin

Chapter 6 - page 4

Life on Earth: What do Fossils Reveal?

Evolution in Populations and Adaptive Radiation

Evolution is a process of biologic change that occurs in populations.

Population - A group of interbreeding organisms that occupy a given area at a given time.

Gene pool - The sum of all of the genetic components of the individuals in a population.

There is no exchange of genes between different populations because they are reproductively isolated.
Barriers keep their gene pools separate (distance, geographic barriers, reproductive barriers, etc.)

Geographic barriers include things such as:

• The Isthmus of Panama, preventing the marine animals of the Atlantic and Pacific from coming into contact with one another
• Separate islands with different populations of land animals that have become isolated from one another
  • Galapagos Island finches
  • Galapagos Island tortoises
  • Hawaiian Island honeycreepers (birds)
• The Grand Canyon separates different species of animals living on opposite sides of the canyon

Reproductive barriers include such things as:

• Ecological isolation - Populations inhabiting the same geographic area, but living in different habitats
• Temporal isolation - Populations that reproduce at different times (such as plants that flower in different seasons)
• Mechanical isolation - Incompatible reproductive organs due to differences in size, shape, or structure
• Gametic isolation - Fertilization is prevented by incompatible gametes

Speciation

In the more limited gene pools of an isolated population, over many generations, genetic differences may accumulate to the point that the different populations are no longer able to interbreed. At this point, the different populations would be considered separate species.

Speciation = The process through which new species arise.

Once a new species is established, segments of the population around the fringes of the population may undergo additional speciation. With successive speciations, diverse organisms arise with diverse living strategies.

Adaptive Radiation

The branching of a population to produce descendants adapted to particular environments and living strategies is called adaptive radiation.

Example of adaptive radiation:
The many species of Hawaiian honey creepers. Bill shapes are adaptations to different means of gathering food.

Diversity of bill shape in the Hawaiian honey creeper as an example of adaptive radiation.

Adaptive radiation also occurs at higher taxonomic levels. For example, there was an adaptive radiation of mammals at the beginning of the Cenozoic Era.

Evolution above the species level is termed macroevolution.

Modeling how evolution occurs

The question is not whether evolution occurs, but rather, exactly how it occurs. What is the mechanism of evolution?

There are several models for how evolution occurs:

1. Phyletic gradualism - gradual progressive change by means of an almost infinite number of small, subtle steps (old idea)
2. Punctuated equilibrium - sudden changes " punctuating" (or interrupting) long periods of little change, termed stasis (Stephen J. Gould and Niles Eldridge, 1977). Most change occurs over a short period of time.

Fossil evidence provides support for both models.

The punctuated equilibrium model for evolution suggests that evolution occurs in isolated areas around the periphery of the area inhabited by the population, referred to as peripheral isolates.

Speciation may occur rapidly in these isolated areas. This has been termed allopatric speciation.

When the new species expands or migrates from the isolated area into new areas, it looks like a sudden appearance in the fossil record.

Phylogeny - The Tree of Life

Phylogeny = the sequence of organisms placed in evolutionary order.

Diagrams called phylogenetic trees are used to display ancestor-descendant relationships.

Branches on the tree are called clades.

Cladograms are diagrams drawn to show ancestor-descendant relationships based on characteristics shared by organisms. It shows how organisms are related but does not include information about time or geologic ranges.

Stratophenic phylogeny arranges organisms in tree-like fashion with the most recently evolved individuals on the upper branches and the older, ancestral forms on the lower branches.

Evolutionary tree of the horses and related organisms (perissodactyls), an example of a stratophenic phylogeny.