CH23 II Flashcards
What is the consequence of natural selection?
The favoring of some alleles over others causes adaptive evolution, organisms well adapted to their environment
What are the two forms of genetic drift?
Founder effect and bottleneck effect
Describe
Founder effect
Individuals become isolated indiscriminately from a large population and establish a new population with a different gene pool
Describe
Bottleneck effect
Sudden change in environment that drastically reduces population size. Low levels of genetic variation for generations are to be expected as a consequence.
What are some other characteristics of genetic drift?
- It is most significant in small populations
- It is unpredictable
- Leads to loss of genetic variation
- Can cause harmful alleles to become fixed
What does gene flow do, in terms of genetic variation?
It reduces genetic differences between populations, if not erase them. Can be good or bad.
Out of the mechanisms that alter allele frequencies the most, which one is responsible for adaptive evolution?
Natural selection. It is actually the only one that consistently causes adaptive evolution
Define
Relative fitness
The contribution an individual makes to the gene pool of the next generation relative to other individuals’ contribution
What does natural selection act on?
It acts on phenotype directly and indirectly, the genotype. This is why recessive traits are hard to select against.
Define
Absolute fitness
Number of offspring that inviduals with a certain trait have
What are the types of natural selection?
Directional, disruptive and stabilizing
Define
Directional selection
Conditions favor a phenotype extreme
Define
Disruptive selection
Conditions favor both phenotype extremes
Define
Stabilizing selection
Conditions act against both extremes; favors intermediates
Define
Sexual selection
Form of selection in which individuals with certain inherited characteristics are more likely to obtain mates
What does sexual selection lead to?
It leads to sexual dimorphism
Define
Sexual dimorphism
Differences between the two sexes in secondary sexual characteristics (size, colour, ornamentation, behavior)
What are the types of sexual selection?
Intrasexual selection and intersexual selection
Define
Intrasexual selection
Competitions between one sex (usually males) for mates of the other sex
Define
Intersexual selection
Individuals of one sex (usually females) are choosy in selecting their mates from the other sex (based on dimorphism).
ex: bright plumage makes a male more visible to predators but if it helps it reproduce, the pros outweigh the cons
Define
Balancing selection
Type of selection that preserves at least two genes at a certain locus
Describe
Heterozygote advantage
Individuals who are heterozygous at a particular locus have a greater fitness than both homozygotes. Could be a stabilizing, directional selection force, depending on genotype-phenotype relationship. Ex: heterozygotes for B polypeptide coding gene (sickle-cell anemia) are protected against malaria
Describe
Frequency-dependent selection
The fitness of a given phenotype depends on how frequent or infrequent it is in the population
Why is natural selection unable to create perfect organisms?
- New alleles cannot be created out of nothing, must be a variant of the fittest phenotype
- Complex structures (ex: wings) cannot be created out of nothing
- Adaptations are often compromises (humans have agility but little structural reinforcement)
- Chance events can affect the evolutionary course, what was “adapted” at one point might not be the next year