Chapter 19 Notes

Patterns of Evolution


Macroevolution: the large-scale evolutionary patterns and processes that occur over long periods of time. 


Parts of Macroevolution:


1.       Extinction: Mass extinctions are caused by a collapse in the food webs which disrupts energy flow through the biosphere. 

·      If the environment begins to collapse, this causes animals at the bottom of the food chain to die, then there is not enough food for animals at the top of the food chain.  This causes extinction of many animals at one time.

·      This has happened several times in Earth’s history, wiping out entire ecosystems.


2.       Adaptive Radiation:  a single species or a small group of species has evolved, through natural selection and other processes, into diverse forms that live in different ways.

·      When dinosaurs were in existence, mammals remained small and scarce.  When dinosaurs died off this opened up new environments for mammals to inhabit.


Example: The Hawaiian Islands Honeycreeper birds

·      They all have distinct plumage and beak shapes but have similar body size and shape.


3.       Convergent Evolution: unrelated organisms come to resemble one another. 

·      Similar environments causing similar selective pressures can cause structures to have traits that are adapted for the same purpose.



Convergent Evolution Support:


Analogous structures:  structures that are not the same in structure but they have the same function. 



a.       The streamline shape of dolphins, penguins and fishes.

b.      Marsupial and placental mammals are similar in body form and in habitat requirements.


c.       Wings of insects, birds and bats.

In order from left to right: insect, pterosaur, bird, bat


4.       Divergent Evolution: species that at one time were all similar to the ancestral species have become more distinct over time

·      Occurs when species begin to adapt to different environmental conditions and then they change, becoming less and less alike, according to the pressures of natural selection.

·      Supported by homologous structures: the same structure used for different purposes.


5.       Coevolution: the process by which two species evolve in response to changes in each other over time.

·      Many flowers can reproduce only if the shape, color, and odor of their flowers attract a specific type of pollinator. 

·      These kinds of relationships can change over time.


6.       Punctuated Equilibrium: pattern of long, stable periods interrupted by brief periods of more rapid change. 

·      Darwin felt that biological change needed to be slow and steady, called gradualism.

·      In some cases the fossil record confirms that populations of organisms did change gradually over time.

·      In other cases the fossil record holds evidence that evolution is a punctuated change, not gradual.


7.       Developmental Genes and Body Plans: the hox genes guide development of major body structures in animals. 

·      We can now perform experiments with gene expression by turning genes on or off and examining the results.

·      Small changes in the activity of control genes can affect many other genes to produce large changes in animals

·      Small changes in the timing of cell differentiation and gene expression can make the difference between long legs and short legs.