WHY OVERPRODUCTION?
•  The Insurance Hypothesis:
–  1) Replacement – excess offspring are
produced to replace any core sib that
disappears (e.g. predation, disease)
–  2) In a very good year, everyone lives
but in a bad year only the
older sibs survive
–  3) Parents do not intercede
to reduce impact of sibling
rivalries A THOUGHT
•  Overproduction of eggs
should only be evolutionary
advantageous when eggs are
cheap to produce
AT THE EXTREME
•  Trophic eggs rarely hatch (or are greatly
delayed) and are eaten by earlier hatching
sibs when other foods are hard to find –
The Icebox Hypothesis
TERRIBLY IMPORTANT
•  The relationship between genotype and
phenotype is critical to our
understanding of natural selection
•  Nature selects for phenotypes but
genes may cause particular phenotypes
to be expressed
CLASSIC MENDELIAN
GENETICS
•  PP x pp gives 3:1 ratio
in F2 generation based
upon 0.25 PP, 0.5 Pp
and 0.25 pp
PHENOTYPE
•  Def’n - the realized expression of the
genotype that results from the
biological activity of the dna molecules!
FACTORS EFFECTING
PHENOTYPE
•
•
•
•
•
•
Multiple alleles
Incomplete dominance
Epistasis
Polygenic traits
Pleiotropy
Environment
IMPORTANT INSIGHT
•  None of the factors on the previous
overhead invalidate the Mendel’s idea
of particulate inheritance and random
segregation they just
complicate it!
MULTIPLE ALLELES
•  E.g. Human blood type alleles A,B,O
•  This increases the number of possible
genotypes in the population: AA, AB, BB,
OB, OA, OO = 6 genotypes
•  The O allele lacks both A and B antigens
•  Phenotypes = Type A, Type B, Type AB
and Type O
MULTIPLE ALLELES
•  E.g. Drosophila wing alleles
•  Wild type, vestigial, antlered, intermediate
•  6 genotypes: –  Ww, Wwv, Wwa
–  wv wv
–  wa wa
–  wv wa
Wild type
Vestigial
Antlered
Intermediate
MULTIPLE ALLELES
•  E.g. Drosophila wing alleles
INCOMPLETE DOMINANCE
•  A single copy of the dominant allele does
not produce the maximum phenotypic effect
INCOMPLETE DOMINANCE
•  ..
EPISTASIS
•  The effect of genes at one locus can effect
the expression of a gene at another locus
_ _ ee
bb E_
B_E_ POLYGENIC TRAITS
aaBbCC
SKIN TONE
AABbCc
aaBBCc
aaBbCc
AaBBCc
AaBbCc
AabbCc
AaBbCC
AabbCC
AaBbcc
AABBcc
AaBBcc
aabbCC
aabbCc
AABbCC AAbbCC
AAbbCC
aaBBcc
aaBbcc
AABBCC AABBCc aaBBCC
AABbcc
AAbbcc
Aabbcc
AaBBCC
DARK
aabbcc
LIGHT
POLYGENIC TRAITS
•
•
•
•
•
•
•
Some examples:
Crop Yield
Some Plant Disease Resistances
Weight Gain in Animals
Fat Content of Meat
IQ Scores
Blood Pressure
PLEIOTROPY
•  A single gene affects many phenotypic
characters e.g. sickle cell allele, cystic
fibrosis!
ANTAGONISTIC
PLEIOTROPY
•  Can it explain finite lifespan and
senescence?!
2.
5
LOG
MR
2
POW’s
1.
5
1
0.
5
AUSSIE’s
0
0
2
0
4
0
6
0
8
0
ANTAGONISTIC
PLEIOTROPY
Late Life
•  George Williams – selection for early
life performance traits may select
against old age performance – an life
history tradeoff due to antagonistic
pleiotropy!
Early Life
A CAUTION
Environmental conditions can mask
antagonistic pleiotropy.
ENVIRONMENT
Gene expression often depends upon the
environment
2 ENVIRONMENTS
YOLK
PLANT
MATERNAL EFFECT
•  Def’n - the effect of the environment
provided by a parent on characteristics
of its offspring!
TEMPERATURE EFFECT
ON SEX EXPRESSION
•  Maternal effect in sea turtles
2 TYPES OF MATERNAL
EFFECTS
•  Common environment
•  Mother-daughter phenotype match
2 TYPES OF MATERNAL
EFFECTS
•  Common environment
2 TYPES OF MATERNAL
EFFECTS
•  Mother-daughter phenotype match
A THOUGHT
•  Since they affect performance in
offspring, maternal effects should be
under selection
EPIGENETICS
•  The study of heritable changes in gene
expression and function that cannot be
explained by changes in DNA sequence.
EPIGENETIC MECHANISMS
•  (i) methylation of cytosine in the DNA, (ii)
remodelling of chromatin structure of
histone proteins and (iii) regulatory
processes mediated by small RNA
molecules
EPIGENETIC IMPACT
•  Epigenetic changes may get passed on to
daughter cells when altered DNA is
replicated during mitosis or meiosis
CAUSES OF EPIGENETIC
CHANGES
•  Exposure to toxins
•  Control during morphogenesis to shut down
specific genes to ensure that their products
are not made in particular cells (e.g. kidney
cells only function as kidney cells even
though they have the software for all other
cells)