Chapter 4 - Evolution of Populations Flashcards
Genetic variation in a population results from:
- Mutations form new alleles
- Chromosomal rearrangement
- Gene duplication
- Sexual reproduction
Crossing over during meiosis I (prophase I)
Effect:
creates new combinations of genes in the gametes that are not found in either parent, contributing to genetic diversity.
Random assortment during meiosis I (metaphase I)
Effect:
produces many variations among daughter cells, giving rise to genetic diversity in offspring.
Random fertilization
any egg (with its unique combination of alleles) can be fertilized by any sperm (with its unique combination of alleles).
Hardy-Weinberg Equilibrium
- No evolution
- is a mathematical formula that calculates the genotype and phenotype of a population. In a large population mating at random allele frequencies will remain constant if there is no mutation, migration, or natural selection
- What is its significance? identifies variables that can influence evolution in real-world populations.
- Natural populations can evolve at some gene loci while being in Hardy-Weinberg equilibrium at other loci
Conditions for Hardy-Weinberg Equilibrium
No mutation
No gene flow
Random mating
Very large population
No selection
Allele Frequency Equation
p + q = 1
freq. of A freq. of a
Genotype Frequencies Equation
p2 + 2pq + q2 = 1
freq. of AA freq. of Aa freq. of aa
Mechanisms Of Evolution
- Mutation
- Gene Flow (migration or emigration)
- Non-random mating
- Genetic Drift
- Natural Selection
Mutation
random change in an organism’s DNA that creates a new allele.
The ultimate source of variation. Individual mutations occur so rarely that mutation alone usually does not change allele frequency much.
Gene Flow (migration or emigration)
exchange of genes from one population to another will contribute to microevolution. A population might gain genes due to migration into the existing population. A population might lose genes due to emigration from the existing population.
A very potent agent of change. Individuals or gametes move from one population to another.
Non-random mating
inbreeding is the most common form. It does not alter allele frequency but reduces the proportion of heterozygotes.
Genetic Drift
Statistical accidents. The random fluctuation in allele frequencies increases as population size decreases.- change in the gene pool of a large population by chance.
- variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce.
Natural Selection
The only agent that produces adaptive evolutionary changes.
- Increases frequencies of alleles that provide reproductive advantage (higher relative fitness)
- genetic drift, mutation, and gene flow lead to microevolution, but it doesn’t necessarily lead to adaptive. Only natural selection is adaptive!
- Natural Selection favors the fittest individuals so they can pass down their alleles to their offspring so the alleles for the fittest phenotype increase over time i.e., the population evolved
- Increases frequencies of alleles that provide reproductive advantage (higher relative fitness)
- 3 conditions:
Variation in population
Variation must be genetically inherited
Variation must result in certain phenotype(s) producing more surviving offspring in following generation
Small population effect
In any generation, by chance, some individuals did not get reproduced.
Founder effect
o colonization of a new location by a few individuals of a population. The gene pool has been reduced because only a few individuals make up the population.
o Allele frequencies different in founder population
Bottleneck effect
o is an event that non-selectively reduces a large population into a small population.
3 modes of Natural selection
Directional selection
Disruptive selection
Stabilizing selection
Directional selection
phenotypes at one extreme have selective advantage over the rest
Disruptive selection
diversifying, both extremes of phenotype are favored over the intermediate.
Stabilizing selection
average phenotype has a selective advantage
Balancing selection can maintain variation in a population
- e.g., Heterozygote advantage
- e.g., frequency-dependent selection
Sexual Selection
- Can lead to sexual dimorphism - distinct difference in size or appearance between the sexes of an animal in addition to differences between the sexual organs themselves.
Evolutionary forces can interact
- E.g., natural selection and genetic drift
- E.g., natural selection and gene flow
Remember:
- There are limits to what natural selection can accomplish
Why?? Selection can only operate on the available genetic variation. It is not guided by a consciousness; it has no foresight and can lead species to evolve down paths that seem advantageous but actually lead to extinction - There is no “goal” in evolution
