Transcript Begin population genetics - April 11

Mitochondrial DNA
Movement of mitochondrial genes
out of Africa
Y chromosome movement out of Africa
Population Genetics
Beginnings of Population Genetics
• From the beginning of Darwin’s formulation of evolution by
natural selection, Darwin was very clear that small heritable
changes provided the continuous variation on which natural
selection acted –many biologists shared his views and were
called selectionists
• However, other biologists thought that continuous variation
could not lead to new species or major evolutionary changes
but felt instead that discontinuous variation (the origin of
“sports” or large mutations) would be necessary to create
evolutionary change
• Nilsson-Ehle (1909) and East (1916) showed that many
evolutionary traits function as quantitative characters (or
continuous traits) where many gene pairs (multiple factors)
interact and affect a single quantitative trait (such as body size
or coat color) and that these traits have many different possible
genotypes each with a different phenotype
Neo-Darwinian Synthesis
• Population genetics and the emphasis of viewing
evolution as changes in gene frequencies within
populations resulted in the so-called Modern or NeoDarwinian Synthesis from which almost all modern
evolutionary thought still comes
• Much of population genetics has been developed by
referring to fairly simple mathematical models which
give much insight into how simplified populations
operate and evolve over time
Architects of the Neo-Darwinian Synthesis
Theodosius Dobzhansky
Ernst Mayr
J.B.S. “Jack” Haldane
Sir Ronald Fisher
George Gaylord Simpson
Architects of the Neo-Darwinian Synthesis
Galesburg’s Own – Sewall Wright
What is a population?
• a population is a group of sexually
interbreeding or potentially interbreeding
individuals
• because Mendelian laws apply to the
transmission of genes among these individuals,
Wright called these groups Mendelian
populations
• the population is usually considered to be a
geographically defined group
CANADA
ALASKA
MAP
AREA
Beaufort Sea
Porcupine
herd range
Porcupine herd
Fortymile
herd range
Fortymile herd
Population Genetic Models
Populations must have two important traits for these
models:
1. Gene frequency – proportion of different alleles of a
gene in a population (should really be called allele
frequency but often is just called gene frequency)
2. Gene Pool – sum total of all the genes in all the
members of a population – often considered to be the
genes in all the gametes of a population
The Gene Pool
Godfrey Hardy
Wilhelm Weinberg
Hardy-Weinberg Equilibrium
Hardy-Weinberg Equilibrium is based on:
1. A very large population where all genotypes
are equally viable
2. Random mating (panmixia)
3. No mutations
4. No gene flow (dispersal of individuals and
their genes)
5. No natural selection
CRCR
CWCW
CRCW
Alleles in the population
Gametes produced
Frequencies of alleles
p = frequency of
CR allele
= 0.8
Each egg:
Each sperm:
q = frequency of
CW allele
= 0.2
20%
80%
chance chance
20%
80%
chance chance
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CW (20%)
CR (80%)
CR
(80%)
64% (p2)
CRCR
Eggs
CW
16% (pq)
CRCW
4% (q2)
CWCW
16% (qp)
CRCW
(20%)
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
+ 16% C R W
(from C C plants)
= 80% CR = 0.8 = p
4% CW
(from CWCW plants)
W
+ 16% C R W
(from C C plants)
= 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
80% CR (p = 0.8)
20% CW (q = 0.2)
CR
Sperm
(80%)
CW (20%)
CR
(80%)
Eggs
CW
(20%)
64% (p2)
CRCR
16% (qp)
CRCW
16% (pq)
CRCW
4% (q2)
CWCW
Sperm
CR (80%)
CW (20%)
CR
(80%)
64% (p2)
CRCR
Eggs
CW
16% (pq)
CRCW
4% (q2)
CWCW
16% (qp)
CRCW
(20%)
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
+ 16% C R W
(from C C plants)
= 80% CR = 0.8 = p
4% CW
(from CWCW plants)
W
+ 16% C R W
(from C C plants)
= 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
A human example
Matsunaga and Itoh
collected the MN blood
types from 741 couples
(or 1,482 individuals) in
a small Japanese town –
this town had genotypic
proportions of:
.274 MM; .502 MN; and
.224 NN – remember
MN is an example of
codominance so it is
easy to score people for
exact genotype
An albino family tree – A not albino allele; a albino allele
Johnny and Edgar Winter
Hardy-Weinberg and albinism
• In a typical human population, albino’s are
present at about 1 in 20,000
• Or q2 = 1/20,000 = 0.00005
• What proportion of the population carry the
albino allele?
Breeding to eliminate
deleterious alleles