Evolution and Population GENETICS. Population Genetics Changes in evolution occur in populations,...
-
Upload
amie-fields -
Category
Documents
-
view
227 -
download
0
Transcript of Evolution and Population GENETICS. Population Genetics Changes in evolution occur in populations,...
Evolution and Population GENETICS
Population GeneticsPopulation Genetics
Changes in evolution occur in populations, not individuals
Population GeneticsBasic Vocaulary
•gene pool
–entire collection of genes in a population
•allele frequency
–number of times an allele appears in a gene pool
•genetic equilibrium
–gene pool which is not changing in allele frequencies
–a population at genetic equilibrium is not evolving
Evolution and Population Genetics
• For a population to evolve, there must be some form of change genetically.
• What are some things that cause change in the gene pool of a population?
4 Basic Evolutionary Mechanisms
1. Mutation
2. Migration
3. Genetic drift
4. Natural selection
Mutation
•introduces of new alleles into gene pool
How Mutation Causes Change
• A mutation (change in DNA code) could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make the genes for brown beetles more frequent in the population.
• Causes of mutations?
Migration
• When individuals of one population move and join a new population
How Migration Causes Change
• Some individuals from a population of brown beetles might have joined a population of green beetles. That would make the genes for brown beetles more frequent in the green beetle population.
Population Genetics•gene flow
–The movement of alleles from one population to another
–this can change the allele frequency of both populations
Genetic Drift
• random change in allele frequency over time
Causes of Genetic Drift
1.The Bottleneck Effect
•Genetic drift that occurs after an event greatly reduces the sizes of a population
•Numbers of individuals in population are greatly reduced
•Loss of diversity
Causes of Genetic Drift
2. The Founder Effect
•Genetic drift that occurs after a small number of individuals colonize a new area
•Gene pool very different from the larger population
Problems with Genetic Drift
• Loss of genetic diversity
• Less likely for some individuals to adapt to a changing environment
• Lethal and/or harmful alleles carried by heterozygotes and become more common in gene pool
Natural Selection
•Organisms that are better adapted to the environment survive and reproduce
•For larger populations natural selection is the significant factor which causes changes in allele frequency
Types of Natural Selection
• Directional- selection for one extreme
• Disruptive- selection for both extremes- against the “middle”
• Stabilizing- selection for the middle- against the extremes
Types of Natural Selection
1. Stabilizing Selection
–selection against extremes, favors the average
Types of Natural Selection
2. Directional Selection
–selection against one extreme, favors other extreme
Types of Natural Selection
3. Disruptive Selection
–selection against average, favors both extremes
Hardy-Weinberg Equilibrium
• Genotype frequencies remain constant in a population as long as certain conditions are met
• Frequencies can be predicted
• Identifies 5 conditions that must be met in order for a population to be in equilibrium
• Populations in equilibrium are not evolving= in Hardy-Weinberg Equilibrium
Hardy-Weinberg Equilibrium
• Very large population (no genetic drift)
• No emigration or immigration (no gene flow)
• No mutations (no new alleles added)
• Random Mating (no sexual selection)
• No natural selection (all traits are equal)
Why Use Hardy-Weinberg?
• Real populations rarely ever meet all 5 conditions!
• Why do you think we use the Hardy-Weinberg model?
Genotype Frequencies
• p2 + 2pq + q2 = 1
• p= frequency of dominant allele
• q= frequency of recessive allele
• p+q = 1 Why?
• What does p2 equal?
• What does 2pq equal?
• What does q2 equal?
Example
• In a population of fish fork tails are dominant over smooth tails.
• You have 1000 fish total in this population.
• 640 of them have forked tails.
• 360 of them have smooth tails.
• What are the genotype frequencies?
Example
• q2= 360 smooth-finned fish out of 1000 total– q2= 360/1000= 0.36
• To find q, take the square root of 0.36= 0.6
• Find P with the equation p + q = 1– So 1- 0.6 = 0.4
• Now we have p= 0.4 and q = 0.6 (allele frequencies)
Example
• Now we have p= 0.4 and q = 0.6 (allele frequencies)
• Calculate the genotype frequencies:– p2 = 0.42 = 0.16 = 16% TT– q2 = 0.62 = 0.36 = 36% tt– 2pq= (2)x(0.4)x(0.6) = 0.48 = 48% Tt
Evolution of a Species
•speciation
–the production of two or more species from one original population
What Causes Speciation?
1. Geographic Isolation
–physical barrier which separates populations
What Causes Speciation?
2. Reproductive Isolation
–groups can no longer sexually reproduce
–ex: behavior, mating patterns, physical differences
Speed of Speciation
1. Gradualism
–species evolve at a slow constant rate
Speed of Speciation
2. Punctuated Equilibrium
–rapid significant changes over a short time
Patterns in Evolution
1. Adaptive Radiation
–new species develop from a single common ancestor
Patterns in Evolution
2. Divergent Evolution
–new species develop from a single common ancestor
Patterns in Evolution
3. Convergent Evolution
–different species develop similar traits
Patterns in Evolution
4. Co-evolution
–different species develop traits as a result of a close relationship with the other species
Two Types of Evolution
• Microevolution: a change in gene frequency within a population. – Ex. Changes is a
beetle population over a short period of time.
– Ex. Antibacterial resistance
• Macroevolution: evolution above the species level. Large Scale!(Patterns, Speciation)– Ex. Changes in all
beetle populations over a long time
– Ex. Origin of mammals