1. 9.6 Evolution as Genetic Change in Populations
Unit 9: Evolution
9.6 Evolution as Genetic Change in Populations

2. Evolutionary “Fitness”
Each time an organism reproduces, it passes copies of its genes on to its offspring.
Evolutionary Fitness: the success in passing genes to the next generation.
Evolutionary Adaptation: any genetically controlled trait that increases an individual’s ability to pass along its alleles.

3. Natural Selection on Single-Gene Traits
Natural Selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.
Example: Lizard Color
Normal lizard color is brown. A mutation causes a red allele and a black allele.
Red lizards are more visible to predators and therefore, are less likely to live and reproduce.
Black lizards might absorb more sunlight and warm up faster on cold days. This allows them to move faster to both feed and escape predators. Black lizards might produce more offspring.
The allele for black color might increase in frequency and therefore the phenotype might increase in frequency.

5. Natural Selection on Polygenic Traits
Remember that the fitness of the individuals varies greatly creating a bell curve.
Because the fitness of individuals is a range, natural selection can affect the relative fitness of phenotypes producing 3 different types:
Directional Selection
Stabilizing Selection
Disruptive Selection

6. Directional Selection
Directional Selection: When individuals at one end of the curve have higher fitness.
Usually happens when the environment changes or a species moves to a new location.
Example: Size and shape of finch beaks based on the availability of certain seeds.

7. Stabilizing Selection
Stabilizing Selection: When individuals near the center of the curve have higher fitness than individuals at either end
Example: Birth weight in human babies

8. Disruptive Selection
Disruptive Selection: When individuals at the outer ends of the curve have higher fitness than individuals near the middle of the curve
Example: Birds with small and large beak sizes with nothing in the middle

9. Genetic Drift Genetic Drift: a random change in allele frequency
In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.
Genetic Bottlenecks: a change in allele frequency following a dramatic reduction in the size of a population.
By chance, the gene pool of this greatly reduced population could be different than the original gene pool thus all future generations will have different allele frequency.
The Founder Effect: a small subgroup of a population migrates resulting in a different gene pool and allele frequency than the original gene pool.
Example: several hundred species of fruit flies on the Hawaiian Islands that evolved from the same mainland fruit fly population.

11. Evolution Vs. Genetic Equilibrium
Genetic Equilibrium: the allele frequency in a population’s gene pool is not changing
Meiosis and Fertilization alone do not change allele frequencies
The Hardy-Weinberg Principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change.
Use it to see if the alleles are in genetic equilibrium.
We can use this genetic equilibrium as a comparison to see how much the gene frequency is changing and therefore, if a population is evolving.

12. The Hardy-Weinberg Equation
You have 2 alleles for a gene: dominant A and recessive a
There are 3 possible genotypes of these alleles: AA, Aa, and aa
Mathematical Equation to predict if evolution is occurring:
(frequency of AA) + (frequency of Aa) + (frequency of aa) = 100%
(Frequency of A) + (Frequency of a) = 100%
p2 + 2pq + q2 = 1 and p + q = 1
p = dominant allele
q = recessive allele

13. p2 + 2pq + q2 = 1
In one generation, the frequency of the A allele is 40% the frequency of the a allele is 60%
p = q= 0.6
If the population is in genetic equilibrium, the chances of an individual in the next generation having genotype:
AA = 16% (p2 = = 0.16 or 16%)
aa = 36% (q2 = = 0.36 or 36%)
Aa = 48% (2pq = 2 (0.4)(0.6) = 0.48 or 48%)
If the genotypes of the next generation are different-we know that evolution is occurring in that population!

14. 5 Things That Disturb Genetic Equilibrium
Nonrandom mating: individuals selecting a mate based on heritable traits, such as size, strength, or coloration. Called sexual selection.
Small Population Size
Immigration or Emigration: individuals who join a population could introduce new alleles and those who leave could remove alleles.
Mutations: introduction of new alleles in the gene pool.
Natural Selection: if different genotypes have different fitness, genetic equilibrium will be disrupted.