BIOL 1020 – CHAPTER 23 LECTURE NOTES
Chapter 23: The Evolution of Population (Microevolution)
I.
Historical perspective – ideas on evolution and related topics up to Darwin
A. divine design and perfection model
1. espoused by Aristotle, the ancient Greek philosopher (284-322 B.C.) whose ideas on biology dominated most thinking on
the subject until the renaissance
2. species were viewed on a scale from simple to complex
3. all organisms were seen as moving toward perfection (which was associated with complexity)
4. based on divine intervention and design (thus supernatural, outside the true realm of science)
5. now discredited in biology, but still part of the social consciousness
B. fossils
1. fossils were known for centuries before Darwin
2. fossils reveal organisms unlike any living today, and the idea that some fossils represent species that had become extinct
was recognized even by Leonardo da Vinci (1452-1519)
C. acquired traits
1. mostly associated with Lamarck (1744-1829)
2. still focused on a model of organisms driven toward complexity, but involved an explanation with natural causes
3. postulated that changes or “acquired characteristics” during an organism’s life could be passed on to offspring
4. famous example was Lamarck’s model for how giraffes developed long necks – he claimed that stretching of the neck in
one generation would lead to offspring with longer necks
5. understanding of genetic inheritance has led to rejection of acquired traits models
D. birth of modern geology – Lyell, uniformitarianism, and the ancient Earth
1. prior to the early 1800s, the world view of most was that the Earth is very young (around 6000 years old)
2. in the early 1800s, geologists began to apply scientific reasoning to studies of geological processes, and quickly
recognized that these processes require that the Earth be very old (billions of years) to occur naturally
3. this “uniformitarian” model of geological processes was made famous by Lyell’s Principles of Geology, which
influenced Charles Darwin
4. the uniformitarian model is essentially the basis of geology today; confirming tests of this model include dating rocks
using radioisotope ratios (more on that later)
E. artificial selection
1. it was well known that domesticated animals and plants had been breed over centuries by humans to produce different
varieties, indicating that the characteristics of a species can be modified by selection
2. some examples are different breeds of dogs and the “wild cabbage” lineage of cabbage, broccoli, cauliflower, Brussels
sprouts, collards, kale, etc.
F. population limits that would allow selection to act naturally were recognized
1. Malthus (1766-1834) wrote the most influential works on this subject
2. mathematically, populations will grow geometrically if unchecked
3. food supplies rarely can be expected to grow faster than arithmetically, thus putting a limit on population growth
II. Darwin’s theory: evolution occurs by natural selection
A. Darwin proposed that species evolve by natural selection; his theory of evolution was based on four general observations:
1. overproduction – each species produces more offspring than will survive to maturity
2. variation – individuals in a population vary, and some of the variation is heritable (this was expanded by others later, as
genetics came to be understood)
3. competition – there is competition among the individuals of a population for limited resources (struggle for existence)
4. differential reproductive success – individuals that possess more favorable characteristics (in the pool of variation) are
more likely to survive and reproduce; those with less favorable characteristics are less likely to survive and reproduce
B. thus, natural selection will produce a population of individuals more suited to their environment through time
C. when populations are separated (such as the geographic separation of islands from each other and a nearby continent), natural
selection on two separate populations can produce two distinct populations with different characteristics – resulting in two
separate species
D. note that for this theory to explain the current variety of species on Earth, there is a need for a long amount of time for natural
selection to produce the variety observed; thus, the idea of an ancient Earth hundreds of millions to billions of years old is
crucial
E. note also that this theory really has two major branches: microevolution, or changes of a population over time, and
macroevolution, or the formation of species
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III. The modern synthesis of evolutionary theory combines Darwin’s concept of natural selection with genetics
A. although Mendel was a contemporary of Darwin, remember that his work was largely unrecognized until around 1900
B. how traits are inherited was central to Darwin’s theory of evolution, and thus Darwin (and others) were keenly interested in
finding working models of inheritance
C. when genetic mechanisms came to be widely understood, they were quickly combined with Darwin’s model in the modern
synthesis, also called Neo-Darwinism or the synthetic theory of evolution
1. this model emphasizes the genetics of populations
2. evolution is seen as working by natural selection on individuals to change the genetic makeup of populations over
successive generations
D. mutations play a key role in providing a source of genetic variation
1. without genetic variation, evolution cannot occur
2. mutations are necessary to produce genetic variation
3. while many mutations have no impact and many others are harmful, it is critical to recognize that some mutations are
advantageous
IV. Microevolution is a change in allele frequencies or genotype frequencies in a population over time
A. population – a group of individuals capable or interbreeding and producing fertile offspring, and that are more or less
isolated from other such groups
B. gene pool – all alleles present in a population at a given time
C. phenotype frequency – proportion of a population with a given phenotype
D. genotype frequency – proportion of a population with a given genotype
E. allele frequency – proportion of a specific allele in a population
1. diploid individuals have two alleles for each gene
2. if you know genotype frequencies, it is easy to calculate allele frequencies
3. example:
 population (1000) = genotypes AA (490) + Aa (420) + aa (90)
 allele number (2000) = A (490x2 + 420) + a (420 + 90x2) = A (1400) + a (600)
 freq[A] = 1400/2000 = 0.70
 freq[a] = 600/2000 = 0.30
 note that the sum of all allele frequencies is 1.0
V. Genetic equilibrium in populations: the Hardy-Weinberg theorem
A. the Hardy-Weinberg theorem describes the frequencies of genotypes in a population based on the frequency of occurrence
of alleles in the population that is in a state of genetic equilibrium (that is, not evolving)
1. the usual case for calculations: if allele “A” is dominant to “a”, and they are the only two alleles possible at the A-locus,
then
 p = freq[A] = the frequency of occurrence of the A-allele in the population
 q = freq[a] = the frequency of occurrence of the a-allele in the population
2. Then p + q = 1 (following the sum rule for probability)
3. Allele associations follow the product rule for probability, so you multiply to predict the genotype frequencies:
 ( p + q ) x ( p + q ) = p2 + 2 pq + q2
 p2 = frequency of homozygous dominant genotypes
 2 pq = frequency of heterozygous genotypes
 q2 = frequency of homozygous recessive genotypes
 note that ( p + q ) x ( p + q ) = 1 x 1 = 1, so p2 + 2 pq + q2 = 1
B. Hardy-Weinberg equilibrium
1. if the Hardy-Weinberg theorem can be used to accurately predict genotype frequencies from allele frequencies for a
population then the population is in Hardy-Weinberg equilibrium or genetic equilibrium
2. in such cases you can use data from one generation to predict the allele, genotype, and phenotype frequencies for the next
generation
3. such populations are not evolving, but are static instead
C. the assumptions of this model are:
1. large population size (due to statistical constraints, to minimize genetic drift)
2. no migration – no exchange of alleles with other populations (no gene flow)
3. no mutations of the alleles under study occur
4. random mating of all genotypes
5. no natural selection
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VI. Microevolution thus can be described as deviation from Hardy-Weinberg equilibrium, where the allele or genotype
frequencies in a population change over time
A. consequences of small population size: genetic drift
1. Consider taking a small sample of individuals from a larger population. If only two individuals were picked they cannot
reflect the allele frequency in the larger population. Neither can 3, 4, or 5 individuals, but as the selected sample gets
larger it becomes more likely that the sample reflects the allele frequency in the larger population.
2. genetic drift is a change in gene frequencies of populations because of small population size
3. genetic drift tends to decrease genetic variation within a population
4. genetic drift tends to increase genetic variation between populations
5. two general mechanisms lead to small population sizes
 genetic bottlenecks are created by dramatic reduction in population size – endangered species face a genetic
bottleneck on a species-wide scale, and suffer lasting effects even if population size later recovers
 founder effect – when a new population is established, typically only a few individuals (founders) are involved in
colonizing the new area; this is common for islands
B. migration – when individuals leave or join a population
1. migrating individuals carry their alleles with them (gene flow), usually resulting in changes in allele frequencies
2. gene flow tends to decrease genetic variation between populations
C. mutations increase variation in the gene pool of a species
1. remember that mutations may be neutral, harmful, or beneficial
2. even at the risk of harmful effects, mutations are necessary to increase variation in the population so that natural selection
can produce organisms more suited to their environment
D. nonrandom mating
1. if individuals do not mate at random, then Hardy-Weinberg equilibrium is not achieved
2. the most common cases of nonrandom mating involve mating between individuals of similar genotypes, either by choice
or location
 such inbreeding does not change allele frequencies, but increases the frequency of homozygous genotypes
 inbreeding depression is seen in some cases, where inbred individuals have lower fitness that non-inbred individuals
 fitness – relative ability of a genotype to contribute to future generations
 fertility declines and high juvenile mortality associated with “unmasking” harmful recessive alleles can reduce
fitness for inbred individuals
 hybrid vigor also leads to higher relative fitness for hybrids
 self-fertilization is the most extreme case of inbreeding
3. assortive mating – mates are selected by phenotypes
 positive assortive mating – selection for the same phenotype; works like inbreeding for the genes governing that
phenotype, and for loci closely linked to those genes
 negative assortive mating – selection for the opposite phenotype
 less common than positive assortive mating
 leads to a decrease in homozygous genotypes for the genes governing the selected phenotype, and for loci
closely linked to those genes
E. natural selection changes allele frequencies in a way that leads to adaptation to the environment
1. fitness is the ability of an organism to compete successfully and pass its alleles on to the next generation
2. populations undergoing natural selection are evolving, with alleles that contribute to better fitness increasing in frequency
over successive generations
3. natural selection only operates based on the current environment – as environmental conditions change, different alleles
will be selected for
4. sexual selection (mate choices based on inherited characteristics) is an aspect of natural selection
5. there are 3 types of natural selection
 stabilizing selection – occurs in populations well adapted to their environments, selecting against phenotypic
extremes
 this is probably the type of selection most commonly faced by populations
 example - human birth weight
 directional selection – permits species to adapt to environmental change by favoring selection of one extreme over
the other; example – peppered moth
 disruptive selection – when more than one extreme phenotype is favored over intermediate phenotypes
 really a special case of direction selection, where there are trends in more than one direction
 can produce a genetic “split” in a population and thus serve as a mechanism for speciation
 example – pocket mice in New Mexico
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VII. Genetic variation must exist for natural selection to occur
A. the ultimate source of genetic variation is mutations
B. once variation exists, it can be affected by independent assortment and genetic recombination during gamete formation
1. consider the cross AaBb x AaBb – 9 different genotypes arise
2. this involves only 2 alleles at 2 loci; if there were 6 alleles possible at just 5 loci, over 4 million genotypes are possible
3. thus, given that there are thousands of genes in an organism, and that many alleles are possible at most of these loci, it
becomes clear that in nature there is great genetic variability
C. the demonstrated presence of two or more alleles at a given locus is genetic polymorphism
1. biologists have produced tools for studying the genetic polymorphism of populations at the molecular level (RFLP, DNA
sequencing, etc.)
2. these tools can be used to demonstrate and study polymorphism in populations without necessarily knowing the specific
genes involved
D. genetic polymorphisms can be maintained by heterozygote advantage or hybrid vigor
1. when either the homozygous dominant or recessive is more suited to an environment than the heterozygote, the
homozygous genotype will be more likely to be fixed in the population
2. but when heterozygous genotypes have advantage over either of the homozygous genotypes, variation tends to increase in
the population
3. example - sickle cell anemia and malaria resistance
E. genetic polymorphisms can be maintained due to frequency-dependent selection
1. there are cases where the frequency of a given genotype affects the degree to which it is or isn't selected in the population
2. example - predator/prey relationships, where individuals with a rare phenotype may be ignored by a predator, but as they
become more abundant the selective advantage decreases because the predator is more likely to notice them
F. much of the genetic variation in a population will produce no selective advantage, i.e. it is neutral
1. the role of neutral variation in evolution is debated today
2. remember that what is neutral in one context may not be neutral in another context, so as environments change some
previously neutral variation may be acted on by natural selection
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