Genetics and Evolution

• Genetics and Evolution
• Mendel explained how traits are inherited.
• Darwin suggested evolution happens through natural selection.
• Genetic variation drives evolution.

Evolution by Natural Selection Explained

• Charles Darwin wrote "On the Origin of Species" in 1859.
• Natural selection explains adaptation as a result of natural laws, not supernatural events.

Adaptive Radiation Demonstrated by Darwin's Finches

• The Galapagos finches exhibit adaptive radiation.
• Each finch species adapted to exploit different food sources.
• Various finches include: small, medium, and large ground finches (differing beak sizes).
• There are cactus-eating finches, sharp-beaked ground finches, vegetarian finches, mangrove finches, Cocos Island finches, warbler finches, and woodpecker finches

Darwin & Wallace: How Evolution Occurs Through Natural Selection

• Unlike Lamarck's theory where acquired traits pass down, Darwin proposed genetic variation leads to evolutionary change.
• Darwin noted giraffes with genetically longer necks are more successful.
• Natural selection favors the passing of long-neck traits; beneficial traits get passed on to successive generations.

Darwin's Theory of Evolution

• Charles Darwin suggested descent with modification, where species evolve and accumulate differences over time.
• Populations have diverse traits inherited from parents.
• Selective pressures lead to survival.
• Favorable traits increase reproductive success.

Genetic Variation

• Genetic variation is the differences in gene alleles within a population.
• Polymorphic variation arises when multiple alleles exist at frequencies higher than due to mere mutation.
• Single nucleotide polymorphisms (SNPs) are assessed to study patterns in populations, which provide the raw information for natural selection

Population Defined

• A population is a group of the same species living in the same area.
• Population genetics studies genes within populations.
• Evolution changes the genetic composition of a population.
• Genetic variation is essential for selection, thus evolution modifies individual fitness.

Change in Allele Frequencies

• Hardy and Weinberg created a model to study mating consequences within a population (Hardy-Weinberg Principle).
• The Hardy-Weinberg Principle looks at an entire population with all individuals and genotypes.
• Each generation's gametes mix randomly in a gene pool.
• Predict offspring genotypes and frequencies within a population

Hardy-Weinberg Equilibrium

• Genotype proportions remain constant if no mutation occurs.
• No genes are transferred in or out of the population (no migration).
• There's random mating.
• The population is large.
• No selection happens.

Allele Frequency

• Alleles p and q in a population determine phenotype, p is for black coat color (B), so black cats are BB or Bb.
• q is for white coat color (b), so white cats are bb.
• Allele frequencies p and q give genotype frequencies BB, Bb, and bb as p², 2pq, and q².
• Two independent events' probability is the product of their individual probabilities.
• Allele frequencies must equal 1: p + q = 1.
• With equilibrium, probability of genotypes is p² + 2pq + q², or BB + Bb + bb, also equal to 1.
• Allele transmission via meiosis and random gamete combination maintains frequency.
• Shifts could mean other factors are at play

Studying Evolutionary Influences

• Use Hardy-Weinberg principle to test the null hypothesis (no change).
• Disagreement with their proportions suggests evolution or nonrandom mating.

Evolutionary Change

• These include nonrandom mating and gene flow.
• Gene flow is when individuals join or leave populations, introducing/removing alleles.
• Mutation continually introduces new alleles, modifying allele frequencies.
• Genetic drift randomly changes allele frequencies, non-adaptive.
• Natural selection increases beneficial allele frequency to facilitate adaptation in a particular environment.

Nonrandom Mating

• Assortative mating happens when phenotypically similar individuals mate, which increases homozygous individuals.
• Dissassortative mating happens when phenotypically different individuals mate, producing more heterozygotes.

Gene Flow

• Gene flow occurs when alleles move between populations.
• Causes include: physical movement of animals, drifting of gametes, or mating between adjacent populations.
• Gene flow reduces species differentiation

Ultimate Genetic Variation

• Ultimate genetic variation is the result of evolution.
• Mutation restores genetic diversity, countering reduction by other mechanisms.

Mutation and Natural Selection's Impact on Evolution

• Evolution via natural selection isn’t goal-oriented; it favors already-better adapted individuals.
• Low selective pressures mean mutations have little effect. Antibiotic resistance in bacteria is one harmful example.
• In bacteria, mutant forms may grow faster or die more slowly than normal cells during antibiotic therapy.

Genetic Mutation

• Mutations create new genetic variations, allowing new alleles
• However allele frequency is inconsequential

Understanding Genetic Drift

• Genetic drift can alter allele frequencies by chance in small populations.
• Magnitude of drift is inversely proportional to population size and can eliminate alleles from isolated groups through the founder and bottleneck

Founder Effects on Genetic Drift

• Founder events occur when small groups start new populations somewhere else.
• Allele frequencies change due to the source population.

Bottleneck Effect

• Genetic drift diminishes alleles in separated populations.
• Uncommon alleles are at greater risk.
• Seal populations were nearly hunted to extinction in the 19th century and now have limited genetic diversity.

Selection

• Selection is impacted by more then one gene
• Selection operates on all the genes for the trait
• Population changes depend on which genotypes are favored
• Natural selection involves types, including disruptive, directional and stabilizing selection

Disruptive Selection

• Disruptive selection results when intermediate traits are at a disadvantage. ex. Birds with intermediate sized beaks cannot open large seeds

Directional and Stabilizing Selection

• Directional Selection: directional selection removes one extreme. ex. Selects for flies towards the light and that behavior increased
• Stabilizing selection removes both extremes, which is when intermediate become more common ex. Human infants survival rate are optimal when an intermediate weight at birth.

Natural Selection Requirements

• Natural selection requires variation among individuals in a population.
• This results in differences in surviving offspring.
• Variation must be inherited

Natural Selection and Evolution

• Natural selection isn't the only mechanism for evolution
• Natural selection is a process; evolution is the culmination of change through time.

Adaptation

• Adaptation involves heritable traits bettering individual fitness in certain environments and increasing offspring survival.
• Evolution changes these traits by modifying that individual's fitness.

Natural Selection Examples

• Pocket mice coat color is an example. Populations vary between dark/light.
• Housefly pesticide resistance involves alleles: (pen decreases pesticide uptake while kdr and dld-r modify target sites).
• In water striders, intermediate sized females are shown to have the highest fitness

Evolutionary Forces

• Genetic drift may reduce a favored gene.
• Selection usually overwhelms drift but gene flow can spread mutations.
• Mutations are important for natural selection
• Frequency-dependent selection means fitness depends on phenotype and population frequency.
• Negative frequency-dependent selection favors rare phenotypes. In positive frequency selection favors common form

Negative Frequency

• Negative frequency (fish eat common colors) occurs in water boatmen.

Heterozygote Advantage

• Heterozygotes are favored over homozygotes to maintain alleles in a population. Sick cell anemia is an example

Sickle Cell Anemia

• This is a hereditary disease affecting hemoglobin, causes severe anemia
• Heterozygotes for sickle cell are at increased survival
• Homozygotes usually die, with need for medical treatment
• Natural selection, combined with particulate inheritance, modifies individual fitness

Natural Selection

• Gene flow and mutations cause natural selection.
• Adaptations that improve individual fitness during reproduction, such as Mutation and gene shuffling contribute to variation

Key Consequences of Evolutionary

• Selection produces adaptation when certain allele combinations increase offspring survival.
• Genetic drift randomly changes allele frequencies. Gene flow equalizes allele frequencies.
• By way of mutations, there are new alleles. Nonrandom mating shifts genotype frequencies.