THE GENETIC THEORY OF NATURAL SELECTION
There are two variations of color in the species- some moths are white with dark speckles, and some moths and dark with light speckles. The peppered moths live in environments with light colored trees- like birch trees.
- Individuals who are best suited for an environment survive longer, reproduce more times, and pass on their genes
Evolution by selection and inheritence
If: There is a correlation between a phenotypic trait ant the number of offspring that individuals leave to the next generation, and Then: the trait will evolve.
- Natural selection and evolation are not the same thing!
- İf selection on a trait occurs but the trait is not inherited, then evolution will not happen.
Fitness: the currency of selection An individual’s absolute fitness is the number of zygotes (offspring) produces over its lifetime. Fitness Components Selection can affect fitness at different points during the life cycle. This sketch shows the life cycle of a sexually reproducing species with four fitness components that affect the number of descendants that an individual leaves to the next generation.
- The rate of evolution is proportional to the strength of selection and the amount of genetic variation. In the absence of either of those two ingredients, there is no evolution by selection.
▪ Natural selection is any consistent difference in fitness among different phenotypes or genotypes. Evolution caused by natural selection has been observed directly many times.
▪ Fitness is the number of offspring that an individual leaves to the next generation, or the average number that an allele, genotype, or phenotype leaves. Selection causes evolution when there is a correlation between a phenotype and fitness, and a correlation for that phenotype between parents and offspring.
▪ The rate at which a beneficial allele spreads through a population is determined by how strongly it
▪ An allele that has no effect on fitness can spread if it is associated (in linkage disequilibrium) with an allele at another locus that is favored by selection. One consequence is that a selective sweep reduces genetic variation in the region of chromosome near the selected locus.
▪ Several kinds of selection can act to maintain genetic variation. One is overdominance, the situation in which heterozygotes have highest fitness. Other kinds are negative frequency dependence, multiple niche polymorphism, and spatial variation in selection.
▪ Positive frequency-dependent selection occurs when fitness increases with the frequency of genotypes or phenotypes in a population. Unlike balancing selection, this situation eliminates variation. Which allele becomes fixed depends on the initial allele frequency.
▪ With underdominance, heterozygotes have lower fitness than both homozygotes. Underdominance does not preserve genetic variation, and one of the alleles will either be lost or spread to fixation, depending on its initial frequency. An allele that is underdominant can spread when rare only if some evolutionary factor other than selection is at work.
▪ The mean fitness of a population evolves as allele frequencies change. Fisher’s fundamental theorem of natural selection states that selection causes the mean fitness to increase. In Wright’s adaptive landscape, allele frequencies change in the direction that increases mean fitness. These conclusions hold only under certain conditions. Evolution can cause a population’s mean fitness to decrease when fitnesses are frequency-dependent.
▪ Deleterious mutations occur frequently. They are maintained in populations by mutation even though selection acts to remove them. Their combined effects across the entire genome contribute to senescence.
Genetic Drift: Evolution at Random
Drift explains features of the living world that natural selection cannot. It also provides us with tools to estimate population sizes, phylogenies, and other important features of nature.
What Is Random Genetic Drift?
Genetic change between generations—that is, evolution—happens even when selection is not at work. We have already seen that mutation changes allele frequencies. Evolution also results from chance events of survival, reproduction, and inheritance. The evolutionary process that results is called genetic drift. - Fate of genes can be influenced by meiosis. When an individual is heterozygous at a locus only one allele is passed to each gamete. So even if all individuals survive and leave the same number of offspring meiosis causes random changes in allele frequencies.
Five Fundamental Features Of Drift
1. drift is unbiased: an allele frequency is as likely to go up as to go down.
2. random fluctuations in allele frequency are larger in smaller populations
3. drift causes genetic variation to be lost.
4. drift causes populations that are initially identical to become different
5. an allele can become fixed without the benefit of natural selection
The Genealogy of Genes The paths of genes inheritance across generations.
Gene tree: A diagram representing the history by which gene copies have been derived from ancestral gene copies in previous generations. Molecular clock Many genes evolve at a relatively constant rate. A constant rate of molecular evolution is called a molecular clock and it can be used to estimate the time since two species shared a common ancestor. Mitochondrial Eve : if we trace back the human mitochondrial genomes backward in time we reach a single genome-most recent common ancestor. Since mitochondria is inherited maternally the person carrying the ancestral mitochondria would necessarily be a woman-called mitochondrial Eve. Likewise tracing the ancestry of all Y chromosomes would lead back to a single male. Please note that we are talking about a group of individuals here even if we say single.
Strength of random genetic drift in a population is measured by effective population size.
• Genetic drift is stronger in smaller populations.
• If most individuals in a population are too young or too old to reproduce, then drift is stronger than it would be if all the individuals were reproductive. o Population botleneckk: A severe, temporary reduction in population size. o Founder effect: the principle that the founders of a new population carry only a fraction of the total genetic variation in the source of population. o Founder event: A population bottleneck that results when a new population is founded by a small number of individuals. Founder event like a bottleneck, reduces genetic variation.
Much of the polymorphism in DNA within species results from random genetic drift acting on selectively neutral mutations. (Please read «Drift and Genetic Variation within Species» title from your reference book.)
▪ Levels of polymorhism also vary systematically along chromosomes. Regions with high recombination rates tend to be more polymorphic.
Genetic Drift And Natural Selection
As an advantageous mutation sweeps through a population, natural selection is not the only process that determines its fate. Genetic drift adds a random component to its trajectory. As a result, the mutation’s frequency can increase more rapidly than is expected from selection alone. But sometimes drift causes its frequency to increase less rapidly, or even to decrease. The fate of beneficial mutations in large populations
• When a beneficial mutation first appears, it is present as only a single copy. The individual carrying it may leave no offspring, or if it does, the mutation may not be passed through meiosis to its offspring. A mutation that increases fitness has a much greater chance of becoming fixed than does a deleterious mutation, but its fate is still not certain.
• Genetic drift can put an evolutionary speed limit on how fast a species can adapt to changing conditions.
Neutral Theory of Molecular Evolution
The hypothesis that most alleles that are polymorphic within populations and that become fixxed do not significantly alter fitness and evolve by genetic drift.
Evolution is a two-step process:
1. Mutation (random)
2. Selection (non-random) Detrimental mutation
=> negative selection => Mutation not seen Beneficial mutation => positive selection => Mutation seen
Selectionist Views of What Drives Molecular Evolution
• Majority of all mutations are detrimental and not seen
• Most observed substitutions have adaptive value
• Classical school:
- Single predominant version of gene ("wild type") present in population
- Natural selection rapidly fixates new, advantageous mutations
• Balance school:
- Appreciable amount of polymorphism in gene pool
- Polymorphism maintained actively by natural selection (e g. sickle cell anemia) Neutralist Views of What Drives Molecular Evolution
• Electrophoretic studies in 1960's showed much higher polymorphism than anticipated by either oclassical or balance school selectionists
• Kimura and others proposed the "Neutral Theory of Molecular Evolution".
Difference Between Selectionist and Neutralist Views of Evolution
• Selectionist view:
- Most observed mutations represent functional innovation
• Neutralist view:
- Most observed mutations represent conservative changes, changes in unimportant regions.om mutations assumed to be deleterious, neutral, and advantageous
