Topic:
Population
Questions/Main Ideas:
What causes genetic
variation in a population?
Name: ____________________________
Date:_________________________
Objective: SWBAT understand what affects the phenotypes of populations.
Population= A group of interbreeding individuals of a species in a given area
Genetic Variations= increase in variability through mutations and gene shuffling
1. Mutations= any change in a sequence of DNA.
- Do not always affect an organism‘s phenotype.
Ex) Point mutation of a DNA codon from GGA to GGU will code for the
same amino acid, glycine, which has no effect on the organism’s phenotype.
- Many mutations produce changes in the organism’s phenotype, affecting its
fitness (ability to survive and reproduce) and eventually changes in the gene
pool.
2. Gene shuffling=bringing together a new combination of genes
- Crossing over which occurs during the production of gametes.
- Sexual reproduction which occurs during the production of the zygote.
What does natural
selection act on?
What are three ways that
natural selection affects
populations?
Natural selection acts on phenotypes, not act directly on genes. It determines which
alleles are passed on from one generation to the next.
Natural selection can affect the distributions of phenotypes in any of three ways:
1. Stabilizing Selection= When individuals near the center of the curve have higher
fitness than individuals at either end of the curve, causing the graph to narrow.
Ex) A mother bird would overheat small eggs and wouldn’t keep large eggs warm
enough. The medium sized babies would survive.
2. Directional Selection= When individuals at one end of the curve have higher fitness
than individuals in the middle or at the other end, causing the graph to shift to one
side.
Ex) When food became scarce, the finches with larger beaks were better able to survive
and reproduce. Thus, the average size of the beaks increased.
3. Disruptive Selection= When individuals at the upper and lower ends of the curve
have higher fitness than individuals near the middle, causing a dip in the graph
Ex) There are only large nuts for birds with large beaks and tiny seeds available for
birds with small beaks.
If disruptive selection is strong enough and long enough, the single curve may split into
two; creating two distinct phenotypes and possibly a different species.
How do populations
maintain genetic
equilibrium?
Gene pool= Common group of genes shared by a population that contains two or more
alleles (or forms) of a certain gene for each trait.
- Ex) A mouse population may have two alleles for fur color (B for black fur; b
for brown fur).
Relative allele frequency= the number of times that an allele occurs in a gene pool
compared with the number of times other alleles occur.
The Hardy-Weinberg Principle= allele frequencies in a population will remain constant
unless one or more factors cause those frequencies to change.
Genetic Equilibrium= allele frequencies remain constant from generation to generation.
- Ex) In generation 1, there are 5 black mice and 3 brown mice; in generation 2,
there are 15 black mice and 9 brown mice
Five conditions are required to maintain genetic equilibrium:
1. Random mating
2. Large population
3. No movement into or out of the population
4. No mutations
5. No Natural Selection
STOP- turn to your
elbow partner and
discuss
How do you calculate
genetic equilibrium?
Summary:
Is this seen in a human population?
p2 + 2pq + q2 = 1 and p + q = 1
p = frequency of the dominant allele in the population Ex) T
q = frequency of the recessive allele in the population Ex) t
p2 = percentage of homozygous dominant individuals Ex) TT
q2 = percentage of homozygous recessive individuals Ex) tt
2pq = percentage of heterozygous individuals Ex) Tt and tT