Evolution of Phenotypic Traits
By: Caleb Scott
What is a phenotype?
According to the book, a phenotype is the morphological, physiological,
biochemical, behavioral, and other properties of an organism manifested
throughout its life; or any subset of such properties, especially those affected by a
particular allele or other portion of the genotype. What a mouthful.
Or, what you see (traits), not necessarily dependant on what you don’t
(genes), but many times affected by it.
Variance in Phenotype
What affects variance in phenotype?
1) Variation in a phenotypic trait (Vp) may include genetic variance (VG) and
variation due to the environment (VE). VP = VG + VE
2) Genetic variance may include both additive genetic variance (VA, due to
additive effects of alleles) and non-additive genetic variance (due to
dominance and epitasis). Only the additive enables a response to selection.
a. Causes of genetic variation are uncertain, but mutation may balance
losses from selection and drift.
b. Example: Lizards
Lizards are classified into two taxon: Squamata and Sphenodon. Squamata can be
further divided into Families Iguania and
Scleroglossa(let’s call them “Glossies”).
From the flow chart, we can see the trait of
lingual feeding. This trait is the ability to
protrude the tongue beyond the jaws to
catch prey. All iguanians and sphendons
use this trait, but the Glossies don’t. They
only use “tongue flicking,” a movement
used to test chemicals in the air. This is
something all squamates and sphenodons
use, not just Glossies. This trait has been
effectively retained in iguanians. This
means iguanians and sphenodons are plesiomorphic, meaning “old- featured.” This
trait was kept by all except the Glossies. The Glossies developed their skulls and
tongues differently than their ancestors. They lost the “functional trade-off” that the
other families kept, or the ESC (evolutionary stable configurations) disintegrated.
Video: Chameleon Eating
http://www.youtube.com/watch?v=lDHCU7FZmns
Video: Chameleons are awesome.
http://www.youtube.com/watch?v=KMT1FLzEn9I
Heritability
Heritability, h2, is the ratio VA/VP.
What is heritable?
 Genotype and phenotype
 Environmental traits
 What about race in humans?
o Two people from the same race can be much more genetically similar
than two people from the same race
o On average, individuals from different populations are just slightly
more different that individuals from the same pop.
o Still, medical implications of racial genetic differences are under
debate.
o Genetic analysis can distinguish groups of people according to their
geographic origin.
o People that have similar skin tones from different origins (i.e. subSahara and Australia) can be genetically very different.
o Genetic racial realism: “If ‘race’ were a mere social construction based
upon a few highly visible features, t would have no statistical
correlation with the DNA markers that indicate genetic relatedness” –
but is does. (Sarich and Miele)
 What about intelligence in humans?
o A study shows that between 40 and 80% of variation in human IQ is
attributable to genetic factors.
o A genome scan for intelligence has revealed 2 regions with evidence
of linkage…which supports the idea that trait variation within the
“normal” range may be used to detect areas of mental retardedness,
like autism and schizophrenia.
Sources:
http://icb.oxfordjournals.org/cgi/content/full/41/3/552
Bamshad, Michael J.; Olson, Steve E. Does Race Exist? Scientific American
Journal, December 2003.
Futuyma, Douglas J. Evolution, 2nd edition. Sinauer Associates, 2009.
Glasgow, Joshua. A Theory of Race. Taylor and Francis 2009.
Posthuma, Danielle, et. al. A Genomewide Scan for Intelligence Identifies
Quantitative Trait Loci on 2q and 6p. Am. J. Hum. Genet. 77:318–326, 2005