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CHAPTER 3
EVOLUTION AND GENETICS
CHAPTER OUTLINE
I. Creationism and Evolution
A. Creationism and Catastrophism
1. Creationism accounts for biological diversity by referring to the divine act of Creation
as described in Genesis.
2. The discovery of fossil remains of creatures clearly unknown to modern humans was
not accountable within the terms of simple Creationism.
3. Catastrophism is a modified version of Creationism. It accounts for the fossil record
by positing divinely authored worldwide disasters that wiped out the creatures
represented in the fossil record, who were then supplanted by newer, created species.
4. Both versions of creationism describe the different species of plants and animals as
essentially different, having distinct, separate moments of creation.
B. Evolution
1. An alternative term for early evolutionism was “transformism.”
2. Darwin was influenced by the geological concept of uniformitarianism.
a. Uniformitarianism states that past geological events can be best explained by
observing the ongoing events of the present and generalizing backward through
time.
b. It further asserts that current geological structures are the result of long-term
natural forces.
3. Transformism had posited the primordial relatedness of all life forms.
4. Darwin posited natural selection as the mechanism through which speciation takes
shape (reaching this conclusion along with Alfred Russell Wallace).
5. “Natural selection is the gradual process by which nature selects the forms most fit to
survive and reproduce in a given environment.”
6. For natural selection to work on a given population there must be variety within that
population and competition for strategic resources.
7. The concept of natural selection argues that organisms that have a better fit within
their environmental niche will reproduce more frequently than those organisms that fit
less well.
II. Genetics
A. The science of genetics explains the origin of the variety upon which natural selection
operates.
B. Heredity is Particulate
1. The study of hereditary traits began in 1856 by Gregor Mendel, an Austrian monk.
2. By experimenting with successive generations of pea plants, Mendel came to the
conclusion that heredity is determined by discrete particles, the effects of which may
disappear in one generation, and reappear in the next.
3. Mendel determined that the traits he observed occurred in two basic forms: dominant
and recessive.
a. Dominant forms manifest themselves in each generation.
b. Recessive forms are masked whenever they are paired with a dominant form of the
same trait in a hybrid individual.
c. It has since been demonstrated that some traits have more than these two forms-human blood type, for example, has several forms, some of which are codominant.
4. The traits Mendel identified occur on chromosomes.
a. Humans have twenty-three matched pairs of chromosomes, with each parent
contributing one chromosome to each pair.
b. Chromosomes contain several genes, or genetic loci, which determine the nature of
a particular trait.
c. A trait may be determined by more than one gene.
d. Alleles are the biochemically different forms that may occur at any given genetic
locus.
e. Chromosome pairs’ loci may be homozygous (identical alleles) or heterozygous
(mixed).
C. Independent Assortment and Recombination
1. Mendel also determined that traits are inherited independently of one another.
2. The fact that traits are transmitted independently of one another, and hence may occur
in new combinations with other traits is responsible for much of the variety upon
which natural selection operates.
3. Mitosis is ordinary cell division, wherein one cell splits to form two identical cells.
4. Meiosis is the type of division particular to sex cells, wherein four cells are produced
from one, each with half the genetic material of the original cell (i.e., twenty-three
chromosomes instead of forty-six).
5. Fertilization allows the products of meiosis from one parent to recombine with those
from the other parent.
6. Because genes sort independently during recombination, the number of possible
combinations is exponentially high (223): a major source of variety.
III. Population Genetics
A. Population genetics looks at changes in gene frequencies at the level of the community or
breeding population.
1. Gene pool refers to all of the alleles and genotypes within a breeding population.
2. Genetic evolution is defined as change in the frequency of alleles in breeding
population from generation to generation.
B. There are four basic mechanisms which produce changes in gene frequency in a
population (i.e., genetic evolution): natural selection, mutation, genetic drift, and gene
flow.
IV. Mechanisms of Genetic Evolution
A. Natural Selection
1. Genotype refers to the genetic makeup of an organism.
2. Phenotype is the expression of the genotype as it has been influenced through
development by interacting with its environment.
3. Environmental influence in this interaction is extremely important, and lends great
plasticity to human biology.
4. Natural selection acts upon phenotypes.
B. Directional Selection
1. Natural selection affects gene frequencies within a population.
2. Adaptive genes are selected for (organisms containing them reproduce more
frequently).
3. Maladaptive genes are selected against (organisms containing them reproduce less
frequently).
4. When specific adaptive genes are selected for over a long time period, causing a major
shift in gene frequency, this is called directional selection.
5. Directional selection continues until equilibrium is reached (due to the effects of
contradictory selective forces, the base mutation rate, or both).
6. Directional selection, in favoring one gene, can reduce variation in a gene pool.
C. Sickle-Cell Anemia
1. Just as directional selection reduce varieties, it can also maintain genetic variety by
favoring a situation in which the frequency of certain alleles remains constant between
generations.
2. Hemoglobin in Africa
a. HbA and HbS are two alleles for a gene that largely determines hemoglobin
production in humans.
b. Homozygous HbA produces normal hemoglobin, homozygous HbS produces lethal
sickle cell anemia, heterozygosity for this gene produces the (in some
circumstances) deleterious but non-lethal sickle-cell syndrome.
c. It was discovered in certain populations in Africa, India, and the Mediterranean that
HbS existed at surprisingly high frequencies.
d. This is largely explained by the fact that the populations noted were in heavily
malarial areas, and that the heterozygous form produced a phenotype that was
resistant to malaria, and was thus the phenotype most fit for that environment.
3. It is important to note that traits that are maladaptive in one environment, such as the
sickle cell would be in a malaria-free zone, can be adaptive in a different environment,
and the reverse of this is also true.
D. Mutation
1. Mutation introduces genetic variation into a breeding population.
2. Chemical alterations in genes may provide a population with entirely new phenotypes,
with possible concomitant selective advantages.
3. The spread of HbS in heavily malarial environments is one example.
E. Random Genetic Drift
1. Random genetic drift is the loss of alleles from a population's gene pool through
chance.
2. There is no set form for this chance; it may simply occur through a statistical fluke in
sexual reproduction patterns, or through the effects of a catastrophe on the population
as a whole.
F. Gene Flow
1. Gene flow occurs through interbreeding: the transmission of genetic material from one
population to another.
2. Gene flow inhibits speciation, the formation of new species.
a. A species is an internally interbreeding population whose offspring can survive and
are capable of reproduction.
b. Speciation occurs when populations of the same species become isolated from each
other (thus stopping gene flow) allowing natural selection and genetic drift to
gradually produce gene pools that are different, to the extent that successful
interbreeding is no longer impossible.
V. Human Biological Adaptation
A. Much of the human biological diversity is the result of human genetic adaptation to
specific environments.
1. The high frequencies of the HbS heterozygote in malarial environments is a good
example.
2. Some alleles that were once maladaptive can lose their disadvantage if the
environment changes.
3. The Human Genome Project is working to map all of the genes and chromosomes
found in humans.
a. Many of today’s incurable hereditary diseases someday may be rendered
evolutionarily neutral through genetic therapies.
b. One downside of this research is that it can also lead to genetic discrimination
(eugenics).
B. In the News: Human Cloning: Yesterday’s Never is Today’s Why Not?
1. Scientists and ethicists are debating whether human beings should be cloned, and if so,
for what purposes.
2. Most researchers see the medical utility of cloning while ethicists are more concerned
with who would control the technology.
C. Genes and Disease
1. Despite the advances in medical research over the last 100 years, diseases still pose a
significant threat to the health of human populations all over the world.
2. Human blood factors play an important role in resistance to some diseases.
a. There is evidence that the various alleles producing human blood types interact
with infectious and non-infectious ailments.
b. For example, the presence of type A blood cells seems to make a person more
susceptible to small pox.
D. Facial Features
1. Thomson's nose rule asserts that noses tend to be longer in colder climates.
2. The Australian Aborigines' sand-permeated diet has selected for larger teeth than seen
in other populations.
E. Size and Body Build
1. Different climates have selected for different body shapes.
2. Bergmann's rule: because of the respective ratios between mass and surface area,
smaller bodies dissipate heat faster, and larger bodies retain heat better; thus more
larger animals are found in colder habitats, while smaller animals have been selected
for in hotter habitats.
3. Allen's Rule: slender bodies with long limbs dissipate heat more efficiently, and are
selected for in tropical climates: heavy, short-limbed bodies retain heat better, and are
selected for in colder climates.
F. Lactose Intolerance
1. The term phenotypic adaptation refers to changes, which occur to an individual
organism during its lifetime, which enhance its reproductive fitness.
2. Individuals from herding populations in Northern Europe and parts of Africa maintain
their ability to digest milk (continue to produce the enzyme, lactase) into adulthood,
whereas people from other populations can digest milk (specifically, milk sugar,
called lactose) only during childhood.
3. The fact that descendents of these herding population who no longer herd continue to
be lactose tolerant as adults indicates genetic adaptation to a milk-rich diet.
4. The fact that lactose intolerance can vary during an individual's adult life, depending
on how much milk is consumed, indicates that some phenotypic adaptation also takes
place.