Unit 7 - Natural Selection

Question 1

What is an adaptation? Give an example.

Answer 1

A heritable characteristic that enhances organisms’ ability to survive and reproduce in specific environments (favored by natural selection)

EX: Desert foxes have large ears, which radiate heat

Question 2

Define evolution

Answer 2

The change in the gene pool of a population over time

Question 3

How does genetic variation lead to evolution of a species?

Answer 3

Genetic variation is the basis of phenotypic variation that can be acted upon by natural selection. Natural selection affects an organism’s ability to survive and reproduce (results from factors like predators) and can lead to an increase in the proportion of favorable traits in a population. As a result, organisms will be more well suited for their environments, a process known as adaptive evolution.

Question 4

Name four pieces of evidence for evolution.

Answer 4

1. Fossil record
2. Biogeography
3. Comparative anatomy (homologous structures, analogous structures)
4. Direct observations

Question 5

How do direct observations serve as evidence for evolutionary change?

Answer 5

Bacteria rapidly evolve to become resistant to antibiotics

Question 6

Describe four examples of comparative anatomy and how they each serve as evidence for evolutionary change.

Answer 6

Homologous structures: originated from same common ancestor but developed different functions over time (EX: forelimbs of mammals are used for flying, swimming, walking, etc in different species)

Vestigial structures: remnants of features that served a function in the organism’s ancestors but are not important anymore (EX: human tailbones)

Embryonic homologies: anatomical homologies in animal embryos during development

Molecular homologies: shared characteristics on the molecular level (similarity in nucleotide/amino acid sequences; same genetic language of DNA and RNA)

Question 7

Convergent evolution

Answer 7

When two unrelated species independently develop similar (analogous) structures due to being exposed to similar selective environmental pressures, NOT because they evolved from a common ancestor

Question 8

Analogous structures

Answer 8

Share similar function but not common ancestry or form; come from convergent evolution

Question 9

Homologous vs Analogous structures?

Answer 9

Homologous structures have common ancestry + similar form but different function while analogous structures have similar function but different ancestry + form.

Question 10

How does the fossil record serve as evidence for evolutionary change?

Answer 10

Fossils = remains or traces of organisms from the past, found in sedimentary rock

They show that evolutionary structures have occurred over time and provide evidence of the origin of new groups of organisms.

Question 11

How does biogeography serve as evidence for evolutionary change?

Answer 11

Biogeography = the distribution of plants + animals in different continents/environments)

Species in discrete geographic areas with different environments are more closely related to each other than species in distant areas with similar environments

Continental drift and the breakup of Pangaea can explain the similarity of species on continents that are distant from each other today

Question 12

Name two sources of genetic variation.

Answer 12

1. Mutations - ONLY source of new alleles

2. Sexual reproduction

Question 13

How does sexual reproduction lead to genetic variation?

Answer 13

Sexual reproduction shuffles existing alleles and combines them at random to produce new genotypes in offspring

Caused by crossing over, independent assortment of chromosomes, and fertilization

Question 14

Population

Answer 14

A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring

Question 15

What does it mean for an allele to be fixed?

Answer 15

All members of a population are homozygous for the same allele. Only that allele exists at that particular locus

Question 16

Name the five conditions for Hardy Weinberg Equilibrium

Answer 16

1. No change in allele frequency due to mutation
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow (emigration, immigration, transfer of pollen, etc)

Question 17

Define Hardy Weinberg Equilibrium

Answer 17

Occurs in a population that is not evolving, in which the allele and genotype frequencies in a population’s gene pool will remain constant from generation to generation

Question 18

Name three mechanisms that alter allele frequencies.

Answer 18

1. Natural selection
2. Genetic drift
3. Gene flow

Question 19

How does natural selection alter allele frequencies in a population?

Answer 19

Individuals with variations that are better suited to their environment will be favored by natural selection, in which they survive and produce more offspring compared to others less suited.

As a result, alleles that result in selected phenotypic characters will increase in frequency in the next generation

Question 20

How does genetic drift alter allele frequencies in a population?

Answer 20

Genetic drift = chance events that cause allele frequencies to fluctuate unpredictably

The smaller the population, the more likely it is to be affected by genetic drift

1. Founder effect: when a few individuals become isolated from a large population, the smaller group may establish a new population whose gene pools differs from the original
2. Bottleneck effect: a sudden change in the environment (fire, flood, etc) could drastically reduce the size of the population, causing certain alleles to be overrepresented/underrepresented/absent among the survivors by chance alone

Genetic drift results in low levels of genetic variation for the population and can even cause certain alleles to become fixed.

Question 21

How does gene flow alter allele frequencies in a population?

Answer 21

Gene flow = the transfer of alleles into/out of a population due to the movement of fertile individuals or their gametes

Reduces genetic differences between populations, thus making populations more similar

Question 22

Relative fitness

Answer 22

The relative contribution an individual makes to the gene pool of the next generation

Individuals that are more fit will survive longer + reproduce more offspring; thus their traits will be more favorable and increase in frequency due to natural selection

Question 23

Describe the three forms of selection (graphs).

Answer 23

1. Directional selection: conditions favor one extreme phenotype; common when a population’s environment changes

EX: Lighter mice are favored, darker mice are not

2. Stabilizing selection: acts against both extreme phenotypes and favors intermediate variants; reduces variation

EX: Human babies are between 3-5 kg; higher/lower results in death

3. Disruptive selection: conditions favor both extremes but not intermediate phenotypes; patchy environment of both light + dark rocks

EX: Lightest mice and darkest mice are favored but not medium shades

Question 24

Sexual selection

Answer 24

Individuals with certain inherited characteristics are more likely than others of the same sex to obtain mates

Question 25

Biological concept of species

Answer 25

Defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to do so with members of other groups

Question 26

Allopatric speciation vs sympatric speciation

Answer 26

Both arise from the formation of reproductive barriers that restrict gene flow and keep the gene pools separate → emergence of new species

Allopatric speciation requires a geographic barrier while sympatric does not

Question 27

Gradualism vs punctuated equilibrium

Answer 27

Gradualism illustrates a gradual divergence in morphology from the original parent species

Punctuated equilibrium illustrates a sudden change

Question 28

Define speciation

Answer 28

The process by which new species arise

Question 29

Define and describe the five prezygotic barriers.

Answer 29

Prezygotic barriers block interbreeding BEFORE formation of the zygote or fertilization occurs

The three following barriers prevent mating from occurring.

1. Temporal isolation - species breed during different times of the day, different seasons, or different years, and thus cannot mix gametes even if they coexist
2. Habitat isolation - species live in the same region but different habitats, so they don’t meet frequently (water vs land)
3. Behavioral isolation - courtship rituals/traits that usually attract potential mates are not present between species; no sexual attraction or mate recognition

The two following barriers occur after mating has succeeded but hinders fertilization.

4. Mechanical isolation - morphological/structural differences reduce compatibility (EX: snail shells spiral in opposition directions)
5. Gametic isolation - sperm and eggs fail to unite during fertilization and form a zygote (EX: aquatic animals release their eggs into the water; some bind poorly to each other)

Question 30

Define and describe the three postzygotic barriers

Answer 30

Postzygotic barriers block interbreeding AFTER formation of the hybrid zygote

1. Reduced hybrid viability - hybrid offspring don’t develop properly and cease to survive, dying before reproductive age
2. Reduced hybrid fertility - hybrid offspring may become adults but they are sterile/infertile and can’t produce functional gametes, thus remaining unable to mate and reproduce
3. Hybrid breakdown - the first generation hybrids are viable and fertile but when they mate, their offspring are weak/sterile