Study Notes: Evolution, Population Genetics, and Community Ecology

Hardy-Weinberg Principle

Genetic Variation and Evolution

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations and the conditions under which evolution does or does not occur.

• Genetic Variation: Refers to differences in DNA sequences among individuals in a population. It is the raw material for evolution.

• Phenotype vs. Genotype: The genotype is the genetic makeup, while the phenotype is the observable trait. Natural selection acts on phenotypes, but only genotypic variation is inherited.

• Sources of Genetic Variation: Mutation, recombination, and gene flow introduce new alleles into a population.

Hardy-Weinberg Equilibrium (HWE)

The Hardy-Weinberg equilibrium describes a non-evolving population where allele and genotype frequencies remain constant from generation to generation, provided certain conditions are met.

• Conditions for HWE: No mutation, random mating, no gene flow, infinite population size, and no selection.

• Allele Frequency Equation:

$p + q = 1$

• Genotype Frequency Equation:

$p^2 + 2pq + q^2 = 1$

• Where: p = frequency of dominant allele, q = frequency of recessive allele.

• p2 = frequency of homozygous dominant genotype, 2pq = frequency of heterozygotes, q2 = frequency of homozygous recessive genotype.

• Application: Used to estimate allele and genotype frequencies in populations and to test if evolution is occurring.

Example: If 9% of a population shows a recessive phenotype (q2 = 0.09), then q = 0.3 and p = 0.7. The frequency of heterozygotes (2pq) is 0.42.

Additional info: Deviations from HWE indicate that one or more evolutionary forces are acting on the population.

Types of Selection

Natural Selection and Its Mechanisms

Natural selection is the process by which certain traits become more common in a population due to differential reproductive success.

• Requirements for Natural Selection: Variation in traits, heritability, and differential survival or reproduction.

• Types of Selection:

  • Directional Selection: Favors one extreme phenotype.

  • Stabilizing Selection: Favors intermediate phenotypes.

  • Disruptive Selection: Favors both extreme phenotypes over intermediates.

• Balancing Selection: Maintains genetic diversity in a population (e.g., heterozygote advantage).

Example: Sickle cell trait in humans is maintained in some populations due to heterozygote advantage against malaria.

Additional info: Selection can act on phenotypic variation, but only heritable genetic variation is passed to offspring.

Speciation

Formation of New Species

Speciation is the evolutionary process by which populations evolve to become distinct species.

• Biological Species Concept: Defines species as groups of interbreeding natural populations that are reproductively isolated from other such groups.

• Other Species Concepts: Morphological (based on physical traits), ecological (based on niche), and phylogenetic (based on evolutionary history).

• Reproductive Isolation: Mechanisms that prevent gene flow between populations, leading to speciation.

Example: Eastern and Western meadowlarks are similar in appearance but do not interbreed due to differences in song.

Additional info: Speciation can occur via allopatric (geographic isolation) or sympatric (without geographic isolation) mechanisms.

Population Ecology

Population Dynamics and Regulation

Population ecology studies the factors that affect population size, density, and structure over time.

• Population: A group of individuals of the same species living in the same area at the same time.

• Population Density: Number of individuals per unit area or volume.

• Population Growth Models:

$\text{Exponential Growth:} \quad \frac{dN}{dt} = rN$

$\text{Logistic Growth:} \quad \frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)$

• Where: N = population size, r = intrinsic rate of increase, K = carrying capacity.

• Carrying Capacity (K): Maximum population size that the environment can sustain.

• Regulation: Populations are regulated by density-dependent (e.g., competition, predation) and density-independent (e.g., weather, disasters) factors.

Example: Logistic growth is observed in populations where resources become limited as population size increases.

Community Ecology

Community Structure and Diversity

Community ecology examines the interactions between species and the structure and diversity of communities.

• Community: All the populations of different species living and interacting in a particular area.

• Biological Diversity (Biodiversity): The variety of life forms in a community, including species richness and evenness.

• Interspecific Interactions: Relationships between species, such as competition, predation, mutualism, commensalism, and parasitism.

• Comparison of Interactions:

Interaction	Effect on Species 1	Effect on Species 2	Example
Competition	-	-	Plants competing for sunlight
Predation	+	-	Lion and zebra
Mutualism	+	+	Bees and flowering plants
Commensalism	+	0	Barnacles on whales
Parasitism	+	-	Tapeworm in humans

• Factors Affecting Diversity: Climate, habitat heterogeneity, disturbance, and interactions among species.

Example: Tropical rainforests have high biodiversity due to stable climate and complex habitats.