UNIT IV: MODERN EVOLUTION
 

I. Post-Darwinian Facts

II. Transition to Post-Darwinian Theory: Population Genetics

   A. Introduction

   B. The Hardy-Weinberg Equilibrium Model

  C.  Violating the Assumptions of "RANDOM MATING"

D. Violating the Assumption of "INFINITE POPULATION SIZE " - Genetic Drift  

   E.  Violating the Assumption of "NO MIGRATION"

F. Violoating the Assumption of "No Mutation"

   G. Violating the Assumption of "No Selection"

  H. Summary

1. Selection and Drift are the most signifigant agents of evolutionary change

2. Mutation is only important as a source of variation and as an instantaneous source of speciation (polyploidy).

 
III. Post-Darwinian Theories

   A. Mutationist School - 1910's

        - As mentioned before, the rediscovery of Mendel's priciples gave geneticists a false sense of the predictive power of gentics, and the importance of mutation. Many geneticists, including T. H. Morgan, thought most evolution was due to mutations changing the gene frequencies.  However, when rates were measured in the 1920's and 1930's, they were found to be too low to account for the morphological change we see in the fossil record.  Something besides mutation had to be more important.  Population Geneticists demonstrated that selection could have dramatic effects (as Darwin had surmised), and it was the only process that could act in a non-random way to cause adaptive change.  The union of Darwinian Theory with the explanatory power of genetics and population genetics created our current model of evolution - the Modern Synthetic Theory of Evolution:

   B. 1930-40's: The Modern Synthetic Theory of Evolution

        1. Overview:

     

SOURCES OF VARIATION	V	AGENTS OF EVOLUTIONARY CHANGE
New Genes:	A	MAJOR:
mutations within a gene	R	Natural Selection - only agent responsible for adaptations to the environment
crossing over between exons	I	Drift - a major agent when population size is small - random change
New Genotypes:	A	MINOR:
mutation	T	Migration - makes populations similar to each other
Recombination:	I	Mutation - does not change gene frequencies much at all
Crossing Over	O	However, polyploidy is an important agent of speciation
Independent Assortment	N	Non-Random Mating - only changes genotypic frequencies

The 1940's saw the unification of genetics with Darwinian evolution (emphasizing the power of selection, and not mutation, in changing teh genetic structure of a population). In addition, the contribution of population genetics formalized several other agents of evolutionary change, and also created a quantitative model for actually MEASURING the rate of evolutionary change within a population.

C. Mayr's Model of Peripatric Speciation

                Evolution occurs when small populations are isolated (Drift), and when populations adapt to new environments (Natural Selection).  Evolution might be FASTEST when small populations are adapting to new environments (BOTH drift and selection...). Ernst Mayr called this "peripatric" evolution, because we should expect it when small subgroups "bud-off" into new habitats along the periphery of a species' range.

D. 1970-1980's: Punctuated Equilibrium and the Fossil Record

        Eldgridge and Gould - 1972: Punctuated Equilibrium: KNOW THIS

         1. - consider a large, well adapted species. (It is large specifically BECAUSE it is well-adapted; that's what adaptation can do -  make a population better suited to its environment so it converts resources to offspring more effectively.  That is what selection maximizes - reproductive success.  So, we should expect populations to become better reproducers, and the offspring to be more successful, as they adapt to their environment.)

            - If the environment is stable, this population will not change much due to either drift (its large) or selection (its already well adapted).  It will persist unchanged for a while (untl env. changes, for instance). It is in 'stasis' or equilibrium.

         2. - However, along its periphery, we can have peripatric evolution  occurring and small subpopulations "budding-off" into new habitats.  These populations may go extinct, or they may change rapidly in response to drift and selection.

         3. - So, they may change rapidly as small, isolated populations.

              - As they adapt to their new environment, the population will grow larger (for the reasons stated above).  As it gets larger and beter adapted, the RATE of evolutionary change will SLOW DOWN.

         4. - eventually, we may have another large, well-adapted species that is in a state of 'stasis' or equilibrium.

         - The large, long-lived populations are more likely to leave a  fossil than the small, short-lived "intermediates".

         - Thus, with what we now know about how evolution works at the  genetic level, we should not expect a complete series of intermediates.  Intermediates will be rare, because this rapid evolutionary change is concentrated in small populations that are changing rapidly.  They are not likely to leave a fossil.

         - THIS SOLVES DARWIN'S LAST DILEMMA.  OUR MODERN UNDERSTANDING OF HOW EVOLUTION WORKS LEADS US TO CONCLUDE THAT INTERMEDIATES IN THE FOSSIL RECORD SHOULD BE RARE.
 

Study Questions

1. Outline The Modern Synthetic Theory of Evolution, listing sources of variation and agents of change.

1. What is peripatric speciation?

2. Why would small populations in new environments evolve more rapidly than large populations in their "old" habitat?  (two reasons, related to agents of change.)

3.  Given the answer the question 2, describe Eldgridge and Gould's conclusions rearding the effects of these changes on the fossil record. - Why are complete sequences of intermediates rare?