Change overtime: how populations evolve

Isn’t Evolution Just A Theory?

• How does the scientific meaning of a term like theory differ from the way it is used in everyday life?

• Can the “facts” of science change over time? If so, how?

Evolution

• Scientific theories– Concepts based on

the conclusions of observations and experiments.

– Reserved for a conceptual scheme supported by a large number of observations and not yet found lacking.

Scientific Theories

Cell All organisms are composed of cells.

Biogenesis Life comes only from life.

Animal Liver cells

Cells of a plant leaf

protistsbacteria

Scientific TheoriesGene Organisms contain coded

information that dictates their form, function, and behavior.

Scientific TheoriesEvolution All living things have a common ancestor.

Genetic changes in a population or species over generations have led to the diversity of life on Earth.

How Do We Know Evolution Happens

• How do fossils give us a picture of change over time?

• What distinguishing feature of the fossil Pakicetus skull identified it as related to a whale? Why was this surprising?

• Why do scientists seek fossils that are intermediate in form and time between modern forms and their probable earliest ancestors?

Fossils from50 mya

Modern

Baleen whale

Chan

ge over man

y generations

Fossils from50 mya

Modern

Baleen whale

Chan

ge over man

y generations

Fossil Called Missing Link from Sea to Land Animals

• What evidence from the fossil fish skeleton suggests it is an intermediate form between fish and land animals?

Molecular clocks

• Comparing differences in DNA and/or protein sequences reveal past mutations

How Does Evolution Really Work?• What are the 4 components of

natural selection?

• What determines an individual hummingbird’s beak length?

• What factors in the environment might select for beak length and shape within the hummingbird population?

• How can hummingbird DNA help Dr. Schindler determine the evolutionary history of hummingbirds?

Natural SelectionNatural selection• The process by which

organisms with favorable variations survive and reproduce at a higher rate.

• The process that results in evolution.

Natural Selection

Natural selection is based on variation:

• Individuals in a population differ from one another

• Many of these differences are genetic– Genetic variation occurs because of

mutations and genetic recombination during meiosis

Natural Selection

• Some variations may – help individuals in a population to survive– Increase the amount of offspring the

individuals have

• These traits if inherited would be passed on to the offspring

• Over time, most individuals in the population will have these traits.

Natural Selection• Examples:

– Peppered moths of England– Snails in England– Flat-tailed horned lizard– Pesticide/antibiotic resistance– Sickle cell disease

• Example of heterozygous advantage – individuals who are heterozygous for sickle cell disease are resistant to malaria. This allows for these individuals to survive better.

2 key points about natural selection

1. Natural selection is more of an editing process than a creative mechanism.

– A pesticide does not create resistant individuals, but selects for resistant insects that were already present in the population

2 key points about natural selection

2. Natural selection depends on time and place

– It favors those characteristics in a varying population that fit the current, local environment.

– Environmental factors vary from place to place and time to time

– An adaptation in one situation may be useless or even harmful in different situation

Microevolution

Gene pool• The total collection of

genes in a population at any one time.

• Consists of all the alleles in all the individuals found in a population.

Microevolution

• A change in the gene pool.• Occurs when the frequency of the alleles in a population

change over a number of generations.

Predict what it will the gene frequency be at Generation 40

MicroevolutionHow do you determine if microevolution is occurring?• The population is changing or in “disequilibrium”

Hardy-Weinberg Equilibrium• States what conditions must exist in a population so

that it doesn’t change (no evolution)

1. The population must be very large

2. Individuals do not migrate into or out of the population.

3. Mutations do not alter the gene pool

4. Mating is random

5. Natural selection does not occurhttp://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf

How does the following Deviate from Hardy-Weinberg equilibrium

• States what conditions must exist in a population so that it doesn’t change

1. The population must be very large

2. Individuals do not migrate into or out of the population.

3. Mutations do not alter the gene pool

4. Mating is random

5. Natural Selection does not occur

• A windstorm blows in hundreds of seeds from a nearby meadow, where nearly all the flowers are yellow. What is wrong?

How does the following Deviate from Hardy-Weinberg equilibrium

• States what conditions must exist in a population so that it doesn’t change

1. The population must be very large

2. Individuals do not migrate into or out of the population.

3. Mutations do not alter the gene pool

4. Mating is random

5. Natural Selection does not occur

• A cosmic ray hits one of the red flowers just as a developing egg cell is replicating its DNA. By chance, a red allele is transformed into a yellow allele. What is wrong?

How does the following Deviate from Hardy-Weinberg equilibrium

• States what conditions must exist in a population so that it doesn’t change

1. The population must be very large

2. Individuals do not migrate into or out of the population.

3. Mutations do not alter the gene pool

4. Mating is random

5. Natural Selection does not occur

• The flowers tend to grow in red or yellow patches. A landslide buries and kills a huge patch of red flowers. What is wrong?

How does the following Deviate from Hardy-Weinberg equilibrium

• States what conditions must exist in a population so that it doesn’t change

1. The population must be very large

2. Individuals do not migrate into or out of the population.

3. Mutations do not alter the gene pool

4. Mating is random

5. Natural Selection does not occur

• The red pigment in the petals of red flowers is poisonous and tends to protect them from beetles that eat the developing seeds. The yellow flowers are not protected in this way. What is wrong?

How does the following Deviate from Hardy-Weinberg equilibrium

• States what conditions must exist in a population so that it doesn’t change

1. The population must be very large

2. Individuals do not migrate into or out of the population.

3. Mutations do not alter the gene pool

4. Mating is random

5. Natural Selection does not occur

• The bees that pollinate the flowers tend to develop a “search image.” Once they start visiting flowers of a certain color, they stick to that color.

• So pollen from red flowers is more likely to be delivered to other red flowers, and pollen from yellow flowers is more likely to fertilize other yellow flowers. What is wrong?

MicroevolutionOne of the possible causes of microevolution isGenetic drift • change in the gene pool of a small population due to

chance

Microevolution

• Bottleneck effect – an event drastically reduces the number of individuals in the population. The surviving individuals do not have the same genetic makeup of the original population

LE 13-9a

Microevolution

• Founder Effect – a small group of individuals from the original population colonize a new location.

13.16 Natural selection can alter variation in a population in three ways– Stabilizing selection: maintains variation for a

particular trait within a narrow range; it selects against extremes in either direction -- ex: human birth size between 6.5-9lbs

– Directional selection: acts against individuals at one of the phenotypic extremes ex: pesticide resistance

– Disruptive selection: favors individuals at both extremes of the phenotypic range ex: light striped snails v. dark snails

Speciation

Species

A population or group of populations whose members can breed and produce fertile offspring.

Speciation

The evolution of a new species

Speciation

Causes of speciation include

1. Reproductive isolation • Prezygotic barriers

prevent mating between populations.

• Postzygotic barriers

prevent the development of offspring that can survive and reproduce

Temporal IsolationSeasonal isolation

Western SkunkFall breeder

Eastern SkunkWinter breeder

East meets WestTerritory overlap

• Courtship ritual in blue-footed boobies is an example of one kind of prezygotic barrier, behavioral isolation

• Many plant species have flower structures that are adapted to specific pollinators– This is an example of

mechanical isolation, another prezygotic barrier

Figure 14.2A, B

• Postzygotic barriers prevent the development of offspring that can survive and reproduce

Speciation

2. Geographical Isolation• Allopatric speciation• A population is separated from other

populations of the same species.• Gene flow is blocked.• Physical separation – mountain range, river.

LE 14-4

• Example: White-tailed antelope squirrel and Harris’s antelope squirrel separated by the Grand Canyon

A. harrisi A. leucurus

• This has been documented by– laboratory

studies (fruit flies)

14.7 Reproductive barriers may evolve as populations diverge

Figure 14.7A

Starch medium

Initial sampleof fruit flies Maltose medium

Results ofmating experiments

FemaleStarch Maltose

Ma

le Sta

rch

Ma

lto

se

22 9

8 20

Mating frequenciesin experimental group

Femalepopulations

Same Different

Ma

lep

op

ula

tio

ns

Sa

me

Dif

fere

nt

18 15

12 15

Mating frequenciesin control group

LE 14-10b

Time

New lineage

Punctuated Equilibrium

Speciation• The fossil record shows

how speciation has occurred over geological time.

• There are four eras (Precambrian, Paleozic, Mesozoic, and Cenozoic) in which mass extinctions were followed by rapid speciation.

Speciation

• This evidence demonstrates punctuated equilibrium – the concept that speciation occurs in spurts followed by long periods of little change.

LE 14-10a

Time

Gradualism

Speciation in the Fossil Record

The gradualist model of species formation

• Populations evolve differences gradually as they become adapted to their local environments

• Speciation occurs by the steady accumulation of many small changes