Transcript population
Chapter 17: Evolution of Populations
Evolution of Populations 1.
When Darwin developed his theory of evolution, he did not understand: •
how heredity worked.
This left him unable to explain two things:
a. source of variation b. how inheritable traits pass from one generation to the next
Evolution of Populations In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular
population
.
This evolution of populations is called
microevolution
.
Vocabulary:
P opulation:
group of individuals of the same species living in the same area that breed with each other.
gene pool
combined genetic info. for all members of a population
Allele:
one form of a gene
2. relative frequency of an allele: # times an allele occurs in the gene pool compared to other alleles (percent) Example Relative Frequency: 70% Allele B 30% Allele b
3. Sources of Variation: a. mutations:
any change in DNA sequence
♦ Can occur because of: ♦
mistakes in replication
♦
environmental chemicals
♦ May or may not affect an organism’s
phenotype
3. Sources of Variation b.
Gene Shuffling
: recombination of genes that occurs during production of gametes ♦ ♦ ♦ ♦ Cause most inheritable differences between relatives Occurs during meiosis As a result, sexual reproduction is a major source of variation in organisms.
Despite gene shuffling, the frequency of alleles does not change in a population.
Explain why this is true
.
Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) are always there.
4. Gene Traits: A) Single gene trait: controlled by single gene with two alleles ♦ Examples : widow’s peak, hitchhiker’s thumb, tongue rolling
(4. Gene Traits:) B) Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles
♦
Examples: height, hair color, skin color, eye color Most human traits are polygenic.
Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain.
type:
single gene
why?
Only two phenotypes possible Example: tongue roller or non-tongue roller
type:
polygenic
why?
Multiple (many) phenotypes possible Example: height range 4feet to 9 feet all
5. Natural selection acts on
phenotypes
, not
genotypes
. Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white? Brown, more hidden.
5. If brown is dominant can a predator tell the difference between:
BB Bb
?
Mouse with highest fitness will have the most
alleles
passed on to the next generation.
White mouse will have low fitness
5. Which mouse will have the lowest fitness?
White, bb (recessive) BB Bb
?
Will the fitness of BB and Bb differ? Why?
No, Both BB and Bb have the same fitness advantage of being brown
Natural Selection
Three ways in which
natural selection affects polygenic traits.
•Directional Selection •Stabilizing Selection •Disruptive Selection
Directional Selection
:
individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait Key
Low
Food becomes scarce.
fitness ♦ Individuals with highest fitness :
those at one end of the curve
♦ Example:
Galapagos finches – beak size
Directional Selection
Directional Selection Food becomes scarce.
Key
Low mortality, high fitness High mortality, low fitness
Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve Stabilizing Selection Individuals with highest fitness: near the center of the curve (average phenotype) Key
Low mortality, high fitness mortality, low fitness
Birth Weight Selection against both extremes keep curve narrow and in same place.
Example: human birth weight
Disruptive Selection
:
individuals at both ends of the curve survive better than the middle of the curve. Disruptive Selection
♦
Largest and smallest seeds become more common.
Individuals with highest fitness:
both ends of Key curve
high fitness Population splits into two subgroups High mortality, low fitness specializing in different seeds.
♦ Example:
birds where seeds are either large or small
Stabilizing Selection
Stabilizing Selection Key
Low mortality, high fitness High mortality, low fitness
Selection against both extremes keep curve narrow and in same place.
Birth Weight
Disruptive Selection
Key
Low mortality, high fitness High mortality, low fitness
Disruptive Selection Largest and smallest seeds become more common.
Population splits into two subgroups specializing in different seeds.
Beak Size Beak Size
However:
No examples ever observed in
animals
A couple examples that may demonstrate speciation exist in plants and some insects.
Genetic Drift
random change in allele frequency that occurs in small populations
Genetic Drift
Two phenomena that
populations and cause genetic drift result in small
1.
Founder Effect
2.
Bottleneck Effect
Genetic Drift
The results of genetic crosses can usually be predicted using the laws of
probability
. In
small
populations, however, these predictions are not always accurate.
Founder effect
Allele frequencies change due to migration of a small subgroup of a population
Founder Effect
Two groups from a large, diverse population could produce new populations that differ from the original group.
♦
2. Bottleneck effect Major change in allele frequencies when population decreases dramatically due to catastrophe
Example: northern elephant seals decreased to 20 individuals in 1800’s, now 30,000 no genetic variation in 24 genes
Bottleneck Effect: Northern Elephant Seal Population ♦ Hunted to near extintion ♦ Population decreased to 20 individuals in 1800’s, those 20 repopulated so today’s population is ~30,000 ♦ No genetic variation in 24 genes
Bottleneck Effect
Original population
Bottleneck Effect
Original population Catastrophe
Bottleneck Effect
Original population Catastrophe Surviving population
Evolution Versus Genetic Equilibrium
What conditions are required to maintain
genetic equilibrium?
According to the equilibrium:
Hardy-Weinberg principle
, five conditions are required to maintain genetic (1) The population must be very large (2) there can be no mutations (3) there must be random mating (4) there can be no movement into or out of the population (5) no natural selection can occur
Genetic equilibrium = no evolution
A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve . The Hardy-Weinberg principle describes the conditions under which evolution does not occur. The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.
Hardy-Weinberg principle
1. Large Population
Genetic drift can cause changes in allele frequencies in small populations.
Genetic drift has less effect on large populations. Large population size helps maintain genetic equilibrium
2. No Mutations
If mutations occur, new alleles may be introduced into the gene pool, and allele frequencies will change.
Hardy-Weinberg principle
3. Random Mating
All members of the population must have an equal opportunity to produce offspring. Individuals must mate with other members of the population at random. In natural populations, however, mating is not random. •Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. •Such non-random mating means that alleles for those traits are under selection pressure.
Hardy-Weinberg principle
4. No Movement Into or Out of the Population
Individuals who join a population may introduce new alleles into the gene pool. (Immigration) Individuals who leave may remove alleles from the gene pool. (emigration) Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population.
Hardy-Weinberg principle
5. No Natural Selection
All genotypes in the population must have equal probabilities phenotype can have a another.
of surviving and reproducing. No selective advantage over
Sexual Reproduction and Allele Frequency
Meiosis and fertilization do not change the relative frequency of alleles in a population. The shuffling of genes during sexual reproduction produces many different gene combinations but does not alter the relative frequencies of alleles in a population.
The Process of Speciation
The formation of new biological species, usually by the division of a single species into two or more genetically distinct one.
Three Isolating Mechanisms
perhaps causing speciation. : Isolate species forming subspecies and
1.
2.
3.
Geographic Isolation Behavioral Isolation Temporal Isolation
Example: Eastern and Western Meadowlark
Male birds sing a matting song that females like, East and West have different songs. Females only respond to their subspecies song.
1. Geographic Isolation
Two populations separated by a geographic barrier; river, lake, canyon, mountain range.
Example: 10,000 years ago the Colorado River separated two squirrel populations.
Kaibab Squirrel Abert Squirrel
Kaibab Squirrel Abert Squirrel
This resulted in a subspecies, but did not result in speciation because the two can still mate if brought together.
Example: Eastern and Western Meadowlark
Eastern and Western Meadowlark populations overlap in the middle of the US
2. Behavioral Isolation
Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.
3. Temporal Isolation
Populations reproduce at different times
January 1 2 3 4 5 6 7 8 9 10 11 12 13
Example: Northern Leopard Frog & North American Bullfrog
Mates in: Mates in: April July
Conclusion:
Geographic, Behavioral and Temporal Isolation are all believed to lead to