BIOB51H3 Lecture 18: LECTURE 18
5 Jul 2018
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LECTURE 18
SPECIATION
Population genetics
-Qualitative traits: characteristics for which phenotypes fall into discrete categories
oControlled by ½ loci (eg. Left handed fish, right handed fish etc.)
-Population genetics focuses on: genotypes, allele frequencies in population (eg. How
does allele S change over time)
oSimplification: assume phenotypes fall into discrete categories determined
strictly by genotypes
Quantitative genetics
-Quantitative trait: characteristic for which phenotypes show a continuous variation
among individuals (eg. Height, bone density, feather length)
oComplex traits that are determined by the environment and by many
mendenlian genes (at many loci)
oFocus on: phenotypes and heritability + statistical properties of traits (eg. Mean,
variance of phenotypes)
How do complex traits evolve?
-Example: the evolution of flight
-study biomechanics, fossil, # of
feathers etc. looked for transition
state -->
-start as bipedal terrestrial (maybe
thermoregulation, mating) then
living in trees (leaping from branch
to branch would be easier if had
some aerodynamic property)
good feather for landing larger
feather for gliding go from larger
feathers to gliding
-all of this is shown in the fossil record
-Bipedal terrestrial (feathers are for
thermoregulation, display)
arboreal (live in trees; feathers are
for aerodynamic leaping)
arboreal elaborated feathers
(allow for safe parachuting from height) arboreal larger feather (allow for gliding)
powered flight; aerial (modern feathers that allow for efficient gliding and true flight)
find more resources at oneclass.com
find more resources at oneclass.com
-Can see the
effects of
directional
selection
(natural
selection)
favors one
extreme of a
phenotypic
distribution
(quantitative
trait) OR
drives allele frequencies in a single
direction (qualitative traits)
-those with feathers for gliding are at an
advantage --> direction selection --> selects
for more elaborate feathers --> eventually
population shifts to more members with feathers
-AFTER SELECTION: there is a shift in the average/mean value/distribution for feather
size/structure from normal feathers to elaborated feathers
-So, for example, we see that
variation occurs in the offspring
generation (f1)
oCan be due to
recombination, Mutation
oAnd as the variation
increases, the mean will
begin to change from the
original mean value
oThen, after selection, the
value shifts even more
towards the mean that
leads to more fit
individuals
- this change in trait value
over generations requires 3
things
find more resources at oneclass.com
find more resources at oneclass.com
a.i.1. Selection
a.i.2. Variation
a.i.3. Heritability
How is this related to speciation?
-Why does this speciation process from
bipedal to aerial birds even start?
o o Env. change caused this selection for
feathers --> might be selection for living in
trees (eg. maybe there might be a good
predator terrestrial)
-Habitat 1
oEg. Efficient terrestrial predator
directional selection for arboreal life
-Habitat 2
oEg. No terrestrial predator no selection for arboreal life. What drives further
change?
Answer: no change in mean trait value
Balancing/stabilizing selection: this favored intermediate
phenotypes (quantitative traits) OR a stable equilibrium with
more than allele present (qualitative traits)
DIVERGENT EVOLUTION
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Qualitative traits: characteristics for which phenotypes fall into discrete categories: controlled by loci (eg. left handed fish, right handed fish etc. ) Population genetics focuses on: genotypes, allele frequencies in population (eg. how does allele s change over time: simplification: assume phenotypes fall into discrete categories determined strictly by genotypes. Study biomechanics, fossil, # of feathers etc. (cid:0) state --> looked for transition. All of this is shown in the fossil record. Can see the effects of directional selection (natural selection) (cid:0) favors one extreme of a phenotypic distribution (quantitative trait) or drives allele frequencies in a single direction (qualitative traits) Those with feathers for gliding are at an advantage --> direction selection --> selects for more elaborate feathers --> eventually population shifts to more members with feathers. After selection: there is a shift in the average/mean value/distribution for feather size/structure from normal feathers to elaborated feathers. This change in trait value over generations requires 3 things a. i. 1. a. i. 2. a. i. 3.
