Chapter 23 terms/concepts Flashcards
Genetic variation
Differences among individuals in the composition of their genes or other DNA sequences. It is necessary for evolution as natural selection requires gene variability to select heritable traits that prove to be advantageous in a particular environment.
Levels of genetic diversity
- Whole-gene level (genetic variability) - which can be quantified as the average percentage of loci that are heterozygous
- molecular level (nucleotide variability) - little of this variation results in phenotypic variation.
Different types of mutations, and where mutations must occur to be heritable?
Types of mutations: variable basepairs (substitutions), insertions (addition of one or more nucleotides), deletion (deletion of one or more nucleotides).
Mutations must occur on exons, which are regions of DNA retained in mRNA after RNA processing. Many nucleotide mutations occur on introns, which are non-coding segments of DNA in between exons.
Sources of Genetic Variation
- Formation of new alleles - arise by mutation (errors in DNA replication, exposure to UV, chemicals, etc).
- Altering gene number or position- duplication of genes due to errors in meiosis (unequal crossing over), slippage during DNA replication, or transposable elements
- Rapid reproduction - while rare, mutations can occur rapidly because of increased or rapid reproduction
- Sexual reproduction - a unique combination of alleles from parents mating (crossing over)
Large changes to genetic sequence that have deleterious effects on the organism will not last, while smaller changes that keep gene intact could prevail for generations.
Neutral variation
Point mutations that occur in noncoding regions (introns) result in neutral variation - differences in DNA sequence that do not confer a selective advantage or disadvantage. Also, redundancy in genetic code can cause a point mutation on an exon to not be expressed, causing neutral variation as well.
How do crossing over, independent assortment, and fertilization contribute to genetic variation?
In crossing over, chromosomes from each parent exchange certain elements of themselves with the other, creating a unique genetic sequence on newly formed chromosomes. These newly recombined (recombinant) chromosomes are then randomly assigned to gametes (independent assortment), then, since random mating patterns occur in nature, random fertilization allows for the random mixing of genetic material.
Four Patterns of Natural Selection
- Directional selection: shifts the overall makeup of the population by favoring variants that are one extreme of the distribution. (Bell curve shifts to either extreme)
- Disruptive selection: favors variants at either extreme (ends of distribution) - bell curve at either extreme with a “dip” in the middle
- Stabilizing selection: favors intermediate variants while removing extreme variants (bell curve in center, narrow)
- Balancing selection: such as frequency-dependent selection (fitness of phenotype depends on how common it is in the population - left or right-mouthed fish as predators, oscillating defenses), Heterozygote advantage (natural selection maintaining two or more alleles at the locus - if heterozygote is intermediate to both homozygotes, it serves as balancing selection. - sickle cell heterozygotes and malaria).
Describe sexual selection (2 different types of sexual selection) and the “good genes” hypothesis
Individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates (intrasexual selection: competition between same sex for mates, intersexual selection: mate choice).
The “good genes” hypothesis theorizes that females select males with certain desirable traits and/or behaviors because those traits are indicative of good genes in other areas, and thus will produce stronger, more viable offspring.
How are gene and allele frequencies calculated using HWE?
Adding up each allele number from the population, and the total # of alleles, and dividing the allele number by the total amount of alleles will give you allele frequency within the population. These frequencies should add up to 1 (p + q = 1)
Calculating the gene frequencies involves using the rule of multiplication: p2 = homozygote genotype frequency, q2 =homozygote genotype frequency, and 2pq = heterozygote frequency.
p2 (expected frequency of genotype) + 2pq (expected frequency of genotype) + q2 (expected homozygote genotype) = 1
Use the Hardy-Weinberg equation to determine if the population is evolving or not with respect to a specific gene.
After calculating genotype frequencies, add p2 (and q2) +1/2(2pq) = original allele frequency. If it matches what you started with, the population is in Hardy-Weinberg equilibrium. If it doesn’t match, the population is evolving.
Differentiate between how natural selection, genetic drift, and gene flow affect gene frequencies in a population
Natural selection promotes individuals with certain heritable traits that provide an advantage to a particular environment. Therefore, natural selection affects gene frequencies as it favors desirable genes in a population
Genetic drift, on the other hand, revolves around chance events that occur and will result in allele frequency changes
Gene flow affects the frequencies of alleles in a population based on the movement of fertile individuals and their gametes. They physically transport varying genetic information to new areas and mate/produce offspring thus affecting allele frequencies.
How does population relate to forces of evolution?
Genetic drift is a type of evolution that occurs in small populations (chance events diminishing a population and thus randomly selecting traits in organisms that survive) while natural selection occurs across large populations given the need for genetic variance and selective pressures on said population to show evolutionary changes.
Consequences of bottleneck and founder effect on populations?
Bottleneck: a sudden change in population that dramatically reduces its size could select certain alleles (and traits) in surviving organisms. Once these surviving organisms mate, they produce offspring with similar genetic make-ups, and thus genetic variance decreases for a long period
Founder effect: When a few individuals become isolated from the larger population and establish a new population that has a differing gene pool from the source population
What is sickle cell anemia and how has it been maintained in the population?
Sickle cell is due to a harmful mutation that causes RBC’s to become “sickle-shaped” and inefficient at carrying oxygen. The reason that it is maintained in the population is heterozygote advantage - people who are heterozygotes for this harmful allele and carry this gene are not as susceptible to malaria due to some of their RBC’s being sickled, which causes the parasite to be unable to attach.
Explain how gene flow can prevent local adaptation
Natural selection selects advantageous traits for a certain environment, thus giving the living organisms advantages in their specific environments. Gene flow between populations in terms of migration can continually re-introduce other alleles that may not be advantageous for the particular environment due to consistent mating, limiting local adaptation.
