Chapter 23: The Evolution of Populations

 

I. Population Genetics

     A. Microevolution: Change in the genetic make-up of a population from generation to generation (Reed)

 

     B. Population: A localized group of individuals that are capable of interbreeding and producting fertile offspring (Reed) 

 

     C. Gene Pool: Aggregate of genes in a popluation at any one time (Includes alleles at all gene loci in all lindividuals of the population). (Reed)

 

     D. Hardy-Weinberg Theorem: 1908 - The grequencies of alleles and genotypes in a population's gene pool remains constant from generation to generation, as long as only      Mendelian segregation and recombination of alleles are at work. (Reed)

 

     E. 5 Conditions for Non-Evolving Populations:

          1. Extremely large population size    

          2. No gene flow     

          3. No mutations

          4. Random mating

          5. No natural selection (Reed)

 

     If the H-W conditions hold true:

Gene frequencies will not change over time.

        Evolution will not occur (Keller). (Griesenauer)

 

     If the H-W conditions are violated:

Microevolution will occur.

           Microevolution is caused by changes of the 5 Hardy Weinberg assumptions:

             1.   Genetic Drift

             2.   Gene Flow

             3.   Mutations

             4.   Nonrandom Mating

             5.   Natural Selection   (Griesenauer)     

     

 

The Equation : p² + 2pq +q² = 1

The letters p and q represent the relative frequencies of the only two possible alleles at a particular locus. p² represents the frequency of the homozygous dominant genotype. q² represents the frequency of the homozygous recessive genotype. 2pq is the frequency of the heterozygous genotype. The Hardy-Weinberg equilibrium applies only if the population is large, mating is random, mutation is negligible, there is no gene flow from other populations, and all individuals have equal reproductive success. (Mroz)

 

Directional, Disruptive, and Stabalizing Selection

Frequency distribution of heritable traits can be altered by natural selection. The three modes of selection are called directional selection, disruptive selection, and stabalizing selection. Directional selection is natural selection that favors individuals at one end of the phenotypic range. It is most common when a population's environment changes or when members of a population migrate to a new habitat with different environmental conditions than their former one. Disruptive selection is natural selection that favors individuals on both extremes of a phenotyic range over intermediate phenotypes. Stabilizing selection is natural selection that favors intermediate variants by acting against extreme phenotypes. This reduces variation and maintains the status quo for a particular phenotypic character. Selection favors certain heritable traits through differential reproductive success. (Mroz)

 http://porpax.bio.miami.edu/~cmallery/150/evol/evolution.htm (Mroz)

This diagram demonstrates the modes of selection using snails. These cases describe three ways in which a hypothetical snail population with heritable variation in snail shell coloration from light to dark may evolve. The graphs show how the frequencies of individuals with various shell colors change over time. The white arrows symbolize natural selection working against certain phenotypes. (Mroz)

 

Mutations and sexual recombination

Mutations, as well as sexual recombination, contribute to variation within a population's gene pool. Mutations are  the origin for new genes and new alleles. Mutations are changes within the nucleotide sequence in DNA. Somatic cells are the most common area for mutations to occur. Only mutations in cell lines produce gametes are passed on to offspring. Point mutations are changes in one base pair, which leads to dramatic changes in phenotype. The most harmful mutations however, are those that delete, disrupt, or rearrange gene loci. The duplication of genes is the key source of variation. (Brandt)

 

Sexual Recombination is also a very significant part in the variations of a species. Nearly all of the phenotype variations are  the direct result recombination shuffling of the present alleles in the gene pool. This process rearranges alleles so that each generation contains fresh pairs of alleles. Bacteria and viruses experience this shuffling much less frequently than animals and plants. (Brandt)

 

Natural Selection

• This is the primary mechanism of adaptive evolution.
• Genetic Variation- Each person has a unique genome, reflected in individual phenotypic variations.
  • Variation Within a Population
    • Discrete characters: either-or basis
    • Heritable quantitative characters: results from the influence of two or more genes on a single phenotypic character (Hill)

 

Genetic Drift

Fluctuations within a population is known as genetic drift. There are two factors that influence the impact of genetic drift, the bottleneck and founder effect.(Brandt)

Bottleneck- Bottleneck effect is when changes in the environment (floods, fires, etc.) greatly reduce the size of a species population. Some of the surviving alleles may be overrepresented, and vice versa. The larger the population is, the less effect bottleneck will have on the allele frequency. (Brandt)

Founder- The founder effect is when certain individuals within a population become isolated, and the group establishes a new population, which is not reflective of the original population. This accounts for much more of the inherited disorders, as opposed to the bottleneck effect. (Brandt)

 

• The net effect of genetic drift on a small population's gene pool can be rapid evolution, as illustrated in the hypothetical inheritance patterns shown below.  Note that the red trait dramatically increases in frequency from generation to generation.  It is important to remember that this can occur independent of natural selection or any other evolutionary mechanism.

 

Gene Flow is the gain or lose of alleles by movement of new or old alleles in or out of a area of population respectively. An example would be a black cat move into an area of white cats and there are now gray kittens. (Ryan)

 

Polymorphism- when individuals (morphs) are different from rest of the population in two or more ways enough to be noticeable. (Ryan)

 

Variation Between Populations

• Geographic Variation- differences between the gene pools of separate populations or population subgroups
• Cline- a graded change in a trait along a geographic axis (Hill) 

  An example of cline is as the longitude increases above the equator, most species of birds and mammals become larger to combat the cooler climate. (Archibald)

 

Gentic Variation (Awan)

Average Heterozygosity- average percent of loci that are heterozygous.

     Geographic Variation- differences between the gene pools of seperatye populations or population subgroups. (Awan)

 

Genetic Basis of Variation

1. Discrete Characters- Menedelian traits with clear phenotypes

2. Quantitative Characters-Multigene traits with overlapping phenotypes; provide most of the heritable variation within a population

(Fernandez)

 

Sexual Dimorphism- is the distintion between males and females on the basis of secondary sexual characteristics. 

Sexual Selection-

• is the selection for traits that may not be adaptive to the environment but do enhance reproductive success.
• such traits may increase an individual's sucess in competing for or attracting mate
• intrasexual selection-individual's sucess in competing for mate
• intersexua selectionl-individual's sucess in attracting a mate
• intersexual selection- may be baed on showy traits that reflect the general health of the male and thus fitness of his alleles.

(Fernandez)

 

this picture shows an example of showy traits to attract mates.

http://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Pfau_imponierend.jpg/260px-Pfau_imponierend.jpg

(Fernandez) 

 

4 Reasons why evolution does not produce perfect organisms:

1. each species has evolved from a long line of ancestral forms, man of whose structures have been coopted for new situations.
2. adaptations are often compromises between the need to do several different things, such as swim and walk
3. chance events affect a population's evolutionary history.
4. natural selection can act only on variations that are available, new alleles do not arise as they are needed.

(Fernandez)

 

Review Questions

1. The average length of jackrabbit ears decreases gradually with increasing latitude. This variation is an example of...(ramey)

a. directional selection

b. discrete variation

c. polymorphism

d. genetic drift

e. diruptive selection

 

2. Sparrows with average-sized wings survive severe storms better than those with longer or shorter wings, illustrating...(ramey)

a. the bottleneck effect

b. stabilizing selection

c. frequency-dependent selection

d. neutral variation

e. disruptive selection

 

3. No two human individuals are alike, except for identical twins. The chief cause of the variation among individuals is...(ramey)

a. new mutations that occurred in the preceding generation

b. sexual recombination

c. genetic drift due to the small size of the population

d. geographic variation within the population

e. environmental effects.

 

4. At a locus with a dominant and recessive allele in Hardy-Weinberg equilibrium, 16% of the individuals are homozygous for the recessive allele. What is the frequenct of the dominant allele in the population?...(Ramey)

a. 0.7

b 0.36

c. 0.6

d. 0.4

e. 0.48

 

answers...a, b, b, c 

 

5) In the gene pool of a population with 100 individuals, a fixed allele for a particular gene locus has a frequency of

     a. 1.

     b. d. 100.

     c. 0.5.

     d. 0.

     e. cannot be calculated based on this information

(Langguth)

 

6) Natural Selection changes allele frequencies in populations because some ___________________ survive and reproduce more successfully than others.

     a. species

     b. gene pools

     c. alleles

     d. individual organisms

     e. gene loci

(Langguth)

 

7) Which of the following is a polymorphic trait in humans?

     a. variation in fingerprints

     b. free versus attached earlobes

     c. variation in the number of fingers

     d. variation in height

     e. variation in intelligence

(Langguth)

 

8) Researchers examining a particular gene in a fruit fly population discovered that the gene can have either of two slightly different sequences, designated A1 and A2. Further tests showed that 70% of the gametes produced in the population contained the A1 sequence. If the population is at Hardy-Weinberg equilibrium, what proportion of teh flies carries both A1 and A2?

     a. 0.36

      b. 0.42.

     c. 0.48.

     d. 0.84.

     e. 0.6.

(Langguth)

 

Answers: 5-8

5) a

6) d

7) b

8) b

 

(Langguth)

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Works Cited

 

Campbell, Neil A.. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.

 

Keller, Amy.  "Chapter 23: The Evolution of Populations."  Avon High School.  Avon, IN.  29 September 2008.