BI158 Chapter 23

Microevolution

Change in allele frequencies in a population over generations.

Natural selection

Genetic drift

Gene flow

What are the three mechanisms that cause allele frequency change?

Natural selection

The process where organisms better adapted to their environment tend to survive and produce more offspring.

Genetic drift

A mechanism of evolution that refers to random fluctuations in allele frequencies within a population.

sampling error

inherited genotype and environmental influence

Phenotype is the product of?

A single gene (classified on either-or bases), influence of two or more genes and vary along a continuum within a population, environmental influences

How are phenotypic differences determined?

Gene flow

The transfer of alleles or genes from one population to another.

Allele frequency

The relative frequency of an allele at a genetic locus in a population.

Hardy-Weinberg equilibrium

A condition under which allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.

Phenotype

The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

Genetic variation

Differences in in genes or DNA sequences among individuals within a population.

mutation

a random change in nucleotide sequence of DNA

Founder effect

Genetic phenomenon where a small group of individuals becomes isolated from a larger population, leading to a reduced genetic diversity.

Bottleneck effect

A sharp reduction in the size of a population due to environmental events leading to a loss of genetic variation.

Adaptive evolution

an improvement in the match between organisms and their environment

Sexual selection

natural selection for mating success

Heterozygote advantage

A situation in which heterozygotes have a higher fitness than either homozygote, maintaining genetic diversity.

Directional selection

A mode of natural selection where a single phenotype is favored, causing the allele frequency to shift in one direction.

Disruptive selection

A mode of natural selection that favors individuals at both extremes of the phenotypic range.

Stabilizing selection

A mode of natural selection that favors intermediate variants and acts against extreme phenotypes.

Genetic variation measurement

Measured as gene variability or nucleotide variability in a population.

Relative fitness

The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals.

Gene variability - average heterozygosity measures that average percent of loci that are heterozygous in a population

Nucleotide variability - measured by comparing the DNA sequences of pairs of individuals.

How is genetic variation measured? (2) and pls define.

gametes

Only mutations in cells that produce ________ can be passed to offspring

point mutations

change in one base in genes

• Mutations that result in a change in protein production are often harmful 

• Harmful mutations can be hidden from selection in recessive alleles 

• Mutations that result in a change in protein production can sometimes be beneficial 

• Point mutations in noncoding regions generally result in neutral variation, conferring no selective advantage or disadvantage 

• Mutations to genes can be neutral because of redundancy in the genetic code 

How can point mutations effects vary?

chromosomal mutations that delete, disrupt, or rearrange many loci

When are chromosomal mutations typically harmful?

duplication of small pieces of DNA

what increases genome size and is usually less harmful?

low; 100,000 per generation

Mutation rates are ______ in animals and plants, on average ______ per generation

lower-prokaryotes, higher-viruses

They have short generation times

Mutation rates are often ________ in prokaryotes and _______ in viruses. But why do mutations accumulate quickly in prokaryotes and viruses?

recombination of alleles

What is more important than mutation in producing the genetic differences that make adaptation possible, in organisms that reproduce sexually?

population

localized group of individuals capable of interbreeding and producing fertile offspring

gene pool

consists of all the alleles for all loci in a population

1) If all individuals in a population are homozygous for the same allele

2) Diploid individuals may be either homozygous or heterozygous

1) When is a locus considered “fixed”?

2) What happens if there are two or more alleles for a locus?

• For diploid organisms, the total number of alleles at a locus is the total number of individuals times 2 

• The total number of dominant alleles at a locus is two alleles for each homozygous dominant individual plus one allele for each heterozygous individual; the same logic applies for recessive alleles

How is the frequency of an allele in a population calculated by?

p + q = 1

Formula used for two alleles at a locus

p^2 + 2pq + q^2 = 1

Hardy Weinberg Equation

the genetic makeup we expect for a population that is not evolving at a particular locus

What does the Hardy-Weinberg Equation describe for?

If the observed genetic makeup of the population differs from expectations under Hardy-Weinberg, it suggests that the population may be evolving

Based on the Hardy-Weinberg equation, how do we know if a population is evolving?

population where gametes contribute to the next generation randomly and Mendelian inheritance occurs, allele and genotype frequencies remain constant from generation to generation

Hardy-Weinberg equilibrium

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• describes a hypothetical population that is not evolving 

• In real populations, allele and genotype frequencies do not change over time 

• Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

What does the Hardy-Weinberg theorem describe?

1. NO mutations 

2. Random mating

3. No natural selection 

4. Extremely large population size 

5. No gene flow

What are the five conditions for nonevolving populations that are rarely met in nature?

1. The PKU gene mutation rate is low 

2. Mate selection is random with respect fo whether or not an individual is a carrier for the PKU allele

3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions 

4. The population is large 

5. Migration has no effect as many other population have similar allele frequencies

In what 5 cases can we assume that locus causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium?

1 per 10,000 births

Occurence of PKU

p = 1 - q = 1 - 0.01 = 0.99

Frequency of normal alleles

• 2pq = 2 x 0.99 x 0.01 = 0.0198 

• Or approximately 2% of the U.S. population

Frequency of carriers

1. Genetic drift is significant in small populations

2. Genetic drift can cause allele frequencies to change at random 

3. Genetic drift can lead to a loss of genetic variation within populations 

4. Genetic drift can cause harmful alleles to become fixed 

Effects of genetic drift (4)

increase and decrease

Gene flow can both _________ and _______ the fitness of a population

They imply direct competition among individuals

Reproductive success is generally more subtle

Why are the terms “struggle for existence” and “survival of the fittest” misleading?

intrasexual selection

direct competition among individuals of one sex (often males) for mates of the opposite sex

Intersexual selection

often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates

directional selection

disruptive selection

stabilizing selection

Three modes of selection

Striking adaptations have arisen by natural selection

Natural selection increases the frequencies of alleles that enhance survival and reproduction

Adaptive evolution occurs as the match between a species and its environment increases

Because the environment can change, adaptive evolution is a continuous process

Key roles of Natural Selection in Adaptive Evolution

They can increase or decrease the match between an organism and its environment

Why do genetic drift and gene flow not consistently lead to adaptive evolution?

sexual dimorphism

marked differences between the sexes in secondary sexual characteristics

Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

What does male showiness do?

if a trait is related to male genetic quality of health, both the male trait and female preference for that trait should increase in frequency.

What does the “good genes” hypothesis suggests?

balancing selection

occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

Heterozygote advantages

Frequency-dependent selection

What does balancing selection include?

Heterozygote advantages

occurs when heterozygotes have a higher fitness than do both homozygotes; result from stabilizing or directional selection

Frequency-dependent selection

the fitness of a phenotype declines if it becomes too common in the population; selection favors whichever phenotype is less common in the population