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Ecology
Lake Tahoe Community College
Fall qtr
Instructor: Sue Kloss
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Chapter 4 - Population Genetics and Evolution
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Darwin first came up with the theory of evolution by natural selection 175 years ago on his voyage around the earth on the
HMS Beagle. Evolution is change over time of an organism.
The genetic variation in a population that allows natural selection to modify a population, so that natural selection occurs at
the population level. Populations respond to selection with changes in allele frequency.
I. Intro
A. there must be genetic variation in a population for natural selection to act upon
1. In order for natural selection to occur in a population, there must be:
a. variation in the pop
b. it must be heritable
c. it must cause differential survival/reproduction
2. Over time, natural selection in action causes evolution
3. Genetic info is stored in DNA (Deoxyribonucleic acid)
4. Structure of DNA is sugar phosphate backbone with nitrogenous bases
5. genetic information is stored in the particular order of bases
6. DNA strand is the code
7. Each set of 3 bases is called a codon
8. Each codon asks for a specific amino acid in the recipe.
9. The particular sequence of amino acids determines the type of protein that will be produced
B. The source of genetic variation is mutation and recombination
1. substitutions most common form of mutation
2. also deletions, additions, rearrangements
3. the new protein may retain the characteristics of the original, or not
4. altered proteins resulting from mutation may or may not be beneficial
5. e.g. sickle cell anemia is caused by a mutation.
6. Mutations are more likely to be beneficial when relationship between the organism and env. changes
7. Mutation is random force in evolution
C. Mutation Rates
1. for any particular nucleotide in a DNA sequence, the rate of mutation is very low
2. low rate x the number of nucleotides in a gene (hundreds or thousands)- each org has mutations
3. the more genes responsible for a particular trait, the more likely mutation in that trait.
D. Recombination
1. when your mom and dad’s gametes were formed, all the chromosomes (23 pairs) got mixed
and matched so that from your mom’s contribution, (the egg), you got an array of her mom’s
genes and an array of her dad’s genes. The same thing happened with your dad’s sperm; in
each gamete, the chromosomes got mixed in a way that has never happened before. This is
called independent assortment.
2. In addition, crossing over occurs in meiosis; you have combinations of genes never seen in
anyone anywhere. Those are the unique sets of genes that you alone will pass on.
3. Recombination is the exchange of homologous sections of mom’s and dad’s chromosomes
a. or in biotech, the splicing of genes to produce new combos
4. recombination – produce genetic variation very rapidly, mutation works very slowly.
5. the new combinations of genetic traits results from sexual reproduction and recombination
provide natural selection with abundant variation to work on.
E. The genotypes of all individuals make up the gene pool of a population
F. Gene Pool - all the genes in all the individuals in a population make up all the genetic variation of the
population
1. When all individuals mate at random in a population, then all combinations of different alleles
are possible
2. many of these combinations are not present in the population at any given time
3. For example, if there are genes A and B in a population, and A1 and B1 are the common
forms, and A2 and B2 are very rare, there may be small numbers of A1B2’s, and of A2B1’s,
but no A2B2’s. The possibility exists that there may be A2B2 individuals in the population in
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the future, however.
4. the more varied the gene pool of a species, in general, the more tools a species has to adapt to
changing environmental conditions (evolve).
G. Hardy-Weinberg law
1. Alleles and genotypes in a population remain constant if:
a. a large number of individuals,
b. random mating
c. no selection
d. no mutation and
e. no migration between populations
2. In other words, natural selection over a period of time causes evolutionary change to occur
Sexual reproduction alone will not change allele or genotype frequencies.
a. additional forces must act on a population to change genotype frequencies in a pop
3. Hardy-Weinberg equilibrium - The particular allele frequency in a population does not change
over time unless acted upon by an outside force
H. Most natural populations deviate from Hardy Weinberg equilibrium due to
1. mutation - a small force in HW, so we will focus on other forces
2. small population size
a. genetic drift
b. founder events
c. bottleneck events- cheetahs are in this situation right now - very little genetic variation
3. nonrandom mating
4. migration - even a very little bit, tends to keep increasing genetic variation
5. natural selection - differential survival/repro
a. stabilizing selection
b. directional selection
c. disruptive selection
I. The genotypes of individuals within a population often vary geographically
1. differences in selective factors or random changes like bottleneck events can cause geographic
differences in allele frequencies
2. such variation is often found between subpopulations divided by a natural barrier like a canyon
3. if natural selection is more of a factor than gene flow, then subpopulations will have differences in allele
frequencies
4. a Swedish botanist, Turesson in the early 1900s, collected seeds from a single species and
grew them in common gardens in difft habitats.
5. plants that maintain original forms, even though they no longer had their normal environments, are
called ecotypes - specific adaptations to particular environment
6. clines- gradual change in a trait in response to environmental variation
a. Yarrow (Achillea millefolium) shows ecotypic variation
b. So does Potentilla glandulosa
7. Evolutionary changes in allele frequencies have been documented in natural populations
J. Ecologists can learn much from population genetics studies
1. every population has some gv that influences fitness; evolution continues in all populations
2. changes in environmental characteristics will cause natural selection to alter the genotype
frequencies in populations; in other words, populations will change to become more well adapted to the
new environment
3. rapid environmental changes (eg. human induced changes) may happen too rapidly for natural
selection to work, in which case decline of a population to extinction is possible
a. eg exotic introductions of competitors or pests/parasites
Lesson Objectives
1. Does natural selection act on the individual, the population, or the community? How does it act?
2. What are 2 sources of genetic variation in life forms? Describe how variation is produced.
3. Define recombination, and explain how it works in sexual reproduction.
4. Distinguish between a genome and a gene pool.
5. What 5 characteristics must be true for a population to maintain its genetic structure?
6. Why are small populations at risk for losing genetic variation? Describe 3 types of events that may cause loss of
genetic variation in a small population.
7. Define an ecotype. Define a cline. Describe the type of experiment that will distinguish between the two conditions.
8. Distinguish between stabilizing, disruptive and directional selection.
9. Describe one case of natural selection that has been documented.
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