HOW POPULATIONS EVOLVE
I.
A sea voyage helped Darwin frame his theory of evolution
a. FOSSILS  the imprints of remnants of organisms that lived in
the past
b. Long believed that Earth was at most 6,000 years old thanks to
Aristotle and religion
c. Late 1700’s, early 1800’s scientists suggested Earth may be
older and that life evolves
d. LAMARCK’S THEORY
i. ACQUIRED CHARACTERSTICS  may develop traits
that can be passed on to offspring
e. Darwin
i. HMS Beagle; 1831
ii. Collected thousands of specimens on shores of South
America
iii. “On the Origin of Species by means of Natural
Selection”
1. DESCENT WITH MODIFICATION  all
organisms related through descent from an
unknown species that lived in the past
iv. As time passed; adaptations were developed within a
specific species that were past to future generations
II.
The study of fossils provides strong evidence for evolution
a. Fossils may be from hard parts of an animal (bones) that are
rich in minerals or may arise from PETRIFICATION
i. PETRIFICATION  minerals dissolved in seawater
seep into tissues
b. PALEONTOLOGISTS  scientists who study fossils
c. FOSSIL RECORD  the ordered array in which fossils appear
within layers, or strata, of sedimentary rocks
d. Oldest known fossils is 3.5 billion years old
e. Fossils indicated that whales evolved from land mammals with
4 legs
f. Fossil evidence shows change of skull shape and size as
mammals arose from reptiles
III.
A mass of evidence validates the evolutionary view of life
a. BIOGEOGRAPHY  geographic distribution of species
i. Organisms appear more similar to organisms from the
same area
b. COMPARATIVE ANATOMY  the comparison of body
structures in different species
i. HOMOLOGOUS STRUCTURES  features that often
have different functions but are structurally similar
because of common ancestry
ii. *Spine and knee joints derived from 4-legged organism,
not designed initially to support bipedal structure
c. COMPARATIVE EMBRYOLOGY  the study of structures
that appear during the development of different organisms
i. Pharyngeal slits present in all vertebrates (fishes, frogs,
snakes, birds, apes)
d. MOLECULAR BIOLOGY  the study of the molecular basis
of genes and gene expression
i. Related individuals have a great amount of similar DNA
IV.
Darwin proposed natural selection as the mechanism of evolution
a. Overproduction of offspring
b. Limited natural resources
c. Heritable variations
d. NATURAL SELECTION  differential, or unequal, success in
reproduction
e. ARTIFICIAL SELECTION  selective breeding of
domesticated plants and animals
f. Two main features of Darwin
i. Diverse forms of life have arisen by descent with
modification from ancestral species
ii. The mechanism for modification has been natural
selection
V.
Scientists can observe natural selection in action
a. A classic and unsettling example of natural selection in action
is he evolution of insecticide resistance in hundreds of insect
species
b. Initial spraying of insecticide is 99 % effective, with each
successive spraying less and less effective
i. Ex. DDT and houseflies
VI.
Populations are the units of evolution
a. POPULATION  a group of individuals of the same species
living in the same place at the same time
b. Darwin could not explain the genetic basis of population
change
c. POPULATION GENETICS  the science of genetic change in
populations
d. SPECIES  a group of populations whose individuals have the
potential to interbreed and produce fertile offspring
VII. Microevolution is change in a population’s gene pool over time
a. GENE POOL  the total collection of genes in a population at
any one time
b. MICROEVOLUTION  the change in the relative frequency
of alleles over time
VIII. The gene pool of a non-evolving population remains constant over
the generations
a. Dominant does not mean dominant in the population
b. HARDY-WEINBERG EQUILIBRIUM  the frequency of
each allele in the gene pool will remain constant unless acted on
by other agents
c. Calculating allele frequencies
i. p + q = 1
p = frequency of dominant allele
2
2
ii. p + 2pq + q = 1 frequencies
homozygotes
and
heterozygotes
IX.
The Hardy-Weinberg equation is useful in public health service
a. Phenylketonuria (PKU) can be quantified with regards to the
percentage of carriers in the population
X.
Five conditions are required for Hardy-Weinberg equilibrium
a. The population is very large
b. The population is isolated; that is, there is no migration of
individuals or gametes into or out of the population
c. Mutations (changes in genes) do not alter the gene pool
d. Mating is random
e. All individuals are equal in reproductive success; that is natural
selection does not occur
XI.
There are several potential causes of microevolution
a. GENETIC DRIFT  a change in the gene pool of a small
population due to chance
i. BOTTLENECK EFFECT  genetic drift that results
from an event that drastically reduces population size
ii. FOUNDER EFFECT  genetic drift that results from
the colonization of a new location by a small sample
population
b. GENE FLOW  when fertile individuals move into or out of a
population, or when gametes are transferred between
populations
c. MUTATION  a random change in an organism’s DNA that
may create a new allele
XII. Adaptive change results when natural selection upsets genetic
equilibrium
a. Natural selection results in the accumulation and maintenance
of traits that adapt a population to its environment
XIII. Variation is extensive in most populations
a. POLYMORPHISM  two or more morphs (different forms of
a phenotypic trait) for a characteristic are present in noticeable
numbers
b. CLINE  a graded change in an inherited characteristic along a
geographic continuum
XIV. Mutation and sexual recombination generate variation
a. The majority of genetic changes are harmful, but on rare
occasions a change may be beneficial; usually as the result of a
changing environment
b. Mostly depend on sexual recombination for variation
XV. Overview: How natural selection affects variation
a. Recessive alleles when found in heterozygotes hide from
natural selection because they don’t impact the phenotype
b. HETEROZYGOTE
ADVANTAGE

heterozygous
individuals have greater reproductive success than homozygotes
XVI. Not all genetic variation may be subject to natural selection
a. NEUTRAL VARIATION  variation in a heritable
characteristic that provides no apparent selective advantage for
some individuals over others
XVII. Endangered species often have reduced variation
a. As populations are reduced, the diversity of the gene pool
declines
XVIII. The perpetuation of genes defines evolutionary fitness
a. DARWINIAN FITNESS  the contribution an individual
makes to the gene pool of the next generation relative to the
contributions of other individuals
b. Production of fertile offspring is the only thing that counts in
natural selection
XIX. There are three general outcomes of natural selection
a. STABILIZING SELECTION  conditions tend to reduce
phenotypic variation
b. DIRECTIONAL SELECTION  shifts the overall makeup of
the population by acting against individuals at one of the
phenotypic extremes
c. DIVERSIFYING SELECTION  environmental conditions
are varied in a way that favors individuals at both extremes of
phenotypic range
XX. Sexual selection may produce sexual dimorphism
a. SEXUAL DIMORPHISM  the distinction in appearance
between a male and a female of the same species
i. INTRASEXUAL SELECTION  same sex, same
species compete for mate
ii. INTERSEXUAL SELECTION  opposite sex (usually
female) gets to choose mate
iii. *Even though trait may be harmful towards long term
survival, if it helps with reproductive success it will
remain in the population
XXI. Natural selection can not fashion perfect organisms
a. Organisms are locked into historical constraints
b. Adaptations are often compromises
c. Not all evolution is adaptive
d. Evolution can only edit existing variation
XXII. The evolution of antibiotic resistance in bacteria is a serious public
health concern
a. Naturally existing resistant bacteria will proliferate if all
antibiotic sensitive bacteria are killed