Evolution: How Populations Evolve (Campbell Biology Ch13 Study Notes)

Evolution: How Populations Evolve

Theme: Evolution Accounts for the Unity and Diversity of Life

Evolution is the central theme in biology, explaining both the diversity and unity of life. It describes how living species are descendants of ancestral species and how adaptation to changing environments leads to the modification of traits over time.

• Biological Diversity: Over 1.8 million species have been identified, with thousands more discovered each year.

• Evolution: The process by which organisms living today are modified descendants of common ancestral species.

• Common Ancestry: Shared traits between organisms are explained by descent from a common ancestor.

• Heritable Changes: Differences among species are due to the accumulation of heritable changes over generations.

Additional info: Evolution is ongoing and influenced by environmental factors, leading to adaptation and speciation.

Mechanisms of Microevolution

Microevolution refers to changes in the genetic makeup of populations over time. These changes are driven by several mechanisms, including mutation, natural selection, genetic drift, and gene flow.

• Mutation: Spontaneous changes in the genetic code that introduce new genetic variation.

• Natural Selection: The process by which individuals with advantageous traits survive and reproduce more successfully.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Gene Flow: Movement of alleles between populations through migration.

Example: Pesticide resistance in insects is a clear example of evolution occurring in real time. Populations exposed to pesticides develop resistance as individuals with resistance genes survive and reproduce.

Evidence for Evolution

Multiple lines of evidence support the theory of evolution:

• Fossil Record: Shows the historical sequence of life and reveals transitional forms between groups.

• Anatomical Homologies: Similar structures in related species indicate common ancestry (e.g., vertebrate forelimbs).

• Analogous Structures: Structures with similar functions but different evolutionary origins, resulting from convergent evolution.

• Embryological Evidence: Similar stages of embryonic development in related species point to shared ancestry.

• Molecular Biology: Similarities in DNA, proteins, and other molecules among species reflect evolutionary relationships.

Adaptation and Natural Selection

Adaptation is a process by which populations become better suited to their environment through the selection of advantageous traits.

• Adaptation: Any structure, behavior, or physiological process that increases an organism's chance of survival and reproduction.

• Natural Selection: Acts on genetic variation within populations, favoring traits that confer a selective advantage.

• Selective Advantage: Traits that improve survival and reproduction become more common in the population.

Example: The evolution of beak shapes in Galapagos finches, allowing them to exploit different food sources.

Population Genetics and Microevolution

Population genetics studies the genetic composition of populations and how it changes over time.

• Gene Pool: The total collection of alleles in a population.

• Microevolution: Changes in allele frequencies within a population over time.

• Hardy-Weinberg Equilibrium: Describes a non-evolving population where allele and genotype frequencies remain constant.

Hardy-Weinberg Equation:

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

Where p and q are the frequencies of two alleles at a locus.

Types of Natural Selection

Natural selection can alter the distribution of phenotypes in a population in three main ways:

• Stabilizing Selection: Favors intermediate phenotypes and reduces variation.

• Directional Selection: Favors one extreme phenotype, shifting the population's traits in one direction.

• Disruptive Selection: Favors both extremes over intermediate phenotypes, increasing variation.

Sexual Selection and Genetic Variation

Sexual selection is a form of natural selection where individuals with certain traits are more likely to obtain mates.

• Secondary Sex Characteristics: Traits that give individuals an advantage in mating.

• Intra-sexual Selection: Competition among members of the same sex for mates.

• Inter-sexual Selection: Mate choice, often by females selecting males with desirable traits.

• Diploidy: Preserves genetic variation by hiding recessive alleles.

• Balancing Selection: Maintains stable frequencies of two or more phenotypes.

• Heterozygote Advantage: Heterozygous individuals have greater reproductive success, maintaining multiple alleles in the population.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Effect on Genetic Variation
Mutation	Spontaneous changes in DNA	Introduces new alleles
Natural Selection	Favors advantageous traits	Increases frequency of beneficial alleles
Genetic Drift	Random changes in allele frequencies	Reduces genetic variation, especially in small populations
Gene Flow	Movement of alleles between populations	Increases genetic variation within populations

Additional info: Evolution is not goal-directed; it is shaped by environmental pressures and genetic variation within populations. Individuals do not evolve; populations do.