Biology Chapter 23: Evolution of Populations

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is microevolution?

Evolution on its smallest scale: the change in allele frequencies in a population over the generations.

What are the three main mechanisms that cause changes in allele frequencies?

Natural selection, genetic drift, gene flow.

What is the only mechanism that is adaptive, improving the match between organisms and their environment?

Natural selection.

Define genetic drift:

Chance events that alter allele frequencies. Signup and view all the answers

Define gene flow:

Transfer of alleles between populations due to the movement of fertile individuals or their gametes. Signup and view all the answers

What are discrete characters?

When there's only two options for a trait. Signup and view all the answers

What are quantitative characters?

Characters that vary along a continuum in a population. Signup and view all the answers

What are two ways of measuring genetic variation in a population, using molecular biology?

Gene variability and nucleotide variability. Signup and view all the answers

What's a cline, what produces it, and why does it suggest natural selection?

Cline is the geographic axis along which genetic variation is graded; external factors produce it. Signup and view all the answers

What is frequency-dependent selection?

The fitness of a phenotype declines if it becomes too common in the population. Signup and view all the answers

What is neutral variation?

Nucleotide differences in noncoding DNA. Signup and view all the answers

Gene variability (average heterozygosity) tends to be greater than _________ because...

nucleotide variability. Signup and view all the answers

When phenotypic effects are neutral, the variation between the populations appears to have resulted from _______

chance events. Signup and view all the answers

Clines probably result from?

Natural selection. Signup and view all the answers

What are the four key points of genetic drift?

<ol> <li>Significant in small populations 2. Can cause allele frequencies to change at random 3. Can cause loss of genetic variation 4. Can cause harmful alleles to become fixed or lost.</li> </ol> Signup and view all the answers

What is translocation, and how could it be beneficial?

The moving of part of one chromosome to another. Signup and view all the answers

How does gene duplication occur and how might it play a role in evolution?

Caused by errors in meiosis. Signup and view all the answers

What are three mechanisms by which sexual reproduction shuffles existing alleles?

Crossing over, independent assortment, random fertilization. Signup and view all the answers

What is a population?

A group of individuals of the same species that live in the same area and interbreed. Signup and view all the answers

What is a gene pool?

All the alleles for all the loci in all individuals of the population. Signup and view all the answers

Define fixed alleles:

When only one allele exists for a particular locus in the population. Signup and view all the answers

What does the Hardy-Weinberg principle state?

That the frequencies of alleles and genotype in a population will remain constant from generation to generation. Signup and view all the answers

What are the five principles of Hardy-Weinberg equilibrium?

<ol> <li>No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow.</li> </ol> Signup and view all the answers

Equations for Hardy-Weinberg equilibrium and what do the parts mean?

p^2 + 2pq + q^2 = 1. Signup and view all the answers

Red flowers (R) are dominant to white flowers (r). In a population of 500, 25% show rr phenotype. How many homozygous dominant and how many heterozygous?

Homo dom = 25%, hetero must = 50%. Signup and view all the answers

64% phenotype of dominant color (red) and 36% rr (white). What is the frequency of the dominant allele?

40% frequency of dominant allele. Signup and view all the answers

What factor results in random, nonadaptive change in allelic frequencies?

Genetic drift. Signup and view all the answers

How might an allele that's slightly disadvantageous increase in frequency?

By hitchhiking. Signup and view all the answers

Explain what happens in the founder effect, an example of genetic drift:

When a few individuals become isolated and establish a new population. Signup and view all the answers

Explain what happens in the bottleneck effect, an example of genetic drift:

Sudden environmental change results in certain alleles being underrepresented or fixed. Signup and view all the answers

Define relative fitness:

The contribution an individual makes to the gene pool of the next generation. Signup and view all the answers

What is the relative fitness of a sterile mule?

Nonexistent. Signup and view all the answers

What is directional selection?

Favors variants at one extreme. Signup and view all the answers

What is disruptive selection?

Favors variants at both ends of the spectrum. Signup and view all the answers

What is stabilizing selection?

Removes extreme variants. Signup and view all the answers

What is often the result of sexual selection?

It causes sexual dimorphism. Signup and view all the answers

What is intrasexual selection?

Individuals of one sex compete for mates of the opposite sex. Signup and view all the answers

What is intersexual selection?

Individuals of one sex are choosy in selecting their mates. Signup and view all the answers

Explain two ways in which genetic variation is preserved in a population.

Diploidy and balancing selection. Signup and view all the answers

What's heterozygote advantage?

When heterozygous individuals have a greater fitness than both kinds of homozygotes. Signup and view all the answers

Give four reasons why natural selection can't produce perfect organisms:

<ol> <li>Selection can act only on existing variations. 2. Evolution is limited by historical constraints. 3. Adaptations are often compromises. 4. Chance, natural selection, and the environment interact.</li> </ol> Signup and view all the answers

A fruit fly population has a gene with two alleles, A1 and A2. What proportion of the flies carry both A1 and A2?

0.42. Signup and view all the answers

There are 40 individuals in population 1, all of which have genotype A1A1, and there are 25 individuals in population 2, all of genotype A2A2. What is the observed genetic variation likely an example of?

Genetic drift. Signup and view all the answers

The chief cause of genetic variation among human individuals is?

The reshuffling of alleles in sexual reproduction. Signup and view all the answers

Study Notes

Microevolution and Mechanisms

Microevolution refers to minor changes in allele frequencies within a population across generations.
Major mechanisms influencing allele frequency changes include natural selection, genetic drift, and gene flow.
Natural selection is the only mechanism that is adaptive, enhancing the fit between organisms and their environment.

Genetic Drift and Gene Flow

Genetic drift involves chance events that can drastically change allele frequencies, especially in small populations.
Example: In a small flower population, the infertility of one allele could lead to its loss.
Gene flow is the movement of fertile individuals or their gametes between populations, which diminishes genetic differences.

Genetic Variation

Discrete characters show only two trait options (e.g., flower color in pea plants).
Quantitative characters exhibit a continuous range (e.g., human skin color).
Genetic variation can be quantified at both the gene level (average heterozygosity) and molecular level (nucleotide variability).

Cline and Natural Selection

A cline is a gradient of genetic variation among populations across a geographic range, influenced by environmental factors.
Example: Temperature affects allele frequency in mummichog fish, indicating natural selection at work.

Types of Selection

Frequency-dependent selection indicates that a phenotype's fitness declines as it becomes common.
Neutral variation consists of nucleotide differences that do not impact an organism's function, often resulting from genetic drift.
The Hardy-Weinberg principle describes the equilibrium of allele and genotype frequencies in a population without external influences.

Hardy-Weinberg Equilibrium Principles

Key principles include: no mutations, random mating, no natural selection, large population size to minimize genetic drift, and no gene flow to maintain allele frequencies.

Effects of Genetic Drift

Small populations are more susceptible to random changes in allele frequencies.
Genetic drift can reduce genetic variation, impacting a population's adaptability and potentially leading to harmful alleles becoming fixed or lost.

Mechanisms of Sexual Reproduction

Sexual reproduction introduces genetic variation through crossing over, independent assortment of chromosomes, and random fertilization.

Definitions and Concepts

A population consists of individuals of the same species residing in the same area and capable of interbreeding.
A gene pool encompasses all alleles across all loci in a population.
Fixed alleles occur when only one allele for a trait exists, leading to homogeneity.

Fitness and Selection Types

Relative fitness refers to an individual's contribution to the gene pool of the next generation compared to others.
Directional selection favors one extreme phenotype, while disruptive selection favors extremes at both ends, and stabilizing selection favors intermediate traits.

Sexual Selection Dynamics

Intrasexual selection involves competition among one sex (usually males) for mates, whereas intersexual selection involves mate choice, often based on appearance or behavior.

Preservation of Genetic Variation

Diploidy protects recessive alleles in heterozygotes, preserving genetic variation that may be beneficial under changing conditions.
Balancing selection occurs when multiple forms are maintained, including heterozygote advantage, where heterozygotes have greater fitness.

Limits of Natural Selection

Natural selection cannot create perfect organisms due to constraints like reliance on existing variations, historical limitations of species, adaptations as compromises, and the influence of chance events.

Genetic Variation in Populations

Genetic variation arises primarily through the reshuffling of alleles during sexual reproduction and is often affected by genetic drift in isolated populations.
For specific genetic distributions in populations, allele frequencies can be predicted using Hardy-Weinberg equilibrium equations.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Test your knowledge of the evolution of populations with flashcards covering microevolution concepts and mechanisms. This quiz includes definitions and key terms essential for understanding how allele frequencies change over generations. Perfect for studying genetics and evolutionary biology.