Download Evolution of populations

Evolution of populations
Ch 21
I. Background
Individuals do not adapt or evolve
Populations adapt and evolve
Microevolution = change in allele frequencies in
a population
Requires inherited traits : genetic variability
II Sources of Genetic Variation
A. gene mutation….only gamete mutations
passed on
 1. point mutation : sickle cell anemia
 2. may be silent….no Δ amino acid
 3. may be neutral…. No change in phenotype

a. introns

b. amino acid Δ makes no difference
 4. may be beneficial or detrimental
 5. may be lethal
B. Chromosomal Mutations– change many loci
 1. duplication of genes (cross over error)
 2. transposable elements

a. transposons just move

b. retrotransposons leave a copy at original
position and move = more copies of gene
 3. large changes often harmful, but a few
extra copies often good
 4. extra copy can take on new function
C. Sexual Reproduction
 1. crossing over
 2. independent assortment
 3. fertilization 2n possible gamete
chromosome combinations

 D. rapid reproduction = higher rate of mutation
III. Hardy-Weinberg Principle
A. detects microevolution of particular genes
 1. determines what gene frequencies would be
if there were NO evolution (null hypothesis)
 2. compare null hypothesis with data collected
from population
 3. null hypothesis supported = no evolution
 4. data differs from null hypothesis = evolution
B. Based on Population Gene Pools
 1. population = all individuals of a species in an
area that reproduce together
 2. gene pool = all copies of every allele at
every locus in all individuals of a population
 3. if all alleles of a gene are the same in the
gene pool the gene is said to be fixed
 4. frequency of that allele would be 100%

C. Genotype Frequencies
 1. If you know the genotypes…..
 2. And there are 500 individuals in population
 3. # individuals/ total population = % of
population with that genotype
D. Allele Frequencies (ƒ)
 1. Number of dominant alleles =
 a. # homozygous dominant x2 plus
 b. # heterozygous
2. do same for
Recessive alleles
3. total frequency =

100%
p = dominant alleleƒ
q = recessive alleleƒ
p + q = 1

(100%)
E. Null Hypothesis : no evolution
 1. frequencies of alleles will remain constant
from generation to generation determined only
by segregation and recombination of alleles
 2. so in next generation each allele is equally
likely to join with any other allele in offspring
 3. mating must be completely random and all
allele combinations must survive equally well
 4. so we use multiplication rule of probability to
predict % of genotypes of offspring
F. Rule of Multiplication (probability 11.9)
 1. A is 80% of gene pool so each egg has 80%
chance to get an A
2. sperm has same
80% chance so..
Chance of AA indiv.
= .8 x .8 or
p x p = p2
3. a is 20% of gene pool so each egg has 20%
chance to get an a
4. sperm has same
20% chance so..
Chance of aa indiv.
= .2 x .2 or
q x q = q2
5. The remainder of population is heterozygous
6. There are 2 ways for a zygote to get
one of each allele
Egg A & sperm a =
 .8 x .2 (p x q)
Egg a & sperm A =
 .2 x .8 (q x p)
So heterozygotes =
 2(.2 x .8) or
2pq
G. Hardy-Weinberg Formula =
p2 + 2pq + q2 = 1
p2 = frequency of homo dominant
q2 = frequency of homo recessive
2pq = frequency of hetero
1 = 100 % of population
H. conditions for Hardy-Weinberg
Equilibrium
 1. no mutations
 2. random mating
 3. no natural selection
 4. extremely large population
 5. no gene flow into/out of population

a. immigration

b. emigration
IV. Causes of Evolution
A. Natural Selection
 1. Differential fitness of one allele
 2. example
 a. 0% of fruit flies had DDT resistance in 1930
 b. 37% had allele for resistance in 1960
 c. null hypothesis rejected
 d. explanation : new mutation then natural
selection favored files with DDT resistance
allele.
B. Genetic Drift : Chance events cause random
change in allele frequency
 1. Small population size maximizes effect
 a. may decrease genetic variation & fix alleles
2. Founder Effect
 a. new population started by few individuals
 b. fewer alleles, different %s than main pop.
 3. Bottleneck Effect
 a. Population nearly wiped out
 b. few remaining ind. have limited gene pool
 c. population may recover but genetic variation
only recovers very slowly with out immigration
Florida Panther
separate subspecies
for 100 years
Population dropped to under 50 in 1970
Inbreeding increased incidence of disorders
kinked tail 88%, but only 27% in other pop.
Desert Bighorn Sheep
Island population of desert bighorn sheep
20 founders in 1975 650 in 1999
Genetic variation significantly less than
mainland
C. Gene Flow
 1. alleles move into or out of population
 2. individuals move

a. emigration – out

b. immigration – in
 3. gametes move
 4. increases genetic diversity
 5. makes populations more similar
 6. prevents speciation
Big horn sexual selection
http://www.youtube.com/watch?v=E6Fx3CaJhg
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