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Chapter 16: Evolution of Population – Notes
Genes and Variation
The study of kinds of number of genes in a populations is called as “population genetics”.
A Gene pool consists of all genes, including all the different alleles, that are present in a population.
The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared
with the number of times other alleles for the same gene occur.
Relative frequency is often expressed as a percentage.
Q 1. The diagram 16-2 shows gene pool for fur color in a population of mice. Calculate. In a total of 50
alleles, 20 alleles are B (black), and 30 are b (brown). How many of each allele would be present in a
total of 100 alleles? ___________
If relative frequency of B allele decreased in the gene pool, what would happen to the relative frequency
of the other allele? Increase or Decrease? ______________________
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Sources of Genetic variation
1. Mutation – change in sequence of DNA due to error in replication, radiation, or chemicals.
Some can affect an organisms fitness(ability to survive & reproduce) others have no effect.
2. Gene shuffling - Most heritable differences are due to gene shuffling that occurs during the
production of gametes.
23 pairs of chromosomes can produce 8.4 million combinations of genes. And crossing over
increases the number of genotypes that can appear in offspring. The sexual reproduction results in
many different phenotypes. But it does not change the relative frequency of alleles in a population.
The number of phenotypes produced for a given trait depends on how many genes control the trait.
Single gene trait - controlled by single gene - 2 alleles - only 2 possible phenotype
Ex Widow’s peak (allele is dominant over the allele for a straight hairline but is less frequent)
Compare by Bar graph.
Polygenic trait – controlled by 2 or more genes – each gene has 2 or more alleles – many possible
genotypes and even more phenotypes.
Ex. Height in Humans. It is represented by symmetrical bell shaped curve.
Evolution as Genetic change - Natural selection does not act on genes. It acts on phenotype and decides
which phenotype is suitable to survive and reproduce.
Natural selection on single-gene traits - leads to changes in allele frequencies and thus to evolution.
Q2. Figure 16-5 shows a population of brown lizards in which mutation produces red & black forms.
If population lives in the dark soil then how does a color affect the fitness of the lizards.
What do you predict the lizard population will look like by generation 50? Explain
Natural Selection on Polygenic Traits - action of multiple alleles on traits such as height produces a range
of phenotypes that often fit a bell curve.
Natural selection affect the distributions of phenotypes in any of three ways:
1. Directional Selection - individuals at one end of the curve have higher fitness than in the middle or
at the other end.
Ex.- increase in the average size of the beaks of finches who compete for food.
Dotted line - original distribution of beak sizes, Solid line - changed distribution of beak sizes.
Peak shifts as average beak size increases.
2. Stabilizing Selection – individuals near the center of the curve have higher fitness than individuals
at either end of the curve.
Ex. Birth weight of human infants
The smaller babies are less healthy, the larger have difficulty being born therefore both are less fit
than the average. The average weighing babies are more likely to survive than smaller or larger.
3. Disruptive Selection - individuals at the upper and lower ends of the curve have higher fitness than
individuals near the middle
The selection acts against an intermediate type & can cause the single curve to split into two (creates
two phenotypes.
The average-sized seeds become less common than larger and smaller seeds. The bird population
splits into two subgroups eating larger & smaller seeds.
Genetic Drift - The random change in allele frequency is called genetic drift.
Most likely to occur in small population or when a small group of organisms
colonize a new habitate.
In small populations, an allele can become more or less common simply by chance.
e.g. individuals that carry a particular allele may leave more descendants than other
individuals do, just by chance.
A situation in which allele frequencies change as a result of the migration of a small subgroup of a
population is known as the founder effect.
Evolution Versus genetic equilibrium
The Hardy-Weinberg principle states that allele frequencies in a population will remain constant
unless one or more factors cause those frequencies to change.
Genetic Equilibrium Occurs when the allele frequencies in the population remain constant &
population does not evolve.
Following Five conditions are required to maintain genetic equilibrium:
1.
2.
3.
4.
5.
There must be random mating
The population must be very large
There can be no movement into or out of the population
No mutations
No natural selection
If the above conditions are not met, genetic equilibrium will be disrupted and the population will
evolve.
Speciation
Speciation - formation of new species
When two populations become reproductively isolated, new species evolve.
Following are Reproductive Isolation - two populations cannot interbreed and produce fertile offspring.
1. Behavioral Isolation - two populations interbreeding but have differences in courtship rituals or
other reproductive strategies that involve behavior.
2. Geographic Isolation - two populations are separated by geographic barriers such as rivers,
mountains.
It does not form new species when it enables to separate populations.
Sometimes tt may separate certain types of organisms but not others.
3. Temporal isolation - two or more species reproduce at different times.
Q3. What type of reproductive isolation do the following situations illustrate?
1. The eastern meadowlark and western meadowlark have overlapping ranges. They do not
interbreed, because they have different mating songs. ______________________________
2. The Kaibab squirrel evolved from the Abert squirrel. The Kaibab squirrels were isolated from the
main population by the Colorado River._____________________________________________
3. The three similar species of orchid living in the same rain forest release pollen on different days
hence can not pollinate one another.________________________________________________.
Testing natural selection in Nature
The bird specimen that Darwin collected looked so different from one another that thought they were
blackbirds, warblers as they differed greatly in the sizes and shapes of their beaks and in their feeding
habits. Later he realized that they were all finches and they had descended from a common ancestor. The
natural selection shaped the beaks of different bird populations as they adapted to eat different foods.
Read Textbook page 406 – 407 to answer the following
Q4. What type of isolation mechanism was responsible for the formation of different finch species?
________________________.
Q5. What was the objective of the Grant’ study?__________________________________________________
___________________________________________________________________________________.
Q6. What type of Natural selection did the Grants observe in the Galapagos? and why? _______________
___________________________________________________________________________________.
Q7. How did Grants’ work reinforce Darwin’s hypothesis about the evolution of the finches? ____________
___________________________________________________________________________________
Q8. What 2 testable assumptions were the basis for Darwin’s hypothesis about the evolution of the finches?
____________________________________________________________________________________
Q9. Explain how the Galapagos finches may have evolved? (page 408-9)_____________________________
____________________________________________________________________________________
Q10. complete the concept map about evolution of population.