1. The Evolution of Populations and Speciation
Reference: Campbell 7th Ed. Chapters 23 & 24
The Evolution of Populations and Speciation

2. Variation of Traits in a Population
Evolution by natural selection gains wide acceptance
Early 1900’s birth of genetics field
Questions resurface about evolution and natural selection
“Population Genetics”: study of evolution from genetic point of view
Involves gradual changes in genetic material over generations, in groups of organisms

3. Variation of Traits in a Population
A Population is the smallest unit in which evolution occurs (“microevolution”)
Individuals may vary in observable traits
Studying variation in a single trait – use a large sample
Quantitative traits in a population (height, weight) show variation in a bell-shaped “normal” curve
Ex. Body length in a population of fish
X axis: fish length (cm)
Y axis: # of fish

4. Variation of Traits in a Population
What causes variation in traits?
Environmental factors & Hereditary can account for different phenotypes within a single family
Genotypes (alleles) come from same parents but in different combinations can account for variations in successive offspring due to formation of gametes & how they fuse (Segregation of Alleles)
Ex: Rr x Rr = ?

5. Causes of Variation Mutation: flawed copies of individual genes
Recombination: reassociation of genes in diploid individual (occurs during meiosis)
Segregation of alleles
Independent assortment (nonhomologous)
Crossing over (homologous)

6. Try this game: The Great Sperm Race
Causes of Variation
Random fusion of gametes: chance game played by gametes
Millions of sperm in mating
“Chosen One” fertilizes egg
Ensures variation in offspring
No exact copies of parents, or other offspring likely
Try this game: The Great Sperm Race

7. Allele Frequencies and Gene Pool
“Gene pool”: total genetic information available in a population
“Allele frequency”: percentage of allele in gene pool (expressed as a decimal)
Ex: If there are ten individuals in a population and at a given locus there are two possible alleles, A and a, then if the genotypes of the individuals are:
Population 1: AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA
Then the allele frequencies of allele A and allele a are:
pA = ( )/20 = 0.7
pa = ( )/20 = 0.3
*remember, gametes are haploid, and carry only one form of allele

8. Predicting Phenotype
Phenotypes are controlled by which alleles are inherited (genotypes)
Phenotype frequency: ratio stating number of times a specific phenotype occurs in a population in a single generation
example, F2:
red 1/6 = 0.17
pink = 3/6 = 0.50
white = 2/6 = 0.33

9. Hardy-Weinberg
British mathematician Godfrey Hardy
German physician Wilhelm Weinberg
Independently showed that allele frequencies in a population “tend to remain the same from generation to generation unless acted on by outside influences” when populations are in “genetic equilibrium”.
Hardy-Weinberg Equilibrium
Based on set of assumptions about ideal hypothetical population that is not evolving

10. Hardy-Weinberg conditions:
1) No mutations occur
Allele frequencies do not change overall
2) Individuals don’t migrate
3) Population is large
4) Individuals mate randomly
5) Natural selection does not occur

11. Hardy-Weinberg Equation:
Equation:     p2+2pq+q2=1.0
p2 = homozygous dominant condition;  AA q2 = homozygous recessive;  aa 2pq = heterozygous ; Aa

12. Hardy-Weinberg
Theoretical state in which allele frequencies remain the same over generations (P = F1 = F2 = F3, etc)
Showed what forces disrupt genetic equilibrium and led to evolutionary change
Real populations usually violate HW conditions, causing gene frequencies to fluctuate

13. Mutation
Evolution results from the change of population’s allele frequencies (genetics) over generations
Any violation of 5 conditions of Hardy-Weinberg Equilibrium results in evolution
Mutagens can cause increase/decrease in allele frequency
Spontaneous mutations occur constantly
Mutations can produce new alleles for trait
Affect genetic equilibrium

14. Mutation
Spontaneously introduces new allele variants into a population
Natural selection is often slow to eliminate harmful recessive mutations
Natural selection operates only when genes are expressed (phenotypes); often not when “carried”
Beneficial mutations are vital to evolution in long run

15. Mutations: beneficial or not?

16. Migration
Gene flow: process of genes moving from one population to another
ex: Baboons
Gene frequency changes
Immigration: movement of individuals into a population
Emigration: movement of individuals out of a population

17. Genetic Drift
Genetic Drift: allele frequencies in a population change as result of random events or chance.
Example:
Small population can be affected by single organism’s ability to reproduce low or high
Small populations are much more susceptible. Why?
Abrupt changes in alleles shows high genetic drift
Large population
Retain stable allele frequencies; low genetic drift

18. Genetic Drift
Small population loses genetic variability and becomes vulnerable to extinction
“Bottlenecking” a population
Northern Elephant Seals
Cheetahs = very little genetic variability left

19. genetic drift population bottleneck Founder effect

20. Nonrandom Mating Most species do not mate randomly
Geographic proximity is a factor
Matings of related individuals can amplify traits & result offspring with disorders
Similar recessive genes (carried, masked) often present in genomes of related individuals

21. Blue and white snow geese
Nonrandom Mating
Physical Characteristics (similar genes)
Assortative Mating: selection of mate based on similarity of characteristics
Nonrandom mating can affect genotypes (combination of alleles) of population
May not affect on overall allele frequencies
Blue and white snow geese

22. Natural Selection Ongoing process in populations
Single most significant factor that disrupts genetic equilibrium
Individuals reproduce more successfully as result of natural selection
Contribution of genes to next generation
Stabilizing, Directional, Disruptive and Sexual all cause evolution in a population (microevolution)

23. Stabilizing Selection
Stabilizing Selection: average form of trait causes organism to have an advantage in reproduction; high fitness
Lizard size
Small lizard runs too slow
Large lizard easily spotted and captured
Selection reduces size range
Most common type of selection

24. Directional Selection
Directional Selection: individuals that display more extreme form of trait have higher fitness than individuals with average

25. Disruptive Selection
Disruptive Selection : individuals with either extreme variation of trait have higher fitness than average form of trait
Limpets
Shell color
Pure white to dark tan
White on rocks with goose barnacles
Dark tan on bare rocks blend in
Intermediate color at disadvantage

26. Selection Charts

27. Sexual Selection
Sexual selection: preferential choice of a mate based on specific phenotypic trait
Females tend to choose males they mate with due to certain traits male expresses
Genes of successful reproducers rather than of merely successful survivors are amplified through natural selection
The Tale of the Peacock

28. Concept of Species
Total # of species today is inaccurate due to numerous undiscovered species
Currently, scientists have named and successfully classified over 1.5 million species. It is estimated that there are as little as 2 million to as many as 50 million more species that have not yet been found and/or have been incorrectly classified.
Remote locations: Rainforests and Oceans
New species discovered while others become extinct at fast rate
One species can become two through process of speciation
Speciation results in many related populations

29. Concept of Species

30. Morphological Species Concept:
Morphology: study of internal and external structure and form of an organism
Using the MSC, species are defined by structure and appearance
Aka “Phenetic” species concept: a species is a set of organisms that are phenotypically similar and that look different from other sets of organisms.

31. Limitations to MSC
Adult & juvenile herring gulls
Mallards (Anas platyrhynchos)
Phenotypic differences may exist among individuals in one population.
American Black duck (Anas rubripes)

32. Biological Species Concept
Organisms may appear different enough to belong to different species. How different do they have to be to be considered a unique species?
Biological Species Concept: A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.
Defines a species as those organisms that can produce viable offspring together. Same chromosome #
Issues:
What about hybrids?
What about plants, etc that reproduce asexually?
What about extinct species?

33. Geographic Isolation
Geographic Isolation: physical separation of members of population
Allopatric Speciation
Populations physically isolated by an extrinsic barrier
Gene flow between them stops
Natural selection and genetic drift cause divergence
Individuals of two populations can no longer interbreed

34. Reproductive Isolation
Reproductive Isolation: results from barriers to successful breeding between population groups in same area
Parapatric Speciation
Two or more separate gene pools form, and eventually these diverge into different species
Two broad types
Prezygotic: before fertilization
Postzygotic: after fertilization

35. Reproductive Isolation
Types of postzygotic isolation
Offspring of interbreeding species are underdeveloped, die early, or are not fertile
If death or infertility occurs parents have wasted gametes from evolution standpoint
Prezygotic
Incompatible behavior
Reduce chance of hybrid formation
Mating times, calls
Frogs, birds

37. Rates of Speciation Gradualism –vs- Punctuated Speciation
Speciation usually takes millions of years, but some species form more rapidly
“Gradualism” - Fossil record indicates many species existed without change for long periods
Fossil evidence seems to indicate that “instant” changes can occurred within few thousand years (Hox genes)
Punctuated Equilibrium: theory that speciation occurs during brief periods of rapid genetic change, interspersed with long equilibrium periods
In 1972 paleontologists Niles Eldredge and Stephen Jay Gould published a landmark paper developing this idea.