1. Chapter 16 Evolution of Populations pg. 393

2. 16-1 Genes and Variation
Darwin’s lack of knowledge about genetics left him with two big gaps.
He had no idea how traits pass from generation to generation.
He had no idea how variation appeared.

3. Connecting genetics to evolution
In the 1930’s evolutionary
biologists began to connect Mendel’s
work on genetics with Darwin’s ideas
about evolution.
Genes became the new focus of experiments aimed at understanding evolutionary change.
Today our knowledge about genetics allows us to test hypotheses about inherited variations and how natural selection works with this variation.
We understand how evolution works better than Darwin ever could.

4. How Common is Genetic Variation?
We know many genes have at least two forms or alleles (Tt, tt or TT).
Some organisms have several alleles for a trait.
Body size, coat color,
flower color, etc.
Some variation is “invisible”
because it deals with
physiological processes.
Immune systems, disease,
health, etc.

5. Variation and Gene Pools
Genetic variation is studied in populations.
A population is a group of individuals of the same species that interbreed.
Because members of a population interbreed, they share a common group of genes called a gene pool.
A gene pool the combined genetic information of all members of a particular population.
Gene pools contain two or more alleles for each gene

6. The relative frequency of an allele is the number of times that an allele occurs in a gene pool when compared with the others.
Example: 36% of the mice are brown and 64% are black.
The percentages have nothing to do with dominant or recessive alleles.
Test: if an allele makes up ¼ of a populations alleles for that given trait its relative frequency is ______%

7. EVOLUTION
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Example: the mouse population now has an relative frequency of 55% brown and 45% black.
The population changed, therefore it is evolving…

8. Sources of Genetic Variation
The two main sources of genetic variation are mutations and genetic shuffling that results from sexual reproduction.
Most heritable differences are due to the gene shuffling that happens during the formation of gametes, not due to mutations.
Our 46 chromosomes can produce 8.4 million different combinations of genes.
Sex produces many different phenotypes but it does not change the relative frequency in a population.

9. Sources of Genetic Variation
Mutation – change in sequence of DNA
Gene Shufffling – most heritable differences are due to this during production of gametes

10. Sex is like a deck of cards…
Each card represents an allele found in a population.
When gametes (sex cells) are made it’s like shuffling the deck.
Shuffling leads to different hands but the number of cards remains the same.
Gene shuffling includes the independent movement of chromosomes during meiosis crossing over.

11. Single – Gene and Polygenetic Traits
The number of phenotypes produced for a given trait depends on how many genes control the trait
Single gene trait - controlled by single gene has two alleles - example a widows peak in humans
The number of phenotypes produced for a given trait depends on the number of genes that control that trait
Polygenetic Traits – traits that are controlled by 2 or more genes

12. The distribution of phenotypes of a typical polygenetic trait can be expressed as a bell shaped curve. .

13. Section 16-2 Evolution as Genetic Change
In genetic terms,
Evolutionary fitness is the ability of an organism to successfully pass its genes to the next generation..
Evolutionary adaptation is any genetically controlled trait that increases an organisms ability to pass on its genes.

14. Natural Selection
Natural Selection affects which individuals survive and reproduce, or die without reproducing. – acts directly on phenotypes
Individuals that die do not
contribute their alleles to the
populations gene pool.
Yellow fish and green fish.
Individuals that reproduce contribute their alleles to their population and over time may increase in frequency.

15. EVOLUTION
Any change over time in the relative frequency of alleles (traits) in a population.
So it is populations, NOT individuals that evolve.

16. Natural Selection on Polygenetic Traits
Natural selection can affect the distribution of phenotypes in 3 ways:
Directional
Stabilizing
Disruptive

17. Directional selection – when individuals at one end of the curve have higher fitness than individuals in the middle or other end

18. Stabilizing Selection – when individuals with an average form of a trait have the highest fitness

19. Disruptive selection – when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle

20. Genetic Drift
Natural selection is NOT the only thing that causes evolutionary change.
Genetic Drift is when an allele becomes more or less common by random chance.
Tends to occur in populations that are small

21. Worm Simulation Genetic Drift
You will need
5 colored pencils for your table
A worm worksheet

22. Worm Simulation Genetic Drift
Why is there no point in adding more generations after the worms are one color (when an allele has been fixed?)
At a population of 5, what is the number of generations until an allele becomes fixed?
What is the mathematical relationship between population size and the number of generations for an allele to become fixed?

23. More on genetic drift…
In small populations, certain individuals could have more offspring than others, by chance. Over time this could cause an allele to become more common in a population.

24. Colonizing new habitats
Genetic drift can occur when a small group of individuals colonize a new habitat.
The change in relative frequencies of the population in the new habitat is NOT due to natural selection, but
due to chance or
genetic drift.

25. Case in point Eastern Pennsylvania is home to beautiful farmlands
and countryside, but it's also a
gold mine of information for
geneticists, who have studied
the region's Amish culture for
decades.

26. The Founder Effect
When allele frequencies change as a result of the immigration (moving in) of a small subgroup of a population, it is known as the founder effect.

27. Sooo………
Can you name three things that will cause a population to evolve over time?

28. Hardy-Weinberg Principle
Alleles in a population will stay the same unless something causes them to change. (HW Principle)
Genetic Equilibrium is when allele frequencies remain the same (no evolution).
5 conditions to maintain genetic equilibrium:
Random mating
Large population
No movement in or out
No mutation
No natural selection

29. Hardy-Weinberg Principle
Genetic Equilibrium of a population can be disturbed by:
Non random mating
Movement into and out of a population
Mutations

30. Section 16-3 The Process of Speciation
How do the changes in allele frequencies in a population form new species?????
Species: a group of organisms
that can breed with one another
and produce fertile offspring.
Speciation is the formation of
a new species.

31. Speciation
In order for new species to arise, the gene pool of a population must be separated.
Reproductive isolation
happens when a population
can no longer interbreed and
produce offspring.
This creates separate gene pools.

32. Geographic Isolation

33. Geographic Isolation
When two populations become separated by a river, mountain or body of water it is called geographic isolation.

34. Behavioral Isolation
Behavioral Isolation occurs when two populations are capable of interbreeding but have differences in mating or courtship rituals.
Example: Western Meadowlark mating calls and Eastern Meadowlark mating calls are different.

35. Temporal Isolation
Temporal Isolation occurs when reproduction happens at different times.
Example: When plants release pollen on different days and can not pollinate one another.
Test Example: when a population of plants whose flowers open at midday and other plants whose flowers open late in day.

36. Back to the finches!
Recall that Darwin found many birds on the Galapagos that differed greatly in their sizes, beaks, and in their feeding habits.
Once Darwin realized that they were all finches, he hypothesized that they had all come from a common ancestor.
He proposed that natural selection
shaped the beaks of the different bird populations as they adapted to eat
different foods.
Good hypothesis, but how do you test it?

37. Speciation – galapagos finch species are an excellent example

38. Testing the hypothesis…
Darwin had two testable assumptions.
In order for beak size and shape to evolve there must be enough variation in those traits.
Differences in beak size must produce differences in fitness.

39. Peter & Rosemary Grant
The Grants tested these hypotheses with the medium ground finch, on an island large enough to support many finches, but small enough to allow them to catch and identify nearly every bird under study.

40. Did enough variation exist?
First, identify and measure as
many birds as possible on the
island.
Record the birds that live and die,
and which have breed or not.
Take measurements of their physical features.
Data indicated that there is great variation among the Galapagos finches!

41. The Grants most interesting discovery…
Individual birds with different sized beaks had different chances of survival during a drought.

42. Evidence of the process of evolution
The Grants research provided evidence of the process of evolution by natural selection.
Found that natural selection takes place frequently.
Sometimes it happens very rapidly.

43. Speciation in Darwin’s Finches
Founders, from species A, arrive on the island from South America.
Some of the birds from species A end up on a nearby island becoming isolated from the others.
Changes in the gene pools on each island, due to different availability of foods allows natural selection to favor certain individuals.

45. Peter and Rosemary Grant learned about mate choice in the Galapagos finches had nothing to do with Genotype
Test – Which is the first step that occurred in the speciation of the Galapagos finches?
Behavior isolation or arrival of the founding population.

46. Video - http://youtu.be/n3265bno2X0