Lecture 21: Sexual selection, Genetic Drift, Gene Flow, Quiz Q's

Allele frequency

% or proportion of a specific allele of a gene in a population of organisms Frequency of "a" allele formula: # of homozygous individuals (aa) × 2) + (# of heterozygous individuals (Aa)) ÷ total # of alleles *Note: x individuals = 2(x) total alleles (double) -Allele frequencies= Positive selection, negative selection and balancing selection -Phenotype frequencies= Directional selection, Disruptive selection and Stabilizing selection

Give an example of species affected by a founder effect and a population bottleneck

A species affected by founder effect is the house sparrow. Around 100 birds were brought to Brooklyn in 1851, and those birds colonized America. Florida panthers were affected by bottlenecks when their wild population was reduced to less than 100 individuals.

maladaptive

A trait that reduces the fitness of individuals.

Founder effect/effect

A type of genetic drift that occurs when only a few individuals establish a new population -New population is likely to have different allele frequencies than source population, by chance -Bottlenecked population is likely to have different allele frequencies than original population, by chance

Under what circumstances can alleles go to fixation, where 100% of the population has the same allele?

Genetic drift can cause beneficial, deleterious, or neutral alleles to become fixed. Directional selection can lead to fixation of alleles that confer a selective advantage. A catastrophic event can lead to a bottleneck effect, where the surviving population is fixed for an allele that was variable in the original population.

sexual selection vs. natural selection

Sexual selection: promotes traits that increase an individual's access to reproductive opportunities (ability to obtain a mate). Natural selection: promotes traits that enhance an individual's fitness.

Consider the example of sickle-cell anemia and the allele for the sickle-cell trait, and answer the following. True or false: This is an example of balancing selection, which will maintain genetic variation at this locus (the position of a gene on a chromosome).

TRUE

Red coloration in wheat plants is a complex trait influenced by three unlinked genes, each with two different alleles: A and a; B and b; C and c. This Punnett square shows the results of a mating between two plants that are both heterozygous at all three loci. Use the Punnett square to answer the following question. True or false: Each dominant allele, represented by an uppercase letter, contributes to the red coloration.

TRUE

Red coloration in wheat plants is a complex trait influenced by three unlinked genes, each with two different alleles: A and a; B and b; C and c. This Punnett square shows the results of a mating between two plants that are both heterozygous at all three loci. Use the Punnett square to answer the following question. True or false: The recessive alleles do not contribute to the red coloration of the seed casing.

TRUE

A storm washed five female and five male tiger beetles from the mainland to a small island that had no previous population of tiger beetles. In tiger beetles, having six spots is dominant to having four spots. All ten beetles had six spots, but one male and one female were heterozygous for the four-spot trait. If six of the beetles died randomly without reproducing, the ratio of the genotypes could be quite different in subsequent generations. This change in allele frequencies would result from which process?

genetic drift

Male peacocks have tail feathers that make up about 60% of their body length. During the mating season they shake these feathers in front of females. Longer feathers produce a larger volume of rustling sound for a given shaking rate, but they are also heavier, and thus require more energy to shake at a given rate than shorter feathers. This suggests that:

there are selective pressures against too long or too short of a tail, which should result in stabilizing selection on tail length.

All of the human phenotypes listed below are determined at least partially by genotype. Which trait is most likely to be influenced by the environment as well?

weight

Trade-off

an exchange in which something is gained at the expense of something lost

Gene pool

collection of all the genes of individuals in a population

Fixed allele

describes a population that exhibits only one allele at a particular gene

The graph can look fixed, lost, or in-between because ...

drift is the result of only a small fraction of the population reproducing in each generation, which can cause large or small changes in allele frequencies over time.

Which of the following is NOT a form of intrasexual selection?

female choice

As a biologist, you are studying a population of moles. In the course of your research, you identify a nearby population of the same species of moles that occasionally comes into contact and interbreeds with individuals in your study population. What best describes the evolutionary process at work?

gene flow

Which types of selection tend to maintain the diversity of alleles in a population with respect to a given gene (or genes), over many generations?

- Balancing selection - Disruptive natural selection - Disruptive sexual selection

Evolution of Color Patterns in Male Guppies

- Enormous variation in color patterns - mosaic of spots/patches vary with respect to size, color & number of spots - color genes only expressed in males (sexual dimorphism) - in some populations, males are typically drab; in others, they are highly conspicuous Hypotheses - colors evolved through natural selection - camouflage males favored by predator avoidance (natural selection) - females favor brightly colored males (sexual selection) - trade-off bw mate attraction and predator avoidance Results: with fine gravel and no predators, males with large spots have higher fitness bc they are noticed by females. With fine gravel and predators, males that blend in with their environment have higher fitness

Sexes can differ in morphology, behavior, and/or physiology

-Morphology -Behavior -Physiology (includes differences in hormones and brain structure) Males and females differ in more than genitalia

In Drosophila melanogaster (fruit flies), genes controlling eye development and eye color are both located on the X chromosome. Assume that these genes are 1.5 cM apart.

98.5% of the gametes have an X chromosome with the same combination of alleles for these two genes as one of the X chromosomes in the parental female's somatic cells. 1.5% of the gametes have a recombinant genotype

Explain how natural and sexual selection can cause fitness trade-offs

A common example of a trade-off between natural and sexual selection is the peacock's tail. It is large, energetically expensive, and attractive to predators. However, peacocks with larger tails are more likely to mate than those with small tails. This results in larger tails due to sexual selection but smaller tails due to natural selection.

Intrasexual selection

A form of sexual selection Competition within a single sex (usually the males) that compete with one another for access to the other sex (usually the females) --compete w/males for traits (e.g. large body size, weapons - tusks or horns) -often produces more than one successful strategy, especially for males, i.e., selection may produce distinct male morphs within populations -can lead to sexual dimorphism in species

Intersexual selection

A form of sexual selection Females choose their male mate --females choose based on traits (colors, patterns, ornamentation) -can lead to sexual dimorphism in species

Give the frequency at which an allele is considered "fixed" within a population and the frequency at which an allele is considered "lost" or "eliminated" from a population

An allele is fixed if its frequency is 1. An allele is lost or eliminated if its frequency is 0.

Bottleneck

An extreme, usually temporary, reduction in population size that may result in marked loss of genetic diversity and, in the process, genetic drift - Affect genetic variation and species survival

Explain how anisogamy leads to the "fundamental asymmetry of sex" and how this influences male and female mating behaviors

Anisogamy is sexual reproduction by fusion of dissimilar gametes. This leads to the "fundamental asymmetry of sex" because females invest more time, energy, and resources into raising their offspring, and thus are choosier about their mating partners

A population of ground beetles is isolated on a mountain top after a volcanic eruption destroys populations that used to live in the valleys. In this isolated mountain population, males prefer to mate with females who have purple eyes. Females mate readily with males of any eye color. Gene E determines eye color: EE homozygotes have red eyes, heterozygotes have purple eyes, and ee homozygotes have blue eyes. What type of selection is acting on alleles of the E gene in the ground beetle population living on the mountain top?

Balancing selection Sexual selection

Which of the statement(s) below is/are accurate? Choose all that apply.

Biologists have conducted thousands of experimental studies on many different organisms that demonstrate evolution occurring in modern populations. Biologists are investigating and further elucidating the processes that produce evolutionary change and the consequences of that change. Biological evolution is still an active area of science and many questions remain unanswered.

Sexual dimorphism

Differences between the sexes in traits often related to attracting & obtaining mates Sexual selection is generally stronger in males, thus sexually selected traits should be more common in males -males often have exaggerated traits that they use in gifting or courtship

Evolutionary theory predicts that alleles causing dominant disorders will be found at lower frequency in a population than recessive disorders (assuming all else is equal). What evolutionary force explains this difference?

Directional selection will more effectively remove deleterious dominant alleles

Explain how directional, stabilizing, and disruptive selection can be applied to sexual selection.

Directional selection would select one trait over another, such as large peacock tails with many eye spots. Stabilizing selection would select for the average trait, such as for female ornaments in a dance fly. Disruptive selection selects for two extremes, such as large males and small, sneaker males.

Which of the following statements about evolution by natural selection is most accurate?

Evolution by natural selection depends upon unequal survival and reproduction of individuals expressing different heritable traits.

Consider the example of sickle-cell anemia and the allele for the sickle-cell trait, and answer the following. True or false: Eventually, natural selection will result in the sickle-cell allele being fixed in the global human population.

FALSE

Genetic drift, acting alone, cannot change allele frequencies in a population. Other evolutionary processes are required to change allele frequencies.

FALSE

You begin an experiment with 2 populations of E. coli that are each composed of 100 cells. The cells are all genetically identical (i.e., they are clones). You grow these populations in flasks on a lab bench under identical conditions with unlimited resources. Is the following statement about the populations true or false? After 100 generations both populations will still be genetically the same, having accumulated the same mutations over time.

FALSE

Which of the following statements about evolutionary processes is ACCURATE?

Genetic drift and gene flow can increase or decrease the mean fitness of the population.

Runaway sexual selection

Female have a preference making the trait advantageous, making the preference for the trait advantageous so you end up with extreme phenotypes Ex. large antlers - they got so big the male couldn't hold his head up

Anisogamy

Gametes of different sizes -Males usually compete more intensely for mates than do females and females are choosier about their mates than are males -Males make many small gametes -Females make fewer, large gametes -Females get to choose, they are more invested --eggs are energetically expensive -Fundamental asymmetry of sex --male-male completion --female mate choice Can lead to intra & intersexual reproduction

Gene flow vs Genetic drift

Gene flow: -requires at least 2 populations of the same species and migration between the 2 populations -can increase or decrease genetic variation in a pop. and this depends on the genotype of the migrating individual Genetic drift: -decreases genetic variation over time -can lead to fixation or loss of a neutral, advantageous, or deleterious allele -has a bigger impact on allele frequencies in small pop. (but occurs in all populations)

INTRAsexual vs. INTERsexual selection

INTRAsexual selection: focuses on interactions between individuals of the same sex (male on male competition). INTERsexual selection: focuses on interactions between individuals of opposite sexes (female's choice).

What does it mean to say that an allele is "fixed" in the population?

It is an indication of no genetic variation at that locus in the population.

Why is it thought that the majority of natural selection is stabilizing selection?

Many mutations are deleterious and produce a changed phenotype that is selected against.

The non-adaptive mechanisms of evolutionary change are:

Migration (gene flow) Mutation Genetic drift

Genetic drift vs. Gene flow vs. Mutation & Genetic Variation

Mutation is a spontaneous change in the DNA sequence of an allele; it increases genetic variation. Gene flow is the movement of alleles from one population to another; it can increase or decrease genetic variation. Genetic drift is the random change in allele frequencies from generation to generation; it decreases genetic variation.

Explain the causes and consequences of stronger sexual selection in males than females

Sexual selection is stronger in males than in females because it is often the males that fight for the females' attention. This results in sexual dimorphism.

In Drosophila melanogaster (fruit flies), genes controlling eye development and eye color are both located on the X chromosome. Assume that these genes are 1.5 cM apart. Wild-type Drosophila melanogaster (fruit flies) have red eyes. When a mutation disrupts the function of the white gene, flies have white eyes. This type of mutant white allele (Xwh) is recessive to the wild-type allele of the white gene (XWH). A white-eyed female is crossed with a white eyed male; both have eyes and are true-breeding for the eyeless gene. Which of the following are accurate genotypes of the parents or possible genotypes of the offspring or possible phenotypes of the offspring? There are 4 correct answers.

Parental genotype: YXwh Offspring genotype: YXwh Offspring phenotype: female with white eyes Parental genotype: XwhXwh

Sexual selection

Process by which individuals that possess certain heritable traits are more successful at attracting & obtaining mates and thus reproduce at a higher rate relative to other individuals in the population - Anisogamy - Intra & Inter - Tradeoffs between natural & sexual selection Sexual selection is generally stronger on males, thus sexually selected traits should be more common in males. Males often have exaggerated traits that they use in fighting or courtship.

Genetic drift

Random change in allele frequencies from generation to generation. By "random," we mean that frequencies go up or down simply by chance -Dramatically affects small populations -Genetic drift often results from population bottlenecks -Leads to allele frequency changes and, in turn, to evolution --leads to loss or fixation of an allele -Does NOT lead to adaptations because the alleles whose frequencies are changing as a result of drift do not affect an individual's ability to survive or reproduce -Impact depends on pop. size -Reduces genetic diversity

Mutation

Rare event Introduces new alleles into a population; already addressed in prior lectures) Generally not important as an evolutionary mechanism that leads allele frequencies to change It is the source of new alleles and genetic variation

One type of natural selection is balancing selection. One scenario that can lead to balancing selection is when a heterozygote phenotype has a fitness advantage. Which of the following are reasonable predictions of balancing selection due to heterozygote advantage? Assume that after 10 generations under balancing selection, the f(A1) = 0.5, where "A1" is one of two alleles of a given gene; "A2" is the other allele.

The f(A1) will remain roughly the same over the following generations.

Gene flow

The movement of alleles from one population to another through interbreeding between members of each population. -Happens when you have migration followed by interbreeding Alleles entering and/or leaving a population

Migration

The movement of organisms from one place to another, including the movement of individuals from one population to another. -Gene flow between populations/change allele frequencies within a population -Results in gene flow -Makes connected populations have more similar allele frequencies --works to homogenize -Can increase or decrease genetic variation -Consequence: homogenization of populations, making them more similar to each other --reducing genetic differences between them

When a founder population colonizes a new habitat or when a population undergoes a bottleneck, genetic drift can dramatically change allele frequencies in the resulting population. This dramatic change happens because:

The population size is very small

Adaptation

the fit between an organism and its environment

In one species of jumping spider, male courtship behaviors are variable. Researchers hypothesized that either genetic drift or intersexual selection could be responsible for variation in male courtship behaviors. What data would allow researchers to determine whether genetic drift or intersexual selection was influencing courtship behavior?

You measure: i) the amount of time between the beginning of male courtship and the time of acceptance by the female and ii) the fitness of males with different courtship behaviors. If male fitness and courtship time are correlated, that would indicate intersexual selection is occurring.

Fitness

a measure of the extent to which an individual's genotype is represented in the next generation

Choose all that apply. The bottleneck effect (which follows a bottleneck event) is usually observed:

after a catastrophic event, such as a storm or flood, which kills many individuals. in extremely small populations.

Deleterious alleles can...

increase in frequency in small populations due to genetic drift, and inbreeding within a small population increases the frequency of individuals homozygous for recessive alleles. Together, these two processes cause individuals in small populations to be more likely to develop problems that can reduce their chance of survival and reproduction.

Contrast the impact of genetic drift on genetic variation in large and small populations

large populations: -genetic drift has a smaller effect -variation -50/50 Small populations: -genetic drift is most prominent -variation -alleles are either fixed or lost due to genetic drift

Reproductive success (fitness) is...

more variable among males than females

Immigration

moving into a population

Emigration

moving out of a population

Consider two isolated populations of lady slipper orchids. These populations are hundreds of miles apart; one is in Northern Minnesota and the other is in the Upper Peninsula of Michigan. They are so far apart that pollinators cannot possibly transfer pollen between the two populations. Based on this information, you expect there to be _________________ between these two populations.

no gene flow

Calculate allele frequencies of a population

ok

In northern elephant seals, body size is determined by at least 8 genes. Males compete for territories on the beach and larger males are more likely to win fights. In this example, inheritance of body size is ________________, intrasexual selection on male body size is ____________________, which results in _______________ in mean male body size in subsequent generations.

polygenic / directional / increases

Genetic drift has an especially significant effect in what kind of populations?

populations that are small

Which of the following IS a form of intrasexual selection?

ritualized displays male combat

Ferrets are unable to withstand long periods of cold weather and must hibernate throughout much of the winter. Hibernating sites are in short supply, and ferrets only survive if they can find holes in trees that match their body size. Very small tree holes are in short supply because they are generally colonized by stinging wasps that chase away ferrets. Very large tree holes are in short supply because they are colonized by snakes that eat ferrets. The limited supply of tree holes should result in:

stabilizing selection on ferret body size

With stabilizing selection, you would predict the mean value of a trait to ____________ and the genetic variation in the trait value to _____________.

stay the same; decrease