Download Big_Idea_1.A.1 Natural Selection

Natural selection is a major mechanism
of evolution.
ESSENTIAL KNOWLEDGE 1.A.1:
What is Evolution?
 Evolution is a change in the genetic makeup
of a population over time, with natural
selection its major driving mechanism..
Evolution
 Natural selection:
populations of organisms can
change over the generations if
individuals having certain heritable
traits leave more offspring than
others (differential reproductive
success)
 Evolutionary adaptations:
a
prevalence of inherited
characteristics that enhance
organisms’ survival and reproduction
November 24, 1859
Evolutionary history
 Linnaeus: taxonomy
 Lyell: uniformitarianism
 Hutton: gradualism
 Darwin: evolution
 Lamarck: evolution
 Mendel: inheritance
 Malthus: populations
 Wallace: evolution
 Cuvier: paleontology
Darwin’s Travels
Galapagos Finches
Factors that led to Darwin’s
theory
 Overproduction
 Variation
 Artificial Selection
Descent with Modification
Descent with Modification
 5 observations:
 1- Exponential fertility
 2- Stable population size
 3- Limited resources
 4- Individuals vary
 5- Heritable variation
Descent with Modification
 3 Inferences:
 1- Struggle for
existence
 2- Non-random
survival
 3- Natural selection
(differential success
in reproduction)
Darwin’s Theory
 According to Darwin’s theory of natural
selection, competition for limited resources
results in differential survival.
 Individuals with more favorable phenotypes
are more likely to survive and produce more
offspring, thus passing traits to subsequent
generations.
Evolutionary Fitness
 Evolutionary fitness: the ability to survive and
Reproduce
 How is fitness measured?
 Evolutionary fitness is measured by
reproductive success.
Two driving forces behind
evolution:
 Genetic variation
and mutations both
play roles in natural
selection.
 A diverse gene pool
is important for the
survival of a species
in a changing
environment.
Environments
 Environments can be more or less stable or
fluctuating, and this affects evolutionary rate
and direction;
 different genetic variations can be selected in
each generation.
Adaptations
 An adaptation is a genetic variation that is
favored by selection and is manifested as a
trait that provides an advantage to an
organism in a particular environment.
 In addition to natural selection, chance and
random events can influence the evolutionary
process, especially for small populations.
Example 1: Rat Snakes
 Rat snakes are found in wide variety of colors,
from yellow striped to black to orange to
greenish, because they adapted to their local
environments.
Photos are courtesy of Phillip Higgins.
http://timberrattlesnake89.tripod.com/ganonvenomous.html
Example 2: Nylon Eating
Bacteria
 Since nylon wasn't invented until the 1940s,
bacteria that can eat nylon can be nothing but new.
The bacterium Pseudomonas is able to metabolize
nylon thanks to certain enzymes it has. However, a
surprising thing happens when you take a non-nylon
eating variety of this bacterium and place it in an
environment where the only type of food available
is nylon. Every single time the experiment was tried,
the bacteria would evolve until it was able to
consume nylon [source: Michigan State University].
This is a very simple example of natural selection,
where the most basic forms of life can adapt to
whatever food the environment offers.
Example 3: Warrior Ants
 The warrior ants in Africa are probably one of the most
impressive examples of adaptation. Within any single
colony, ants emit a chemical signal that lets the others
know they all belong to the same compound. Or, put more
simply, a signal that says "Don't attack me, we're all
family." However, warrior ants have learned how to
imitate the signal from a different colony. So if a group of
warrior ants attacks a colony, they will be able to imitate
that colony's signal. As a result, the workers in the colony
will continue on, now under the direction of new masters,
without ever realizing an invasion has taken place.
 Warrior ants in Africa can imitate another ant colony's
chemical signal so they can go undetected.
Law of Superposition
Hardy-Weinberg equilibrium
 Conditions for a population or an allele to be
in Hardy-Weinberg equilibrium are:
 (1) a large population size,
 (2) absence of migration,
 (3) no net mutations,
 (4) random mating and
 (5) absence of selection.
These conditions are seldom met.
5 Agents of evolutionary change
Hardy-Weinberg Equations
Problem 1 The allele for
black coat is recessive. We
can use the HardyWeinberg equation to
determine the percent of
the pig population that is
heterozygous for white
coat.
1. Calculate q2 :Count the individuals
that are homozygous recessive in
the illustration above. Calculate the
percent of the total population they
represent. This is q2.
2. Find q. Take the square root of q2
to obtain q, the frequency of the
recessive allele.
3. Find p. The sum of the frequencies of
both alleles = 100%, p + q = l. You know
q, so what is p?
4. Find 2pq. The frequency of the
heterozygotes is represented by 2pq.
This gives you the percent of the
population that is heterozygous for white
coat:
Sample Problem 2
In a certain population of 1000 fruit flies,
640 have red eyes while the remainder
have sepia eyes. The sepia eye trait is
recessive to red eyes. How many
individuals would you expect to be
homozygous for red eye color?
Answer:
160
Sample Problem 3
 If 9% of an African population is born with a
severe form of sickle-cell anemia (ss), what
percentage of the population will be more
resistant to malaria because they are
heterozygous(Ss) for the sickle-cell gene?
 42% are heterozygous
Microevolution
 A change in the gene
pool of a population over
a succession of
generations
 1- Genetic drift:
changes in the gene
pool of a small
population due to
chance (usually reduces
genetic variability)
Microevolution
 The Bottleneck
Effect: type of
genetic drift resulting
from a reduction in
population (natural
disaster) such that
the surviving
population is no
longer genetically
representative of the
original population
Microevolution
 Founder Effect:
a cause of genetic
drift attributable to
colonization by a
limited number of
individuals from a
parent population
Microevolution
 2- Gene Flow:
genetic exchange due
to the migration of
fertile individuals or
gametes between
populations (reduces
differences between
populations)
Microevolution
 3- Mutations:
a change in an organism’s
DNA (gametes; many
generations); original source
of genetic variation (raw
material for natural
selection)
Microevolution
 4- Nonrandom
mating: inbreeding
and assortive mating
(both shift
frequencies of
different genotypes)
Microevolution
 5- Natural Selection:
differential success in
reproduction;
only form of
microevolution that
adapts a population
to its environment
Population variation
 Polymorphism:
coexistence of 2 or more
distinct forms of
individuals (morphs)
within the same
population
 Geographical
variation: differences
in genetic structure
between populations
(cline)
Variation preservation
 Prevention of natural selection’s
reduction of variation
 Diploidy
2nd set of chromosomes hides
variation in the heterozygote
 Balanced polymorphism
1- heterozygote advantage
(hybrid vigor; i.e.,
malaria/sickle-cell anemia);
2- frequency dependent
selection (survival &
reproduction of any 1 morph
declines if it becomes too
common; i.e., parasite/host)
Natural selection
 Fitness: contribution
an individual makes
to the gene pool of




the next generation
3 types:
A. Directional
B. Diversifying
C. Stabilizing
Sexual selection
 Sexual dimorphism:
secondary sex
characteristic distinction
 Intersexual and Intrasexual
selection
 Sexual selection:
selection towards
secondary sex
characteristics that leads to
sexual dimorphism
Homework:
 Hardy Weinberg
Practice Problems:
Natural Selection of Straw Fish
“Strawfish” live in freshwater ponds. In this simulation, we will investigate
how different natural selection factors in the environment can influence
the colors of Strawfish. We will also look “underneath the skin” and
measure how these natural selection factors also affect the inheritance of
the genes that code for the color of Strawfish. In Strawfish, there are
three scale/skin colors (phenotypes)— blue, yellow, green. These three
colors are controlled by a color gene that comes in two versions (two
alleles) — the blue allele and the yellow allele. The blue and yellow alleles
do not show a classical dominant / recessive interaction. Instead when
they are inherited together they show an incomplete dominance
interaction, therefore the heterozygote will be a green colored fish.
 Each lab group (working in pairs) will be given a bag of alleles (straws) —
20 yellow and 20 blue straws. These represent the collection of genes in
our population of fish — the fish gene pool. As in nature, Strawfish are
diploid organisms — they have two copies of every gene. The color of
each fish is always determined by the interaction of the two copies (the
two straws).
 Follow with College Board Lab: Mathmatical Modeling: Hardy-Weinberg
H-W Practice
 In humans, albinism is a recessive condition
caused by mutations in genes involved in the
production of melanin, a skin pigment. In
Americans of European ancestry, albino
individuals occur at a frequency of about 1 in
10,000 (or 0.0001). If you assume HardyWeinberg equilibrium in this population, what
percentage of them do you expect to be
carriers (heterozygotes) for albinism?
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