BIOA01H3 – Lecture 25 th
Microevolution: is a heritable change in the genetic makeup of a population.
Population: includes all individuals of a single species that live together in the same
place and time.
16.1 Variation in Natural Populations
Phenotypic variation: differences in appearance or function that – if based on
heritable information – are passed from generation to generation.
16.1a Evolutionary Biologists Describe and Quantify Phenotypic Variation
Microevolutionary studies often begin by assessing phenotypic variation
Most characters exhibit quantitative variation individuals differ in small,
Other characters exhibit qualitative variation they exist in two or more
discrete states, and intermediate forms are often absent
Polymorphism: existence of discrete variants of a character.
16.1b Phenotypic Variation Can Have Genetic and Environmental Causes
Phenotypic variation within populations may be caused by genetic
differences between individuals, by differences in the environmental factors,
and a combination of both
Therefore genetic and phenotypic variations may not be perfectly correlated
16.1c Several Processes Generate Genetic Variation
Genetic variation has two potential sources:
1. The production of new alleles
2. The rearrangement of existing alleles
Most new alleles arises from small-scale mutations in DNA
Rearrangement of existing alleles into new combos result from larger-scale
changes in chromosome structure or number and from several forms of
16.1d Populations Often Contain Substantial Genetic Variation
Identify biochemical polymorphisms in diverse organisms using gel
1 Technique separates two or more forms of a given protein if differ
significantly in shape, mass, or net electrical charge
Identification of protein polymorphism allows researchers to infer genetic
variation at locus coding for that protein
16.2a All Populations Have a Genetic Structure
Populations are made up of individuals of the same species, each w/ its own
In diploid organisms an individual’s genotype includes two alleles at every
Sum of all alleles at all gene loci in all individuals called population’s gene
To describe structure of gene pool, scientists first identify genotypes in
representative sample and calculate genotype frequencies, the percentage
of individuals possessing each genotype
Knowing that each diploid organism has two alleles (either same alleles or
diff alleles) at each gene locus, scientist can then calculate allele
frequencies, the relative abundances of the different alleles
For locus w/ two alleles, use symbol p to identify frequency of one allele, and
q to identify frequency of other allele
16.2b The Hardy-Weinberg Principle Is a Null Model That Defines How Evolution
Does Not Occur
Hardy-Weinberg Principle: specifies the conditions under which a population of
diploid organisms achieves genetic equilibrium the point at which neither allele
frequencies nor genotype frequencies change in succeeding generations.
Also showed that dominant alleles needn’t replace recessive ones, and
shuffling of genes in sexual reproduction doesn’t itself cause gene pool to
According to this model, genetic equilibrium only possible if all following conditions
1. No mutations are occurring.
2. The population is closed to migration from other populations.
3. The population is infinite in size.
4. All genotypes in the populations survive and reproduce equally well.
5. Individuals in the population mate randomly with respect to genotypes.
If these conditions are met, allele frequencies of population for identified
gene locus will never change, and genotype frequencies will stop changing
after one generation microevolution will not occur
2 16.3a Mutations Create New Genetic Variations
Mutations: heritable changes in DNA; may be neutral, deleterious, or beneficial.
Deleterious mutations example; several simple mutations in humans cause
forms of Ehlers-Danlos syndrome, a disruption of collagen synthesis that may
result in loose skin; weak joints; or sudden death from the rupture of major
blood vessels, the colon, or the uterus.
16.3b Gene Flow Introduces Novel Genetic Variants into Populations
Gene flow: immigration of organisms/gametes that reproduce and introduce novel
alleles into populations they’ve joined.
16.3c Genetic Drift Reduces Genetic Variability within Populations
Genetic drift: chances events causing allele frequencies in a population to change
Particularly common in small populations b/c only few individuals
contribute to gene pool and b/c any given allele present in very few
Leads to loss of alleles and reduced genetic variability
Two general circumstances:
1. Population Bottlenecks
2. Founder Effect
Stressful factor i.e. droughts/disease/starvation kills individuals & eliminates
some alleles from population, producing population bottleneck
When few individuals colonize distant locality and start new population,
carry only small sample of parent population’s genetic variation
By chance, some alleles may be totally missing from new population, whereas
alleles that were rare back home, might occur at relatively high frequencies
Change in gene pool called founder effect
Genetic drift has improtatn implications for conservation of biology
By definition, endangered species experience severe population bottlenecks,
resulting in loss of genetic variability
3 Small number of individuals available for captive breeding programs may not
fully represent species’ genetic diversity; w/o variation, population more
subjective to diseases & environmental changes no