Evolutionary Biology: Hardy-Weinberg and Selection

Hardy-Weinberg Principle

• A null model used to identify evolutionary forces that change gene frequencies.
• Assumes random mating and the absence of evolutionary forces.
• States allele frequencies will remain constant in a population meeting these conditions.
• Conditions necessary for Hardy-Weinberg equilibrium:
  • Random mating
  • No mutations
  • Large population size
  • No immigration
  • Equitable fitness between all genotypes.
• It is likely that at least one of these conditions won't be met, leading to changes in allele frequencies.
• Indicates the potential for evolutionary change in natural populations is significant.

Genetic Drift

• Change in allele frequencies due to chance or random events.
• Reduces genetic diversity over time by increasing some alleles and reducing or eliminating others.
• More impactful in smaller populations.
• Examples include endangered species, founder effects, and bottlenecks.

Natural Selection

• Differential survival and reproduction among phenotypes.
• Some individuals possess phenotypic characteristics that provide higher survival rates and increased offspring production.
• The fitness of an individual is measured by their relative contribution of offspring, or genes, to future generations.
• Natural selection can favor, disfavor, or conserve the genetic makeup of populations.

Stabilizing Selection

• Acts to impede changes in the population by favoring average phenotypes and acting against extreme phenotypes.
• Extreme phenotypes have lower reproductive and survival rates, resulting in the average phenotype remaining the most common across generations.

Directional Selection

• Drives changes in phenotypes by favoring one extreme phenotype over others in the population.
• Extreme phenotypes experience higher rates of reproduction and survival, causing the population average to shift in that direction over time.

Disruptive Selection

• Creates a bimodal distribution by favoring two or more extreme phenotypes over the average phenotype.
• Average phenotypes have lower reproductive and survival rates compared to the extremes.
• Over time, average phenotypes become less common, leading to increased phenotypic diversity in the population.

Heritability and Evolution

• Natural selection can alter genotypic and phenotypic frequencies in populations, potentially leading to adaptation.
• Heritability of traits is crucial for evolutionary change:
  • h^2 = G/(G+E), where G is the genetic variance and E is the environmental variance.
  • Phenotypic variance is influenced by both genetics and environmental factors.

Examples of Evolutionary Processes

• Soapberry bugs:
  • A classic example of directional selection through beak length adaptation based on fruit size.
• Darwin's finches:
  • Demonstrate disruptive selection where two distinct beak sizes are favored, highlighting the importance of beak morphology for food acquisition.
• Chihuahua Spruce:
  • Shows evidence of genetic drift in smaller populations, illustrating the impact of reduced genetic diversity.

Island Populations

• Tend to harbor lower genetic variation compared to mainland populations.
• This is likely due to limited gene flow and founder effects, which can lead to inbreeding and reduced diversity.

Genetic Diversity and Extinction

• Inbreeding can drive extinction in small populations.
• Reduced fecundity, juvenile survival, and lifespan are associated with inbreeding, leading to population decline.
• The Glanville fritillary butterfly study highlights the relationship between inbreeding and extinction risks.

Evolution and Agriculture

• Artificial selection is the breeding of domesticated organisms, selecting for desirable traits.
• Genetic engineering involves the introduction or deletion of genes, creating genetically modified organisms (GMOs).

Unintended Evolutionary Consequences

• Agricultural practices can have unintended evolutionary consequences.
• Plant and animal pests may evolve resistance to pesticides and herbicides, leading to increasing challenges in controlling populations.
• The emergence of antibiotic-resistant strains like MRSA illustrates the rapid evolution of populations in response to selective pressures.

Understanding Populations

• A population refers to all individuals of a single species occupying a specific area at a particular time.
• Key aspects of population study:
  • Number of individuals
  • Density
  • Distribution
  • Abundance.
  • Age structure
  • Growth rates

Environmental Constraints

• The geographic distribution of species is limited by both physical and biotic factors.
• Organisms have limits in their ability to compensate for environmental variations.
• Niche: A concept describing the biophysical environmental factors influencing a species' growth, survival, and reproduction.

Fundamental vs. Realized Niche

• Fundamental niche: The theoretical range of conditions where an organism could live.
• Realized niche: The actual range of conditions occupied by a species, which is often limited by biotic interactions such as competition, predation, or diseases.

Distribution on Small Scales

• Random dispersion: Individuals have an equal probability of occurring anywhere within an area.
• Regular dispersion: Individuals are evenly spaced throughout the environment, often indicating competitive interactions.
• Clumped dispersion: Individuals are grouped in areas of high local abundance, suggesting positive interactions or resource concentration.

Examples of Distribution

• Desert Shrubs:
  • Traditionally, it was thought that desert shrubs were regularly spaced due to competition.
  • Research by Phillips and MacMahon suggests that clumped distribution is more common, potentially driven by factors such as seed germination at safe sites, limited seed dispersal, and nurse plant relationships.

Population Dynamics

• Population size (Nt) can be described by:
  • Nt = Nt-1 + B + I - D - E
    • Nt is the population size at a given time.
    • Nt-1 is the population size at the previous time step.
    • B is the birth rate.
    • I is the immigration rate.
    • D is the death rate.
    • E is the emigration rate.

Geometric Population Growth

• Occurs when populations have abundant resources and experience minimal limiting factors.
• Successive generations differ in size by a constant ratio.

Exponential Population Growth

• Models unlimited population growth when generations are not discrete.
• Change in population size over time (dN/dt) is a function of per capita increase and the number of individuals.
• Paleo- ecological examples:
  • Receding glaciers exposing new habitats with limited competition and abundant resources.

Logistic Population Growth

• Occurs when exponential growth slows down due to environmental limitations such as resource depletion, competition, predation, or disease.
• The carrying capacity or K represents the maximum population size that the environment can support.
• Population growth slows as it approaches the carrying capacity, leading to an S-shaped or sigmoidal growth curve.

Survivorship Curves

• Type 1: High survival rates for young and middle-aged individuals with low survival rates for older individuals.
• Type 2: Constant survival rates across all ages.
• Type 3: Extremely low survival rates for juveniles with high survival rates for older individuals.

Age Distribution

• Can be assessed by aging individuals in a population.
• Provides insights into population stability or growth.
• For example, a population with many young individuals suggests future growth.

Life Tables

• Used to track mortality and reproduction rates over time for a population.
• Calculations include:
  • l x m = lm (Proportion of individuals surviving to a certain age x reproduction rate at that age = net reproductive rate)
  • log(net reproductive rate)/ generation time

Summary

• Understanding population dynamics requires considering factors such as dispersal rate, metapopulation structure, survivorship curves, age distribution, and life tables.
• Population growth models, including geometric, exponential, and logistic growth, provide insights into the relationship between population size and environmental factors.
• Evolutionary processes shape populations through mechanisms like genetic drift, natural selection, and artificial selection.
• The study of population biology is essential for managing populations, conserving biodiversity, and addressing challenges related to environmental change.

Explore key concepts in evolutionary biology, including the Hardy-Weinberg principle, genetic drift, and natural selection. This quiz will test your understanding of how these mechanisms affect gene frequencies and genetic diversity in populations. Perfect for students studying genetics and evolution.