Genetic Variation and Evolution
Chapter 20: Genes Within Populations
20.1 Genetic Variation and Evolution
Genetic Variation
Differences in alleles (1) of genes found within individuals in a population (2).
Natural populations contain much more variation (3).
Evolution
How an entity changes over time (4).
Development of modern concept traced to Darwin: “Descent with modification” (5).
Evolution is defined as: “Through time, species accumulate differences; as a result, descendants differ from their ancestors. In this way, new species arise from existing ones.” - Charles Darwin.
Many processes lead to evolutionary change.
Darwin was not the first to propose a theory of evolution; however, he proposed natural selection (6) as the mechanism of evolution.
Natural selection can lead to changes in allele (7) frequencies – frequencies of alleles of a gene from generation to generation (8).
Population Genetics
Study of frequencies and distributions (9) of genes in a population.
Evolution results in a change in the genetic structure (10) of a population.
Natural populations contain substantial genetic variation (11).
Genetic variation is essential (12) for evolution to occur.
20.2 Changes in Allele Frequency
Genetic Variation Measurement
Genetic variation in populations is now measured using increasingly sophisticated tools.
Examples of variation due to genetic differences include:
Human blood groups, SNPs (Single-Nucleotide Polymorphisms).
Variation in a gene (13) that affects only one (14) nucleotide and that variation occurs in more than 1% (15) of the population.
In total, 100,000 human genomes have been partially or wholly sequenced.
Extensive genetic variation has been documented.
Hardy–Weinberg Principle
Principle 1: Predicts allele frequencies (16).
Hardy-Weinberg Equilibrium: Proportions of genotypes do not change in a population as long as:
No mutations (17) take place.
No genes are added (18) to or from other sources (no immigration or emigration).
Mating is random (19).
The population size is large (20).
No selection (21) occurs.
Hardy–Weinberg Principle 2
Can be expressed as an equation used to calculate allele frequencies:
Frequency of first allele is p (22), second allele is q (23).
Equation: p + q = 1
For example, in cats:
p = B for black coat color (BB or Bb).
q = b for white coat color (bb).
Making Hardy–Weinberg Predictions
If all 5 assumptions for Hardy-Weinberg equilibrium are true, allele and genotype frequencies remain constant (24) from one generation to the next.
In reality, most populations will not (25) meet all 5 assumptions.
Primary use of the equation is to determine whether forces of evolution (26) are operating in a population and identify (27) what those processes are.
20.3 Five Agents of Evolutionary Change
Mutation
Rates generally low (32).
Other evolutionary processes usually more important in changing allele frequency.
Ultimate source of genetic variation (33) (mutation leads to variation).
Makes evolution possible.
Gene Flow
Movement of alleles (34) from one population to another.
For example: Animal migration (35) into a new population, or drifting of gametes or immature stages of plants or animals into an area.
Pollen and seeds can travel long distances.
Nonrandom Mating
Genetic Drift
In small populations, allele frequency may change by chance (36).
Population must be large to be in Hardy-Weinberg equilibrium.
Magnitude of genetic drift is inversely related to population size (37).
Can lead to the loss of alleles in isolated populations; uncommon alleles are more vulnerable.
Example #1 - Founder effect: One or a few individuals disperse and become the founders (38) of a new, isolated population.
Some alleles are lost (39), while others change in frequency.
Common in organisms on islands; for instance, self-pollinating plants.
Example #2 - Bottleneck effect: Drastic reduction (40) in population size due to drought, disease, or other natural forces.
Survivors may constitute a narrow (41) genetic sample of the original population.
Results in loss (42) of genetic variability.
Case Study: Northern Elephant Seal
Nearly hunted to extinction in the 19th century.
As a result, this species has lost almost all of its genetic variation.
Population now numbers in tens of thousands.
20.4 Quantifying Natural Selection
Fitness
Individuals with one phenotype leave more viable offspring (56) in the next generation than individuals with an alternative phenotype.
A relative concept; the most fit phenotype is simply the one that produces, on average, the greatest number (57) of offspring.
Components of Fitness
Parental Investment: Refers to the time (58) and energy (59) each sex invests in producing and rearing offspring.
Females have a higher (60) parental investment.
Sexual Selection
Males best increase their fitness by mating with as many (61) females as possible.
Females are limited by the number of offspring (62) they can produce, so a female should be choosy in picking the male that can provide the greatest investment (63).
Types of Sexual Selection
Intrasexual selection: Competitive interactions between members of one sex (64), such as males fighting.
Intersexual selection: Mate choice (65).
Secondary sexual characteristics: Features like antlers and horns that are used to contest other males, or long tail feathers and bright plumage that attract the opposite sex.
Sexual Dimorphism: Differences between sexes (66); for example, males tending to be larger due to intrasexual selection.
Sperm Competition: Selects for features that increase the probability that a male’s sperm will fertilize (67) the eggs.
20.5 Natural Selection’s Role in Maintaining Variation
Frequency-Dependent Selection
Fitness of a phenotype depends on its frequency within the population – selection favors how common (68) or rare (69) a phenotype is.
Negative Frequency-Dependent Selection:
Rare (70) phenotypes favored by selection.
Rare forms may not be in “search image”; preyed upon less frequently.
Positive Frequency-Dependent Selection:
Favors the common (71) form.
Tends to reduce (72) variation.
“Oddballs” stand out.
Examples
Negative Frequency-Dependent Selection:
Male common side-blotched lizard – orange-throated: strongest but do not form a bond with females.
Blue-throated: middle-sized and do form strong bonds with females.
Yellow-throated: smallest and can mimic females, letting them approach females near orange-throated lizards.
Positive Frequency-Dependent Selection:
Oscillating Selection:
Selection favors one phenotype at one time and another phenotype at another time, resulting in maintenance of variation (74) in the population.
Example: Medium ground finch of Galápagos Islands - Birds with big bills favored during drought, birds with smaller bills favored in wet conditions.
Environmental changes (75), NOT fitness, lead to change in selection.
Heterozygote Advantage
Heterozygotes are favored over homozygotes, working to maintain both (76) alleles in the population.
Example: Sickle cell anemia.
Hereditary disease affecting hemoglobin that causes severe anemia.
Homozygotes for the sickle cell allele usually die before reproductive age (77) (without medical treatment).
Why is the Sickle Cell Allele Not Eliminated?
Leading cause of death in central Africa is malaria (78).
Heterozygotes for the sickle cell allele do not suffer anemia and are much less susceptible to malaria.
20.6 Selection Acting on Traits Affected by Multiple Genes
Polygenic Traits
Many traits are affected by more than one gene (79).
Selection operates on all the genes for the trait, changing the population depending on which genotypes are favored.
Types of Selection
Disruptive Selection: Acts to eliminate intermediate (80) types.
Example with African black-bellied seedcracker finch: Available seeds fall into two (81) categories.
Favors bill sizes at either extreme; birds with intermediate-sized beaks are at a disadvantage.
Directional Selection: Acts to eliminate one extreme (82).
Often occurs in nature when the environment changes, as seen in Drosophila eliminating flies that moved toward the light.
Stabilizing Selection: Acts to eliminate both extremes (83).
In humans, infants with intermediate weight at birth have the highest survival rate.