1. Population Genetics

2. What do we know? Evolution is:
A change in species over a very long period of time
Heredity is:
Passing of alleles through generations of organisms

3. How are Evolution and Genetics Related?
Mutations produce variations that can be passed on to offspring. Natural selection “works” on these variations.
Genetic variation is studied in populations.
Because members of populations reproduce together, they share a common group of genes called a gene pool.

4. Relative frequency of alleles: The number of times a particular allele occurs in a gene pool
In a population of 100 slugs, there are 50 yellow slugs (YY), 35 yellow-brown (YB), and 15 brown slugs (BB).

5. For Y, there are 50+50+35= 135 Y alleles.
Remember: in a population of 100 slugs, there are: 50 yellows (YY), 35 yellowish-browns (YB), and 15 browns (BB).
To figure out relative frequencies, we must see how many times the alleles for Y and B occur in our population.
There are 100 slugs. Each has two alleles for color. That means there are 200 alleles total.
For Y, there are = 135 Y alleles.
Divide 135/200=.675 x100 = 67.5%!
Do the same thing for B. You should get…

6. = 65 B alleles 65/200=.325 x 100 = 32.5%!
So, we have a relative frequency of 67.5% for the Y allele and 32.5% for the B allele.
Let’s say that a species of frog started to eat these slugs. The yellow ones were much easier for the frogs to see! What might happen?
If the relative frequencies change, EVOLUTION has occurred!

7. If the allele frequency stays the same…
No change in allele frequency = NO EVOLUTION
This never happens, but if it did, the population would be in Hardy-Weinberg Equilibrium

8. Conditions that need to be met for HW Equilibrium
Population is infinitely large: small populations can lead to genetic drift (random evolution), such as the Bottleneck and Founder Effects
No Gene Flow: no emigration and immigration (which allow for gene flow into/out of gene pool)
No mutations: which normally allow for evolution to occur, via natural selection
Random Mating: no sexual selection; leads to inbreeding or assortative mating
No natural selection: the environment cannot select for/against any traits in the population
If all of the above conditions are met, allele frequencies will stay the same from generation to generation

9. Hardy-Weinberg Equation
A = dominant allele; a = recessive allele
p = (freq)A
q = (freq)a
So p + q = 1
Frequency of dominant allele + Frequency of recessive allele = 100% (1)
If the population is in EQUILIBRIUM:
p2=(freq)AA (homozygous dominant)
q2=(freq)aa (homozygous recessive)
2pq=(freq) Aa (heterozygous)
So p pq + q2 = 1

10. p + q = 1 AND p pq + q2 = 1
You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that knowledge, calculate the following:
Frequency of “AA” genotype
Frequency of “a” allele
Frequency of “A” allele
Frequencies of genotype “Aa”

11. p + q = 1 AND p pq + q2 = 1
The ability to taste PTC is due to a single dominant allele “T”. Sample the class, then calculate the following:
Frequency of “TT” genotype
Frequency of “t” allele
Frequency of “T” allele
Frequencies of genotype “Tt”

12. STOP
Let’s practice

13. If not in HW Equilibrium, the population is evolving!
Microevolution: changes in allele frequencies from generation-to-generation
Macroevolution: the compounded effects of microevolution that lead to new species (origin of new taxonomic groups)!

14. Genetic Drift
Random, unpredictable changes in allele frequency. Refers to statistical drift in allele frequencies over time.
Drift Events:
1. Population Bottleneck: When a population’s size decreases dramatically in a short period of time (due to environmental one-time disaster)

16. Drift Events, cont.
2. Founder Effect: when a small group in a population splinters off from the original population and forms a new one.
Ex: Afrikaner population in South Africa; Huntington’s Disease

17. Genetic Drift is most likely to affect small populations
Those populations left after a bottleneck or founding event become the basis for the gene pool in every successive population
When there are few copies of an allele (because of small population), the effect of drift is larger
Drift Events may cause gene variants to disappear completely, thereby reducing genetic variety

18. Predictable Changes in Allele Frequency
Natural Selection: phenotype is not determined by genes alone, but by the interactions of the phenotype with the environment (survival of the “fittest”) Fitness: Measured by relative contribution an individual makes to the gene pool of next generation (will they survive until reproductive age and make babies?)

19. Types of Variety that Natural Selection will act upon
Within Populations
Polymorphism: 2 or more phenotypes exist for a particular gene in a population
Between Populations
Geographic Variation: variety may be different depending on factors like climate and predators (that differ between populations)

20. Three types of natural selection:
Stabilizing selection
Disruptive selection
Directional selection

21. Stabilizing Selection
Average traits are favored; Genetic Diversity decreases as population stabilizes
Example: Human Birth Weight
Babies that are too large have a hard time fitting through the birth canal.
Babies that are too small tend to lose too much heat and have more illnesses.
Average weight babies (around 8 lbs.) tend to be the healthiest

22. Favors traits at both extremes
Disruptive Selection
Favors traits at both extremes

23. Directional Selection
Allele frequency shifts in one direction, over time.
Example: Peppered moths
So, the darker colored moths were selected for, since now they blended in with the soot-covered trees!
Peppered moths, found in England, were originally light colored and hid on light colored trees.
Then came the industrial revolution, covering England with soot…

26. Ways to prevent extinction, despite natural selection
Diploidy: Genes are duplicated (to make homologous pairs) so that they duplicate version can mutate without knocking out essential functions
Neutral Variation: variation of a trait that has no advantage over another; ex: eye color

27. Mechanisms of Evolution part deux

28. Macroevolution
Changes in allele frequency that lead to new species (speciation)
Speciation = formation of new species

29. Biological Species Concept
Species: population whose members interbreed to produce fertile offspring
Cannot produce viable offspring with other species
Hybrids are often infertile, therefore inter-species breeding is disadvantageous
Gene flow can occur between populations of same species
* Different species are isolated by barriers that block genetic mixing*

30. Speciation: how does it happen?
Over time, populations become reproductively isolated from each other, resulting in separation of gene pools.
If gene pools no longer mix, each population is likely to evolve into a new species over time.

31. Reproductive Isolation
Any structural, functional, or behavioral characteristic that prevents successful reproduction (could happen before or after fertilization)
Gene flow must not occur

32. Isolating Mechanisms in INDIVIDUALS
Pre-Zygotic Barriers: differences that prevent mating or make fertilization unlikely
Post-Zygotic Barriers: prevent development of embryo/hybrid after mating has happened

33. Habitat (Geographic) isolation: Populations are physically separated by geographical barriers like water, canyons, or mountains; continental drift

34. Temporal isolation (Species reproduce at different times)
Because of timing in breeding cycles (seasons) or the time of day/night that species search for mates

35. Behavioral isolation (Species have different reproductive behaviors)
Courting individual (usually male) does not display to members of other species
Signal and response are species-specific

36. Mechanical Isolation
The parts don’t fit

37. Gamete Isolation
Certain species of eggs will only accept sperm of the same species

38. Post-Zygotic Barriers
Zebroid
Zygote Mortality
Hybrid sterility
Cama
Africanized honey “killer” bees

39. Reproductive Isolation is a prerequisite of speciation Once separated, there are 2 modes for speciation…

40. Modes (scenarios) of Speciation
Allopatric Speciation: new species result from populations being separated by geographic barriers
Sympatric Speciation: when members of a single population develop a genetic difference that prevents them from reproducing with parent type; more common among plants than animals

41. Ring Species = allopatric speciation in progress
Species occur along a continuum of interbreeding.
Species at both “ends” of ring cannot interbreed.

42. How speciation occurs, depending on mode
Phyletic: a new species completely replaces an older species (sympatric)
Divergent: when species form, existing at the same time, in different places (allopatric)

43. We call phyletic speciation “anagenesis” and divergent speciation “cladogenesis”

44. Anagenesis: evolution within a lineage
Cladogenesis: evolution that results in the splitting of a lineage

45. WORD ATTACK

46. What is an analogous structure, and how is it different from a homologous structure?
Analogous structures have a SIMILAR function but DIFFERENT evolutionary origin.
Homologous structures are similar in structure AND show common ancestry!

47. Take-Away from “Why Sex?”
Organisms will go to extreme lengths to reproduce
Features that may make reproduction more likely could also be harmful to survival (attract predators)
Sexual Selection is 2-way street
Male competition
Female choice
No offspringno genes passed to next generation
Therefore, that individuals “fitness” = ZERO

48. Review: Phyletic or Divergent?

49. Review: Ways Evolution Happens
Divergent Evolution
Convergent Evolution
Coevolution

50. Divergent Evolution (Adaptive Radiation):
Diversification of a single ancestral species into several forms that are each morphologically specialized to a particular environmental niche.
Starting with a single ancestor, it results in the speciation and phenotypic adaptation of an array of species with different morphological traits which allow them to survive in their respective environments
*Evidence: homologous structures

51. *Evidence: analogous structures
Convergent Evolution
Occurs when organisms that are not closely related independently evolve (acquire)similar traits
Wings evolved separately in:
insects (arthropods)
pterosaurs (extinct flying reptiles)
birds (birds)
bats (mammals)
*Evidence: analogous structures

52. Sometimes, two or more species can evolve in response to each other
Sometimes, two or more species can evolve in response to each other. This is called coevolution.
Predator and prey evolve in response to each other
Rawwr…

53. “Speed” of Speciation Events
2 “tempos” at which speciation can occur
Gradualism=
constant, gradual evolution
(smooth and continuous)
Punctuated Equilibrium= speciation is rapid/sudden; long periods of stasis

54. Gradualism Punctuated Equilibrium
Slight changes between
speciation events
Punctuated
Equilibrium
HUGE morphological changes
between speciation events

55. Thoughts on Punctuated Equilibrium
“Sudden” appearance of dramatically different organisms is less due to rapid evolution and more due to lack of fossil evidence of intermediates
Long periods of stasis are illusions
Really just large gaps in fossil record

56. Large Morphological Trends
Have evolved in small increments over the course of MANY speciation events (Macroevolution)
Examples:
Limb Growth (25:15-27:05)
Eye/Optic Complexity

57. How is embryonic development related to evolution?
All living organisms have a similar set of developmental genes that control body plan
Homeobox “Hox” genes
We all start out with these basic instructions (evidence of evolution)
How instructions are used/interpreted depends on environment and how organism has adapted

59. SHIFTING GEARS: GEOLOGIC TIME
Thanks, fossils!

60. Essentials from Geologic Timeline
Eras: PrecambrianPaleozoicMesozoicCenozoic
Major Extinctions:
Permian (250 MYA)
Killed >90% marine species
Cretaceous (65 MYA)
Killed ~1/2 marine and terrestrial species including DINOSAURS!
Understand scale!