Download Chapter 16: The Evolution of Populations and Speciation

Chapter 16:
“Population
Genetics and
Speciation”
Aka: Natural Selection
meets Genetics - What
Darwin Never Knew
16.1: Genetic Equilibrium
Amongst any population, there are always
variations
Bell Curve
• Graph showing frequencies of occurrences with
the majority in the middle and curving out
towards the extremes.
• Gene frequencies within a population follow this
curve.
Population Genetics
Study of how genetics plays a role in
evolution
**New definition of evolution:
– A gradual change in the genetic material within a
population over time.
**Old definition of evolution
– Development of new types of organisms from preexisting types of organisms over time
So what causes these variations in
populations?
• Variations – caused by:
– Environmental factors (food availability,
amount of sunlight, etc.)
– Mutations – gene and chromosomal
– Recombinations (crossing over, independent
assortment)
– Random fusion of gametes - each brings their
own set of genes
Gene Pool
A gene pool is
the sum of all
the individual
genes (alleles)
in a given
population.
A
population
is the
smallest
unit of
living
organisms
that can
undergo
evolution.
Peccaries are small, tough relatives of the modern pig,
whose lineage diverged about 40 million years ago. They
live in southern Texas, Arizona, and New Mexico.
Determining Allele Frequency
If we could take inventory of the gene pool
and find out all of the alleles present, then
we could apply a simple mathematical
formula to make predictions about how the
gene pool might change over time – you will
see WHY we do this in the next section!!!
Suppose…
• A certain population has two alleles,
B and b in 10 gametes.
• Let’s say 3 of the gametes carry the B
allele, and 7 carry the b allele – what is the
allele frequency of B and b?
• Answer: 0.3 or 30% and 0.7 or 70%.
• So, how do we determine allele
frequency?
Applications of Allele Frequency
If two parents are each pure breeding for
a certain characteristic:
RR x rr
Then their offspring will have these
genotypes:
4Rr
Out of 8 total alleles:
R = 4/8 = 0.5 = 50%
r = 4/8 = 0.5 = 50%
Notice that 0.5 + 0.5 = 1; must be that
way because you are dealing with
frequencies
Applications of Allele
Frequency, cont.
If two parents are hybrids for a certain
characteristic:
Rr x Rr
Then their offspring will have these genotypes:
RR 2Rr rr
Out of 8 total alleles:
R = 4/8 = 0.5 = 50%
r = 4/8 = 0.5 = 50%
Notice that 0.5 + 0.5 = 1; must be that way
because you are dealing with frequencies
So what, big deal, who cares?
• Did the phenotype frequencies change
from the P to the F1 generation?
• Did the genotype frequencies change from
the P to the F1 generation?
• So, did evolution actually occur, and will
it occur with this same population given
the same circumstances?
• From here, we proudly introduce…
Hardy and Weinberg
* A mathematician and a physician who in 1908 developed
a mathematical model to predict gene frequencies in
future populations in an attempt to explain
microevolution (change in genetic material of a
population). Macroevolution (chapter 15) is change on a
phenotypic level.
• This is ONE equation they used: p + q = 1
where p = dominant allele
where q = recessive allele
p equals all of the alleles in individuals who are homozygous dominant (AA) and half of
the alleles in people who are heterozygous (Aa) for this trait in a population.
So, if 30% of a population had a certain trait, you could
calculate what % must have the other (70%). You would
not know, however, what % was heterozygous righthanded with this equation.
Applications, cont.
• We will use the 4 o’clock flower as an
example
• Remember – this flower has an incomplete
dominance inheritance pattern
In a population, there are 4 red four o’clock
flowers and 4 pink ones (incomplete
dominance)
RR RR RR RR
Rr Rr Rr Rr
• Phenotype frequency =
4/8 Red 4/8 Pink
0.5 0.5 or 50% 50%
Allele frequency =
R 12/16 r 4/16
0.75 0.25
HW call R = p and r = q and state: p + q = 1
Second generation: 5 Red, 2 pink and 1 white
RR RR RR RR RR Rr Rr
rr
Phenotype frequency =
5/8 Red 2/8 Pink 1/8 White
.625 .25 .125
Allele frequency =
R 12/16 r 4/16
.75 .25
.75 + .25 =1
**So the allele frequencies remain the same from
generation to generation in a closed population because
you are dipping into the same gene pool!
So what is the likelihood of two “R” genes
(RR) being pulled from the gene pool?
• If p = .75
– then it’s p2 = .75 x .75 = .5625 – WHY?
•
•
•
•
p2 = homozygous dominant allele frequency
If q = .25
- then it’s q2 = .25 x .25 = .0625 – WHY?
q2 = homozygous recessive allele frequency
• WAIT – why don’t these two values add up to 1?
WHAT is missing and how do we find it?
How about them hybrids?
• Frequencies of all types must add up to 1.0
p+q=1
• So, 1.0 – frequency of RR – frequency of rr =
frequency of Rr
• 1.0 - .5625 - .0625 = 0.375 (frequency of Rr)
Hardy Weinberg Genetic
p is defined as the
Equilibrium
equation
frequency of the
dominant allele and q as
the frequency of the
p+q=1
recessive allele for a trait
(p + q) x (p + controlled
q) = 1 by a pair of
alleles (A and a). In
other words, p equals all
of the alleles in
individuals who are
homozygous
dominant
Hybrid frequency
(AA) and half of the
Pure Dominant alleles in people who are
frequency
heterozygous (Aa) for
this trait in a population.
2
p
+ 2pq +
2
q
=1
Pure Recessive
frequency
Let’s Practice!
• p2 + 2pq + q2 = 1 and p + q = 1
• p = frequency of the dominant allele in the population
• q = frequency of the recessive allele in the population
• p2 = percentage of homozygous dominant individuals
• q2 = percentage of homozygous recessive individuals
• 2pq = percentage of heterozygous individuals
Practice, cont.
• SHOW ALL WORK ON A SEPARATE PIECE OF
LOOSE LEAF PAPER
•
You have sampled a population in which you know
that the percentage of the homozygous recessive
genotype (aa) is 36%. Using that 36%, calculate the
following:
• - The frequency of the "aa" genotype.
– The frequency of the "a" allele.
– The frequency of the "A" allele.
– The frequencies of the genotypes "AA" and "Aa."
– The frequencies of the two possible phenotypes if "A"
is completely dominant over "a."
Practice, cont.
1. Brown hair (B) is dominant to blond hair (b). If
there are 168 brown haired individuals in a
population of 200, calculate all phenotypic and
allele frequencies.
2. If 81% of a population is homozygous recessive
for a given trait..........
• What is the predicted frequency of homozygous
dominant?
• What is the predicted frequency of
heterozygotes?
Hardy Weinberg Genetic
Equilibrium
Hardy and Weinberg showed that allele
frequencies in a population remain the same
over time unless acted upon by outside
influences
Assumptions
The allele frequencies of generations within
a population will remain the same if:
– large population - to insure no sampling
error from one generation to the next
– random mating - no assortative mating or
mating by phenotype
– no mutations – allele frequencies don’t
show a net change due to mutations
– no migration between populations
– no natural selection – all genotypes
reproduce with equal success
Disruption of Genetic Equilibrium
(results in evolution!)
1. Mutations – occur spontaneously
Introduce new alleles into population
– Harmful mutations are slowly selected out
while beneficial mutations drive evolution
2.Migration
• Constant movement of animals within a
population, both in and out
–Immigration – movement into a
population
–Emigration – movement out of a
population
These conditions are referred to as
gene flow – movement of genes from
one population to another
3.Genetic Drift
•
•
•
•
Random events or chance
Small populations
Disease
Natural disasters
• Genetic Drift occurs when a small
population is isolated.
• Dipping in to the same gene pool.
• Gene frequency changes but not due to
natural selection or adaptive advantage.
Due to purely chance.
• Danger occurs when only 1 allele is left.
No variation w/in a population
• Simulation
4.Nonrandom Mating
• What if mating only occurs with people
within one’s own village?
• Dipping into the same gene pool could
expose disorders due to the repetitive
appearance of recessive genes
• Assortative mating – organisms tend to
mate with others that are similar to them
• Sexual selection - attractive phenotypes
such as brightly colored feathers
(peacock); human pheromones
5.Natural Selection
• Most significant factor to disrupt genetic
equilibrium
• Peppered moths in Industrial England –
White trees became soot covered
White on white – > in numbers White on dark - < in numbers Sooty on light - < in #’s
Patterns of Natural Selection
Stabilizing
Directional
Disruptive
16.3 Formation of a Species –
Speciation
• Species –
– a population of organisms that can
successfully interbreed and can
produce a viable, fertile offspring with
others out of their species
– Morphologically (internal & external
structures) they are all similar; can be
confusing because of phenotypic
variations
What drives speciation?
•
Isolation – separates breeding
populations
– 1) Geographic isolation – physical
separation from original habitat = allopatric
speciation
Development of a deep cannon, river
changes, land floats, volcanic activity, etc
– Isolation stops gene flow between two
subpopulations
– Populations diverge and can no longer
interbreed
What drives speciation?
2) Reproductive Isolation- caused by barriers to
successful breeding between population groups
in the same area such as disruptive selection –
two types:
Prezygotic isolation – occurs before
fertilization
* Incompatible behaviors - different mating
seasons and mating calls
* Sympatric speciation – within the same
area
Prezygotic isolation
Leopard Frog
Wood Frog
Wood frogs mate in late March, Leopard frogs
mate in mid April; since both are of the genus
Rana, they can interbreed but do not
What drives speciation?
• Post zygotic isolation
• occurs after fertilization
• due to a failure of zygote to develop or
infertility
• wastes gametes
Rates of Speciation
•
•
Gradualism – evolution is
a constant process and
occurs at a steady rate
Punctuated equilibriumPeriods of stability
separated by accelerated
changes
–
Species arise abruptly then
have long periods of little
change
Chapter Closure
• How might the following scientists have
responded if they knew about the modern
day approaches to evolution as described
in this chapter? Why?
• Cuvier
• Lyell
• Lamarck
• Mendel
• Darwin