Lecture 10: Natural Selection Flashcards
non-adaptive mechanisms of evolution (3)
- Genetic drift
- Migration
- Mutation
an adaptive mechanism of evolution
Natural Selection
What was Darwin’s contribution regarding population speciation?
Natural selection driving population speciation.
Why do organisms produce too many offspring?
To increase chances of survival in competitive environments.
What causes limited resources and competition in populations?
Scarce environmental resources lead to competition among individuals.
What is the significance of variation in a population?
It provides the raw material for natural selection to act upon.
How do better-adapted individuals fare in natural selection?
They are more likely to survive and reproduce.
What happens to the average character of a population over time due to natural selection?
It changes as better-adapted traits become more prevalent.
What does natural selection entail?
Differential survival and reproduction of genetically-based traits.
What determines the fitness of different phenotypes in natural selection?
Their suitability to the environment and their heritability.
Natural selection leads to __, NOT random, but __
- adaptation
- deterministic (favoring traits best suited to the environment)
What distinguishes traits subject to natural selection from plastic phenotypes?
Genetically-based inheritance, rather than environmental influence.
Traits subject to natural selection are distinguished from __ __ primarily by their mode of inheritance. Traits subject to natural selection are primarily determined by __-__ __.
- plastic phenotypes
- genetically-based inheritance
important factor in natural selection
fitness
An organism’s ability to survive and reproduce in a particular environment
Fitness
Higher fitness means that greater numbers of __ survive in the next generation
offspring
What happens when mutations arise in the reverse transcriptase gene of HIV?
The enzyme gains the ability to recognize and evade the effects of AZT.
How do drugs like AZT (Azidothymidine) impose selection on viruses?
They create pressure favoring mutations that confer resistance.
What role does genetic variation play in the evolution of drug resistance in viruses?
It allows the virus to respond to selection pressures by generating mutations that confer resistance.
How does natural selection act in the presence of AZT?
It favors the survival and reproduction of HIV variants that can resist the effects of the drug.
How can the direction of selection affect the evolution of virulence?
It can change with shifts in the environment.
Phenomenon: Traits that enhance a pathogen’s fitness in one environment or under certain conditions may be disadvantageous in others, leading to shifts in the direction of selection.
Evolutionary tradeoffs
Selection imposed by Transmission Rate on Virulence of HIV
Explain the evolutionary tradeoff: Need to keep host alive long enough to get passed on to the next host
Evolutionary tradeoff between fast population growth and keeping the host alive
High Transmission rate : __ __
High Virulence
Low Transmission Rate : __ __
Low Virulence
What advantage does a virus with a high transmission rate have?
It is more likely to spread to new hosts efficiently.
Why does natural selection favor more virulent strains in viruses that have a high transmission rate?
- Faster-growing and more virulent strains are likely to outcompete slower strains, especially if transmission to new hosts is common.
- It’s ok to kill the host since the chances of jumping to a new host are high
Why does natural selection favor less virulent strains in viruses that have a low transmission rate?
It’s not ok to kill the host since the chances of jumping to a new host are low. If the virus kills the host, it will kill itself
What are alternative reasons for trait variation besides adaptation?
Phenotypic plasticity or nonadaptive genetic causes like genetic drift or linkage.
How does phenotypic plasticity contribute to trait variation?
It allows organisms to produce different phenotypes in response to environmental cues, even without genetic changes.
What could cause phenotypic change that is NOT due to genetic changes?
Environmental factors or phenotypic plasticity
What could cause phenotypic changes that are due to genetic changes, but NOT due to selection (adaptation)?
Genetic drift, mutation, linkage
Definition: Experiment where individuals from different genetic backgrounds are grown in a common environment.
Purpose: Minimize environmental variation to assess differences in traits attributed to genetic differences.
Method: Compare traits of individuals from different genetic backgrounds under identical conditions.
Outcome: Differences observed are attributed to genetics rather than environment.
Common garden experiment
Definition: Experiment where individuals from different genetic backgrounds are exchanged between different environments.
Purpose: Assess how genetic backgrounds respond to different environments.
Method: Individuals from one habitat are transplanted to another, and vice versa.
Outcome: Performance in native vs. foreign environments determines the influence of genetics vs. environment.
Reciprocal transplant experiment
What does adaptation require? (3)
- natural selection
- polymorphism in a population
- an effect on fitness
refers to the presence of multiple forms of a trait within a population.
polymorphism
a frequency (%) change in a population due to selective forces.
adaptation
any factor in the environment that affects the survival and reproduction of individuals with certain traits.
selective force
How can you tell if a trait evolved as a result of adaptation (and due to natural selection)? (3)
(1) The trait must be heritable.
(2) The differences between populations are genetically based (in the genetic code) differences rather than inducible differences (plasticity).
(3) The trait has fitness consequences (promotes survival, performance, and number of offspring).
refers to differences in phenotype that a genotype exhibits across a range of environments, due to changes in gene expression.
phenotypic plasticity
What causes changes in gene expression in phenotypic plasticity?
environmental cues instigating signal transduction, as well as by epigenetic modifications.
Can you provide examples of traits with a plastic component? (5)
- intelligence
- height
- temperature tolerance
- salinity tolerance
- muscle mass.
refers to the reversible physiological or behavioral changes in response to environmental fluctuations within an organism’s lifetime.
acclimation
What is the result of acclimation?
phenotypic plasticity
Is acclimation heritable?
No, acclimation is not heritable, except in cases of epigenetic inheritance.
What is the temporal scope of acclimation?
short-term or developmental responses within a single generation.
How does acclimation arise compared to adaptation?
Acclimation arises through differential gene expression or other regulatory mechanisms, whereas adaptation arises through natural selection.
With selection, what matters is the __ __ of different genotypes.
relative fitness
Recessive deleterious alleles are harder to remove by negative selection because their phenotype is hidden in the __ state
heterozygous
Dominant beneficial alleles are more difficult to drive to fixation by positive selection because they are dragging __ along in the __ state.
- recessive alleles
- heterozygous
if recessive beneficial alleles are very rare they will take longer to fix because most will be in the __ (rather than __ state)
- heterozygous
- homozygous
What is the impact of selection on recessive alleles?
- As a recessive allele becomes rare, the rate of its disappearance slows down.
- Homozygous recessive alleles become rare, and most are in the heterozygous state, masked from selection
What is the impact of selection on dominant alleles?
A dominant allele disfavored by selection is removed quickly from the population.
Modes of selection on Loci (3)
- Positive selection
- Negative selection (Purifying selection)
- Balancing selection
Selection favoring an allele or a heritable trait
positive selection
Selection disfavoring an allele or a heritable trait
Negative selection (purifying selection)
type of selection that acts to maintain genetic variation in a population
Balancing Selection (on discrete or continuous traits)
Different mechanisms of balancing selection (3)
- Fluctuating selection
- selection favoring heterozygote (overdominance)
- frequency-dependent selection
Examples of balancing selection: (3)
■ Selection for heterozygotes (sickle cell anemia, and cystic fibrosis)
■ Selection for different traits in different environments
■ Selection for different traits at different times (fluctuating selection)
genetic diversity decreases as the population stabilizes on a particular trait value-phenotype (often a multilocus quantitative trait, like body size, height, clutch number, finger number, intermediate color –typically of a multilocus nature)
Stabilizing selection
In sickle cell anemia, what genotype is favored in the presence of malaria and why?
What mode of selection is exemplified by the presence of malaria and the favoring of the heterozygous genotype?
- the heterozygous genotype (HAHs) is favored due to the protective effect against malaria conferred by the presence of one normal and one mutant allele.
- balancing selection or overdominance, as both alleles are maintained in the population at relatively high frequencies due to the heterozygote advantage.
In sickle cell anemia, what genotype is favored in the absence of malaria and why?
What mode of selection is exemplified by the absence of malaria and the favoring of the homozygous genotype with two copies of the normal allele?
- the normal homozygous genotype (HAHA) is favored because individuals with two copies of the normal allele do not suffer from sickle cell anemia and have normal hemoglobin function.
- directional selection, as there is a clear advantage for one extreme of the allelic spectrum.
The __ __-__ __ (__) is disfavored in malaria-endemic regions because it lacks the protective effect against malaria. Without the resistance to malaria conferred by the abnormal hemoglobin, individuals with this genotype are at a disadvantage in areas where malaria is prevalent.
homozygous wild-type genotype (HAHA)
The __ __ __ (__) is disfavored because it is lethal. Individuals with this genotype typically do not survive to reproductive age due to severe health complications associated with sickle cell disease.
homozygous mutant genotype (HsHs)
What is sexual selection for heterozygosity at MHC loci?
Sexual selection for heterozygosity at Major Histocompatibility Complex (MHC) loci influences mating preferences, often mediated by preferences based on body odor.
How is mating preference influenced by MHC in various species?
Studies on rodents, fish, and humans have reported a tendency to prefer MHC-dissimilar mates.
What is the evolutionary significance of preferring MHC-dissimilar mates?
Preferring MHC-dissimilar mates is a form of sexual selection that favors the production of MHC-heterozygous offspring. These offspring are more resistant to pathogens, representing a type of balancing selection that favors heterozygotes.
Modes of selection on traits (3)
- directional selection
- stabilizing selection
- disruptive selection
A character that falls into distinct, non-overlapping categories or classes.
- These traits are often controlled by one or a few genes (loci) with distinct alleles.
Discrete traits
Selection can act directly on the genotypes underlying __ traits, favoring certain alleles over others.
discrete
A character that exhibits a range of values along a continuum without clear-cut boundaries.
- These traits are usually controlled by multiple genes (polygenic inheritance) and influenced by environmental factors.
Continuous traits
Selection on __ traits leads to changes in the distribution of phenotypic values within a population.
continuous
What are continuous traits encoded by?
Continuous traits, such as beak shape, body size, and hair color, are encoded by many loci and are called “quantitative traits.”
Can you provide an example of a discrete trait?
Mendel’s yellow vs. green peas.
Selection favors an extreme phenotype and causes a shift in allele frequencies toward that direction
directional selection
Examples of directional selection (3)
- Drug-resistance in tuberculosis and HIV
- Agriculture: artificial selection for specific traits
- Sexual selection for a trait (e.g. for bright plumage, peacock tail)
■ Selection favoring intermediate trait values
■ The average trait value stays the same
■ Genetic diversity is reduced at the relevant loci
Stabilizing Selection
Examples of stabilizing selection (3)
- Selection for an optimal number of fingers
- Selection for optimal body size
- Selection for an optimal number of offspring
■ Selection favors the extremes
■ Genetic diversity is increased (favor novel beneficial
alleles; knock out alleles that code for intermediate traits)
■ Can lead to formation of new species
Disruptive selection
Examples of disruptive selection (3)
■ Niche Partitioning (Specialization on different resources, food)
■ Differences in habitat use
■ Sexual selection for different traits (blue birds mate with blue, red birds mate with red)—intermediate colors selected against
Why is the selection pressure imposed by drugs like AZT considered directional?
drug selects for HIV variants with mutations that allow them to resist its effects, resulting in a directional shift towards drug resistance.
created the world’s first gene edited babies to have the delta32 allele
He Jiankui
Sexual Directional Selection
- Most of the __ do not mate, so their genotypes (and phenotypes) are removed from the population
- genetic diversity reduced at the relevant loci
males
also known as the dN/dS ratio or the ratio of non-synonymous to synonymous substitutions, is a simple and widely used method for detecting natural selection at the molecular level.
Ka/Ks test
What does Ka represent? Ks?
- Nonsynonymous substitution rate
- Synonymous substitution rate
What does a Ka/Ks ratio greater than 1 indicate?
Positive selection - indicating a higher rate of amino acid-changing mutations compared to neutral mutations.
What does a Ka/Ks ratio equal to 1 suggest?
Neutral evolution - indicating similar rates of nonsynonymous and synonymous substitutions, expected in the absence of selection pressures.
What does a Ka/Ks ratio less than 1 imply?
Purifying selection - indicating a higher rate of synonymous substitutions compared to nonsynonymous substitutions, suggesting conservation of amino acid sequences.
