Understanding Natural Selection

Question 1

Fitness = ?

Answer 1

Fitness = Survival, finding mates, number of healthy fertile offspring

Question 2

Fitness is…

Answer 2

Fitness is measured relative to other members of population
-Best reproductive success = fittest

Question 3

Stabilizing Selection

Answer 3

-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

Question 4

Directional Selection

Answer 4

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

Question 5

Disruptive (Diversifying) Selection

Answer 5

-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

Question 6

Frequency-Dependent Selection

Answer 6

-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

Question 7

Sexual Selection

Answer 7

-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

Question 8

Intersexual Selection

Answer 8

-Can result from female choice
–Can even select for detrimental traits
–Handicap principle vs. Good Genes Hypothesis
Ex) Red male cardinals
Ex) Showy Peacocks

Question 9

Handicap Principle

Answer 9

Proposes females prefer males with mating characteristics that reduce survival chances because handicaps are indicators of heritable variability

Question 10

Good Genes Hypothesis

Answer 10

Traits females choose when selecting a mate are honest indicators of males ability to pass on genes

Question 11

Intrasexual Selection

Answer 11

-Can result from direct competition between males
Ex) Elephant seals
Ex) Gorilla harems

Question 12

If natural selection is so important, why do populations continue to have variation after so many years?

Answer 12

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

Question 13

Limitations of Natural Selection

Answer 13

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