Principles of Biology II - Leege
Georgia Southern University
Evolution of Populations


I. What is Evolution?
Change in genetic composition of a population over time

What is evolution in Genetic terms?
Changes in proportions of alleles present in a population for a particular trait across generations

II. Where does variation come from?

Two processes, mutation and sexual recombination, produce the variation in gene pools that contributes to differences among individuals
Mutation
Mutations are changes in the nucleotide sequence of DNA
Mutations cause new genes and alleles to arise

Point Mutations
A point mutation is a change in one base in a gene
It is usually harmless but may have significant impact on phenotype

Mutations That Alter Gene Number or Sequence
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Gene duplication is nearly always harmful

Mutation Rates
Mutation rates are low in animals and plants
The average is about one mutation in every 100,000 genes per generation
Mutations are more rapid in microorganisms

Sexual Recombination
Sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible

 

III. How do we measure evolution?
Hardy Weinberg Equation

Gene Pools and Allele Frequencies
Population: localized group of individuals capable of interbreeding and producing fertile offspring
Gene pool: all genes in a population at any one time
Gene pool consists of all gene loci in all individuals of the population

The Hardy-Weinberg Theorem
Describes a population that is not evolving

Frequencies of alleles and genotypes in a population's gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work

Mendelian inheritance preserves genetic variation in a population
LE 23-7

Preservation of Allele Frequencies
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes a population in which random mating occurs
It describes a population where allele frequencies do not change

Describing populations

Allele frequencies: p + q = 1
p = frequency of dominant allele

q = frequency of recessive allele

Practice:
If p = 0.5, then 1/2 the alleles in the whole population are dominant alleles

 

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
p2 + 2pq + q2 = 1

p2 = freq. of homozygous dominant genotype
q2 = frequency of homozygous recessive genotype
2pq = frequency of the heterozygous genotype

LE 23-7

Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population
In real populations, allele and genotype frequencies do change over time

 

The five conditions for non-evolving populations are rarely met in nature:
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection


Practice:

Population Genetics and Human Health
We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease

 

III. What are the agents of change in populations?
Three major factors alter allele frequencies and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow

Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

Genetic Drift
The smaller a sample, the greater the chance of deviation from a predicted result
Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next
Genetic drift tends to reduce genetic variation through losses of alleles
LE 23-8

The Bottleneck Effect
The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population
The resulting gene pool may no longer be reflective of the original population's gene pool
LE 23-9

Understanding the bottleneck effect can increase understanding of how human activity affects other species

The Founder Effect
The founder effect occurs when a few individuals become isolated from a larger population
It can affect allele frequencies in a population

Gene Flow
Gene flow consists of genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes
Gene flow causes a population to gain or lose alleles
It tends to reduce differences between populations over time