9. Evolution of Populations Flashcards
What is Population Genetics?
Population genetics is the study of how populations change genetically over time
A change in gene frequency over time means that the population has evolved
What is Microevolution?
Microevolution is a change in allele frequencies in a population over generations
What is a Population?
A population is a localized group of individuals (a species in an area) capable of interbreeding and producing fertile offspring
How do we count Alleles?
How do we count Individuals?
Determine the frequency of White Cats, The Reccesive Allele
Determine the frequency of Black Cats, The Dominant Allele (Homozygotes Only)
Determine the frequency of Black Cats, The Dominant Allele (Heterozygotes Only)
What is the Hardy-Weinberg Equation?
The Hardy-Weinberg equation can be used to test whether a population is evolving
If the predicted and observed frequencies are not a match, then population is evolving
The Hardy-Weinberg principle describes a population that is NOT EVOLVING!!!
What five conditions must be satisfied if a population is to remain Hardy-Weinberg Equilibrium?
- No mutations (no genetic change – i.e., no introduction, no loss or no modification of genes to alter the gene pool)
- Random mating (no sexual selection)
- So no mate choice; gametes combine randomly - No natural selection (no alleles/genotypes selected over other alleles/genotypes)
- All members of the parental generation contribute equal numbers of gametes to the gene pool - No genetic drift, or random allele frequency, affects the gene in question due to the extremely large population size
- Large breeding population minimizes fluctuations in the gene pool (which means allele/genotype frequencies do not change over time) - No gene flow (i.e., no migration)
- No new alleles can come into the population, and no alleles can be lost
Assumptions of the Hardy-Weinberg Equilibrium Summary
What Five major factors (or mechanisms) can alter allele/genotype frequencies and bring evolutionarily change?
- Mutation
- Natural selection
- Genetic drift (especially with small population sizes)
- Gene flow
- Non-random mating (sexual selection)
What is a Mutation?
Mutations are changes in the nucleotide sequence of DNA
– Can cause new genes and alleles to arise
- Only mutations in cells that produce gametes can be passed to offspring, and can potentially change the gene pool of a population
What is a Point Mutation?
Point – single nucleotide change in the DNA
- Can be a substitution of one nucleotide to another
- Can be a removal of a nucleotide (deletion)
- Can be an addition of a nucleotide (insertion)
What is a Large Scale Mutation?
Large scale (chromosomal) – covered in a previous class
- Chromosomal mutations are changes in the number or structure of chromosomes
- Mutations that delete, disrupt, or rearrange many loci are typically harmful
What is Natural Selection?
Differential success in reproduction and survival results in certain alleles being passed to the next generation in greater proportions
If individuals having certain genes are better able to produce mature offspring than those without them, the frequency of those genes will increase
Natural selection accumulates and maintains favourable genotypes in a population
What is Genetic Drift?
Allele frequencies fluctuate unpredictably from one generation to the next (i.e., causes allele frequencies to change at random)
– Random events = sudden death due to environmental catastrophes, habitat destruction, predation or indiscriminate hunting
- Genetic drift can cause genetic traits to be lost from a population or become widespread in a population without respect to the survival or reproductive value of the alleles involved
- Genetic drift can occur only in small, isolated populations in which the gene pool is small enough that random events can change its makeup substantially
What is the Bottleneck Effect?
The bottleneck effect is an extreme example of genetic drift that happens when the size of a population is severely reduced
– Events like natural disasters (earthquakes, floods, fires) can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors
– Allele frequencies in this group may be very different from those of the population prior to the disaster event, and some alleles may be missing entirely
How can a bottleneck event reduce genetic diversity?
– Imagine a bottle filled with marbles, where the marbles represent individuals in a population
– If a bottleneck event occurs, a small, random assortment of individuals survive the catastrophic event and pass through the bottleneck (and into a cup), while vast
majority of the population is killed off (remains in bottles)
– Genetic composition of random survivors is now the genetic composition of the entire population
What is the Founders Effect?
The founder effect is another extreme example of drift, one that occurs when a small group of individuals breaks off from a larger population to establish a colony
- The new colony is isolated from the original population, and founding individuals may not represent the full genetic diversity of the original population
– That is, alleles in the founding population may be present at different frequencies than in original population, and some alleles may be missing altogether
What is Gene Flow?
Non-random movement of genes into or out of a population by migration
- Members of one population may breed with occasional immigrants from an adjacent population of the same species
– This can introduce new genes or alter existing gene frequencies in the residents
- Gene flow tends to reduce differences between populations over time
What are the 3 Modes of Natural Selection?
- Directional selection
- Disruptive selection
(diversifying selection) - Stabilizing selection
What is Directional Selection?
Directional selection favors individuals at one end of the phenotypic range
– Peppered moth (originally white, but dark phenotypes survived with the introduction of dark smog)
– Antibiotic resistance
– Evolution of horse size
– More sickle cell genes in areas that are high in malaria
– Giraffe neck size
What is Disruptive Selection?
(diversifying selection)
Disruptive selection (or diversifying selection) favours individuals at both extremes of the phenotypic range (rarest mode of natural selection)
– Black-bellied seed crackers in Africa Species has two sizes of the beak – no intermediates
What is Stabilizing Selection?
Stabilizing selection favours intermediate variants and acts against extreme phenotypes
– Human birth weight
– Medium-height plants
If too short, then tall plants can outcompete for sunlight…if too tall, then more susceptible to wind damage
– Siberian Husky (medium dog)
These dogs have strong pectoral and leg muscles, allowing it to move through dense snow…the muscles are not too heavy or else they would sink deeper into the snow and become slower…the muscles are not too light or else they would not be strong enough to pull sleds
