Targets: Alleles and Genes Evolution Population Genetics 1. I can explain how genetic variation in a species increases chances for survival 2. I can write an example of how lethal alleles are stored in a gene pool 3. I can define allele frequency 4. I can explain why natural selection acts on phenotype rather than genotype Vocabulary Review Population all of the individuals of a single species that live together in one place. Natural Selection populations changing in response to their environment as individuals adapt and leave more offspring. Allele alternative forms of a gene that code for the same trait (Ex: eye color has blue, brown, green,.) You have two alleles for each trait, one from your mom and one from your dad. Phenotype the physical traits expressed for each allele Ex: Genotype = BB Phenotype = Brown eyes Genetic variation increases the chance of survival for a species Natural Selection selects for favorable phenotypes of a population The more variety there is within a population, the more likely that population will survive changes. Examples: - during cold winters, penguins that are short and round might survive the cold better. - during famine, tall and slender penguins might be able to dive deeper for additional food sources. Genetic variation is stored in a population's gene pool A gene pool is the combined alleles of all the individuals in a population. The more alleles for a trait, the greater the genetic variation Allele frequency is the relative amount of an allele in a population. 1
Allele Frequency How to calculate allele frequency in a population: Count the number of times a specific allele occurs in a gene pool Divide by the total number of alleles Answer expressed as a percentage (%) What is the allele frequency for b? What is the allele frequency for B? 7/14 = 50% How are lethal alleles stored in a gene pool? Most lethal alleles are recessive and do not appear until later in life *Most people don t know they are carriers *Don t know they have it till later in life (have already passed on the gene to kids) *Both parents must carry the recessive gene (only 25% chance of passing on) Targets: Effects of Mutations 5. I can explain how mutations can increase genetic variation. 6. I can evaluate the impact that mutations can have in a gene pool. How is genetic variation created? Two main sources of new variation in a population: Mutations Recombination (during meiosis) Mutations A mutation is a random change in DNA New mutations are constantly being generated, adding new alleles to the gene pool, thus increasing genetic variation in the population Change can be positive, negative or neutral 2
Mutations alter genotype DNA is changed add variety to a population Natural Selection acts on phenotype. DNA is NOT changed New variations ARE NOT added Favorable traits (either from mutations or meiosis) are passed on Natural Selection acts on the distribution of traits within a population Within a population there is a range of phenotypes for each trait. These ranges often have more of some phenotypes and less of others. Natural Selection can change the distribution in 3 ways: Stabilizing Directional Diversifying/Disruptive Stabilizing Selection Extremes from both ends of the frequency distribution are eliminated. Most common form of natural selection Example: birth weight of babies Directional Selection Favors phenotypes at one extreme of a trait s range. What we usually think of as natural selection. Example: increase of drug-resistant bacteria Disruptive Selection Both extreme phenotypes are favored at expense of middle phenotype. Favoring both extremes can lead to new species formation. Example: small-medium-large beak size of finch Targets: Gene Flow 7. I can define gene flow 8. I can define and give examples of genetic drift. 3
Gene Flow The movement of alleles from one population to another (migration). - can occur in plants and animals - increases variation in receiving population ** a lack of gene flow between 2 populations increases the chance they will evolve into 2 different species. Genetic Drift Change in allele frequency due to chance. - causes a loss of diversity - greatly affects smaller populations Two types of genetic drift: Founder Effect Bottleneck Effect Founder Effect A small group of individuals break away and colonize a new area. - gene pool of small group different from original group. Example: Ellis-van Creveld Syndrome Amish population in Pennsylvania Bottleneck Effect Occurs after an event greatly reduces the size of a population: - overhunting - predation - catastrophe New population no longer has many alleles that old population has. 4
Effects of Genetic Drift Loss of genetic variation Lethal alleles become more common Founder Effect Video Types of Isolation Geographic Isolation Barriers divide a population: rivers, mountains, dried lake beds. Reproductive Isolation Members of a population are unable to mate successfully with each other. Could be due to different mating seasons, different mating calls, variations that favor different environmental factors. Last step in becoming different species. 5