Mechanisms of Evolution

Chapter 16, 17.4, 32.3Chapter 16, 17.4, 32.3

Vocabulary1. gene pool

2. relative frequency

3. single-gene trait

4. polygenic trait

5. directional selection

6. stabilizing selection

7. disruptive selection

8. genetic drift

9. founder effect

10. Hardy-Weinberg principle

11. genetic equilibrium12. speciation13. reproductive isolation

14. geographic isolation15. behavioral isolation16. temporal isolation17. anatomical isolation18. macroevolution19. adaptive radiation20. convergent evolution21. coevolution22. gradualism23. punctuated

equilibrium24. opposable thumb25. hominid26. bipedal

I. Genes & VariationA. Genetic Variation is Common

1. Many genes have at least 2 forms (alleles), some traits are controlled by multiple genes

2. Individual organisms are heterozygous for many genesB. Variation & Gene Pools

1. Population = group of individuals of the same species that interbreed

2. Gene pool = all the genes, including alleles, present in a population

3. Relative frequency = # of times an allele occurs in a gene pool

4. Evolution is any change in the relative frequency of alleles in a population

5. The more variation in a population the more likelihood some members of the species will survive if the environment changes

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

C. Single-gene & Polygenic Traits1. # of phenotypes produced for a trait depends on how many

genes control the trait

a. Single-gene trait = trait controlled by one gene only

1) Ex: widow’s peak

DRAW Figure 16-3, pg 395

b. Polygenic trait = trait controlled by two or more genes

1) Ex: height

DRAW Figure 16-4, pg 396

II. Evolution is Genetic ChangeA. Genetic Change

1. Populations, not individuals, evolve over time

2. Any change in allele frequency evolution

3. Three causes of genetic change in a populationa. Mutations – any change in the sequence of DNA

1) If a mutation changes an organism’s phenotype, ability to survive and reproduce may be affected

b. Natural selection

c. Chance events – allele frequency changing due to genetic drift or founder effect

B. Types of Natural selection1. Directional selection = entire curve moves;

individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

a. Ex: average beak size increases

DRAWDRAW

2. Stabilizing selection = individuals near the center of the curve have higher fitness than individuals at either end of the curve

a. Ex: birth weight

DRAW

3. Disruptive selection = individuals near the upper and lower ends of the curve have higher fitness than individuals near the middle of the curvea. selection creates 2 distinct phenotypesb. ex: small and large beak types dominate

DRAW

C. Chance Events1. Genetic Drift = in small populations, individuals

that carry a particular allele may leave more descendants than other individuals, just by chance

a. over time, a series of chance occurrences can cause an allele to become more common in a population

b. Ex: a forest fire isolates a group of rodents in the remnants of the forest

2. Founder effect = situation in which allele frequencies change as a result of the migration of a small subgroup of a population

a. Ex: fruit flies in Hawaii

D. Evolution vs. Genetic Equilibrium1. To figure out the factors that can cause a

population to evolve over time look at what happens when no change (no evolution) takes place

a. genetic equilibrium = allele frequencies remain constant

2. Hardy-Weinberg principle = allele frequencies in a population will remain constant unless a factor causes those frequencies to change

III. SpeciationA. species = a group of similar organisms that can

interbreed to produce fertile offspringB. speciation = formation of a new species

C. Types of Reproductive Isolation 1. Behavioral isolation = 2 populations do not interbreed due

to differences in courtship or behaviora. Ex: Western and Eastern meadowlarks use different mating songs to

attract mates and so they do not interbreed

2. temporal isolation = 2 populations reproduce at different times

a. Ex: orchids that release their pollen on different days

3. Geographic isolation = 2 populations are separated by geographic barriers such as mountains or bodies of water so that they cannot interbreed

a. ex: Ensatina salamanders in mountains1) populations in the north breed2) slow evolutionary changes as you move south3) two southern species are too different to breed they are now

separate species

4. Anatomical isolation = 2 populations that physically can not reproduce due to their anatomy

1. Ex: chihuahua and great dane could be close to separate species

QuickTime™ and a decompressor

are needed to see this picture.

Ensatina salamanders in California exhibit subtle morphological and genetic differences all along their range. They all interbreed with their immediate neighbors with one exception: where the extreme ends of the range overlap in Southern California, E. klauberi and E. eschscholtzii do not interbreed.

Salamander SpeciationSalamander Speciation

IV. Patterns of EvolutionA. Extinction

1. More than 99 % of all species that have ever lived have gone extinct

2. Reasons: competition or environment changes

3. Effect of mass extinctions – disappearance of many species leaves habitats wide open providing opportunity for organisms that survived a. Ex: burst of the evolution of mammals

and birds after Cretaceous Period

B. Adaptive radiation1. Process by which a single species evolves

into several different species (ex: Galapagos finches)

C. Convergent evolution1. Produces unrelated organisms that look similar

2. When organisms face similar environmental changes, natural selection favors adaptations that have similar functionsa. Dissimilar appendages with similar functions: flippers of a

dolphin, wings of a penguin, and fins of a shark

Convergent EvolutionConvergent EvolutionDRAWDRAW

D. Coevolution1. 2 species evolve in response to changes in each

other over timea. Ex: flower types only being able to reproduce if their

shape, color, and odor attracts the right type of pollinator

Hummingbird Speciation Video @ pbs.orghttp://www.pbs.org/wgbh/evolution/library/05/2/l_052_04.html

E. Punctuated equilibrium1. Darwin thought evolution was only slow,

gradual change (gradualism), but we now have evidence this is not always the case

2. Punctuated equilibrium = pattern of long, stable periods interrupted by brief periods of more rapid changea. Ex: surviving organisms evolving rapidly after

mass extinctions

DRAW Fig. 17-25, pg 439

F. Developmental genes & body plans

1. Genes for growth & differentiation during embryological development can produce transformations of body shape & size

2. Small changes to “control genes” can affect many other genes

a. Ex: if a control gene for insect wings is turned off in a body segment, the insect will have one less pair of wings

that are

can undergo can undergo can undergo can undergo can undergo

in underunderform inin

Species

Unrelated CloselyRelated

relationships Similar environments

Intense environmental

pressure

Small populations

Different environments

Coevolution Convergent evolution

ExtinctionPunctuated equilibrium

Adaptive radiation

G. Macroevolution graphic organizerG. Macroevolution graphic organizer

Primate Relatedness

QuickTime™ and a decompressor

are needed to see this picture.

V. Human EvolutionA. Trends in Primate Evolution

1. five digits with an opposable thumb

2. both eyes in front improved depth perception

3. color vision4. a reduction in the sense of

smell5. longer period for learning6. uprightness

a. freed the hands to develop and use tools

b. gave better vision (higher up and straight forward)

B. Evolution of Hominids1. 6-7 million years ago, the ancestors of apes and humans

diverged 2. ~ 20 different species of hominid (humanlike species) fossils

categorized so far3. Many hominid species we’ve found so far are not direct

ancestors of modern humansRemember: the tree of life has many branches that go extinct

4. Trends in Physical Changes of hominids

a. jaws and teeth get smaller

b. cranium (skull) is getting larger

c. forehead of skull is getting less sloped

d. hips are wider 5. Dispelling misconceptions about Homo

sapiens

a. Did not evolve from monkeys

b. Did not co-exist with dinosaurs

QuickTime™ and a decompressor

are needed to see this picture.

C. Some Hominid Species1. Australopithecus afarensis (3.2

million years ago)a. 300 recovered fossils, “Lucy” is the most famous fossilb. Location: eastern Africa, savanna woodlandc. Features

1) apelike2) one meter tall and weighed 50 lb.3) bipedal, lived in groups4) diet: fruits, nuts, seeds, tubers

d. a direct ancestor of modern humans

2. Homo habilis (2.2 – 1.5 m.y.a.)

a. Location: eastern Africa, savanna woodland

b. Features:1) more humanlike2) smaller jaw and teeth3) diet: generalized diet, may have

included meat4) used crude simple tools for

scavenging large animals (bone marrow) and preparing vegetables

c. a direct ancestor or close “cousin”

3. Homo erectus (1.8 m.y.a. - 300,000 y.a.)

a. Location: originated in Africa, were nomadic and migrated into Asia and Europe

b. Features: more humanlike but still had some apelike features

1) larger brain, worked cooperatively and hunted large animals

2) adult males up to 6 feet tall

3) diet: included vegetables and meat

4) used advanced tools (hand ax and fire)

c. a close cousin

4. Homo neanderthalensis (150,000 – 30,000 years ago)

a. Location: Europe & Middle East, cold woodlands and tundra (Ice Age)

b. lived at the same time as early Homo sapiens

c. Features1) short, stocky bodies

2) massive brain, large brow bone, no chin

3) reliance on meat, some vegetables

4) used simple “language”

5) purposeful burial of dead

d. Although our DNA is 99.5% similar, Neanderthals are not direct ancestors of modern humans (the two groups only lived at the same time)

Skull Comparison of Hominids

Australopithecus afarensis Homo erectus

Neanderthal Cro-Magnon Modern Homo sapiens

Large braincase

Large browridge

Large nose

Face protrudesforward

Large canineteeth

Weak browridge

Inflatedcheeks

Largenose

Strong chin

Even teeth

Round, highskull

5. Homo sapiens (100,000 – present)

a. Location: from Africa then migrated, reached Australia 60,000 years ago, North & South America 20,000 - 12,000

b. Features:1) lighter skeletal build

2) large brain

3) eyebrow ridge is small

4) prominent chin

5) larynx in the throat is lower allowing for complex language

c. FOXP2 gene occurred 50K-100K y.a. allowing for complex speech and language

d. Cro-Magnon man (40,000 years ago)1) “caveman”

2) gene microcephalin, which increases brain size, shows up at 37K y.a.

3) sophisticated culturea) tools of bone and antlers, clothing

b) cave paintings, sculptures, musical instruments, jewelry

Cro-magnon cave paintingsCro-magnon cave paintingsMammoth Mammoth ivory fluteivory flute

D. Continuing Homo sapien Evolution1. DNA evidence

a. Mitochondrial DNA (mtDNA) is passed from mother to child

b. Y chromosome is passed from father to sonc. Areas of both the Y chromosome and mtDNA get

occasional harmless mutations at a constant rates that are called “markers”1) We can calculate when and where a genetic marker

occurred 2) By comparing genetic markers to those of native

peoples today, it tells us when in time the mutation occurred and the migration patterns of hominids

2. Examples of recent evolutiona. Ex: Skin color

1) mutations of at least 3 genes led to light-colored skin

2) light skin more easily absorbs UV rays needed to produce vitamin D

3) advantageous where sunlight is limited

QuickTime™ and a decompressor

are needed to see this picture.

b. Ex: Lactase evolution1) Lactase (enzyme enabling

digestion of milk) stops being produced after infancy

2) Dairy farming societies (northern Europe, Middle East, & East Africa) developed a mutation (~9,000 y.a.) that allowed them to make lactase into adulthood a) a major nutritional advantage

3) Many people are lactose intolerant because they did not inherit this mutation (75%)

QuickTime™ and a decompressor

are needed to see this picture.