Download 29th Feb and 1st March

Natural Selection
• Who or what selects if an organism survives?
• What is the “Selective pressure?”
• What organism is selected?
Lamarck vs. Darwin
• Lamarck
• Environment causes
giraffes to stretch
their necks over their
lifetime to better
survive.
• Darwin
• Environment selected
that giraffes born
with longer necks
had a greater chance
of survival
Natural Selection
1.
2.
3.
4.
Variation.
Inheritance.
Differential reproduction / “Struggle for existence”
Differential adaptedness
Types of Selection
1. Directional
• One extreme form of a trait is favoured
2. Stabilizing
• Intermediate form of a trait is favoured
3. Disruptive
• Both extreme forms of a trait are favoured
Research time:
Examples of the different types of
selection?
Genes: Unit of Variation
• Genes are the carriers of inheritable traits
• Mutations create new traits (new genes)
• Mutations to germinal cells (i.e. sperm & egg) are hereditary
• Somatic mutations (i.e. skin cells) are not
• Sexual reproduction shuffles existing traits to create new combinations.
• Environment selects for or against these new traits or combinations
So, how does Selection work on genes?
• Population
• A group of organisms of the same species and living in
the same area
• Therefore, sharing the same gene pool
• Evolution works on populations not on individual
organisms
• Selection shifts the relative frequency and abundance
of alleles in a gene pool
The Genetic definition of evolution:
Any change in the relative frequency of
alleles in the gene pool of a population.
Artificial selection
• Also called “Selective Breeding”
• Selective pressure = ?
• Variation increased or decreased?
• Problems?
Humans selected:
Humans selected:
Do the FIT necessarily get to reproduce
more?
NON-RANDOM MATING
Non-random Mating
• Also known as “Sexual Selection”
• Some organisms are more desirable mates than others.
1.Make themselves attractive to the opposite sex
2.Fighting off same-sex rivals
• Organisms with this desirable trait are more likely to mate and
produce offspring with similar characteristics
• “Inbreeding”
• Desirable trait may not improve fitness
Sex mixes things around!
• Invented 2 billion years ago
• Increased variation: unique individuals produced
• Shuffling of DNA
Mendel’s Experiment
Gregor Mendel (1822-1884).
1. Experimented with Pea Plant breeding.
2. Focused on 7 well-defined garden pea traits one at a time
Genotype = collection of genes (and alleles) in an organism
Phenotype = observable properties of an organism
Seed shape
Seed color
Pod shape
Mendel’s 7
garden pea
characters.
Pod color
Flower color
Flower location
Plant size
Monohybrid Cross
When he bred a purebred
tall plant with a purebred
short plant, all off the
offspring were tall
Purebred tall
× Purebred short
F1 generation = all tall
“Filius:” Latin for “son”
Then, when he bred two
plants of the new
generation together, he
would get 1 short plant for
every 3 tall plants.
F1
×
F1
F2 generation = 3 tall : 1 short
Mendel’s Conclusions
1. Hereditary particles (genes) exist in all living
organisms and remain unchanged in an organism
even if they are not expressed or seen.
•
“Shortness” disappeared in F1 and reappeared in F2.
Shortness must therefore have still been present in F1
2. Each organism must contain two gene copies for
each trait.
3. One form of the gene is Dominant, the other is
Recessive.
•
The F1 were all tall even though one parent was short. Tall
must therefore be dominant
Monohybrid Cross - explained
Parental
generation:
“Homozygous”
F1-generation:
“Heterozygous”
This is called a
Punnett’s
Square
TT
×
tt
F1 = all Tt
Tt
Tt
×
T
t
T
TT
Tt
t
Tt
tt
F2 phenotypic ratio = 3:1; genotypic ratio = 1:2:1
Practising Punnet Squares
1. Hh × Hh
2. YY × yy
Try these if you can:
AABB × aabb for F1 & F2
y
Y
Y
Yy
Yy
y
Yy
Yy
• Phenotypic Ratio 4:0
• Genotypic Ratio 4:0
H
H
h
HH
Hh
h
Hh
hh
• Phenotypic Ratio 3:1
• Genotypic Ratio 1:2:1
More practice for punnett squares
• Phenotypic Ratio 3:1
• Genotypic Ratio
4 AA’s
8 Aa’s
4 aa’s
4 BB’s
8 Bb’s
4 bb’s
4:8:4 or
1:2:1 for both A and B
Mendel’s Theories
• Genes for different traits are inherited independently of each
other.
• Some exceptions, of course, occur
• Incomplete Dominance (blending) for some traits
• Codominance (random mixing)
• Some genes are sex-linked