Transcript Ch. 16 - LBHS Biology

Chapter 16
GENES AND
VARIATION
16.1 - Variation of Traits in a
Population (gene pool)
Population: all the members of the
same species that live in a particular
location at the same time and have
the potential to interbreed.
Members of a population, even
though of the same species, are
somewhat different genetically.
Why?
•mutations in DNA
•recombination of genes during
meiosis
•random fusion of gametes
•A difference in genotype usually
results in a difference in phenotype
Often, variation in traits are
due also to environmental
factors as well
Example: Why are some of the
perch in a pond bigger/longer
than others? Why do snowshoe
hares change fur color with the
seasons? (environmental triggers)
Allele Frequencies and Genetic
Equilibrium: Population Genetics
Variations in genotype arise through
mutation,
recombination and
crossing-over
Causes of variation
Mutation – change in DNA sequence,
if change in gene then there may be a
change in protein (phenotype)
Recombination – due to meiosis and
and fertilization (it’s how you get two
siblings that are not anything alike)
Crossing –over - happens in meiosis,
pieces of chromosomes switch place
Gene shuffling
Mark in notes that recombination and
crossing-over are considered – GENE
SHUFFLING
Some important words
Gene Pool: the collection of genes for all
the traits in a population (contains all the
alleles for all the genes)
Allele Frequency: the percentage of a
specific allele of a gene in the gene pool
A population in which allele frequencies do
not change from one generation to the next
is said to be in Genetic Equilibrium
16.2 - The Hardy-Weinberg
Principle
-no mutations
-no migrations
-large populations
-random mating
-no selection of alleles
DISRUPTION OF GENETIC
EQUILIBRIUM
When gene frequencies
change over time
(Hardy-Weinberg does
NOT hold true) then
EVOLUTION happens.
Factors that cause change in gene
pools:
Mutation
Migration
Nonrandom mating
Genetic Drift: allele frequencies in a
population change as a result of
random events or chance
Selection: some alleles are more
favorable to have than others
TYPES OF NATURAL
SELECTION
Stabilizing Selection
Directional Selection
Disruptive Selection
Sexual Selection: mate preferences
based on traits, usually some physical
feature
Stabilizing Selection (read this)
Stabilizing selection favors the norm, the common,
average traits in a population. Look at the Siberian
Husky, a dog bred for working in the snow. The
Siberian Husky is a medium dog, males weighing
16-27kg (35-60lbs). These dogs have strong
pectoral and leg muscles, allowing it to move
through dense snow. The Siberian Husky is well
designed for working in the snow. If the Siberian
Husky had heavier muscles, it would sink deeper
into the snow, so they would move slower or would
sink and get stuck in the snow. Yet if the Siberian
Husky had lighter muscles, it would not be strong
enough to pull sleds and equipment, so the dog
would have little value as a working dog. So
stabilizing selection has chosen a norm for the
size of the Siberian Husky.
Directional selection
favors those individuals who have extreme
variations in traits within a population. A useful
example can be found in the breeding of the
greyhound dog. Early breeders were
interested in dog with the greatest speed.
They carefully selected from a group of
hounds those who ran the fastest. From their
offspring, the greyhound breeders again
selected those dogs who ran the fastest. By
continuing this selection for those dogs who
ran faster than most of the hound dog
population, they gradually produced a dog
who could run up to 64km/h (40mph).
Disruptive Selection
Disruptive selection, like directional selection, favors the
extremes traits in a population. Disruptive selection differs in
that sudden changes in the environment creates a sudden
forces favoring that extreme. Think about the changes in the
environment when that meteor crashed into Earth 65mya. A
sudden decrease in light levels as the dust rose over large
portions of the Earth. Extremely large tidal waves washing
miles over the land. Increased seismic activity. The sudden
lost of food along the coast, possible plague due to the high
initial death rate, dust filling the lungs of animals would have
been the most stressful on larger animals. Large animals
need a large oxygen supply to supply energy to their
muscles. They also need a large, constant supply of food.
The sudden drop of oxygen due the dust, and the drop in
fresh food, large animals would be stressed. If a plague
started by the high death rate also hit these stressed
animals, they would have been sorely pushed to survive.
Evidence shows that they did not. So disruptive selection
occurs quickly, selecting for those extreme traits that help
organisms survive in the new environmental conditions.
APPLY YOUR KNOWLEDGE
Look at the graphs in your notes and
decide which type of selection they
are showing
GENETIC DRIFT – Disruption by
Chance
Founder Effect – allele frequency
changes due to the migration of a
small sub group
EX. Amish community from the 32
original Swedish descendents
Genetic drift continued
Bottleneck effect – elimination of
alleles in a population due to a
disaster
EX - the elephant seal population was
hunted down to just 20 individuals.
Then the population rebounded to
30,000 BUT little genetic variation
between animals.
16-3 FORMATION OF SPECIES
How do species form?
A "species" is a group of like organisms
that are capable of reproducing viable
offspring in nature.
Morphological species:
based on similarities/differences in structure;
easy to observe
Biological species:
can interbreed; are chromosomally and
genetically similar enough to be considered the
same.
FORMATION OF SPECIES
"Speciation"
Isolated Populations:
become two (or more) separate gene
pools, where different selection and
mutations (DNA alterations) make them
very different over time...so different that
they will no longer interbreed.
Two main causes: geographical isolation
or reproduction isolation
1. Geographic Isolation (pg 405)
"Allopatric speciation", also known as
geographic isolation , occurs when
populations physically isolated by a
physical barrier
These populations evolve genetic
reproductive isolation such that if the
barrier between the populations breaks
down, individuals of the two populations
can no longer interbreed.
ex: Galapagos finches
ex: Grand canyon squirrels
The Abert squirrel inhabits the Coconino Plateau, just to the
south of the Grand Canyon, and what is called the Kaibab
squirrel inhabits the Kaibab Plateau, just to the north of the
Grand Canyon. The Grand Canyon, 200 miles long, 5,000 feet
deep, and 12 to 15 miles across, with the Colorado River
running through it, acts as a barrier to terrestrial animal
movement.
2. Reproductive Isolation
"Parapatric speciation": no specific
extrinsic barrier to gene flow. The
population is continuous, but
nonetheless, the population does not
mate randomly. Two (or more)
separate gene pools form, and
eventually these diverge into different
species .
prezygotic mechanisms –
before zygote formation
(no fertilization occurs)
EX 1 – BEHAVIORAL
ISOLATION – different
mating calls (songs in
birds, chirps in crickets) or
different rituals such as
dances, etc (pg 404)
EX 2 – TEMPORAL
ISOLATION – reproduction
at different times, frogs lay
eggs at different time of yr,
pollen released diff. time of
yr, etc (405)
postzygotic mechanisms – fertilization
occurs but zygote dies or is sterile
Ex - mule
Polyploidy – DON’T NEED TO
KNOW!
17-4 Rates of Speciation
How quickly do new species form? Is it a
long, slow, even process over time
("Gradualism")
or does it happens in spurts ("Punctuated
Equilibrium") of rapid genetic change?
Extinction – more than 99% of all species
that have ever lived are now extinct, many
mass extinctions have occurred
Gradualism – probably not
Puncuated Equilibrium – takes into
account the mass extinctions (pg 439)
TOPICS FROM CHAPTER 17.4
that should be included NOW!
Coevolution: the change in two or
more species that are closely
associated with one another
Pollinators
Camouflage
Predator/prey
In this snowy environment, the
polar bear is white to avoid being
noticed as it approaches the seal,
and the seal pup is white to avoid
being noticed by the bear.
Convergent Evolution: organisms
that appear to be similar but are not
closely related at all,
Despite their very different evolutionary
paths – one as a mammal, the other as a
bird – both have evolved and adapted to
the aquatic environment they now inhabit.
Divergrent Evolution: two or more
closely related organisms become
more and more dissimilar over time
2 important types
1) adaptive radiation
2) artificial selection
HARDY WEINBERG PRACTICE
(16.2 in text)