Topic 4: Evolution of a Population

Question 1

Explain if an individual can evolve

Answer 1

• populations are able to evolve, but individuals cannot
• the process of selection acts on individuals (causing certain individuals with certain traits to survive and reproduce) but the outcome of natural selection is seen at the population level

Question 2

What is a gene?

Answer 2

• It is a combination of two alleles (in diploid individuals)
• one allele per chromosome, one from mom and one from dad

Question 3

What is genetic variation?

Answer 3

the diversity of alleles in a population

Question 4

How is genetic variation measured?

Answer 4

It is measured by determining the proportion of heterozygotes/ or number of alleles at various loci

Question 5

What does less genetic variation mean for the survival of species?

Answer 5

• less genetic variation means there is less flexibility in species being able to adapt to their environment when it changes, which decreases their chance of survival
• endangered species often have less variation

Question 6

What is the phenotypic variation?

Answer 6

• the variation in the outward expression of the genotype and its interaction with the environment
• it can fall into categories or types like male or female or be continuous eg: height

Question 7

How do the environment, genotype, and phenotype interact?

Answer 7

• environment dictates natural selection which operates on the genotype, which results in the expression of the phenotype
• the environment selects based on the phenotype (based on the expression of the trait)
• the expressed phenotype depends on both the genotype and the environment eg: without proper nutrition, will not reach max potential height (phenotype is very context-dependent)

Question 8

What is the statistical variation?

Answer 8

• measures the difference between individuals and the average condition (the difference from the central condition)
• can graph certain phenotypes

Question 9

What is micro-evolution?

Answer 9

-the change in the frequencies of alleles in a population (gene pool) between generations over time

Question 10

What are the 5 factors of evolution? (factors that can change allele frequency)

Answer 10

• natural selection
• sexual selection
• mutations
• gene flow
• genetic drift

Question 11

Explain how sexual selection influences evolution

Answer 11

• when mating is non-random there is an increase and decrease in the probability that a specific individual will mate based on different factors
  eg: due to preferred phenotypes (colorful monkeys), inbreeding (like self-fertilization), etc

Question 12

What can be the effect of sexual selection on genetic variation?

Answer 12

-it may decrease genetic variation because individuals will only mate with certain characteristics which will be passed on to offspring or will only mate with their own/themselves

Question 13

What is a mutation?

Answer 13

• change in an individual’s DNA

- can be caused by an error in DNA replication or by structural damage to DNA eg: radiation

Question 14

Can the environment cause differential production of mutations?

Answer 14

No, it cannot, mutations are random and the environment cannot cause the “right” mutations to arise

Question 15

How do we know if a mutation is beneficial?

Answer 15

-is context-dependent, depending on if that certain mutation promotes survival in that particular enviroment

Question 16

What are stop codons and how do they cause mutations?

Answer 16

-causes mutations because they make shorter protein sequences, this can be detrimental

Question 17

What are synonymous mutations?

Answer 17

-results in no change in amino acids, therefore the proteins should be the same

Question 18

What are nonsynonymous mutations?

Answer 18

-subtle changes in amino acids which causes changes in the way proteins fold

Question 19

What can we generalize about how beneficial mutations are?

Answer 19

• usually neutral or negative, very rarely are they actually beneficial
• however whether or not they are beneficial is context-dependent

Question 20

What is gene flow?

Answer 20

-the transfer of genes (alleles) between populations eg: interbreeding or migration

Question 21

How does gene flow affect genetic variation in a population?

Answer 21

• more variation within a population

- less variation between populations

Question 22

What is genetic drift?

Answer 22

• changes in allele frequency due to chance (regardless of natural selection)
• random fluctuations in allelic frequencies due to chance events
• allele frequencies drift from one generation to the next
  eg: introduction of a brown frog that passes on its traits
  eg: bottleneck effect and founder effect

Question 23

How does population size relate to genetic drift?

Answer 23

-the impact of genetic drift is more pronounced in smaller populations

Question 24

What is the founder effect?

Answer 24

• small populations who break off from large populations have the potential to rapidly change the allelic frequencies in that population
• results in a small fraction of the total genetic variation compared to the ancestral population

Question 25

What is the bottleneck effect?

Answer 25

• a catastrophe happens (a bottleneck) that results in the reduction in a population
• only a few individuals survive and only these few reproduce in the next generation
• this results in gene frequency in the next generation being different than in the previous generation

Question 26

How do the bottleneck effect and founder effect influence genetic variation?

Answer 26

• reduce genetic variation, because usually the smaller population only has a handful of traits
• rare alleles are more likely to be lost due to genetic drift

Question 27

What is directional selection?

Answer 27

• the extreme phenotype is favored (highest fitness)
• it is a response to a steady change in the environment
• the frequency distribution of alleles shifts which can result in the loss of genetic variation (favoring of more extreme allele)
• it is a directional shift in the mean of the population while the variance will stay the same (differences of individuals within a population still exist)

Question 28

What is disruptive selection?

Answer 28

• extremes are favored
• results in polymorphism (2 or more divergent phenotypes)
• maintains genetic variation (may not cause changes in allelic frequency)
• can create 2 different species from one species

Question 29

What is stabilizing selection?

Answer 29

• intermediate or common phenotypes are favored (average condition is favored)
• selection against extremes (eliminates harmful mutations)
• the mean of the population stays the same (variance decreases)
• little or no evolutionary change (maintains genetic variation)

Question 30

What are the 8 ways in which genetic variation can be maintained in spite of natural selection?

Answer 30

• mutation
• recombination (crossing-over)
• independent assortment (of alleles)
• fertilization (sexual reproduction)
• disruptive selection (type of natural selection)
• gene flow (between populations)
• negative frequency-dependent selection (alternating direction of selection) eg: left mouth and right mouth fish
• heterozygote advantage eg: being heterozygous for sickle cell disease=protection against malaria-stabilizing selection (rare genotype has an advantage)

Question 31

Why can natural selection not fashion perfect organisms?

Answer 31

1. selection can act only on existing variations (can’t create perfection from the absence of variation required to do so)
2. evolution is limited by historical constraints eg: humans cannot regain tails even if it is useful to us in the future
3. adaptions are often compromising (what is good for one thing may be bad for another thing) eg: frogs mate by serenading which causes ripples in water which increases the chance of being eaten by predators
4. chances, natural selection, and the environment interact (sometimes modification will work out in this combined context, and sometimes it won’t)