SYLLABUS

TOPIC SCHEDULE

GRADES

ASSIGNMENTS

ACTIVITIES

BACK TO LECTURES

RETURN TO A105
 

 A105 Human Origins and Prehistory
A105 Lecture 2 Spring 2002

History of Evolutionary Thought and Basic Genetics

Review:  Evolution -- defined as genetic change over time, which can often be seen in the physical appearance of organisms, due to differential survival of organisms best suited to their environment.
     
               Two main obstacles to evolutionary theory: Fixity of species  and Young Earth.  Basically, the idea of change in species was not accepted.
 
 

  • Plato, eidos -- unchanging essence of species, existing in ideal world.
  • Medieval clerics -- added mind of God, value differences.
  • Great Chain of Being, in which each organism had a fixed place.  Immutable and perfect.


         The idea of a young earth also has a long history:

  • Bishop James Ussher -- world begins 4004 B.C.
  • Buffon -- early scientific calculations
The 18th Century.
         The study of nature was seen as a Godly pursuit, through the 19th century.
         Early attempts at classification:

        John Ray, 1691.

       Linnaeus, 1707-1778.  Binomial nomenclature, killing feature.

Fossils continued to be discovered.  This change had to be accounted for.

       Cuvier, 1769-1832.  Catastrophism.  Worldwide extinctions followed by special creations.

       Lamarck, 1744-1829.  Mechanism for change: inheritance of acquired characteristics.  

      Lyell, 1797-1875.  Geologist.  Uniformitarianism -- idea that processes we observe today also operated similarly in the past.

Charles Darwin, 1809-1882.

Influences: Lyell's uniformitarianism, Beagle voyage, observations of domestic animals, Malthus (Essay on Population).
 
 
 
 
 
 

Natural Selection: formulated 1838; published 1859.
3 key observations:
1. Variation is the norm in nature
2. Variation is inherited
3. More offspring are born than survive to reproduce

4 Principles making up natural selection
1. There is a struggle for survival
2. Certain variations give individuals an edge
3. Differential survival and reproduction -- these individuals leave more descendants, who are like them.
(4. Over time, this accumulation of variation leads to the origin of new species)

Selection works on individuals, but populations evolve.

Darwin lacked a mechanism for natural selection.  Blending inheritance a problem.

Mendel: the Gardening Monk.
Did experiments with pea plants.  Kept very careful records.  Studied 7 traits, each with two variants; worked with true-breeding lines.

Experiments on single traits revealed that each trait was controlled by two "particles", now called alleles.  Law of Segregation.

Also revealed the concept of dominance, where one form of a trait is expressed over another.  Recessive traits are the opposite -- can be masked.

More terms: heterozygous (different alleles for a trait); homozygous (two copies of same allele); genotype (individual's alleles); phenotype (physical appearance).

Law of Independent Assortment.  States that traits are inherited independently from each other.
 

Mendel was missing the discovery of DNA (he called them "particles," not genes), as well as chromosomes.  DNA codes for proteins, which build bodies, among other things.

Genetics was not immediately helpful to the theory of evolution:  the workings of DNA seemed so regular that it was hard to explain how change would occur.  Then, Muller X-rayed his fruit flies, producing mutations.
 

Hence, the Modern Synthesis: Mendel + Muller + Darwin = Evolution as genetic change over time.  More specifically, it is change in the proportion of certain alleles in a population.