BIOL 214 - Topic 7

Microevolution; Genetic Changes in Populations

Microevolution

heritable change in the genetics of a population

Population

all the individuals of a single species that live together in the same place and time

Variation

• the mechanism underlying natural selection - differences in the population

• microevolution

Natural Selection

• most successful portion of population (most viable offspring) → greater proportion of their genes in the next generation

• changes the pattern of variation in a population from one generation to the next

Phenotypic Variation

Causes = genetic differences, environmental factors, environment-genetic combination → genetic and phenotypic variations are not perfectly correlated

• differences in appearance or function that exist within populations of all organisms

• Quantitative Variation, Qualitative Variation, Polymorphism

Quantitative Variation

• phenotypic variation is quantified and assessed in microevolutionary studies

  • bell curve width = amount of variation

Qualitative Variation

• phenotypic variation is qualitatively assessed in microevolutionary studies

  • Mendel’s work (pea plants with distinct traits)

Polymorphism

• existence of discrete variants of a character among individuals in a population

  • variability; e.g. blood type

• nearly half the gene loci in plant and animal populations are polymorphic'

  • coding & non-coding regions of DNA → extensive genetic variation (changes can occur here)

    • most protein coding genes are polymorphic in their DNA sequences → 2+ alleles

• may appear within:

  • heterozygous individuals

  • between individuals in a single population, same species populations, and related species populations

Genetic Variation

2 sources of variation:

• production of new alleles

  • small-scale mutations in DNA

  • large scale mutations tend to be more dangerous & unviable

• rearrangement of existing alleles

  • larger changes in chromosome structure/number

  • several forms of genetic recombination

  • crossing over, independent assortment, random fertilization

Population Genetics

• look at genetic structure → quantify what exists before seeing how it changes

• genotype frequencies

Genotypic Frequencies

• frequencies/percentages of individuals in a population possessing a particular genotype

• genotype = 2 copies of every gene

• population gene pool = sum of all gene copies at all gene loci in all individuals

Allele Frequencies

• abundance of one allele relative to others in the same gene locus in the individuals of a population

• diploid organism → 2 copies of each gene → allele frequency

• 2 alleles = p and q to represent frequencies of each allele

Frequency Problems

1000 individuals = 2000 alleles

• genotype frequency = # / 1000

• allele frequency = # / 2000

  • p = homo #1 + ½ het / total

  • q = homo #2 + ½ het / total

genotypic frequencies in the offspring generation (hardy weinberg)

• (p+q) * (p+q) = p² + 2pq + q²

Hardy-Weinberg Principle

• Null Model

• an evolutionary rule of thumb that specifies the conditions under which a population of diploid organisms achieves genetic equilibrium → point where neither allele frequency nor genetic frequency will change over successive generations

  • how genotypic frequencies are established within sexually reproducing organisms → dominant alleles don’t replace recessive allele; genetic shuffling isn’t all that causes the gene pool to change

Null Model

defines how/when evolution does/will not occur

• no control for observational data so we use null models

• predict what would happen if a particular factor had no effect → theoretical reference point against which observations can be evaluated

  • investigated factor matches model’s prediction → factor has no effect

Genetic Equilibrium

Possible if: (microevolution will not occur - null model)

1. no mutations occur

2. the population is closed to migration from other populations

3. the population is infinitely large

4. all genotypes in the population survive and reproduce equally well

5. individuals in the population mate randomly with respect to genotypes

Mutations (Microevolution)

• spontaneous heritable change in DNA - very rare

• introduces new genetic variation into population

• does not change allele frequencies quickly

• unpredictable effect on fitness → most mutations in protein-coding genes lower fitness

4 types

1. Deleterious mutations - alter an individual’s structure, function, or behavior in harmful ways

2. Lethal mutations - cause great harm to organisms carrying them

3. Neutral mutations - neither harmful nor helpful

4. Advantageous mutation - some benefit on an individual that carries it

Gene Flow (Microevolution)

• change in allele frequencies as individuals join a population and reproduce

• introduces novel genetic variants into populations (from another population)

  • movement of organisms/gametes & reproduction within joined population

  • shifts away from Hardy-Weinberg model predictions

• unpredictable effect on fitness

• may introduce beneficial or harmful alleles

Evolutionary Importance of Gene Flow

1. degree of genetic differentiation between populations

2. rate of gene flow between them

Genetic Drift

• random fluctuations in allele frequencies because of chance events → reduced genetic variability within populations

  • more pronounced within small populations (less diversity)

  • genotype and allele frequency differs from predicted by Hardy-Weinberg model

2 common circumstances

• Population bottlenecks

• Founder effects

Population Bottlenecks

• rare alleles/genes in the population may not be present among surviving individuals in the case of a drastic reduction in population size → reduction in diversity

• severe with endangered species (lack of variation - less resistant)

Natural Selection

• shapes genetic variability (favors some traits over others) → successful traits become more common in subsequent generations

• can lead to allele frequency changes in the population

• phenotype is what is successful, not the allele

• individuals that survive and reproduce → favorable and unfavorable alleles passed to next generation

• reproductive success → relative fitness

Relative Fitness

number of surviving offspring that an individual produces compared with the number left by others in the population

3 Modes (Natural Selection)

• Directional Selection → phenotype moves towards either end favored by natural selection

• Stabilizing Selection → favors the mean phenotype over extremes

• Disruptive Selection → favors the extreme phenotypes over the mean

Non-Random Mating

• sexual selection

  • unequal parental investment - evolution of showy structures or elaborate courtship behavior is favored

  • intersexual selection (mate choice) vs intrasexual selection (competition)

  • causes sexual dimorphism

• inbreeding → genetically related individuals mate with each other

  • increased frequency of homozygous genotypes

  • decreased frequency of heterozygous genotypes

  • increased expression of recessive phenotypes

Sexual Dimorphism

• distinct physical/behavioral differences between sexes

• similar process to directional selection (one extreme has advantage)

• can influence genotype frequencies

• caused by sexual selection

Diploid Condition

• heterozygotes - “hide” the recessive allele

• reduced effectiveness of natural selection removing harmful recessive alleles

  • natural selection works on phenotype → recessive alleles cannot be seen in heterozygotes

Balanced Polymorphism

• maintenance of 2+ phenotypes in fairly stable proportions over many generations

Natural selection preserves balanced polymorphism when:

1. heterozygotes have higher relative fitness

2. different alleles are favored in different environments

3. rarity of a phenotype provides a selective advantage

Heterozygote Advantage

• heterozygotes have higher relative fitness than either homozygote

  • crosses between two homozygous strains that exhibit a robustness → hybrid vigor

• e.g. sickle cell disease

Environments

• selection varies in environments

• genetic variability can also be maintained within a population when different alleles are favored in different places or at different times

Frequency-Dependent Selection

• rare phenotypes have a selective advantage because they are rare

• increases in frequency until it loses that advantage

• altruistic (A) vs selfish (S) example

  • many A → a few S thrive

  • many S → all out for themselves → S don’t thrive

• common allele → fitness drops; rare allele → fitness rises; cycle

Neutral Variation Hypothesis

• some variation at gene loci coding for enzymes and other soluble proteins → neither favored nor eliminated by natural selection (selectively neutral)

• e.g. alleles → diff AA → all different forms of the proteins still function equally well → no allele favored

• cannot assume every genetic variant that exists has been preserved by natural selection → hard to test (fitness effects are very subtle)

Adaptation/Evolutionary Constraints

• environments change over time → no organism can be perfectly adapted to its environment

• selection preserves alleles successful under prevailing conditions → adapted to conditions that parents lived under

  • adaptation has a “time lag”

  • acts on new mutations (rare) and existing genetic variation

  • works mostly with alleles present for many generations or small modifications of existing structures