Evolutionary Principles and Population Genetics

Constraints to Evolution

• Natural selection can only act on existing variations within a population.

• New structures do not arise de novo; they are modifications of ancestral structures.

• Adaptations are frequently compromises, as organisms face multiple, sometimes conflicting, selective pressures.

• Evolution is influenced by the interaction of chance events, natural selection, and the environment.

Genetic Variation: The Foundation of Evolution

• Definition: Genetic variation refers to the genetic differences observed among individuals within a population.

• Sources of new alleles and genes:

  • Mutation: Random changes in the nucleotide sequence of DNA. This is the ultimate source of all new genetic variation.

  • Gene duplication: The accidental copying of genes, which can lead to new genetic material upon which evolution can act.

• Rate of variation production:

  • Organisms with short generation times (e.g., bacteria, viruses) can produce new genetic variants rapidly.

• Reshuffling of existing alleles in sexually reproducing organisms:

  • Crossing over: Exchange of genetic material between homologous chromosomes during meiosis, creating new combinations of alleles on chromosomes.

  • Independent assortment of chromosomes: Random orientation of homologous chromosome pairs during meiosis I, leading to a vast number of possible chromosome combinations in gametes.

  • Fertilization: The random combination of male and female gametes, further increasing genetic diversity.

• Phenotypic impact of nucleotide variability:

  • Often, much of the nucleotide variability within a genetic locus does not affect the phenotype. This can be due to mutations occurring in non-coding regions, or silent mutations in coding regions that do not change the amino acid sequence, or changes that do not significantly alter protein function.

The Hardy-Weinberg Equation and Population Evolution

• Population Definition: A localized group of organisms of the same species that are capable of interbreeding and producing fertile offspring.

• Gene Pool: The aggregate of all copies of every type of allele at all loci in all members of a population.

• Hardy-Weinberg Equilibrium: A theoretical state where allele and genotype frequencies in a population remain constant from generation to generation. This indicates that the population is NOT evolving.

• Conditions for Hardy-Weinberg Equilibrium:

  • No mutations: Alleles do not change into other alleles.

  • Random mating: Individuals mate without preference for genotype.

  • No natural selection: All individuals have equal survival and reproductive rates.

  • Extremely large population size: Prevents random fluctuations in allele frequencies due to chance (genetic drift).

  • No gene flow: No migration of individuals or gametes into or out of the population.

• Hardy-Weinberg Equation: For a locus with two alleles, $A1$ and $A2$, where $p$ is the frequency of $A1$ and $q$ is the frequency of $A2$, the allele and genotype frequencies are:

  • Allele frequencies: $p + q = 1$

  • Genotype frequencies: $p^2$ (frequency of $A1A1$ homozygotes) $+ 2pq$ (frequency of $A1A2$ heterozygotes) $+ q^2$ (frequency of $A2A2$ homozygotes) $= 1$

• Application: The equation is used to compare observed genotype frequencies in a population with those expected under Hardy-Weinberg equilibrium. Significant deviations indicate that the population is evolving, and at least one of the equilibrium conditions is not being met.

  • It is not circular reasoning to calculate $p$ and $q$ from observed genotype frequencies and then use them to test for equilibrium. The initial $p$ and $q$ are derived from the observed state, and then these values are used to predict what genotype frequencies should be if the population were in equilibrium. The comparison of observed vs. predicted reveals whether evolution is occurring.

Mechanisms Altering Allele Frequencies

Natural Selection

• Mechanism: Individuals possessing certain heritable traits tend to survive and reproduce at higher rates compared to others due to those traits.

• Outcome: Consistently increases the frequencies of alleles that enhance survival and reproduction, leading to adaptive evolution—organisms become better suited to their environment.

Genetic Drift

• Mechanism: Unpredictable, chance fluctuations in allele frequencies from one generation to the next, especially prominent in small populations.

• Outcome: Tends to reduce genetic variation within a population by leading to the loss or fixation of alleles.

  • Example: If two small, geographically isolated populations exist in very different environments, they are unlikely to evolve in similar ways. Genetic drift will cause random changes, and different selection pressures will drive distinct adaptations, leading to divergence.

  • Initial population differences (e.g., one $AA$, another $aa$) in isolated settings can be attributed to genetic drift causing allele fixation in their founding populations.

Gene Flow

• Mechanism: The transfer of alleles between populations, typically through the movement of fertile individuals or gametes.

• Outcome: Tends to reduce genetic differences between populations over time, making populations more similar genetically.

  • Mussel (Mytilus edulis) example: Larvae that disperse long distances before settling on rocks contribute to gene flow, which can counterbalance localized natural selection pressures related to salinity.

  • Fire-bellied and yellow-bellied toads (Figure 24.13) show gene flow can occur across species boundaries within a hybrid zone, influencing allele frequencies in adjacent populations.

Modes of Natural Selection

• Relative Fitness: The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population.

• Directional Selection: Favors individuals at one extreme end of the phenotypic range, shifting the population's phenotypic distribution in that direction.

• Disruptive Selection: Favors individuals at both extremes of the phenotypic range over intermediate phenotypes.

• Stabilizing Selection: Favors intermediate variants and acts against extreme phenotypes, reducing variation and maintaining the status quo.

Sexual Selection

• Mechanism: A form of natural selection where individuals with certain inherited characteristics are more likely to obtain mates.

• Outcome: Can result in secondary sex characteristics (e.g., peacock's tail, larger size, brighter coloration in males, or competitive behaviors) that provide advantages in mating, even if they pose risks to survival.

  • If females compete for mates, they would likely possess more elaborate ornamentation, be larger, more colorful, or engage in behaviors to attract or secure mates, mirroring typical male characteristics in species where males compete.

Balancing Selection

• Mechanism: Occurs when natural selection maintains two or more forms (alleles) in a population.

• Examples:

  • Heterozygote advantage: When heterozygotes have higher fitness than both homozygotes (e.g., sickle-cell allele in regions with malaria).

    • Sickle-cell (Concept 14.4): Heterozygotes produce both normal and abnormal hemoglobin. While some red blood cells may be prone to sickling, the overall advantage against malaria at the individual level maintains the sickle-cell allele in the population's gene pool, demonstrating emergent properties from molecular (hemoglobin composition) and cellular (RBC sickling) events to individual (malaria resistance) and population (allele frequency) levels.

  • Frequency-dependent selection: The fitness of a phenotype depends on how common it is in the population.

Speciation Concepts and Mechanisms

Biological Species Concept

• Definition: A species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups.

• Central role of gene flow: This concept defines species based on the absence of gene flow between distinct species, making gene flow a crucial factor.

• Limitations: Cannot be applied to asexual species (like bacteria or extinct organisms) because they do not interbreed.

Morphological Species Concept

• Definition: Distinguishes species by body shape and other structural features.

• Practicality: Easiest to apply in the field as it relies solely on observable appearance, without requiring information on reproductive habits.

Allopatric Speciation

• Mechanism: A new species forms while geographically isolated from its parent species.

• Key factor: Geographic isolation extensively reduces gene flow between populations.

• Likelihood: More common than sympatric speciation due to the strong reduction in gene flow.

  • Example: An island far from a mainland is more likely to experience allopatric speciation because the reduced gene flow allows for greater genetic divergence to occur compared to a nearby island with ongoing gene flow from the mainland.

Sympatric Speciation

• Mechanism: A new species forms without geographic isolation, within the same geographic area as its parent species.

• Reduction of gene flow: Even in the absence of a physical barrier, gene flow can be reduced by:

  • Polyploidy: Changes in chromosome number, especially common in plants, can block gene flow and establish reproductive isolation in a single generation (e.g., Tragopogon species, where new polyploid species arise from parent species).

  • Habitat differentiation: Subpopulations exploit different habitats or resources within the same area (e.g., apple maggot flies (Rhagoletis) specializing on different host plants).

  • Sexual selection: Mate choice based on specific traits can isolate gene pools (e.g., cichlid fish in Lake Victoria, where female preference for specific male colors inhibits interbreeding, but in murky waters, color distinction is poor leading to increased interbreeding and gene pool fusion).

Time Frame of Speciation

• Speciation events involve two main phases:

  1. The time it takes for populations of a newly formed species to begin diverging reproductively from one another.

  2. The time it takes for speciation to be complete once this divergence has begun.

• Divergence can start rapidly, but it may take millions of years for the initial divergence to begin.

Hybrid Zones

• Definition: Regions where members of different species meet and mate, producing offspring of mixed ancestry (hybrids).

• Stability of Hybrid Zones:

  • If hybrids are selected against: The hybrid zone can persist if individuals from the parent species regularly migrate into the zone, continually producing new hybrids.

  • If hybrids are not selected against: Large numbers of hybrid offspring may be produced, but natural selection favoring specific traits in the different environments of the parent species can keep their gene pools distinct, preventing fusion and maintaining a stable hybrid zone.

• Fate of Hybrid Zones:

  • Reinforcement: If hybrids are less fit than parent species, natural selection strengthens prezygotic barriers, reducing hybrid production over time.

  • Fusion: If hybrids are as fit as parent species, extensive gene flow can occur between hybrids and parent species, potentially leading to the fusion of the two parent species into a single species.

  • Stability: Continual production of fit hybrids can lead to a stable hybrid zone.

Evolution in Action: Modern Examples

• Speciation is an ongoing process observed today (e.g., goatsbeard plant, Bahamas mosquitofish, apple maggot fly).

• Any event that reduces gene flow between populations of a parent species can initiate speciation. This includes:

  • Colonization of a new, geographically isolated area by a few individuals.

  • Utilization of a new habitat or resource by some members of a population.

  • Sexual selection isolating formerly connected populations or subpopulations.

Specific Examples and Concepts Elaborated

• Nucleotide variability vs. Gene variability (Question 3): If nucleotide variability at a locus is $0\%$, it means all individuals have the exact same nucleotide sequence for that gene. Therefore, gene variability is also $0\%$, and there can only be $1$ allele at that locus.

• Fruit Fly Hardy-Weinberg (Question 5): Given A1 frequency ($p$) = $0.7$, then A2 frequency ($q$) = $1 - 0.7 = 0.3$. The proportion of flies carrying both A1 and A2 (heterozygotes) in Hardy-Weinberg equilibrium is $2pq = 2 \times 0.7 \times 0.3 = 0.42$.