Chapter 17 Evolutionary Processes Flashcards
Explain the statement “it is the population, not the individual, that evolves.”
- Evolution involves a change in allele frequencies in a population.
- Natural selection increases the frequency of certain alleles that contribute to increased survival and reproduction, i.e. increased fitness
Distinguish between natural selection, genetic drift, gene flow, and mutation
- Natural selection increases the frequency of certain alleles that contribute to increased survival and reproduction, i.e. increased fitness
- Natural selection is not the only process that can bring about changes in the allele frequencies within populations; there are 3 additional processes:
- Genetic drift
• Involves random changes/fluctuations in allele frequencies. - Gene flow
• Produces allele frequency changes when new individuals enter an existing population or individuals leave an existing population for elsewhere. - Mutation
• Modifies allele frequencies by continually introducing new alleles that may be either detrimental, beneficial, or have no effect on fitness.
Define the terms “population” and “gene pool”.
What is a population?
A group of individuals of the same species in the same area that regularly interbreed.
Gene Pool?
The total of all the alleles for all of genes for all of the individuals in a population is referred to as the population’s gene pool.
List the five conditions that must be met for a population to remain in Hardy -Weinberg equilibrium.
- No mutations: Alteration of alleles, or deletion or duplication of genes, will modify the gene pool.
- Random mating with respect to the locus (gene) in question: If individuals pick mates with certain genotypes, or preferentially mate within a subset of the population, the mixing of gametes will not be random and genotype
frequencies will change. - No natural selection: Differential survival or reproductive success among genotypes will alter allele frequencies.
- Extremely large population size: In small populations, chance fluctuations in the gene pool will cause allele frequencies to change over time. These random changes are called genetic drift.
- No gene flow: Gene flow, the transfer of alleles due to the migration of individuals or gametes between populations, will change the frequencies of alleles.
Explain the following statement: “Only natural selection leads to the adaptation of organisms to their environment.”
Natural selection consistently increases the frequency of alleles that provide survival and reproductive advantages and therefore leads to adaptive evolution.
Each of the 4 evolutionary processes has different consequences (i.e. distinct effects on the genetic composition of populations); however, only natural selection lead to adaptation.
Define locus
a particular position, point, or place.
Directional selection
- Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range.
- Shifts a population’s frequency curve in one direction or the other, changing the average value of the trait.
- Most common during periods of environmental change or when members of a population migrate to a new habitat with different environmental conditions.
- Tends to reduce the genetic diversity in populations by favoring one extreme over another.
- If directional selection continues over time, then the favored alleles will eventually reach a frequency of 1 (are “fixed”), while the disadvantageous alleles will reach a frequency of 0 (are “lost”).
- When disadvantageous alleles decline in frequency = purifying selection.
- HOWEVER:
- Directional selection is rarely constant throughout a species’ range and throughout time – does not always result in fixation + loss.
- Common to find that one cause of directional selection in a trait is counterbalanced by a different factor that causes selection in the opposite direction for the same trait (think trade-offs!).
- Example: Larger swallow body size better for storing fat, but smaller body size more maneuverable for catching insects in flight.
Explain disruptive selection and give an example
- Disruptive selection occurs when conditions favor individuals at both extremes of the phenotypic range over those with intermediate phenotypes.
- Relatively rare.
- Sometimes plays a role in speciation (later in lecture).
Example: Large bills more efficient for hard seeds where as small bills fore efficient for soft seeds, intermediate bills relatively inefficient for both
Explain stabilizing selection and give an example
- Stabilizing selection, in contrast to disruptive selection, favors intermediate variants and acts against extreme phenotypes.
- Reduces genetic variation.
- No change in average value of trait over time.
Example: very small and very large babies are more likely to die leading to narrower distribution of birth weights
Define sexual selection
- Sexual selection is a special case of natural selection.
- Occurs when individuals within a population differ in their ability to attract mates.
- Favors individuals with heritable traits that enhance their ability to obtain mates.
Explain why sexual selection acts more strongly on males than on females.
• Tends to act on males much more strongly than females, and as a result traits that attract mates are more elaborate in males than in females.
Because…
• Eggs are large and expensive
• A female’s fitness is primarily limited by her ability to gain the resources needed to produce more eggs and healthier young – not by the ability to attract a mate.
• Sperm are small and cheap
• A male’s fitness is not limited by the ability to acquire resources needed to produce sperm, but by the number of females he can mate with.
• Females invest more in their offspring than males do - should protect that investment by being choosy about their mates.
—-> A female tries to select a mate that will confer a fitness advantage on their mutual offspring.
• If males invests little in offspring, should be more willing to mate with almost any female.
—–> If there are an equal number of males and females in the population, and males are trying to mate with every female possible, then males will have to compete for mates.
Distinguish between intrasexual selection (i.e., mate choice) and intersexual selection (i.e., male-male competition), providing an example for each.
• Intersexual selection (mate choice) occurs when members of one sex (usually females) are choosy in selecting their mates from individuals of the other sex.
intErsexual = EVERYONE
• In many cases, the female chooses a male based on his
showy appearance or behavior. Why would females show
this preference?
• “Good gene hypothesis”
• Trait preferred by females is indicative of a male’s health.
• Example: Colorful feathers or beaks may suggest that the individual is well nourished and healthy.
• By choosing a male that is colorful, a female is choosing a male with alleles that enable it to acquire nutrients and fight off disease, and will likely have offspring with
the same qualities.
• Example: Elaborate songs and dances and other courtship displays send the message “I’m healthy and I’m well fed, because I have good alleles. Mate with me!”
• Females may also choose mates on the basis of parental care or resources provided by the male.
• Several species of female prefer to mate with males that protect a nest site and care for eggs until they hatch.
• Key point: Every time a female chooses a mate based on appearance or behavior…
1. She perpetuates (preserve from extinction) the alleles that caused her to make that choice.
2. She enables a male with that particular phenotype to perpetuate his alleles.
Distinguish between intrasexual selection (i.e., mate choice) and intersexual selection (i.e., male-male competition), providing an example for each continued..
• Intrasexual selection involves direct competition among
individuals of one sex for mates of the opposite sex.
intrAsexual = AMOUNG US
• Competition may take the form of direct physical battles between individuals, usually males.
• Example: a single male may patrol a group of females and prevent other males from mating with them by defeating them in combat.
• Males that lose fights produce fewer offspring alleles of territory owning males increase in frequency in population.
Explain one hypothesis for why females may show preferences for showy males.
• “Good gene hypothesis”
• Trait preferred by females is indicative of a male’s health.
• Example: Colorful feathers or beaks may suggest that the individual is well nourished and healthy.
• By choosing a male that is colorful, a female is choosing a male with alleles that enable it to acquire nutrients and fight off disease, and will likely have offspring with
the same qualities.
• Example: Elaborate songs and dances and other courtship displays send the message “I’m healthy and I’m well fed, because I have good alleles. Mate with me!”
Define sexual dimorphism.
Marked differences between the traits of males and females that are not directly associated with reproduction.
(google definition: distinct difference in size or appearance between the sexes of an animal in addition to difference between the sexual organs themselves.)
