Assumptions of Natural Selection

• Darwin’s description
• – Traits passed from parents to offspring
• – Important differences between individuals
• Even same species

· Darwin & Mendel

• • Questions existed between relationship of Darwinian evolution and Mendelian Genetics
• • Reconciled in 1930s – Gave rise to population genetics

Population Genetics

• – Study of how genes of entire populations change over time
• – Describes how groups of organisms evolve

Variation

• Central to both theories

– Darwin noticed that

• Individuals in natural populations show
• Slightly different forms of a given trait or characteristic
• Without variation, nature would have nothing to act on

– Mendel focused on traits that show only two distinct forms

How Do We Characterize Variation?

• Smooth or discontinuous

– Evolution made sense to naturalists
– Mendel described individuals with

• Discrete form of a trait, nothing between

Survival & reproducibility depended on

• Traits falling within some certain range
• Contradicted Mendel’s laws

Experimentalists chose to focus on

– Sudden changes resulting from mutations
• New forms or traits from muted alleles
• Evolution seen as progressing leaps & bounds

How Do We Characterize Continuous Variation?

– Determined by two or more genes

– Many evolutionary traits not traced to single gene
– Polygenic or quantitative traits

Each single gene

• Has its own chromosomal address
• Called locus (loci, plural)

A person can carry at most 2 alleles for 1 gene

How Do Populations Differ?

• Brachydactyly
• – Human trait in which
• – Terminal bones of fingers & toes do not grow to normal length
• Understanding genetics of entire population
• – Requires knowledge – alleles in pop.
• Populations are collections of alleles
• – Gene pool – all alleles in a pop.
• • Alleles occur at certain frequencies

How Do Populations Differ?

• Hardy-Weinberg Principle, 1908
• – Relates genotypes & allelic frequencies

– Stated that:

• The frequencies of genotypes for a gene with two different alleles are a binomial function of the allelic frequencies (p² + 2pq +q² = 1)

•Hardy-Weinberg Assumptions

• Populations are very large
• Individuals mate at random
• Populations do not gain or lose individuals to immigration or emigration

ie: populations are at genetic equilibrium

Allelic Frequencies - How Can they Change Over Time?

• Natural selection
• – Changes allelic frequencies
• – Heterozygote advantage
• Microevolution
• – Changes in population allelic frequency
• – Can occur in short time
• – Reversible
• Macroevolution – Larger-scale changes that
• • Lead to the origin of higher taxa or
• • Categories of organisms
• • Favoring different parts of a phenotypic range
• – Directional selection
• • Mean value for trait shifts in particular direction
• – Stabilizing selection
• • Sometimes middle of variation range is most advantageous
• • Favoring different parts of a phenotypic range
• – Disruptive or diversifying selection
• • Sometimes middle of range is problem while either end is advantage
• Genetic drift
• – In a small population, chance can be important in determining
• – Without influences of selection
• • Flux (emigration, immigration), or mutation
• • Passing alleles from parent to offspring is random process

Genetic drift

• – Is the alteration in allele frequencies
• – Results from chance variation in survival and/or reproductive success

Neutral selection

• • Microevolutionary changes that are independent of natural selection

Founder effect

• • Occurs when there is difference in gene pool allelic makeup
• • Due to population initiation by small number of individuals
• Primary factor in abnormally high genetic disease levels in small, closed human populations
• • Example – Ellis-van Crevald syndrome in Amish of Lancaster, PA

Bottlenecks

• Population undergoes temporary decline to low numbers from which survivors of all future generations are derived
• Genetic drift plays role in determining gene pool composition

e.g – 19th century – Northern elephant seal heavily hunted - Population dropped to <20

Mutation

• – Introduce new alleles into gene pool
• – Three arbitrary categories
• • Lethal or near-lethal
• • Most common – selectively or nearly neutral mutations
• • Advantageous mutations – small #

Gene flow - Movement between populations

• • Results in introduction of alleles
• • From one population to another
• • Via immigration or the loss of alleles
• • From one population to another via emigration

– Significant when coupled with natural selection

• • Example – Mosquitoes & DDT
• • After World War II, DDT used to control mosquitoes with malaria
• • Mutant mosquitoes had ability to detoxify DDT
• • Alleles (detoxifying) flowed from one population to another
• • Today, most mosquito populations are DDT resistant
• • Thus, malaria is now on rise in many African & Asian countries
• • Pest control programs must consider principles of pop. Genetics

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