Transcript Chapter 23 - Bio-Guru

The Evolution of Populations
Chapter 23
Populations and Gene Pools
• A population is a group of individuals in a
certain time and place that belong to the
same species – that is: they can interbreed
and produce viable offspring
• A Gene Pool is the total number of genes
within a population at a particular time and
place.
Alleles and Genetic Variation
• Germline Mutations in an original gene will give rise to
new alleles in a population.
• Alleles will create the variations in a population that
Darwin talked about
• Some alleles will be beneficial to an organism, others will
be detrimental
• The alleles that contribute to the survival of a species in
a particular environment, will be passed on to
subsequent generations.
• Non beneficial alleles will perish with the individuals.
What else (other than mutations) creates and
maintains Genetic Variation?
• Sexual Recombination- sex reshuffles alleles –
that is better for a population
• Diploidy – two alleles better than one
• Balanced Polymorphism – The ability of natural
selection to maintain diversity in a population
- Heterozygote Advantage – heterzygotes are
naturally selected
• Neutral Variation – a trivial variation like human
fingerprints
Counting Alleles
• The Hardy Weinberg Theorem allows a
population geneticist to count the alleles of a
particular gene in a population
– How many dominant alleles?
– How many recessive?
– How many homozygous or heterozygous individuals?
• But one can ONLY count alleles in a nonevolving, fixed-size population – one with a
closed gene pool.
Get Out! The Pool is Closed!
• The Amish for example – the
only variations in the
population arise from
independent assortment and
crossing over during meiosis
and nonrandom fertilization
(mating within the same
community)
• No gene flow! (immigration, for
example)
My cousin’s a hottie!
Nonrandom mating / fertilization
• Nonrandom Mating – “mating in the same
neighborhood” or inbreeding
– Also known as “selfing” in plants
• Gene Flow – the flow of new genes into a
population from neighboring populations –
in other words - migration
Gene Flow
Populaitons used to be
isolated from each
other. But migration of
people has increased
gene flow between
populations.
Gene flow
homogenizes the gene
pools of a population
and reducing
geographic variation in
appearance.
Problems with Inbreeding
• Leads to an excess of homozygotes
• Many diseases are recessively inherited
and one must be homozygous recessive
to have the disease
• Closely related “healthy” individuals could
be heterozygous since the allele is
already in the family
• They are not be affected by the disease,
but will pass it on to offspring if they mate
Non-Evolving Populations
The Hardy-Weinberg theorem applies to non-evolving
populations – a population in equilibrium.
Hardy-Weinberg Rules:
1.
2.
3.
4.
5.
6.
7.
Mutations must not occur
Natural selection must not be occurring
The population must be infinitely large
All members of the population must breed
All mating must be totally random
Everyone must produce the same number of offspring
NO GENE FLOW - there must be no migration in or out
of the population
The formula is : p2 + 2pq + q2 = 1
Where:
• p = the dominant allele
• q = the recessive allele
• p2 = homozygous dominant
• q2 = homozygous recessive
• 2pq = heterozygous
Sample Problem
1 in 1700 US Caucasian newborns have cystic fibrous. C for normal is
dominant over c for cystic fibrous.
ALLELE FREQUENCY CALCULATIONS:
•cc = cystic fibrosis.
•Therefore q2 = 1/1700, q2 = 0.00059 or 0.059% (with both recessive alleles)
•If q2 = 0.00059, then q = square root of 0.00059 = 0.024 or 2.4% for cystic fibrosis
allele
• if p = 1-q, then 1- 0.024 = 0.976 or 97.6% for normal, dominant allele
Now that you know that p =.976 and q = .024. The following genotypes can be found.
(f)CC- Normal homozygous dominant = p2 = ?
p2 = (0.976)2 = 0.953 or 95.3% frequency for both normal alleles (CC)
(f)Cc -carriers of cystic fibrous = 2pq = ?
2pq = 2(0.976 x 0.024) = 0.0468 or 4.68% heterozygous or carriers (Cc)
So remember the formula: p2 + 2pq + q2 = 1
Plug in the numbers – do they work?
Practice Problem 1
1.In a population with two alleles for a
particular locus, B and b, the allele
frequency of B is 0.7. What would be the
frequency of heterozygotes if the
population is in Hardy-Weinberg
equilibrium?
Practice Problem 2
2.In a population that is in Hardy-Weinberg
equilibrium, 16% of the individuals show
the recessive trait. What is the frequency
of the dominant allele in the population?
Microevolution
• When the actual values deviate from the expected
values of the Hardy-Weinberg equilibrium, it means
the population is evolving.
• Usually, evolution on a grand scale is not visible – we
did not see any of the current species evolve.
• But small generation-to-generation changes in one
population’s alleles and genotypes CAN be
observable (mixing of races from colonization, for
example)
• This is evolution on a very small scale - Microevolution
Causes of Microevolution
Genetic Drift – changes in the gene pool of a
population due to chance. Two situations can
lead to genetic drift:
1. Bottleneck Effect - natural disasters/hunting
may kill many, leaving a few whose genetic
makeup does not reflect that of the original
population
2. The Founder Effect – refers to the loss of
genetic variation when a new colony is
established by a very small number of
individuals from a larger population.
Bottleneck Effect
Founder Effect
• The Amish population in PA.
stemmed from a small number of
German immigrants -- about 200
individuals.
• The Amish carry unusual
concentrations of gene mutations
that cause a number of otherwise
rare inherited disorders – such as
dwarfism.
•
One form of dwarfism, Ellis-van
Creveld syndrome, involves short
stature, polydactyly (extra fingers
or toes), abnormalities of the nails
and teeth, and, in about half of
individuals, a hole between the two
upper chambers of the heart.
Geographical Variation
• Geographic variation results from differences in
phenotypes or genotypes
– Variation Within a Population – Subtle difference between
members of the same race that live in the same geographic area
(between 2 Caucasians)
– Variation Between Populations - between populations or
between subgroups of a single population that inhabit different
areas (human races)
• Geographic variation in the form of graded change in a
trait along a geographic axis is called a cline. For
example, as you travel towards the equator, people
have more pigment in their skin (they’re darker)
Variations within Populations
• Cline – a graded change in a particular trait along a geographic axis.
Types of Natural Selection
Directional Selection
• When conditions
favor one extreme
phenotype, so the
allele frequency is
shifted to one side.
• The best example for
this the phenomenon
of peppered moths
and industrial
melanism
Stabilizing Selection
• Removes extreme phenotypes or variants from a
population. So population looks more
homogenous. In this case, the darker and lighter
snail probably got eaten, so the “middle” colored
shells survived.
Disruptive or
Diversifying
Selection
Phenotypes at the opposite
ends of the spectrum are
naturally selected.
Intermediate phenotypes
are lost.
Disruptive Selection, cont’d.
• Darwin observed disruptive beak sizes in certain seedeating finch species:
This variation appeared to be adaptively related to the
seed size available on the respective islands
- big beaks for big seeds
- small beaks for small seeds.
Medium beaks had difficulty retrieving small seeds and
were also not tough enough for the bigger seeds.
So medium beaks were lost as a phenotype.
Sexual Selection
• A form of natural selection where
individuals with certain inherited traits are
more likely to finds mates.
• Sexual selection causes sexual
dimorphism, where there are clear
differences between the two sexes (Like in
peacocks and peahens)
Intrasexual Selection
– A form of sexual
selection in which
members of one sex
compete with each other
to mate with the opposite
sex. Males will frequently
fight each other for a
female or for a desirable
territory to attract a
female. Mammals
Intersexual Selection – A form of sexual
selection in which members of one sex are
differentially attracted to members of the opposite
sex, and so competition is for the attention of the
opposite sex. Birds
END OF CHAPTER 23