Selection and Maintaining variation

What is balancing selection?

• NS can maintain alleles at intermediate frequency between 0-1

• maintains two or more alleles in a population

• Ex: allele favored in dry area and different allele favord in a wet area

  • in species as a whole, both alleles maintained by NS at intermediate frequencies

Why are alleles said to be balanced (balanced selection)?

stable equilibrium state is reached

What happens when the equilibrium is disturbed?

• selection will return them back to this state

• (unless environment has too strong of an affect —> causes genetic drift)

What is balanced polymorphisms?

• maintaining heterozygotes and keeping recessive allels in the population even if other is more fit

What are examples of balanced polymorphisms

1. heterozygote advantage

2. Frequency-dependent selection

What is heterozygote advantage

• when heterozygote fitness > homozygote

• Balancing selection allows both alleles to remain in population

What is sickle cell Anemia

• • Red blood cell disorder in which there aren't enough healthy red blood cells to carry oxygen throughout your body. •

• Pain, anemia, chest syndrome, infections, organ damage

What did Dr. William Warrick Cardozo discover (1905-1962)?

• Sickle cell anemia was inherited. Almost exclusively among

people of African descent.

• Not all people having sickle cells were anemic.

• Sickle cell disease wasn’t always fatal, but no successful

treatment found.

What did Dr. Roland Scott discover (1909-2002)?

• Pediatrician, treated many children with sickle cell anemia.

• Published on incidence of red cell sickling in newborns, lead

to implementation of screening tests.

• Had Congress pass Sickle Cell Anemia Control Act of 1971

providing federal funding for research and treatment

What is Malaria?

• Parasite, Plasmodium, causes disease in humans and transmitted by

mosquitoes. Extreme flu-like symptoms can be reoccurring and fatal.

• Endemic to same region as sickle cell anemia

What are those with AA genotype venerable to?

• Malaria likes circular blood cells

• Don't get any sickle cell disease --> but plasmodium that causes symptoms of malaria are most complete in circular blood cells 

What is heterozygote in Malaria and Sickle Cell?

• A= normal hemoglobin, S= distorted hemoglobin

• AS heterozygote has advantage

  • No sickle cell disease and some protection form Malaria

• AA: Lack sickle cell, vunerable to malaria

• aa: protected from malaria, have sick-cell disease

Why does Aa (AS) more protected from sickle cell disease and Malaria?

• Don’t get malaria as bad: some sickle cell that can’t attack the plasmodium

• Fewer symptoms of sickle cell anemia because they have circular blood cells

• In regions: 40-50% of people are heterozygotes 

What is frequency-dependent selection?

• The relative fitness of genotypes are not constant but vary with their frequencies in the population

• Fitness of a genotype increases as its frequency decreases

What is the highest form of fitness in negative frequency-dependent selection?

• • Highest fitness when rare

• Can maintain a stable balanced polymorphism

How do Right-mouthed and Left-mouthed perissodus show negative frequency-dependent selection?

• Right mouthed fish always bites prey on the left

• Left mouth bites fish on right side

• If phenotype is more common in a year --> victims can learn behavior to be ready when one side is more common --> when prey react faster to one side, the one side decreases in population from more food

  • Other side: prey less likely to expect them --> fish with left mouth is less common but survive more because fish doesn't expect them

• Causes shift in phenotype dominance and strategy

• Negative slection: Maintains different states in population

If a population is not in Hardy-Weinberg equilibrium, we can conclude that:

1. nonrandom mating has occurred

2. natural selection has occurred

3. One of the assumption of the Hardy-Weinberg equilibrium has been violated?

4. Evolution has occurred because one or more of the assumptions of Hardy-Weinberg equilibrium has been violated

D : the most complete answer

What is one way to solve competition?

Forming a niche

What are galls in plants?

• expansion of the stem of plants

• protective structure that fly creates in plants by exuding chemical in plants that changes how it grows

• Eggs hatches and grows --> releases chemicals that protect the eggs inside the gall

How did evolution impact the wasp in reference to galls?

• wasp: created specialized structure to puncture egg inside of gall —>eats the fly larvae

What does this graph show?

Higher likelihood of fly being killed by parasite wasp when it produces a gall that is smaller in diameter 

Which directions are maintenance of variation and natural selection pressures?

• comes from all sides

• rarely from one direction of a long time

What does this show about gall preferences in the bird (Downy woodpecker)?

Too small gall --> birds can land on them --> birds prefer larger galls 

What do you think will be the effect on the flies that survive and emerge from the galls?

1. A. Stabilizing selection will maintain the same average size of fly

2. B. Directional selection will shift the mean towards larger flies

3. C. Directional selection will shift the mean towards smaller flies

4. D. Disruptive selection will result in a bimodal distribution with two distinct means, one for small flies and one for large flies.

• wasp selecting for smaller, birds selection for larger

• A: two pressures on the sides push fly size to an optimum middle

• opposing forces can lead to stabilizing selection

Are selective pressures constant?

• No, environment is always chaning what is fit

• Ex: Shifts year to year depending on population of wasps/birds which can affect which side has more pressure 

What can selection be based on?

Sexual selection

What is sexual selection?

• Competition for mates

• form of NS

• Competition can be between rivals for the affection of a potential mate

What does sexual selection promote?

• promotes traits that increases chance of reproductive opportunities

How are males more likely to be impacted by sexual selection?

• more common in males to gain phenotypes for sexual selection

Relationship between NS and sexual selection

• Balance and compromise is created between NS and Sexual selection

• Ex: peacock feathers getting too long and being predated on --> drives selection towards smaller feathers (other direction) 

  • Sexual selection can act opposite of NS

What is intersexual selection?

• one sex (usually males) competing with one another to acsess of other sex (females)

• focuses on competition between individuals

What is sexual dimorphism?

distinct difference in size or appearance between the sexes of an animal in addition to difference between the sexual organs themselves.

When does sexual cannibalism occur?

Adaptive male strategy if benefits of being eaten exceed male's expected future reproductive value

What possible benefits would cause selection of sexual cannibalism"?

• Increased paternity

• Decreased likelihood of female remating

• Ex: Evolution led to mating strategy --> male being eaten exceeds the males future reproductive value (low chance of meeting another female) 

  • Matched fitness --> goal met 

Negatives of sexual cannibalism

• death and lost opportunity for future matings

• Ex: male black widows unlikely to mate more than once, even in they survive copulation

Do females select males based off good genes?

• Long male calls better?

• or do females invest more in the offspring of more attractive males?

• difficult to disentangle

What is the the pressure in most sexual selection?

phenotype: features under selection by environment (usually the female)

What factors are important to having successful offspring?

• early survival (ex: tadpole survival)

• Organism growth

• Organism development

Why is faster organism growth better?

Lower chance off being eaten in youth

Why is there still variation when selection is so strong?

• Not all females have strong preferences •

• Females cannot always mate with the ‘best’ male •

• Thus, short call males still have reproductive success •

• Long call males have higher relative fitness, but this is not absolute

What can cause females to not be able to choose the best males?

• physical barriers

Why is higher relativeness fitness not always absolute?

In future new predator/environment could change what is fit —> could be valuable in the future

What are adaptive traits?

Products of selection that increase relative fitness

What is adaptation?

• Accumulation of adaptive traits over time

• Adaptation hypotheses must be tested

Adaptation where do current structures come from?

• Current structures came from previous structures

How do some traits arise in adaptation?

• some arise by chance and not selection

Is Natural selection is a process which ensures that organisms are optimally suited to their environment?

• False

• Natural selection does not necessarily lead to an absolute optimum in a particular trait

• Environment in the future can change --> no constant optimum 

What does natural selection favour?

A competitive optimum (as good or better than alternatives)

Why doesn’t natural selection create perfect organisms?

• Selection can act only on existing variation

  • evolution is limited by historical constraints

• Adaptations are often compromises

• Chance, natural selection, and the environment all interact

Example of compromise in Pandas?

• human thumb bones corresponds to sixth finger (false thum) on panda hand

Why are adaptations often compromises?

• Most environments have competing selective pressures

• Environments constantly change over time

What are vestigial structures?

• currently useless structures = ‘evolutionary leftovers’

Examples of vestigial structures in humans

1. Hair stands up to make furry animals look bigger —> humans lose hair —> hair still stands up

2. Wisdom Tooth: need for larger jaw (eating tough, uncooked food) no longer needed due to different diet

3. Appendix: Used to harbor microbiome and used to be important in past environment and diet

What are traits observed today a result of?

result of natural selection acting in the past

Which environment do generations adapt to?

each generation adapts to environment of parents

What can natural selection never anticipate?

• environmental change

• most studies evaluate phenotype not genotype

how does Urban evolution impact genetic diversity?

• Humans change environment drastically --> too fast for species to adapt to 

• Can measure evolution due to urbanization 

What is genetic isolation?

members of one species cannot exchange genetic material with members of another species

What is the molecular clock?

The observation of rate constancy in molecular evolution. The extent of genetic divergence at a gene in two taxa is thus a reflection of the time since the taxa last shared a common ancestor.

• measures the changes in genome over time

What is molecular evolution?

• When differences in population accumulate and diverge genetically (DNA sequences)

• Evolution at the level of DNA, which in time results in the genetic divergence of population

What is the molecular clock determined from?

• uses fossil record to set clock

• uses region in DNA or protein that has a known rate of mutations over time (usually mitochondrial)

Is the molecular clock always constant?

• no, varies from gene to gene

• molecular clock data should be interpreted cautiously

Why is the molecular clock not always reliable?

• rate of mutations depend on like generation time and population size

Example of molecular clock

• Scientist knows gene knows gene changes bases by 2 bases every 25 million years

• Observe species differs in DNA by four bases —> diverged from a common ancestor 50 million years ago