Transcript Genes and Populations chp 20

Genes and populations
Chp 20
Generation to generation
change in allele frequency
Evolution
 Populations
of living things slowly
adapt and change over time
 Does not explain origin of
the very first living thing
Evolution theory is a way to show
the connection of all life forms
Evolution also explains the variety
within a kind
Carolus Linnaeus
 1707-1778
 Father
of modern taxonomy
(classification)
 Binomial nomenclature
 genus species
 Canis Lupus
James Hutton - 1788
 Gradualism
– Profound change is the
product of slow, continuous
processes
 “The present is the key to the past”
 Ex. Mountains grow slowly now
so they always grew slowly!
Jean Baptiste Lamarck 1744-1829
 Evolution
through acquired traits
 Based on fossils, relics or
impressions of dead organisms
 Sedimentary rock, ice, tar, amber
Charles Darwin 1809-1882
 Evolution
by natural selection
 Also cited fossil evidence
 Anticipated that intermediates
would be found
 Travelled extensively to
observe diversity of life
Published “origin of species”
In 1859
Evolutionary theories
Lamarck
VS
Darwin
Lamarck - Inheritance by acquired
characteristics
 Individual
organisms change.
 Ex Giraffes – stretched their necks to
get food and passed longer necks on
to offspring
Darwin – Natural selection
 .Survival
of the fittest.
 The ones best naturally adapted to
survive, have more offspring and
pass on the traits to those offspring
Darwin’s version of giraffes
 Some
giraffes were born with longer
necks and better able to get food.
These ones survived and passed
longer necks to their offspring
s
Population Genetics
 Studies the genetic variations
within a population
Species
A group of organisms capable of
breeding to produce fertile
offspring

Different species can not
reproduce
and have offspring that can
reproduce
Variations
 When
different species members
have differences in
characteristics
 Ex. Dogs – one species but many
varieties
Population
A
localized group of individuals of
the same species
Gene pool
 All
of the genes in a population
Why does the dominant trait
take over?
 Hardy
and Weinberg stated the
genes in a population will remain
stable if under certain conditions
Assumptions of Hardy Weinberg
1)
2)
3)
4)
5)
There are no mutations.
No genes transferred
(No immigration or emigration)
Mating is random.
The population should be large.
No selection is occurring
Hardy-Weinberg theorem
An equation used to identify a
non-evolving population.
 Looks at the frequency of each allele


HARDY WEINBERG EQUILIBRIUM = There is no
change in gene frequency in a population

p2 + 2 pq + q2= 1
Mendel genetics – Apply to alleles in
one gametes of one pair
Mate two individuals heterozygous
(Bb) for a trait.
 25% offspring are homozygous for the
dominant allele (BB)
 50% are heterozygous like their parents
(Bb) and
 25% are homozygous for the recessive
allele (bb) and express the recessive
phenotype

populations have random
alleles
The frequency of two alleles in an
entire population of organisms is
unlikely to be exactly the same.
Ex. population of hamsters:
 A) 80% of all the gametes in the
population carry a dominant allele
for black coat (B) and
 B) 20% carry the recessive allele for
gray coat (b).

hamsters
MENDEL
monohybrid cross
Results of random union of
the two gametes produced
by two individuals, each
heterozygous for a given
trait. As a result of
meiosis, half the gametes
produced by each parent
with carry allele B; the
other half allele b.
0.5 B
0.5 b
0.5 B
0.25 BB
0.25 Bb
0.5 b
0.25 Bb
0.25 bb
RANDOM POP
Results of random union of
the gametes produced by
an entire population with a
gene pool containing 80%
B and 20% b.
0.8 B
0.2 b
0.8 B
0.64 BB
0.16 Bb
0.2 b
0.16 Bb
0.04 bb
Allele frequency
P
= frequency of dominant allele
 q = frequency of recessive allele
 Brown
eyes vs blue eyes
 Brown (B) = P
 Blue (b) = q
Mind your Ps and Qs
 From
old English pubs, be careful
how many pints and quarts you
consume
 Also from old typesetters, not mixing
up ps and qs
Total frequency of alleles in
population = 1
THEREFORE
p+q=1
(dom + res = 1)

q =1 – p
(res = 1 – dom)

p=1–q
(dom = 1 – res)
 Ex. R = red
r = white there are
20% white flowers in a field
 q freq =.2 (20%) white
then p freq = 1 - .2 = .8 (80%) red

Allele frequency of a dominant
and recessive trait
 Similar
to punett square
Ex. Frequency alleles of
Red (R) and white (r) flowers
 p2
+
Frequency
of RR
genotype
2 pq
freq
of Rr
genotype
+
q2
freq
rr
genotype
=1
p2
+
2 pq
+
q2
=1
Given: 4% of the population = white
flowers (rr)
 What is the frequency of r? (q)
 What is the frequency of R? (p)
 What % of pop. = Rr?
q2 = .04
so
q = .2
so
p = .8
4% rr
2(.8)(.2) = .32 Rr =
32% Rr
64% RR
NOT Hardy Weinberg equilibrium
 Change
of allele frequency in 3
generations
5 agents of evolutionary change
 Things
that CHANGE equilibrium of
gene pool
1) Mutation
 Change
in DNA code
 Mutagen
Mutations
 The
origin of new alleles
2) Gene Flow
 Migration
– Individuals move from
one population to next
 Bring genes into new population
3) Non-Random Mating
 Self
fertilization
 Inter breeding
4) Genetic drift
A
change in frequency due to chance
Bottleneck effect
 Genetic
drift due to a reduction in
population size
 Ex skittles
Tsunami bottle neck
Founder effect
 Genetic
drift due to formation of a
new colony with organisms with
distinctly different phenotypes
5) Natural selection
Darwin’s idea
 Survival of the
fittest
 The environment
influences who
passes on their
DNA

Fitness - ability to pass on traits to
offspring

1)
2)
3)
The individuals in a population that
are most fit are the ones that
survive
Attract mates better
Catch prey better
Hide better from predators
Polymorphism –
 When
there are two or more forms of
one character
 aids natural selection by increasing
possible phenotypes
Geographic Variation
Differences in gene pools between
populations
 Can aid natural selection

Cline
A
graded change in a trait over a
geographical area
Heterozygote advantage
 When
it is advantageous to be
heterozygous
 Ex Sickle cell anemia
Types of Natural
Selection
Directional selection
 One
extreme is better
 Ex. length of an anteaters tongue
Diversifying selection
 Opposite
extremes are favored
 Ex. White shell or Dark shell
Stabilizing selection
 The
average is best
 Ex. Field mouse size
Types of natural selection
Sexual selection
 Picking
a mate based on secondary
sexual characteristics
 EX. Tail of peacock

Sexual dimorphism
 Difference
in appearance between
males and females
 Males tend to be more colorful
Males tend to be shorter
 Ex.
Praying mantis
Diamond back terrapins
 Males
smaller
Or just weird to us humans
More sexual dimorphism
Intrasexual selection
 Competition
between individuals of
same sex
 Ex. Rams, elephant seals
Fight for Harem
Inter sexual selection
 (mate
Choice)
 One sex is choosy about selecting
mates
 Ex Bower bird makes fancy bower to
attract mate