how populations evolve and population genetics
TRANSCRIPT
How Populations Evolve and Introduction to Population
Genetics
05-23-2016
Chapter 19 sections 1, 2, and 3
Learning Goals for Today:
• Explain the connection between populations, genes, and evolution
• Predict how populations may change over time
How Are Populations, Genes, and Evolution Related?
• Evolutionary change occurs over time to populations
• A population is a group that includes all members of a species living in a given area
• Individuals live/die, reproduce/not …depending on their traits
Population genetics - concepts
• A gene is a segment of DNA located at a particular place on a chromosome
• Different members of a species may have slightly different nucleotide sequences for the same gene, these are called alleles
– Different alleles code for slightly different versions of the same protein
Population genetics - concepts
• In a population, there are usually two or more alleles of each gene
• If the population is diploid,
– an individual with the same two alleles of a gene is homozygous
– an individual with two different alleles of a gene is heterozygous
• Example: human height
– Genes
– Childhood nutrition
– Growth hormones
Genes and the environment interact to determine traits
Example of Tracking Alleles in a Population
• Hamster coat color
• The dominant allele encodes for an enzyme that catalyzes black pigment formation (B)
• The recessive allele encodes for an enzyme that catalyzes brown pigment (b)
• How many alleles will 1 hamster have?
• What color is a Bb hamster?
B B B b
BB Bb bb
b b
chromosomes
genotype
phenotype
Coat-color allele B is
dominant, so heterozygous
hamsters have black coatsEach chromosome
has one allele of the
coat-color gene
Alleles, Genotype, and Phenotype in Individuals
Gene Pools
• Sum of the genes in a population
– Population genetics deals with the frequency, distribution, and inheritance of alleles in populations
– A gene pool consists of all the alleles of all the genes in all individuals of a population
Gene Pools
• The number of copies of each allele in a gene pool is equal to:
1. The number of heterozygous individuals that carry one copy of the allele
2. Twice the number of homozygous individuals for that allele since they carry two copies of the allele
Gene Pools
• Sum of the genes in a population
– If we counted all of the alleles present for one gene in a population we could determine allele frequency for each different allele
– Allele frequency the proportion of one allele relative to all the alleles of that gene
How Are Populations, Genes, and Evolution Related?
• Determining allele frequency• A population of 25 hamsters contains 50 alleles of
the coat color gene
• If 20 of those 50 alleles code for black coats (are B), then the frequency of the black allele (B) is
• 20/50 = 0.4 = 40%
• What is the frequency of the b allele?
BB BB BB
B B B B B B B B
BB
Bb Bb Bb
B B B B b b b b
Bb
Bb Bb Bb
B B B B b b b b
Bb
bb bb bb
b b b b b b b b
bb
bb bb bb
b b b b b b b b
bb
bb
b b
Bb Bb Bb
B B B B b b b b
Bb
Population: 25 individuals Gene pool: 50 alleles
The gene pool for the
coat-color gene contains
20 copies of allele B and
30 copies for allele b
A Gene Pool
Fig. 15-2
Example: calculating allele frequencies
Population of 50 hamsters (25 BB, 25 bb)
Frequency of B allele? Of b allele?
Population of 50 hamsters (all Bb)
Frequency of B allele? Of b allele?
What do the allele frequencies always add up to?
The Big Picture
• Evolution is the change of allele frequencies within a population
– Frequencies change (over generations) = evolution
– Frequencies stable (over generations) = no evolution
How can we tell if a population is evolving?
• The Hardy-Weinberg principle: a mathematical model for population biology
• Evolution: change in allele frequencies
• Mathematician Godfrey H. Hardy and physician Wilhelm Weinberg came up with a way to track allele frequencies
• No change = no evolution– Said to be in “equilibrium”
Conditions required to have no allele change (no evolution occurring)
Conditions1. No mutations must occur in the population 2. There must be no gene flow between
populations• No movement of alleles in or out of the population
3. The population must be very large 4. Mating must be completely random 5. There can be no selection
• All genotypes have equal reproductive success
Under these conditions, allele frequencies in a population will remain the same indefinitely
• Without evolution, alleles in a population will
quickly reach equilibrium
• Once the frequency of alleles in a population is
known, we can use the Hardy-Weinberg equation
• p = frequency of the dominant allele in the population
• q = frequency of the recessive allele in the population
Hardy-Weinberg Principle
• Assume that 2 heterozygous individuals
produce offspring
Where does it come from?
p
p
q
q
pp
(p2)
(q2)
pq
pq
p
p
q
q
pp
(p2)
(q2)
pq
pq
• We can calculate what proportion of individuals in
the next generation will have a given genotype and
phenotype
• You will do this in Lab 9
What does it mean?
• Two alleles exist for color in a certain type of beetle. Red (R) is dominant to blue (r).
• In a specific population of beetles, 51% are red and 49% are blue.
• Assuming the population is in H-W equilibrium, what are the frequencies of the red and blue alleles in the gene pool?
Example
= RR and Rr = rr only
p2 and pq q2 only
q2 = 0.49
q = 0.7
p + q = 1
1 – 0.7 = p
p = 0.3
Example
• Cystic fibrosis is an autosomal recessive disorder that affects 1/2500 (0.0004) Caucasians.
• Assuming the population is in H-W equilibrium, what percentage of Caucasians are carriers?
Example
= CC and Cc = cc only
p2 and pq q2 only
q2 = 0.0004
q = 0.02
p + q = 1
1 – 0.02 = p
p = 0.98
Example
= CC and Cc = cc only
p2 and pq q2 only
Frequency of heterozygotes = 2pq
2(0.98)(0.02) = 2pq
0.0392 or 3.92%
Example
p2 + 2pq + q2 = 1
p = 0.98 q = 0.02
(0.98)2 + 2(0.98)(0.02) + (0.02)2 = ?
1
Check Your Work!
Practice calculating allele frequencies from phenotypes
Population of 100 hamsters
Year 1: 25 brown, 75 black
Year 2: 36 brown 64 black
Did the allele frequencies change?
Evolutionary forces
• Mutations
• Gene flow
• Genetic drift
• Non-random mating
Mutations
• Changes in DNA sequence– Usually occur during DNA replication
– Rare
• A new mutation will appear in an allele
– 1 of every 100,000 human gametes
– 1 of every 50,000 human babies
– Causes very small changes in the frequency of any allele
• But we have
– 20 to 25,000 genes
– 50,000 alleles
– Most newborns have one or two mutations
– New alleles new variation evolution
Mutations
• Changes in DNA sequence
– Usually occur during DNA replication
– Rare
– Source of new alleles
– Spontaneous
– Can be neutral, harmful, beneficial
– Provides a potential for evolutionary change
Mutations
• Changes in DNA sequence
– Usually occur during DNA replication
– Rare
– Source of new alleles
– Spontaneous
– Can be neutral, harmful, beneficial
– Can be inherited (IF gametes carry the mutation)
Gene Flow
• Movement of alleles from one population to another
– Individuals move around
– Gametes (like pollen) can also move
Fig. 15-4
– Baboons live in troops, juvenile males move from troop to troop to rise in social status
– Changes the alleles of the destination population to be more like the source population
• Examples:
• A migrating bird changes flocks
• Grolar bears (gene flow between species)
Gene Flow
Genetic Drift
• Random change in allele frequencies over time brought about by chance alone
– Only makes a difference in small populations
• Examples of bad luck:
– Seeds can fall into a pond or parking lot and never sprout
– Animals can be killed by floods, fires, volcanic eruptions
Genetic Drift
• Imagine you have a population of 20 hamsters
• B = 0.5 and b = 0.5
• If all animals are allowed to breed randomly and yield 20 new hamsters, the frequencies would not change
• But if you chose two randomly and let only them breed and yield 20 new hamsters, allele frequencies would change dramatically
– Two causes that lead to:
• Population bottleneck
• Founder effect
Alleles are more likely to disappear
due to random chance in small
populations
Genetic Drift by Population Bottlenecks
Fig. 15-7
Natural catastrophes, over-hunting
Some examples:
Elephant sealsCheetahsCalifornia condors
Limited genetic variation leaves populations vulnerable to extinction
• A bottleneck event is random
• A plague that kills off individuals lacking a
particular allele is natural selection
• Natural selection kills individuals due to genetic
make-up
• Bottlenecking kills indiscriminately
IMPORTANT!!
Genetic Drift by the Founder Effect
• Small number of individuals leave a large population and establish a new isolated population
• By chance, not all alleles are present in this new smaller population
http://beacon-center.org/blog/2012/01/03/beacon-researchers-at-work-how-the-cricket-lost-its-song/
Genetic Drift by the Founder Effect
• Example:
• All Amish in Lancaster, PA
descended from ~200
mostly Swiss individuals
that migrated in 1744
• Ellis-Van Creveld
syndrome is common
among old order Amish
• Amish: 1 or 2 in 200
• General population: 1 in
1000
Non-Random Mating
• Mating within a population is almost never random
– Lack of mobility
• Many organisms inbreed (self-fertility is common in plants)
– Can increase frequency of homozygous recessive individuals
• Mates have preferences
Fig. 15-9
• Snow geese mate with partners of the same color
– Assortative mating
• In large population, neither inbreeding nor assortative mating will alter allele frequencies
Evolutionary forces
– Mutation
– Gene flow (between populations)
– Small population size (genetic drift)
– Non-random mating
– Selection occurs
Natural selection
• When an allele provides “some little superiority”, the individuals with that allele are favored by natural selection
• Natural selection favors traits that increase an individual’s survival only to the extent that the individual’s survival leads to improved reproduction
• Reproductive success determines the fate of the alleles
Natural Selection and Antibiotic Resistance
• Penicillin: antibiotic, widely-used starting around WWII
• Killed most bacteria• Some individual bacteria were naturally
resistant (rare resistance allele)
Before penicillin treatment
Penicillintreatment
After penicillin treatment
Natural Selection and Antibiotic Resistance
• Penicillin killed most bacteria• Some individual bacteria were naturally resistant
(rare resistance allele)• Natural selection:
– Did not cause genetic changes in individuals– Acted on individuals (killed them)– Caused the population to evolve/change allele
frequency
• Evolution by natural selection is not progressive; it does not make organisms “better”
Patterns in evolutionary change
• Directional selection
• Stabilizing selection
• Disruptive selection
Directional Stabilizing Disruptive
How selection can alter population structure
Directional selection
• Occurs when natural selection favors one extreme of continuous variation
• Over time, the favored extreme will become more common and the other extreme will be less common or lost
Dark-peppered moths were prevalent in the early 19th century
• Historical case in England: peppered moth
Directional Selection
Pre-Industrial revolution
Post Industrial revolution
Stabilizing selection
• Removes individuals from both ends of a phenotypic distribution, thus maintaining the same distribution mean
• Occurs when natural selection favors the intermediate states of continuous variation– Common when there are opposing forces
acting on a trait
Goldilocks
Human Birth Weights: an Example of Stabilizing Selection
Disruptive or diversifying selection
• Removes individuals from the center of a phenotypic distribution and thus causes the distribution to become bimodal
• Occurs when natural selection favors both extremes of continuous variation
• Disruptive selection can lead to two new species
Balanced polymorphism
• Often occurs when environmental conditions favor
heterozygotes
• Normal and sickle-cell hemoglobin alleles coexist in malaria-prone regions of Africa
Types of selection
• Natural
– Kin selection
– Sexual selection
– Frequency-dependent selection
• Artificial
Kin selection
• A type of selection that involves altruistic behavior, e.g., the protection of offspring
• Occurs when natural selection favors a trait that benefits related members of a group
Kin selection
• Worker bees exhibit altruistic behavior• In terms of simple fitness, the worker bee
does not reproduce • However, all of the bees in the hive are close
relatives, a worker bee's genes will be passed to the next generation indirectly
Young bee-eaters remain with parents when breeding opportunities are low
• Habitat is saturated or no more suitable nesting sites
• More beneficial to assist family with siblings
Sexual Selection• In many species, males and females look very
different
• Sexual dimorphism
Sexual selection
• A type of selection in which the forces determined by mate choice act to cause one genotype to mate more frequently than another genotype
Sexual selection in Long-tailed widowbird
• The evolutionary fitness of an organism not only depends upon its ability to survive but also its ability to reproduce
• To reproduce, an individual must obtain a mate and produce viable offspring
• Natural selection favors traits that maximize the ability of an individual to compete for and attract mates, and/or the ability to produce offspring
Sexual Selection
• Some of these differences give a competitive
advantage when competing with each other for
mates
• Intrasexual selection
Sexual Selection
• Some of these differences give a competitive
advantage when attempting to attract mates
• Intersexual selection
Sexual Selection: one explanation for traits that appear
disadvantageous
Fig. 14-14
Frequency-dependent selection
• Individuals with either the common (positive frequency-dependent selection) or the rare (negative frequency-dependent selection) phenotype are selected for
• Negative frequency-dependent selection serves to increase the population’s genetic variance
• Positive frequency-dependent selection usually decreases genetic variance
Lizards in the PNW show negative frequency-dependent selection
• The common side-blotched lizard
• Alternate male strategies leads to frequency-dependent selection
Artificial Selection
People have bred many varieties of animals and
plants
Selective breeding of organisms by humans is
artificial selection
These organisms have been changed to look very
different from the original parent stock
Types of Selection:
• Natural
• Occur in nature (no human intervention)
– Directional selection
– Stabilizing selection
– Disruptive selection
• Artificial
• Organisms bred by humans for specific traits
– Directional
– Stabilizing
– Disruptive
Crop breeding: artificial selection in action
http://the10000yearexplosion.com/pictures/evolutionary-business/1720176
Teosinte: wild ancestor of corn
What traits were selected for in the selective breeding of corn?
Brassica oleracea: many different forms from one ancestor
A cultivar is a plant or grouping of plants selected for desirable characteristics that can be maintained by sexual or vegetative propagation.
Varieties of Canis familiaris
http://www.blueberrybasket.com/catalog/home/home_KDOG.htm
Ancestor to all dogsCanis lupus
Domestic cat breeds: recent example of artificial selection
http://en.wikipedia.org/wiki/List_of_cat_breeds
Closest relative
What happens when artificially selected organisms return to
nature?
Crop and animal breeding
• Genetic variance is the diversity of alleles and genotypes within a population
• Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population
• Breeders attempt to increase a population’s genetic variance to preserve as much of the phenotypic diversity as possible and to reduce inbreeding
• Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding
Upcoming lab:
• Lab 9, population genetics
• Hardy-Weinberg
• Equation for calculating allele frequencies from phenotypes