Understanding Natural Selection Flashcards
Exam 1
Fitness = ?
Fitness = Survival, finding mates, number of healthy fertile offspring
Fitness is…
Fitness is measured relative to other members of population
-Best reproductive success = fittest
Stabilizing Selection
-Environmental conditions stable over time
-Intermediate variants more fit than extremes
–Reduces genetic diversity
Ex) Brown mouse in forest
Ex) Leaf size in plants
Ex) Birth weight in humans
Directional Selection
Continuous variation under stable conditions but…
-Introduced selective pressure favors one or more extreme variants
-Mean value of variation shifts
Ex) Peppered moths in industrial England
Ex) Antibiotic-resistant bacteria
Disruptive (Diversifying) Selection
-Extreme variants on both ends are more fit than intermediates
-Maintains diversity
Ex) African Seedcracker finches
Ex) Alpha males vs. Sneaky males in animal populations
Frequency-Dependent Selection
-Rise in frequency of one variant in response to frequency of another can reduce genetic diversity through positive frequency-dependent selection
–more offspring resemble common variant
-Can increase genetic diversity through negative-dependent selection
–More offspring are not like common variant
Ex) Male side-blotched lizards and throat color
-orange > blue; blue > yellow; yellow > orange
Sexual Selection
-Leads to sexual dimorphism
–Condition where sexes of same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction
-Success based on obtaining mates rather than environment
Intersexual Selection
-Can result from female choice
–Can even select for detrimental traits
–Handicap principle vs. Good Genes Hypothesis
Ex) Red male cardinals
Ex) Showy Peacocks
Handicap Principle
Proposes females prefer males with mating characteristics that reduce survival chances because handicaps are indicators of heritable variability
Good Genes Hypothesis
Traits females choose when selecting a mate are honest indicators of males ability to pass on genes
Intrasexual Selection
-Can result from direct competition between males
Ex) Elephant seals
Ex) Gorilla harems
If natural selection is so important, why do populations continue to have variation after so many years?
Diploidy
-Less successful recessive alleles masked in heterozygotes
Disruptive Selection
-Extreme phenotypes that survive will carry different alleles
Heterozygote Advantage
-Selection may favor heterozygotes
Ex) Sickle cell allele and malaria (recessive allele is masked)
Frequency-Dependent Selection
-Fitness of phenotype decreases as its frequency increases in population
–Visual predators have a mental model of suitable prey
Limitations of Natural Selection
1) It acts on phenotype of entire individual
-“Compromise” in form due to competing needs
2) It can act only on existing variation
-When adaptation is impossible = Extinction possible