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Biology II Notes
Chapter 13 pp. 264-275
V. continued
E. Scientists can observe natural selection in action.
1. Natural selection is a work in progress and has been documented more
than 100 times. (insects, bacteria)
2. Natural selection is an editing process, not a creative mechanism. It is
also timely, regional, and can occur rapidly.
VI. Population Genetics and the Modern Synthesis
A. A population is a group of individual organisms living in the same place at the
same time.
B. Evolution is the measured frequency of a given characteristic within a
population over a succession of generations.
C. Population genetics concentrates on the population as the unit of evolution
and natural selection’s role in the evolution.
D. A population’s gene pool consists of all the alleles in all the individuals
making up the population.
E. During microevolution, the relative frequencies of the alleles governing
characteristics change.
VII. The Gene Pool of a nonevolving population remains constant over the generations.
A. The Hardy-Weinberg equation shows that allele frequencies are stable in a
population not undergoing macroevolution.
p2 + 2pq + q2 = 1
p2 homozygous dominant genotypes;
2pq heterozygous genotypes
q2 homozygous recessive genotypes
B. Five conditions are required for Hardy-Weinberg equilibrium:
The population is very large.
The population is isolated (no gene flow).
Mutations do not alter the gene pool.
Mating is random.
All individuals are equal in reproductive success (no natural
C. Hardy-Weinberg rarely exists in natural populations. It gives us a basis for
understanding how populations evolve.
VIII. In addition to natural selection, genetic drift and gene flow can contribute to
A. Genetic Drift is a change in a gene pool of a small population due to chance.
The smaller the population, the greater the effect.
Bottleneck effect – genetic drift resulting from a disaster that
reduces population size.
Founder effect- genetic drift resulting from colonization of a new
area by a small number of individuals.
B. Gene Flow- a gain or loss of alleles from a population due to immigration or
emigration of individuals or gametes.
C. Endangered Species often have reduced variation.
1. Becoming worse as human activity endangers wild populations.
2. These animals suffered bottlenecks due to disease, hunting, and
drought. There is an extremely high degree of genetic uniformity.
VIX. Variation and Natural Selection
A. Variation is most extensive in most populations.
B. Variation may be caused by one or more genes or the action of the
C. A population is polymorphic for a characteristic if two or more morphs
(contrasting forms) are noticeably present.
D. Most populations exhibit geographic variation in the distribution of
characteristics. This type of variation is called a cline.
X. Mutation and Sexual Recombination Generate Variation.
A. Mutation is the ultimate source of variation. Only mutations in gametes maybe
passed to offspring.
B. Sexual recombination shuffles to mixture of alleles in diploid organisms.
1. independent assortment
2. crossing over
3. random fertilization of sperm and egg
XI. Diploidy and Balancing Selection Preserve Variation
A. An ancestral population is varied, with individuals having characteristics
suited for many types of environments.
B. Over successive generations, those individuals with the characteristics best
suited for the environment leave more offspring. These characteristics
increase in the subsequent generations.
C. Those individuals with characteristics not suited for the environment leave
fewer offspring. These characteristics decrease in subsequent generations.
D. Heterozygote advantage is a situation in which the heterozygote is favored
over either homozygote.
XII. The perpetuation of genes defines evolutionary fitness.
A. It is the survival of genes and the phenotypic traits expressed by genes, not
individual organisms that are important.
B. Fitness is the relative condition that the individual makes to the gene pool of
the next generation.
XIII. Natural Selection can alter variation in a population in a three ways.
A. Stabilizing selection
B. Directional selection
C. Disruptional Selection
XIV. Sexual selection may produce sexual dimorphism: two examples of secondary
sexual characteristics.
XV. Natural Selection cannot fashion perfect organisms.
A. Historical constraints
B. Adaptations are often compromises
C. Chance and natural selection interact
D. Selection only edits existing variations