Evolution of Phenotypic Traits
What is phenotype:
A. describes any observed quality of an organism, such as its morphology,
development or behavior
B. is not simply a product of the genotype, but is influenced by the environment
to a greater or lesser extent (phenotypic plasticity).
C. characteristics that can be made visible by some technical procedure since the
concept of the phenotype has some hidden subtleties
What is Phenotypic variation?
A. due to underlying heritable genetic variation
B. fundamental prerequisite for evolution by natural selection
C. Richard Dawkins in The Extended Phenotype
D. So when it comes down to it a phenotype is any detectable characteristic of an
organism (i.e., structural, biochemical, physiological and behavioral) determined
by an interaction between its genotype and environment.
** Do all organisms with a different phenotype possess a different genotype as well?
- phenotypic plasticity:
Diversity of phenotypes for skin pigmentation in humans:
Why?
i. Correlations between environment and external traits
ii. Natural selection in the case of skin pigmentation and body hair

Homo ergaster (early African Homo erectus) – first hominid to be
strongly affected by hot, dry environment.
How?
i. At some point in human evolution, the thick body hair seen in other primates was
lost.
 Happened in hot environments, why?


ii. At some point in human evolution, different phenotypes for skin color developed.


Also hot environments.
Influence of UV radiation:
UV radiation- good for synthesis of vitamin D3, dangerous in excess.
melanization: increase in density of melanocytes in epidermis and
production of darker skin.
o More melanin in hot climates with a lot of UV radiation is advantageous:
- More pigment to block out radiation, but constant UV would
still assure synthesis of vitamin D.
- Protects from folate lysis:
-causes infertility in males
-causes neural tube defects
-problems with bone marrow maturation
-problems with red blood cell development
*folic acid is crucial to reproductive success
So…in warm climates…
Homo ergaster with (More or Less?) melanin = more viable
offspring.
Homo ergaster with (More or Less?) melanin = less viable
offspring
…and this is what would have led to darker skin colors being
associated with warmer climates.
o As populations spread from Africa into cooler climates, less melanin
would have become more advantageous, why?:


So…in cool climates…
People with MORE melanin = less viable offspring, increased chance
of death.
People with LESS melanin = more viable offspring, lesser chance of
death.
How does this relate to evolution of phenotypic traits?
i.
ii.
Variation in phenotypic characters is based on several or many variable gene loci,
as well as the environment. (We used skin color as an example of this).
The strength of natural selection of phenotypic traits:
- Tendency for selection involving mating success to be stronger than survival
selection:
- In our example of evolution of human skin pigmentation, this
would mean that sterility in males as a result of folate lysis
would have more of an effect in regards to natural selection
than would individuals dying from vitamin D deficiency.
How much and by what manner is the phenotype intelligence affected by genes and
not by the environment?
a. first set of experimental observations done by Galton
b. Sir Francis Galton and Charles Spearman and the g factor
c. Nervous system, protein evolution and increase in brain size and complexity
(picture)
References
Futuyma, Douglas J. (2005). Evolution. Sinauer Associates, Inc.
Galton, F. (1869/1962). Hereditary Genius: An Inquiry into its Laws and Consequences.
London, McMillan/Fontana.
Plomin R. and Spinath F.M. (2004). Intelligence: Genetics, Genes, and Genomics.
Journal of Personality and Social Psychology. 15, 112-129.
Salgado (n.d.). Evolution of Phenotypic Traits. Retrieved
November 2, 2007 from Christian Brothers University, Biology
Department Web site: http://www.cbu.edu/~esalgado/BIOL246/ch13.doc
Smithsonian Institution (n.d.). What are the differences between
races, and how long ago did these differences evolve? .
Retrieved November 2, 2007 from Human Evolution at the Smithsonian
Institute, Web site:
http://anthropology.si.edu/humanorigins/faq/race.htm