1. Evolution of Populations
Chapter 16
Evolution of Populations

2. 16-1
Genes and Variation

3. Darwin’s Ideas Revisited
Darwin’s disadvantage was he did not understand the mechanism for heredity
he did not know the source for variation
he did not know how inheritable traits were passed from one generation to the next
it was fifty years later that biologists combined Mendel’s work with Darwin’s work
Gene Pools
gene pool: the combined genetic information of all members of a particular population
contains two or more alleles for each inheritable trait
relative frequency: the number of times an allele occurs in a gene pool compared with the number of times other alleles occur
expressed in percents

4. Sources of Genetic Variation
mutations and genetic shuffling are two main sources of genetic variation
mutations
a change in the genetic sequence of DNA occurring through replication mistakes, radiation, or mutagens
can be a few bases or large segments of chromosomes
most mutations have a negative affect on organism’s survival
gene shuffling
most variation comes from genetic shuffling
occurs during the production of gametes because of independent assortment and crossing over
it may change the various combinations of alleles but does not affect the relative frequency of alleles

5. Single-Gene and Polygenic Traits
the number of phenotypes produced for a given trait depends on how many genes control the trait
single-gene traits have two distinct phenotypes (eg. widow’s peak, attached ear lobes, etc.)
sometimes the dominant phenotype may be less frequent in a population
most traits are polygenic traits controlled by two or more genes
the larger of variations produced are generally grouped in a symmetrical bell-shaped curve

6. The dominant phenotype may be less frequent since allele frequencies may not match Mendelian ratios
100 80 60 40 20
Frequency of Phenotype
(%)
Widow’s peak
No widow’s peak
Phenotype

7. Frequency of Phenotype
Various phenotypes of polygenic traits can be grouped into a bell-shaped curve
Frequency of Phenotype
Phenotype (height)