Darwin, Mendel, DeVries: Understanding Inheritance

• Evolution of Populations & Population Genetics is covered in Chapter 19.

Genes, Mutations, and Inheritance

• Contrast today's understanding of genetics with Darwin's inheritance assumptions.
• Describe phenotypic and genetic variation causes and types in a species.
• Define "population" using allelic and genotypic frequencies.
• Solve basic population genetics using the Hardy-Weinberg equation.
• Relate microevolution to the Hardy-Weinberg Principle.
• Identify and describe microevolution mechanisms and their expected outcomes.
• Selection forms and natural selection limits are explained.

Darwin and Genetics

• Darwin (1859) noted continuous variation in species and accumulation of differences in offspring.
• Mendel (1866) and DeVries (1890-1900) found discrete genetic factors, no blending/accumulation, and the importance of mutations.
• Sutton-Boveri (1902) proposed the chromosome theory of inheritance.
• Morgan (1910's) studied mutations and modern genetics.

Phenotypic Variation

• Phenotypic variation is primarily genetic, but the environment affects expression, creating non-heritable variation.

Genetic Variation

• Discrete genetic variation involves a single gene locus with two or more alleles.
• Continuous variation results from the combined effects of two or more genes, producing a range of phenotypes.

Sexual Recombination

• Sexual recombination leads to crossovers, independent assortment, and random fertilization in offspring.

Mutations

• New alleles come from DNA mutations in cells that ultimately make gametes.
• Point mutations involve 1 to 3 base pairs.

Chromosomal Alterations

• Chromosomal alterations include deletions, duplications, and translocations.

Gene Duplications

• Gene duplications expand the size of the genome; duplicates can mutate into new alleles.

Alleles

• Most new alleles are harmful, or "deleterious," yet harmful effects can be "hidden" in heterozygotes.
• New alleles may be neutral, not affecting the likelihood of leaving offspring concerning selection.
• If the environment changes, harmful or neutral alleles may become adaptive.

DNA Variability

• Most DNA variability does not affect phenotype because protein translation/gene expression is unaffected.

Hardy-Weinberg & Microevolution: Population

• Population pertains to a group of interbreeding individuals in the same area, but may be somewhat isolated from other groups.
• Populations differ in genetic makeup.
• Gene pool constitutes all the alleles of all the genes in a population.

Alleles

• Many genes have "fixed" alleles (homozygous in individuals).
• Other genes containing two or more alleles.
• Genotypic frequency = genotype percentage in the population, or %AA, %Aa & %aa.
• Allelic frequency = percentage of each allele in the population, or %A allele and %a allele.

Hardy-Weinberg Principle

• Allele frequencies are the same, but genotypic frequency is different.
• Some allele and genotypic frequencies are different.
• A random mutation alters allele/genotype ratios and produces a new phenotype.
• A new phenotype introduces this gene with three alleles in this population.
• A population allelic or genotypic frequency change is microevolution. It has the smallest/fundamental evolution unit.

Hardy-Weinberg Equilibrium

• If a large population reproduces sexually at random, then genetic frequencies should not change.
• It remains in equilibrium if there are no mutations, mating is random, there is no selection, there is a very large population size, and no gene flow.

Hardy-Weinberg Equation

• Given a population of 500 flowers with 320 red (CRCR), 20 white (CWCW), and 160 pink (CRCW),
• Red flowers have CRCR, with 640 CR and 0 CW.
• White flowers are CWCW, with 0 CR and 40 CW.
• Pink flowers have CRCW, having 160 CR and 160 CW.
• The totals being 500 flowers and 800 CR and 200 CW.
• Genotypic frequencies are:
  • CRCR at 320/500 = 64% or 0.64.
  • CWCW at 20/500 = 4% or 0.04.
  • CRCW at 160/500 = 32% or 0.32.
• Allele frequencies are:
  • CR at 800/1000 = 80% or 0.8.
  • CW at 200/1000 = 20% or 0.2.
• When populations meet H-W random conditions, any generation will have the same ratios.
• p = frequency of CR allele = 0.8.
• q = frequency of CW allele = 0.2.
• p + q = 1.
• The same frequency of alleles and genotypes stay in subsequent generations.
• Allelic frequencies remain also the same.

The H-W Equation

• H-W equation regards the population and its equilibrium.
• If p = dominant allele frequency, and q = recessive allele frequency, while p + q = 1, then: p² + 2pq + q² = 1.
• p² = homozygous dominant genotype frequency.
• 2pq = heterozygous genotype frequency.
• q² = homozygous recessive genotype frequency.
• Use the equation to determine population genetic makeup if a HW equilibrium is known or can be assumed.

Hardy-Weinberg Example

• 64% have black stripes on an island butterfly population in H-W equilibrium since it's a trait with the autosomal dominant allele B.
• Solve the allele frequencies:
  • p² + 2pq + q2 = 1.
  • 36% are homozygous recessive: q² = 0.36.
  • q = √0.36 = 0.6 (60% b)
  • p = 1-q = 0.4 (40% B).
• What % of the population is homozygous dominant?
  • %BB = p² = (0.4)² = 0.16 = 16%.

H-W Equation Example 2

• Isolate the human population while assuming H-W.
• The Tay Sach's allele frequency is 0.02.
• What is the individual carrier chance?
• Carriers are Tt. Percentage of heterozygotes is 2pq.
• q = 0.02
• p+q = 1.0
• p = 0.98
• 2pq= 2(0.98)(0.02)
• = 0.04 or 4%

H-W & Microevolution: H-W Equilibrium as Basis

• H-W detects microevolution.
• H-W equilibrium is the "null hypothesis."
• Observed ratios ≠ expected H-W ratios equals population.
• Microevolution regards a population showing genetic change over periods.

H-W Example

• One population of 1000 ducks: 500 AA, 200 Aa, 300 aa.
• freq a = (300 x 2 + 200)/2000 = q = 0.4.
• p = 1- 0.4 = 0.6.
• Expected H-W ratios:
  • p² = (0.6)² = 0.36 (360).
  • 2 pq = 2(0.6)(0.4)= 0.48 (480).
  • q² = (0.4)² = 0.16 (160).
• Actual (observed) ratios:
  • 0.50 (500/1000).
  • 0.20 (200/1000).
  • 0.30 (300/1000).
• Observed ratios are not expected, meaning it's evolving.

Microevolution Mechanisms

• Microevolution happens because of natural selection, genetic drift, and gene flow.

Natural Selection

• Natural selection affects individuals' phenotypes non-randomly.
• Population allelic/genotypic frequencies change non-randomly.
• Always adapts population to the current environment.
• Resistance to DDT:
  • chromosome with gene conferring resistance to pesticide.
  • nonrandom selection of an individual's phenotype.
  • population is more resistant to DDT, or population adaptation.
  • frequencies of the alleles change. -DDT-R freq. = 0% before DDT (1930). -DDT-R freq. = 37% after DDT (1960+).

Genetic Drift

• Genetic drift is the random genetic frequency changes, often in small populations, like "sampling errors" in statistics.
• Genetic drift changes allele frequencies in either way, reduces diversity, and one allele becomes "fixed," losing any other alleles.
• Two places are important for genetic drift in microevolution.
  • The founder effect: genetic drift in a few founders starts a new isolated population.
    • Founder gene differs from the original source.
    • Small-size populations leads to the loss of better alleles.
  • An event drastically cutting a population creates a bottleneck effect and genetic drift.
    • a random gene pool of random survivors contains lost alleles.
    • it leads to more genetic drift.
  • Example: High rate of inherited blindness on Tristan da Cunha. There is an increased maladaptive allele frequency since about 250 descendants from 15 settlers, where retinitis pigmentosa is autosomal recessive.
  • The gene pool of survivors becomes more random, causing lost alleles.
  • Reduced genetic variation from the overhunting of elephant seals and cheetahs.
• The prairie chicken habitat loss example involves:
  • harmful alleles that increased.
  • reduced genetic variation.
  • 75% of original prairie land habitat has been lost.

Gene Flow

• Gene flow pertains to alleles that move in/out of a population.
• Gene migration of adults and dispersal of seeds, gametes, and larvae.
• The water strider Aquarius remigis has allelic differences among populations that are more significant between streams than within them.

Natural Selection Understanding: Relative Fitness

• Fitness is relative to other individuals.
• "Fittest" is the best reproductive succes.
• Fitness includes survival, finding mates, and the number of healthy and fertile offspring.

Forms of Natural Selection

• Directional selection shifts a character's mean value in one direction.
• Disruptive or diversifying selection, where intermediate types are less fit, maintains diversity. One example being seedcracker finches because there are 2 sizes to the beak since seeds are mostly available in one of two sizes.
• Stabilizing selection, where intermediate types are more fit and the diversity of traits are reduced.

Sexual Selection

• Sexual selection refers to success based on mate-obtaining traits that are not environmentally based.
• It leads to sexual differences.
• The difference in genders, the red attracts females is a malr trait, but the camouflage allows females to stay safe from prey or predators.
• Only males compete/have big noses for direct species interaction.

The Maintenance of Genetic Variation

• Through diploidy, less successful recessive alleles are hidden.
• Through disruptive selection, extreme phenotypes may carry different alleles since patchy resources may lead to disruptive selection.
• Selection favors heterozygotes as long as it maintains both allies.

Natural Selection & Hb Examples

• HbAHbA = regular/normal RBC, but the individual is likely to die from malaria.
• HЬАНЬB = Malaria resistant.
• HbBHbB = Sickle cell disease.
• Natural selection does not involve an intermediate phenotype. Heterozygotes are fitter.
• The fitness of a phenotype decreases with the frequency of frequency-dependent selection as its frequency increases.
• Rare traits get to eat more and leave more offspring. The more common it is, the less fit its phenotype is.

Limits of Natural Selection

• Acts on the phenotype.
• Adaptation happens with compromising with its form to reduce competing needs.
• Selection can only act on existing variation.
• Extinction occurs since adaptation happens when adaptation isn't possible.
• Form is constrained by ancestors.
• Chance, environment, and current selection has a historical history.

Explore the differing views on inheritance between Darwin and later scientists like Mendel and DeVries. Analyze genotype-phenotype relationships and genetic variation through flower color examples. Investigate body size variations in beetle populations and larval development based on diet.