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Chapter 23 concept 23.4 contd.
Evolutionary fitness is NOT direct competition between individuals.
Adaptive advantage = fitness.
(Contribution to the gene pool of next generation RELATIVE to the
contribution by other individuals of same population).
Relative fitness = contribution of a genotype cf another genotype for the
SAME LOCUS. Ex. Red flower plants produce 50% offspring cf white flower
plants that produce 100% (equal number of offspring). Relative fitness of
red flower plant = 50/100 = 0.5.
7b – 120/200 = relative fitness of brown markings to bright blue markings.
Note: Natural selection acts on phenotypes NOT genotypes. Therefore
whole organism acted on by Natural selection.
For relative fitness of a specific allele, take into account:
a) complete genetic context
b) complete environmental context allele is expressed in.
Alleles that do no harm/benefit may be perpetuated in individuals with high
overall fitness. Ex. Transposable elements and old retroviruses that have
been carried along generation to generation.
Survival necessary but need to be fertile and produce enough offspring .
Survival alone no use for fitness.
Variations in sexual maturing time and longevity of individual are also
factors.
Sexual selection is natural selection for mating success.
It results in sexual dimorphism (distinct differences in the sexes in
secondary sex characteristics not directly related to reproduction).
a) Intrasexual selection- selection within a sex for a mate. Ex. Members of
one sex competing amongst themselves for a mate of the opposite sex.
b) Intersexual selection – selection between sexes for a mate. Ex. Females
select a mate based on mating rituals etc.
Reproductive handicap of sex?
a) Produce far fewer offspring cf asexual reproduction.
b) Half males and half females so not all can produce offspring.
c) Benefit is meiotic recombination.
d) Benefit is short term – generation to generation time scale (here and
now).
Q: Does Evolution fashion “perfect” organisms?
A: NO!
a) Limited by historical constraints – modifying what came before, not
from scratch. Descent with modification from ancestral forms.
b) Adaptations are often compromises. Organisms need to do many
things.
c) Chance and Natural selection act on individuals.
d) Selection can ONLY edit existing variations. Phenotypes in current
population may not be ideal traits.
Because of a) to d) Evolution cannot perfect individuals, it can only, through
Natural selection, work on a “better than “ basis.
Types of selection: Fig. 23.12.
a) Directional (most common).
b) Disruptive.
c) Stabilizing.