Lecture 19 Flashcards
Fitness
Genetic contribution of individuals to next generation
relative to others as a result of differences in
viability and fertility
= Darwinian fitness
– A relative quantity, not absolute survival or offspring
number
Selective advantage:
The amount by which some individuals of a given
genotype are better adapted to a given environment
– Reflects relative differences in fitness
Adaptation
A trait that contributes to fitness by making an
organism better able to survive or reproduce in a
given environment [noun]
– Compared to the prior ancestral state
– Link between trait and environment makes it adaptive
* The evolutionary process that leads to the
origin and maintenance of such traits [verb]
– Natural selection
Artificial Selection
Selection by humans toward a goal
– Domesticated plants and animals
– Selection experiments in genetics
– Evolution of attenuated vaccines
Natural Selection
Selection by abiotic & biotic environment
– No “goal”
– Affects all organisms (including humans)
How to Study Adaptation?
Monitor correlations of alleles or traits with
environment over space and time
* Analyze genomic diversity
– Genes targeted by selection ought to show
distinctive patterns
* Experimental manipulations in field & lab
Stabilizing selection
on human birth weight
Infant mortality is lowest at intermediate birth weight 9
Directional Selection
On Beak Size in Galápagos Finches
Disruptive Selection
on Beak Size in
African Finches
- Disruptive selection leads to
trait divergence - In some cases, may lead to
speciation - Requires spatial heterogeneity
or discrete resources
The Struggle to Determine
the Agents of Selection
Research through today shows:
– Thousands of measurements of selection
– Demonstrates fitness differences &
evolutionary change in traits
* And yet:
– Many fewer convincing cases document
the mechanisms (agents) of selection in
natural populations
– Linking evolution to ecology is difficult!
Evolution by Pollution
Evolution of industrial melanism in
peppered moths
The Peppered Moth (Biston betularia)
and Industrial Melanism
Light and dark forms of species that rest on trees
* In UK, before 1850 dark moths rare
– Dark (melanic) form caused by single dominant allele of Cortex gene
* Industrial pollution blackened tree trunks near cities
– Resulted in increase in dark form
* Dark variant replaced light form in polluted areas
– Light form predominated in rural unpolluted areas
* Mechanism of selection due to predation by birds
– Differences in moth crypsis (camouflage) depend on trunk coloration
* Experiments in the field test this
DNA Variation at G6PD
G6PD gene shows evidence of the recent,
rapid spread of resistance allele
– Consistent with hypothesis of selection for
malaria resistance
Ancient DNA can be used to study
natural selection
Historical
genomes
allow the
tracking of
alleles over
time
* Natural
selection will
cause much
faster allele
frequency
change than
random
chance (drift)
Evidence of natural selection on vitamin D
deficiency from ancient to modern genomes in
the UK
Vitamin D synthesis requires UV radiation, and northern,
cloudy skies in the UK reduce UV
-DHCR7 gene involved in vitamin D metabolism
