1. Evolution Notes 1
Chapters 22-23
Evidences and Causes of Evolution

2. Evolution
Evolution is genetic change in a population over time.
Charles Darwin was the first scientist to propose the theory of evolution, in 1859.

3. The Voyage of the Beagle
EVIDENCE OF EVOLUTION
The Voyage of the Beagle
Darwin wrote Origin of Species
In 1831, he was on a boat that was mapping coastlines, the HMS Beagle.
He studied plants and animals on the Galapagos Islands.

4. Darwin observed: similarities between living and fossil organisms
the diversity of life on the Galápagos Islands, such as finches (birds) and giant tortoises
Figure 13.1A

5. Darwin concluded that:
The Earth was old and constantly changing (4.6 billion years old)
Living things also change (evolve) over generations.
Living things are related to animals and plants that used to exist but are now extinct.

6. Evidence for Evolution
Fossils
Biogeography
Comparative Anatomy
Comparative Embryology
Molecular Biology

7. Fossils Fossils are the preserved remains of dead organisms.
They show how life has changed over time.
Examples:
Hominid skull: an early relative
Petrified trees: trees turned to stone
Figure 13.2A, B

8. Ammonite casts: 375 million year old aquatic organisms
Fossilized organic matter in a leaf: molecular and cellular structures are preserved.
Figure 13.2C, D

9. Scorpion in amber: 30 million years old, intact DNA
“Ice Man”: 5,000 years old, cells and DNA preserved.
Figure 13.2E, F

10. Use Index Fossils- which existed over large range for a long period
The fossil record shows that organisms have appeared in a historical sequence
Relative Dating: Use layer of rock and surrounding fossils to date (Not absolute)
Use Index Fossils- which existed over large range for a long period
Figure 13.2G, H

11. Dating Fossils
Radioactive Dating is Absolute Age (Exact age of a fossil)
Uses half lives
Amount of time it takes for half of the isotopes in a radioactive sample to turn to stable
Carbon-14 and Pottasium-40 commonly used
5,730 yrs for C and 4.6 billion yrs for P

12. Many fossils link early extinct species with species living today
Vestigial Structures: no longer have a purpose in the current form of an organism of the given species but were used in the past. (Leg bones in whales)
Figure 13.2G, H

13. Do we have any fossil evidence for intermediate species.
Transitional Fossils
Land Mammal ?
Do we have any fossil evidence for intermediate species.

14. Punctuated Equilibrium: long stable periods of change, interrupted by brief periods of rapid change
Accounts for gaps in fossil record

15. Biogeography
Biogeography is the geographic distribution of species (where animals live).
Plants and animals in different parts of the world are related because they share common ancestors.

16. Comparative Anatomy
Anatomical similarities among many species show signs of common descent.
Humans, cats, whales, and bats have the same skeletal elements because we all evolved from a common ancestor.
Human
Cat
Whale
Bat

17. Comparative Embryology
Closely related organisms often have similar stages in their embryonic development.
Fish, frogs, snakes, birds, apes, and people all have pharyngeal slits as embryos which develop into either gills or lungs.
We are all related!

18. Molecular Biology
Scientists can compare DNA sequences and amino acid sequences between species to see how closely related we are.
Humans and chimps share 98.5% of their DNA.

19. Natural Selection Darwin observed that
organisms produce more offspring than the environment can support
organisms vary in many characteristics
these variations can be inherited

20. Natural Selection
Darwin concluded that individuals best suited for a particular environment are more likely to survive and reproduce than those less well adapted
Aka: Survival of the Fittest (giraffe example)

21. Darwin saw natural selection as the basic mechanism of evolution
As a result, the proportion of individuals with favorable characteristics increases
Populations gradually change in response to the environment
Phenotypes that are better reproduce more, eventually, better genotypes become more common.

22. Darwin also saw that when humans choose organisms with specific characteristics as breeding stock, they are performing the role of the environment
This is called artificial selection
Example of artificial selection in plants: five vegetables derived from wild mustard, dog breeding
Figure 13.4A

23. Thousands to millions of years of natural selection
These five canine species evolved from a common ancestor through natural selection
African wild dog
Coyote
Fox
Wolf
Jackal
Thousands to millions of years of natural selection
Ancestral canine
Figure 13.4C

24. Populations are the units of evolution
A species is a group of populations whose individuals can interbreed and produce fertile offspring
People (and animals) are more likely to choose mates locally.
Figure 13.6

25. Adaptations
Adaptation: is a trait with a current functional role in the life of an organism that is maintained and evolved by means of natural selection.

26. Exaptation
A structure that evolved in one context and was later adapted to another function is referred to as exaptation.
Example: Dinosaurs evolved feathers as insulation against extreme temperatures, as birds evolved from small dinosaurs, feathers changed through natural selection to give flight.

27. Microevolution
A gene pool is the total collection of genes in a population at any one time
Microevolution is a change in the relative frequencies of alleles in a gene pool
New mutations are constantly being generated in a gene pool, by accident or as a response to environmental changes.

28. Variation in Gene Pools
Relative frequency = number of times an allele appears in the gene pool compared with other alleles for same gene
Gene flow– individuals and their genes enter or leave a pop’n

29. Evolution is the change in the relative frequency of alleles in a population over time.

30. Two sources of genetic variation
1. Mutations
2. Genetic shuffling from sexual reproduction
Homologous chromosomes move independently during meiosis
Crossing -over

31. Natural Selection Acts on?
NS never acts on a single gene because it is the entire organisms that survives to reproduce or fails to do so
NS acts on phenotype
If gene has no effect on fitness, it is not under pressure from natural selection

32. Natural Selection on Polygenic Traits
Affects distribution of phenotypes in one of 3 ways:
Directional Selection
Stabilizing Selection
Disruptive Selection

33. Directional Selection
Individuals at one end of the curve have higher fitness than those in middle or other end
Ex. Beak size for finches when seeds are large

34. Stabilizing Selection
Individuals near center of curve have higher fitness than those at either end
Ex. Mass of human babies at birth

35. Disruptive Selection
Individuals at either end have higher fitness than those in the middle
Could form 2 distinct phenotypes with strong enough pressure
Ex. Finch beak size
when middle- sized
seeds are rare

36. What causes evolution? Genetic drift Bottleneck Effect Founder Effect
Gene Flow
Mutation

37. Genetic Drift Genetic drift is a change in a gene pool due to chance
Genetic drift can cause the Bottleneck Effect: an event that drastically reduces population size (fire, flood, earthquake)
Original population
Bottlenecking event
Surviving population
Figure 13.11A

38. Genetic drift…
If a population is very diverse and something bad happens, at least a few individuals will survive.
These individuals will then reproduce and the species will evolve, or change.

39. The founder effect is when some individuals leave a population and start living somewhere new.
Only a few people or animals leave, and the new population will be closely related to due lack of genetic diversity.
Figure 13.11B, C

40. Gene flow can change a gene pool due to the movement of genes into or out of a population (new organisms move in or leave)
Mutation changes alleles, these are random changes in DNA that can create new proteins or new characteristics.

41. Hardy-Weinberg Equilibrium
Showing that evolution has to happen by showing that characteristics in nature are always changing...
Hardy-Weinberg equilibrium states that the shuffling of genes during sexual reproduction does not alter the proportions of different alleles in a gene pool
Populations are always evolving and not usually in equilibrium.
Figure 13.8A

42. Five conditions are required for Hardy-Weinberg equilibrium:
The population is very large
The population is isolated
Mutations do not alter the gene pool
Mating is random
All individuals are equal in reproductive success
***This does not happen in nature!

43. The Equation p2 + 2pq + q2 = 1 p + q = 1
p = frequency of the dominant allele in the population (A) q = frequency of the recessive allele in the population (a) p2 = percentage of homozygous dominant individuals (AA) q2 = percentage of homozygous recessive individuals (aa) 2pq = percentage of heterozygous individuals (Aa)

44. Practice Problem: p2 + 2pq + q2 = 1 p + q = 1
You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:
The frequency of the "a" allele.
The frequency of aa is 36%, which means that q2 = If q2 = 0.36, then q = 0.6. Since q equals the frequency of the a allele, then the frequency is 60%
The frequency of the "A" allele.
Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is by definition equal to p, so the answer is 40%

45. The frequencies of the genotypes "AA" and "Aa."
The frequency of AA is equal to p2, and the frequency of Aa is equal to 2pq. So, using the information above, the frequency of AA is 16% (i.e. p2 is 0.4 x 0.4 = 0.16) and Aa is 48% (2pq = 2 x 0.4 x 0.6 = 0.48).