BIOL 2040 Lecture Notes - Lecture 11: Medium Ground Finch, Geospiza, Heritability
27 Apr 2018
School
Department
Course
Professor
Lecture 11 - Adaptation (end), Heritability
April 9, 2018
8:33 PM
Darwin on Finches
• Bird beak sizes vary depending on the species of Geospiza
• Bird beak sizes have a huge variability - it's not an either/or basis
Beak size in medium ground finch
• Question: do offspring beak size correlate with parent beak size?
• Offspring in parents nest were watched
• Researchers then averaged parent beak size on one axis, and averaged offspring beak size on
other axis
o A strong correlation was revealed - points are not scattered, but followed a positive linear
pattern
• Beak size shows high heritability
Drought in 1976-1977
• Food supply of medium ground finches dropped (not enough water to water the plants that
produced their food source)
o Remaining seeds were markedly larger and harder to crack
• Birds ended up having to eat larger and harder seeds
o Birds with larger beaks were strong enough to crack the seeds
o Birds with smaller beaks were unable to crack seeds - their fitness drops
• 90% of the birds in the population died
o Most who survived had larger beaks - evidence of selection
• Before selection:
o Mean beak size was 9.5mm
• After selection:
o Mean beak size was 10mm
o Same generation after selection
Response to Selection
• Young finches before the drought had a mean beak size of 8.9mm
• Young finches after the drought (offspring of survivors) had a mean beak size of 9.7mm
• Offspring after drought were more likely to have larger beaks than offspring before drought
Long-Term trend
• Drought made a major change - eventually environment went back to normal and selection
favoured smaller beaks again (selection favoured the original size range)
find more resources at oneclass.com
find more resources at oneclass.com
• After environment was returning to its original state, selection selected for a smaller body size
instead of its' original range before drought
• Drought strongly favoured deeper beaks but weakly favoured narrower beaks
Wide-shallow beaks have the lowest fitness, deep-narrow beaks have the highest fitness.
Correlated traits
• Beaks vary in size
o Deep beaks are also wide
o Narrow beaks are also 'shallow'
o None are both deep and shallow
• This forms what we will come to see as a 'trade-off' - an inescapable compromise between one
trait and another
Heritability
• Genetics of continuously varying traits
find more resources at oneclass.com
find more resources at oneclass.com
• There is no definite variation:
o Not either tall or short people, tall or short plants
o How tall something is doesn't fall into discrete categories - it shows continuous variation
Environmental effects
• Traits such as height are often subjected to environmental effects
• The same genotype can produce different phenotypes, depending on what environment it is
grown in
Genetic and Environmental Variance
• 3 genotypes are different
• Genotypic variance - the variations due to genotype of the individual
• Environmental variance - the variations due to environmental effects
• Total variance - the sum of both genotypic and environmental variance
Quantitative genetics
• P = G + E
o P = individual phenotype
• Average phenotype
o G = average value of the individual's genotype in population
• Non-environmental effects
o E = environmental effect specific to the individual
• Environmental effects
• VP = VG + VE
o VP = phenotypic variance
o VG = variation of genotype
o VE = variation of environment
• Broad-sense heritability, HB = VG/VP
A line cross in tobacco
• Cross a long parent and a short parent
find more resources at oneclass.com
find more resources at oneclass.com
