Chapter 23: The Evolution of Populations

Distinguish between *intrasexual* selection and *intersexual* selection.

*Intrasexual Selection-* Individuals of the *same* sex compete for mates of the opposite sex (Typically males) (KANGAROO FIGHTS) *Intersexual Selection-* Mate Choice; individuals of one sex (Typically females) are choosy about selecting their mate from the opposite sex. (PEACOCKS)

In a population of plants, 64% exhibit the dominant flower color (red), and 36% of the plants have white flowers. What is the frequency of the dominant allele? (There are a couple of twists in this problem, so read and think carefully. A complete solution for this problem is at the end of this Reading Guide.)

0.4 (2/5) is the frequency of the dominant allele.

Summarize the events of Genetic Drift

1) Genetic drift is significant in small populations since it can cause a particular allele to be disproportionately over or under represented in the future generation(s) 2) Genetic drift can cause allele frequencies to change at random. 3) Genetic drift can lead to loss of genetic variation 4) It can also cause harmful alleles to be fixed, as they can be fixed entirely by chance events.

Three major factors alter allelic frequency and bring about evolutionary change. List each factor, and give an explanation.

1) Natural Selection: Individuals in a population exhibit variations in heritable traits 2) Genetic Drift: Chance events causing allele proportions in a population to change/fluctuate in small populations especially. 3) Gene Flow: Due to the movement of fertile individuals or gametes, alleles can be transfered in and out of a population.

If the frequency of alleles in a population remains constant, the population is at Hardy- Weinberg equilibrium. There are five conditions for Hardy-Weinberg equilibrium. It is very important for you to know these conditions, so enter them neatly into the box below.

1) No mutations in any allele in the gene pool 2) Random mating within individuals 3) No natural selection 4) Large population size 5) No gene flow, since it can alter allele frequencies

Finally, give four reasons why natural selection cannot produce perfect organisms.

1) Selection can act only on existing variations, since it favors the fittest phenotypes. 2) Evolution is limited by historical constraints since it doesn't scrap the ancestral anatomy and build each new complex structure from scratch, but it acts based on old characteristics of organisms that it acts on. 3) Adaptations are often compromises since each organism must perform several different functions. 4) Chance, nat selection, and environment all interact together.

List the *five* conditions that must be met for a population to remain in Hardy-Weinberg equilibrium.

1. No mutations 2. Random mating 3. No natural selection 4. Large population size 5. No gene flow (No migration)

Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait? (A complete solution for this problem is at the end of this Reading Guide.)

375 individuals, STEPS: P^2 + 2PQ + Q^2 = 1 Q^2 = 0.25 Q = 0.5 P^2 + P = 0.75 P = 0.5 (P + P^2) * 500 = 0.75 * 500 = 375 individuals, 125 HomDom and 250 Heterozygous

What is a gene pool?

A gene pool is all the alleles that characterize a particular population.

Species

A group of individuals capable of interbreeding and producing fertile offspring.

Population

A group of individuals of the same species that live in the same area, interbreed, and produce fertile offspring.

Explain how sexual recombination generates genetic variability

A random physical exchange of regions of homologous chromatids creates a completely unique chromosome. This chromosome can contribute to new genetic expression and adds to genetic variability. Honorable mention- Independent assortment Mutation

Gene Pool

All of the alleles for all genes within a population.

(Recall to Chapter 21) Why are point mutations very unlikely to affect polypeptide formation?

DNA is 99% non-coding sequences. Point mutations that happen here will have no effect on the proteins that are formed.

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

Diploidy (Recessive alleles can persist by propagation in heterozygous individuals) and Balancing selection (Natural selection maintaining two or more forms in a population)

Figure 23.13 is important because it helps explain the three modes of selection. Label each type of selection, and fill in the chart to explain what is occurring.

Directional selection: Shifts overall makeup of population by favoring extreme variants of the species. Disruptive selection: Where individuals that are at both extremes of a phenotypic range are favored, e.g black-bellied seed cracker finches in Cameroon whose members display two distinctly different beak sizes. Stabilizing selection: Acts against both extreme phenotypes and favors intermediate variants, reduces variation and maintains status quo for particular phenotypic character.

Distinguish among *directional*, *disruptive*, and *stabilizing selection*. Give an example of each mode of selection.

Directional- Favors a specific type of mouse... Shifts in a specific direction. Disruptive- Favors both extremes. Splits graph Stabilizing selection- Favors intermediate types. (Heterozygotes increase)

Explain the role of population size in genetic drift. Describe how genetic drift can lead to a loss of genetic variation in a population, or can cause harmful alleles to become fixed in a population.

Every population has genetic drift. - It is only visible in smaller populations. (Changes in allele frequency in a population due to a chance event) (Includes founder and bottleneck effect)

How can geographic variation contribute to genetic composition of separate populations?

Evolution in isolation from each other is caused by geographic relocation of certain species. Cline: Graded change in a character along a geographic axis.

Distinguish between the *bottleneck effect* and the *founder effect*.

Founder effect: Individuals become isolated from a larger population. (Ex: Island population migrations) Less genetic variation Bottleneck Effect: A very large population turns into a very small population due to a change in the event. (Flood, fire...) Less genetic variation

Which of the factors above results in a random, nonadaptive change in allelic frequencies?

Genetic drift, as it is based on chance events

Describe how *heterozygote advantage* works, and it represent a mechanism of *balancing selection*.

Heterozygotes have a greater fitness that homozygotes. This is because they can produce offspring or exhibit in themselves both dominant and recessive traits that may produce survivability. By having such an advantage, it promotes the preservation of balancing selection which selects for two or more forms in a population.

Can we predict when a mutation will happen or what it will alter? Where in animals do most mutations occur?

In animals, most mutations occur in somatic cells and are lost when the animal dies. However, mutations are unpredictable.

Explain what happens in each of these examples of genetic drift: founder effect bottleneck effect

In the founder effect, a smaller group of individuals that has been isolated from a larger population may establish a new population with a different gene pool from the source population. This can account for the high frequency of certain inherited disorders among isolated populations, such as the retinitis pigmentosa that spread from UK to Tristan da Cunha island. In the bottleneck effect, population size is drastically reduced by a natural disaster. Survivors have the best traits and certain alleles are overrepresented among the survivors of the disaster.

What is the difference between intrasexual selection and intersexual selection? Give an example of each type of selection.

Intrasexual: Occurs within same sex, individuals of one sex are competing for mates of opposite sex. Intersexual selection is called mate choice, where individuals are choosy about mates from other sex.

How can Average Heterozygosity (Genetic variability within a specific population) be estimated?

It can be estimated by surveying the protein products of genes using gel electrophoresis.

The Hardy-Weinberg principle is used to describe a population that is not evolving. What does this principle state?

It states that the frequencies of genotypes will remain constant from generation to generation in a particular population even with Mendelian crosses and genetic recombination via sexual reproduction occurs.

Describe how *gene flow* can act to reduce genetic differences between adjacent populations.

Migration. (Movement of alleles from one population to another.) - When individuals and gametes move across populations. Can either reintroduce lost alleles or reduce genetic variability (Typically counteracts genetic drift)

Which organisms have more mutations?

Mutations happen very rarely in plant and animal somatic cells, while due to the short lifespans/generation spans of prokaryotic organisms, mutations can happen quickly. Additionally, mutations can happen quickly in viruses as well.

Discuss what is meant by heterozygote advantage, and use sickle-cell anemia as an example.

Natural selection can maintain two or more alleles at that locus. The recessive allele coding for an improper Beta polypeptide subunit of hemoglobin

Example of mutations in evolution

Olfactory genes: When mice evolved and humans evolved separately, human olfactory genes were mostly inactivated by several mutations, while mice only lost about 20% of theirs.

What is a population?

Organism group of the same species, live in the same area and "mate" to produce offspring.

Explain how Mendel's theory of inheritance provided necessary support for Darwin's theory of evolution by natural selection.

Parents contribute and mix their genes to create offspring with a blend of the parents genes? Given a lot of time, if certain traits are favorable, there will be an adaptation and an increase in the favored trait. (Idk if this is the answer...)

How can silent mutations be included in estimates of average heterozygosity?

Researchers use approaches that may be PCR-based and are also restriction fragment analyses.

What is often the result of sexual selection?

Result of Sexual Selection is sexual dimorphism, or genetic/phenotypic differences between the two genders in secondary sexual characteristics.

Define *relative fitness*.

The contribution that an organism makes to the gene pool of the next generation, relative to other organisms in the population.

Why does meiosis and random fertilization alone not alter the frequency of alleles or genotypes in a population?

The frequency is also determined by natural and sexual selection.

Define *neutral variations*. Explain why these alleles are not acted upon by natural selection.

Variations that have little to no effect on reproductive fitness. Not affected by natural selection.

Explain how *diploidy* can protect a rare recessive allele from elimination by natural selection.

We have two alleles for every gene. So some alleles may be hidden by the dominant and not subject to selection. This allows infrequent alleles to be passed on, even when they are not beneficial to survival.

The greater the number of fixed alleles, the lower the species' diversity. What does it mean to say that an allele is fixed?

When an allele is fixed, it means that there is only one allele that exists for a particular locus in a population.

In evolutionary terms, fitness refers only to the ability to leave offspring and contribute to the gene pool of the next generation. It may have nothing to do with being big, or strong, or aggressive. Define relative fitness.

contribution an individual makes to the gene pool of future generations of individuals in a positive way, so that future generations will survive and produce offspring in the same environment