Unit 7 - Natural Selection Flashcards
What is an adaptation? Give an example.
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
Define evolution
The change in the gene pool of a population over time
How does genetic variation lead to evolution of a species?
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.
Name four pieces of evidence for evolution.
- Fossil record
- Biogeography
- Comparative anatomy (homologous structures, analogous structures)
- Direct observations
How do direct observations serve as evidence for evolutionary change?
Bacteria rapidly evolve to become resistant to antibiotics
Describe four examples of comparative anatomy and how they each serve as evidence for evolutionary change.
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)
Convergent evolution
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
Analogous structures
Share similar function but not common ancestry or form; come from convergent evolution
Homologous vs Analogous structures?
Homologous structures have common ancestry + similar form but different function while analogous structures have similar function but different ancestry + form.
How does the fossil record serve as evidence for evolutionary change?
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.
How does biogeography serve as evidence for evolutionary change?
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
Name two sources of genetic variation.
- Mutations - ONLY source of new alleles
2. Sexual reproduction
How does sexual reproduction lead to genetic variation?
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
Population
A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
What does it mean for an allele to be fixed?
All members of a population are homozygous for the same allele. Only that allele exists at that particular locus
Name the five conditions for Hardy Weinberg Equilibrium
- No change in allele frequency due to mutation
- Random mating
- No natural selection
- Extremely large population size
- No gene flow (emigration, immigration, transfer of pollen, etc)
Define Hardy Weinberg Equilibrium
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
Name three mechanisms that alter allele frequencies.
- Natural selection
- Genetic drift
- Gene flow
How does natural selection alter allele frequencies in a population?
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
How does genetic drift alter allele frequencies in a population?
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
- 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
- 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.
How does gene flow alter allele frequencies in a population?
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
Relative fitness
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
Describe the three forms of selection (graphs).
- Directional selection: conditions favor one extreme phenotype; common when a population’s environment changes
EX: Lighter mice are favored, darker mice are not
- 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
- 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
Sexual selection
Individuals with certain inherited characteristics are more likely than others of the same sex to obtain mates
Biological concept of species
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
Allopatric speciation vs sympatric speciation
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
Gradualism vs punctuated equilibrium
Gradualism illustrates a gradual divergence in morphology from the original parent species
Punctuated equilibrium illustrates a sudden change
Define speciation
The process by which new species arise
Define and describe the five prezygotic barriers.
Prezygotic barriers block interbreeding BEFORE formation of the zygote or fertilization occurs
The three following barriers prevent mating from occurring.
- Temporal isolation - species breed during different times of the day, different seasons, or different years, and thus cannot mix gametes even if they coexist
- Habitat isolation - species live in the same region but different habitats, so they don’t meet frequently (water vs land)
- 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.
- Mechanical isolation - morphological/structural differences reduce compatibility (EX: snail shells spiral in opposition directions)
- 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)
Define and describe the three postzygotic barriers
Postzygotic barriers block interbreeding AFTER formation of the hybrid zygote
- Reduced hybrid viability - hybrid offspring don’t develop properly and cease to survive, dying before reproductive age
- 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
- Hybrid breakdown - the first generation hybrids are viable and fertile but when they mate, their offspring are weak/sterile
