Name:
Date:
Class Period:
Notes: Variation, Diversity, and Lethal Allele Maintenance
What have we learned about evolution so far?
•Evolution is change in the traits of a population over time
•Natural selection is the mechanism of evolution
•Evidence of evolution comes from the fossil record, DNA and proteins, comparative anatomy, and embryology
•Isolation can lead to speciation. The major types of isolation are geographic, behavioral, temporal, and reproductive
What are we going to learn about evolution today?
•Variation within a species increases the probability that some members of the species will survive
•A great diversity of species increases the chance that at least some organisms will survive great changes in the environment
•Natural selection acts on the phenotype of an organism, not its genotype
•How do lethal alleles stay in the gene pool?
Variation and Gene Pools
Genetic ______is studied in ______ of species. Because populations ______ (mate with each other), they share a gene pool.
A ______ is made up of all the genes, including all the different ______, that are present in a ______.
Allele Frequency
______is the ______ of an allele in the gene pool. For example, if a group of pea plants is 50% ______(Bb), 30% homozygous ______(BB), and 20% homozygous ______ (bb), the allele frequency of the dominant allele would be ______% and the allele frequency of the recessive allele would be ______%.
Calculating Allele Frequency
Here’s how I calculated that:
We start with the 30% of the total that is homozygous dominant (BB). We know 50% of the population is heterozygous (Bb). Only half of the alleles in the heterozygotes are dominant, so we add half of 50%, or 25%, to 30%.
Here is the formula I used to calculate the allele frequency of dominant alleles:
% BB individuals+ half the % of the Bb individuals = allele frequency
30% + (½ x 50%) = allele frequency
30% + 25% = 55%
Then, to find the frequency of the recessive allele, just subtract 55% from 100%
100% - 55% = 45%
Another Definition of Evolution
Using genetic terms, ______ can be defined as any change in the allele frequency in a population.
For example, in beetles, if the ______ of the recessive green allele was 75%, but over time the frequency ______to 71%, ______ has occurred.
Practice Problem
In pea plants, purple flowers (P) are dominant over white flowers (p). In one population of pea plants, 15% of the plants are homozygous dominant, 60% are heterozygous, and 25% are homozygous recessive.
What is the allele frequency of the dominant purple allele?
What is the allele frequency of the recessive white allele?
Variation Within a Species Increases Chance of Survival
Let’s say you are studying a bird that is very well ______ to its environment, but all of the individuals are genetically very similar. If a deadly ______ starts infecting these birds, they could all die because they are all ______ to the virus!
Now, if there was more genetic ______ in this population of birds, there is a chance that some of the birds might have a natural ______to the virus. These ______birds would ______ and be able to pass on their genes.
Biodiversity Increases the Chance of Survival
We just looked at how variation ______ species can increase the chance that some members of the species will survive even if there is a change in the environment.
Having a lot of ______ (many different species present) increases the chances that some species will survive a ______in the ______ .
For example, the ______ that scientists believe was the reason ______went extinct did not kill all of the life on earth because there was a great ______of living ______ . If large dinosaurs were the only species on earth at the time, all life on earth might have been killed! But, since there were many different species all adapted to different ______and ______ , some animals were able to survive and reproduce.
Natural Selection acts on the Phenotype of an Organism, not its Genotype
Natural selection never acts directly on ______. ______, not genes, survive and reproduce or die without reproducing, not genes. ______contribute to fitness, not ______. For example, a green beetle might be more ______ than a brown beetle, but if the green color is ______, the green beetle could have the genotype GGorGg and still be equally fit. The green beetles that survive and reproduce may pass on either the green allele (G) or the brown allele (g).
How are Lethal Alleles Maintained in the Gene Pool?
If there are alleles that can cause ______(deadly) genetic disorders, why are they still in the gene pool? Wouldn’t they be removed because organisms that have them die before they can reproduce?
Not necessarily! We are about to find out how!
Lethal Allele Maintenance
Example 1: Some lethal alleles are ______, so an organism would have to have ______of the alleles to get the ______effect.
Example 2: Other lethal alleles are ______, but are only lethal in an individual that is homozygous dominant. Heterozygotes can survive and pass on their genes. An example of this is coat colors in mice. Mice with two dominant alleles for coat color die before they are born, but heterozygotes survive.
Example 3: Huntington’s disease is a lethal, ______genetic disorder in humans. People with Huntington’s disease always ______from the disease (unless something else killed them first). However, the symptoms of Huntington’s disease do not appear until the affected person is ______-______years old. Many people have already had children by this age, so they have already had a chance to ______the lethal allele to their offspring.
Lethal Allele Maintenance and the Heterozygote Advantage
Sometimes, alleles that cause genetic disorders can give an ______to ______.
Sickle cell anemia is a genetic disorder that causes red blood cells to be shaped like sickles. People who are ______for the sickle cell allele have a ______form of the disease and have a life expectancy of about ______-______years with treatment. Sickle shaped cells usually live about ______-______days. ______red blood cells live about______days.
People who are ______for the sickle cell gene have some red blood cells that are normal, and some that are sickle-shaped. They do ______have a severe form of the disease.
______is a disease that is caused by a parasite, and it can be ______. The malaria parasite infects red blood cells. When the parasite infects normal red blood cells, it has enough time to reproduce, creating many baby parasites that then burst out and infect more red blood cells. When the parasite infects ______shaped cells, the cells die before the parasite has a chance to reproduce.
This gives the ______an advantage: ______from malaria!
1