Chapter 16 - Population Evolution

• Chapter 16 - How Populations Evolve

Genes, Populations, and Evolution

• A population is a group of organisms of a single species occupying a particular area at the same time.
• Diversity exists among members of a population.
• Population genetics is the study of this diversity in terms of allele differences; it studies genotype and phenotype frequencies over time

Microevolution

• Population geneticists describe variations in a population in terms of alleles
• Microevolution pertains to evolutionary changes within populations
• The gene pool of a population consists of all alleles at all gene loci in all individuals
• The gene pool of a population can be described in terms of genotype frequencies and allele frequencies

Natural Selection

• Natural selection is the adaptation of a population to the living (biotic) and non-living (abiotic) environment as it favors the variant that is most adaptive to the present environmental conditions
• Natural selection needs variation (members of a population differ from one another), inheritance (genetic differences are heritable), differential adaptiveness (some differences affect survivability), and differential reproduction (some differences affect likelihood of successful reproduction)

Polygenic Traits and Selection

• Most traits are polygenic and show variations that result in a bell-shaped curve
• Three types of selection exist: stabilizing, directional, and disruptive selection
• Stabilizing selection occurs when an intermediate phenotype is the most adaptive for the given environmental conditions, thus the peak of the curve increases and tails decrease.
• As an example of stabilizing selection, human babies with low or high birth weight are less likely to survive

Directional and Disruptive Selection

• Directional selection occurs when an extreme phenotype is favored.

• The curve shifts in one direction during directional selection.

• Bacteria becoming resistant to antibiotics is an example of directional selection

• Disruptive selection occurs when two or more extreme phenotypes are favored over the intermediate phenotype.

• During disruptive selection, the curve has two peaks.

• As an example of disruptive selection, British land snails vary because a wide geographic range causes selection to vary.

• Chapter 17 - Speciation and Macroevolution

How New Species Evolve

• Macroevolution is evolution on a large scale; it involves the origin of species also called speciation
• Macroevolution can be best observed within the fossil record
• Speciation is the splitting of one species into two or more species and is the final result of changes in the gene pool's allelic and genotypic frequencies

Reproductive Isolating Mechanisms

• Reproductive isolating mechanisms inhibit gene flow between species.
• Two general types of reproductive isolating mechanisms exist: prezygotic and postzygotic

Prezygotic Isolating Mechanisms

• Prezygotic isolating mechanisms prevent mating attempts or make it unlikely that fertilization will be successful.
• Types of prezygotic isolation include habitat isolation (species occupy different habitats), temporal isolation (each reproduces at a different time), behavioral isolation (courtship patterns for recognizing mates differ), mechanical isolation (incompatible animal genitalia or plant floral structures), and gamete isolation (gametes that meet do not fuse to become a zygote)

Postzygotic Isolating Mechanisms

• Postzygotic isolating mechanisms prevent hybrid offspring from developing or breeding.

• Types of postzygotic isolating mechanisms include hybrid inviability (hybrid zygote is not viable and may die), and hybrid sterility (hybrid zygote may develop into a sterile adult).

• For example, mules, the offspring of a cross between a female horse and a male donkey, are usually sterile and cannot reproduce

• Chapter 18 - Origin and History of Life

Geological Factors that Influence Evolution

• Continental drift affects geological factors that influence evolution; the positions of continents and oceans are not fixed.

• Earth's crust consists of slablike plates.

• Tectonic plates float on a lower, hot mantle layer.

• Movements of plates result in continental drift.

• Modern mammalian diversity results from isolated evolution on separate continents.

• Chapter 20 - Viruses, Bacteria and Archaea

Heterotrophic Bacteria

• Most prokaryotes are chemoheterotrophs that take in organic nutrients.
• Aerobic saprotrophs decompose most large organic molecules to smaller molecules.
• Heterotrophic bacteria are essential components of a healthy ecosystem
• Heterotrophic bacteria may be free-living or symbiotic (two different species live together in an intimate way)
• Mutualism is when both species benefit from association like mutualistic bacteria that live in human intestines and release vitamins K and B12, which help produce blood components.
• Commensalism is when one population modifies the environment in such a way that a second population benefits; ex: obligate anaerobes live in our intestine because bacterium E. coli uses up oxygen.
• Parasitism is when a parasite benefits at host expense; disease-causing bacteria are called pathogens that often form endospores.