Chapter 20 Flashcards
(28 cards)
Genetic Variation and Evolution
-Genetic variation
Differences in alleles of genes found within individuals in a population
Raw material for natural selection
-Evolution
How an entity changes through time
Development of modern concept of biological evolution can be traced to Darwin
“Descent with modification”
Darwin was not the first to propose a theory of evolution
Unlike his predecessors, however, Darwin proposed natural selection as the mechanism of evolution
B
Rival theory of Jean-Baptiste Lamarck was evolution by inheritance of acquired characteristics
-Ex. If a short-necked giraffe stretched its neck to eat leaves, it could pass on this to its offspring (but stretching is not genetically based so there is no way to pass it on to its offspring)
a
How do you determine if a population
is undergoing evolution?
-Population genetics
-Study of properties of genes in a population
-Evolution results in a change in the genetic composition of a population—a change in the allele frequencies in a population—natural selection favors individuals with certain alleles
-Genetic variation is the raw material for selection
In nature, genetic variation is the rule
Loosestrife showing genetic variation
-Polymorphic loci
More than one allele at frequencies greater than mutation alone
-Heterozygosity
Probability that a randomly selected gene will be heterozygous in a randomly selected individual
Hardy–Weinberg principle
Hardy–Weinberg equilibrium
- Proportions of genotypes do not change in a population if:
1. No mutation takes place
2. No genes are transferred to or from other sources (no immigration or emigration takes place)
3. Random mating is occurring
4. The population size is very large
5. No selection occurs
-Principle can be written as an equation
-Used to calculate allele frequencies
-For 2 alleles, p and q
p = frequency of B for black coat color
-Black cat is BB or Bb
q = frequency of b for white coat color
-White cats are bb
p + q = 1
Genotype p2 + 2pq + q2 = 1
frequencies
BB + Bb + bb = 1
d
- If all 5 assumptions for equilibrium are true, allele and genotype frequencies do not change from one generation to the next
- In reality, most populations will not meet all 5 assumptions
- Look for changes in frequency
- Suggest hypotheses about what process or processes at work
a
5 agents of evolutionary change
1.Mutation Rates generally low Other evolutionary processes usually more important in changing allele frequency -Ultimate source of genetic variation Makes evolution possible Rate = 1/100,000 cell divisions
- Gene flow
Movement of alleles from one population to another
Animal physically moves into new population
Drifting of gametes or immature stages into an area
Mating of individuals from adjacent populations
3.Nonrandom mating
– Inbreeding
Assortative mating
Phenotypically similar individuals mate
Increases proportion of homozygous individuals
Disassortative mating
Phenotypically different individuals mate
Produces excess of heterozygotes
4.Genetic drift
-In small populations, allele frequency may change by chance alone
Magnitude of genetic drift is negatively related to population size
-Founder effect—small group migrates—don’t carry all the alleles present in the larger pop.—some alleles lost. Amish—dwarfism allele retained
-Bottleneck effect—due to natural disaster
- Genetic drift can lead to the loss of alleles in isolated populations
- Alleles that initially are uncommon are particularly vulnerable
Northern Elephant Seal
- Bottleneck case study
- Nearly hunted to extinction in 19th century
- As a result, species has lost almost all of its genetic variation
- Population now numbers in tens of thousands—but species remains vulnerable to extinction
- Because of lack of genetic variability
Selection
Selection—favors some genotypes over others—changes allele frequencies in a way that increases adaptation
-Some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior
Artificial selection
Natural selection
breeder selectsdesired characteristics
environmentalconditions determine which indiv.
in a pop. produce the most offspring
3 conditions for natural selection to occur and to result in evolutionary change
- Variation must exist among individuals in a population
- Variation among individuals (phenotype or behavior) must result in differences in the number of offspring surviving in the next generation
- Variation must be genetically inherited
Natural selection and evolution are not the same
-Natural selection is a process
Only one of several processes that can result in evolution
-Evolution is the historical record, or outcome, of change through time
-Result of evolution driven by natural selection is that populations become better adapted to their environment
Selection pressure—predation
Selection “for” those adaptations that decrease the probability of capture by a predator:
Common sulphur butterfly
Caterpillar usually pale green
Excellent camouflage
Bright yellow color morph rare and kept at low frequency by predation
Selection to match climatic conditions
-Some enzymes function better at certain
temperatures—vary with latitude
-There is selection “for” the alleles that are
better catalysts in colder temps. in the north.
Selection “for” alleles that are better catalysts
in warmer temps. in the south.
-Fish along eastern coast—geographic
variation due to an allele for an enzyme
that functions better at lower temps.
Fitness and its measurement
Fitness—reproductive success
- Selection “for” individuals with a phenotype that leaves more surviving offspring in the next generation than individuals with an alternative phenotype
- Relative concept; the most fit phenotype is simply the one that produces, on average, the greatest number of offspring
Fitness is determined by several components —combination of:
- Survival of fittest
- Sexual selection – some individuals more successful at attracting mates (mating success)
- Number of offspring per mating
Selection favors phenotypes with the greatest fitness
Phenotype with greater fitness usually increases in frequency
Maintenance of variationHow is variation maintained in a population?
- Frequency-dependent selection
-Fitness of a phenotype depends on its frequency within the population
-Negative frequency-dependent selection
-Rare phenotypes favored by selection—maintains variation
Rare forms may not be in “search image”
-Predators may not have search image for rare forms
-Positive frequency-dependent selection
Favors common form. Rare form selected against.
Tends to eliminate variation
“Oddballs” stand out—attract attention of predator
Oscillating selection
-Selection favors one phenotype at one time and another phenotype at another time
-Effect—maintains genetic variation in the population
-Medium ground finch of Galapagos Islands
-Birds with big bills favored during drought (fewer small soft seeds)
-Birds with smaller bills favored in wet conditions
(small seeds return and small bills are favored)
-Does not depend on its frequency in
the population—environmental changes
lead to oscillation
Heterozygote advantage–Sickle cell anemia
-Heterozygotes are favored over homozygotes because the are less susceptible to malaria
-Parasite goes into blood cells and causes low oxygen which makes
the cell sickle. Spleen filters these cells out and gets rid of the parasite.
-Homozygotes who have the sickle cell alleles usually die before reproducing (without medical treatment)
-Homozygotes who do not have the sickle
cell allele will die of malaria.
-Works to maintain both alleles in the population
