Quantitative Genetics Pages 73-94, 11-15, 319-355
A growing body of evolution, concluded by Darwin, 3.2 EVOLUTION BY NATURAL SELECTION
indicated both fossilized and living organisms were
derived from a common ancestor or a few. Darwin proposed that the descent of modification is the
outcome of 4 postulates:
3.1 ARTIFICIAL SELECTION: DOMESTIC ANIMALS AND 1) Individuals within population are variable
PLANTS 2) The variations among individuals are, at least in part,
passed from parents to offspring
Darwin studied mechanism of nature under 3) In every generation, some individuals are more
domestication; method plant and animal breeders use to successful at surviving and reproducing than others
modify crops and livestock 4) The survival and reproduction of individuals are not
random; instead they are tied to variation among
In order to refine a particular breed of pigeon that has, individuals. The individuals with the most favourable
for example, taller feathers or feathers curl more variations, those who are better at surviving and
elegantly, breeders employ artificial selection reproducing, are naturally selected
They choose pigeons from the flock with the desirable If these 4 postulates are true, then the composition of the
traits, let them mate to produce next generation population changes from one generation to the next.
(The stronger individuals survive at higher rates) (pg.76
If the desirable traits are passed from parent to offspring, fig.3.4)
the next generation will show the desirable traits in a
higher proportion than existed in the previous -Darwinian evolution: gradual change in populations over
generations flock time
This theory helps to prove the descent of giant tomatoes -Selection happens to individuals, but what changes is
versus small tomatoes populations.
On chromosome 2 of tomatoes, a gene called fw2.2 is -When a particular trait survives and is passed on to the
carried. This gene encodes a protein that represses cell next generation and the next generation has a higher
division; high production of this protein, smaller the fruit percentage of this trait in the present generation than
the one before, this is evolution by natural selection
Changes in the fw2.2 promoter (on/off switch) alter the
total production of protein -Darwinian fitness is the ability of an individual to survive
and reproduce in its environment
Large tomatoes fw2.2 promoter OFF, less protein
production, cell division not repressed -Adaptation is a characteristic that increases the fitness
of an individual compared to individuals without the trait
Small tomatoes- fw2.2 promoter ON, more repressor
protein production, cell division repressed
Farmers take seeds of larger tomatoes and replant them,
eliminating smaller tomatoes Quantitative Genetics Pages 73-94, 11-15, 319-355
3.3 THE EVOLUTION OF FLOWER COLOUR IN AN Testing Darwins Prediction: Did the Population Evolve?
EXPERIMENTAL SNAPDRAGON POPULATION
-Since bees selected white flowers over yellow and white
-48 snapdragons were tested have higher reproductive success, and genes determine
flower colour, there would be a higher proportion of
Postulate 1: There is Variation among Individuals white flowers in the next generation
-Snapdragons varied in flower colour (75% white with 2
yellow spots, 25% full yellow) -Experiment proved Darwins theory of evolution, proving
all 4 postulates hold.
Postulate 2: Some of the Variation is Heritable
-Variation in colour was due to different genotypes.
Genes SS and Ss have white flowers with 2 yellow spots,
ss have full yellow flowers; 12/48 SS, 24/48 Ss, 12/48 ss
Testing Postulate 3: Do Individuals Vary in Their Success
at Surviving or Reproducing?
-Bees pollinated these snapdragons and to monitor
plants success at reproducing, the researchers counted
the seeds produced from each fruit.
-Consistent with Darwins 3 postulate, plants showed
considerable variation in reproductive success (as pollen
donors and seed mothers)
Testing Postulate 4: Is Reproduction Nonrandom?
-Bees were more attracted to white flowers wit 2 spots,
and white flowers/2spots had higher success of pollen
donation and seed production
-Flowers that donated more pollen, had higher seed
-Consistent with Darwins 4 postulate, reproductive
success was no random. Through pollen donation and
seed production, white plants had higher reproductive
success than yellow plants Quantitative Genetics Pages 73-94, 11-15, 319-355
3.4 THE EVOLUTION OF BEAK SHAPE IN GALAPAGOS -Darwins 2 postulate is met; a substantial fraction of
FINCHES the variation in beak size is due to variation in genotype
-The beak is the primary tool used by birds in feeding, and -It is experimentally seen that BMP4 (bone morphogenic
the enormous range of beak morphologies among the protein 4), a signaling molecule that helps sculpt the
Galapagos finches reflects the diversity of foods they eat shape of bird beaks, are in larger amounts and are
produced earlier in development for birds with larger
-Medium ground finches were studied in an experiment. beaks.
These finches are mostly seedeaters, in general, birds
with bigger beaks eat bigger seeds and birds with smaller
beaks eat smaller seeds due to efficiency
Testing Postulate 1: Is The Finch Population Variable?
-Scientists weighed each finch, measured its wing length,
tail length, beak width, beak depth and beak length. All
these traits are variable.
-1 postulate is met; variation among individuals within
populations is practically universal
Testing Postulate 2: Is Some of the Variation among
-Variation in the amount of food that individual birds ate
as a chick can lead to variation in beak depth among
adults. Injuries or abrasion against hard seeds or rocks
can also affect beak size and shape
-To determine if variation among finch beaks is
genetically based, scientists estimated heritability
-Heritability of a trait is the proportion of the variation
observed in a population that is due to variation in genes.
Heritability ranges from 0-1 (because it is a proportion)
-Based on collective data (fig.3.10, pg. 83), parents with
shallow beaks tend to have chicks with shallow beaks and
parents with deep beaks have chicks with deep beaks.
-This proves large proportion of the observed variation in
beak depth is genetically based and can be passed onto