Why is Daphne Major suitable for finch analysis?
- It remains relatively pristine. No invasive species have been introduced. No farming has been done. Thus, it can confidently be said that there have been no anthropogenic extinctions on the island.
- It is ecologically simple; There aren’t many plant species and small numbers of finches are born every year.
- No finches emigrate or immigrate.
- Beak size is highly heritable.
- All finches could be captured and measured to track reproduction and survival.
Explain Darwin’s finch example:
- Rosemary and Peter Grant have been observing finches on Daphne Major for more than 40 years.
- They’ve noticed that finches with different beak shapes and sizes (heritable trait) prefer different kinds of seeds.
- After a drought, natural selection had caused the average beak size of the medium ground finches to increase, as their primary food source (smaller seeds) had vanished.
- 5 years later, after heavy rain, small seeds flourished again, giving a selective advantage to smaller beaks. Afterwards, beak size reduced again.
- Through this study, the Grants observed the heritability of beak size, the strength and direction of several episodes of natural selection, and the evolutionary response of the population.
What is artificial selection?
Selection resulting from human activity. When breeders non randomly choose individuals with economically beneficial traits, they impose strong selection on those traits.
Why are long term studies of evolution such as the Daphne Major study important?
They show fluctuations in the direction and magnitude of selection.
What is gene flow?
- The movement or the migration of alleles from one population to another.
- Although many alleles that undergo gene flow are neutral, some may be deleterious or advantageous. Alleles that are advantageous in one population may be deleterious in another.
- Example: Being brightly colored for mimicry of coral snake in scarlet kingsnakes is beneficial in the south but increases predation in the north. Gene flow occurs between the two populations, but results in varying fitness.
What is aposematism?
Antipredator strategy used by prey to signal danger or a lack of palatability.
Does selection mean evolution?
- No, not all types of selection lead to evolution.
- If heritability is low, populations won’t evolve even if they experience selection.
- The difference in the mean of a trait after selection and before selection.
- Mean of reproducing individuals- mean of entire population
- Measure of the strength of phenotypic selection.
- The proportion of within-population variation in a trait that comes from genetic factors.
- (h2) is a number between 0 and 1 which represents how heritable a given trait is.
- The higher the heritability, the stronger the selection will be.
- If heritability is 0, evolution will not occur.
- Favors individuals at one end of a trait distribution. Shifts to left or right.
- Mean increases.
- Variation doesn’t necessarily change.
stabilizing selection (purifying)
- Favors individuals with a trait near the population mean. Decreases variance (inceliyor mean’de)
- Mean doesn’t change.
- Variation decreases.
- Favors individuals at either end of the trait distribution. Here, if selection is strong enough, populations begin to diverge in phenotype and become dimorphic.
- Mean doesn’t change.
- Variation increases.
- Example: Small fish are better at hiding from predators. Larger fish are too big to be targeted by predators.
What determines how quickly a population evolves in response to selection?
- Amount of variation there is in a phenotypic trait in the population.
- How much of the variation is heritable.
What is the response to selection (R) ? How do we calculate this?
- Mean of offspring minus mean of parent generation.
- Directional selection occurs when the mean phenotype of breeding individuals differs from the mean of the individuals in the parent population.
- If the difference is large, selection is strong.
What is the Breeder’s equation?
Evolutionary response ( R ) = h2 (heritability) x S (selection differential)
frequency dependent selection
Occurs when the fitness of a genotype depends on its frequency in the population
Negative frequency dependent selection
- A phenotype has the greatest selective advantage if it is rare.
- Viruses are an example. We have immunity to common strains.
Positive frequency dependent selection
- A phenotype has the greatest selective advantage if it is common.
- Aposematism is an example of this.
Explain Darwin’s inferences:
- Since more individuals are produced than can be supported by the available resources but population size remains stable, there must be fierce struggle for existence among the individuals of the population, resulting in the survival of only a very small part of the progeny of a generation.
- Survival in the struggle for existence is not random but depends in part on the hereditary constitution of surviving individuals. This unequal survival constitutes natural selection.
- Over the generations, this process of natural selection will lead to continuing, gradual change in the population.
Explain Darwin’s 3 Postulates:
- Natural selection will occur when:
- Individuals are variable in some traits.
- At least some of this variation is heritable.
- There is a struggle for survival or reproduction due to limited resources, and some individuals fare better than others.
What phenotypic traits can selection operate on?
- Morphological traits: Distinctive paper wasps fare better if they can recognize each other.
- Physiological traits
- Behavioral traits: Altruism, socialization, how well a bird can sing.
What causes phenotypic differences?
- Genetic differences.
- Environmental differences
- Interactions between genes and the environment.
- Measurement error.
- Ontogenetic (developmental stage) differences
What are ontogenetic variation?
Variation in phenotype across development; occurs mostly before maturation.
What is natural selection?
The non-random process by which biological traits become more or less common in a population as a result of differential reproductive success of their bearers.
- Published joint manuscript with Darwin in 1858.
- However, Darwin published On the Origin of Species in 1959 (before Wallace).
What two features on the midparent-midoffspring graph indicate strong heritability?
- Positive slope (close to 1)
- Low scatter of values around the line of best fit.
Natural selection in humans: Lactose tolerance
- Producing lactase to digest milk after childhood is unnecessary and costly.
- 10,000 years ago, cattle domestication occurred in NW Europe and East Africa
- The allele for lactose tolerance arose, spread through populations where milk digestion was common,
any variation that can help an organism survive in an environment is called
Life history traits
- Life history refers to the timing and duration of key events during an organism’s lifetime.
- Life history traits include an organism’s age of first reproduction, the duration and schedule of reproduction, the number and size of offspring produced, and life span.
- A change in the environment can lead to a rapid evolution of life history traits.
Life history theory
- Life history theory explores how the schedule and duration of key events in an organism’s life span are determined by natural selection.
- It explains variation in the age at which organisms first begin to reproduce, the size and number of offspring produced, the amount and type of parental care invested and the onset of senescence.
Extrinsic mortality rate
The rate at which external events such as predation lead to mortality in the population. (predation, starvation, infectious disease)
Intrinsic mortality rate
- The rate at which internal events (aging, disease, mutations) lead to mortality in the population.
- Different kinds of organisms tend to experience different kinds of mortality.
Explain the effect of mutations at different ages.
Deleterious or advantageous mutations at an old age do not affect the fitness of the organism. Thus, selection operates more strongly at younger ages (before reproduction and during reproduction) than it does at older ages.
Explain brood size under low predation and high predation?
- Low predation: Fewer offspring, invest less in reproduction, more in growth
- High predation: More offspring, invest more in reproduction, less in growth
- They arise when allocation to one life history trait reduces investment in another trait.
- For example, investment in reproduction often comes at the expense of growth or body maintenance.
- Natural selection favors specific energy investment strategies depending on context.
- Reproduction vs. growth and maintenance
- Size of offspring vs. number of offspring
What is fitness?
- An individual’s proportional representation in the gene pool of subsequent generations.
- Fitness is assessed relative to the fitness of other individuals. (not absolute)
What are the two traits that determine a species’ fitness?
Reproduction and survival. (how many times it can reproduce is based on survival)
- Includes both direct and indirect fitness.
- Can help us explain the evolution of altruism.
- Results from organism's own success in transmitting alleles to future generations.
- Number of organisms that an organism produces over its entire lifetime.
- dIrect fitness is determined by life history traits.
The fitness that an individual gains by helping a relative raise their offspring.
- When a genetic variant with a beneficial effect on one trait also has a detrimental effect on another trait. (Pleiotropy means a gene with multiple effects.)
- Within the context of senescence, this could be a gene that is advantageous in one part of an organism’s life, and deleterious in some other part of an organism’s life. For example, what happens when you have a gene that allows you to have higher fitness early in life but shortens your lifespan?
- A mutation that is beneficial at an earlier age but is deleterious at an older age may be selected for due to the mutation not affecting fitness.
- Occurs when parents have an evolutionary conflict of interest over the optimal strategy for parental care.
- Example: In the case of the Eurasian penduline tit, abandoning the nest for the other parent to incubate can boost one parent’s total number of offspring. 1) If the father leaves too early, the mother may not be able to lay her legs / may mate with another male bird 2) If the father leaves too late, the mother might leave before him, forcing him to incubate the eggs, or there might not be any more female birds to mate with. Thus, females have evolved some behaviors (such as laying some eggs at the bottom of the nest or attacking the male if he enters the nest) to deter the male from leaving the nest.
Why do we age?
- Antagonistic pleiotropy. Selection can strongly favor traits that help organisms survive and reproduce early in life. Selection tends to be weaker later in life than in early on in life.
- Example: Fewer CAG copies are associated with increased risk of cancer in males and females. However, fewer CAG copies are also associated with higher fecundity. (fertility)
- Example: More than 37 CAG repeats leads to Huntingtin’s disease, typically by 45 years old. However, people with Huntintin’s disease display higher fecundity.
- Occurs when parents benefit from withholding parental care or resources from some offspring and invest in another offspring.
- To maximize lifetime fitness, a parent may not provide the maximum resource for her earlier broods, leading to lower survivorship.
Siblicide, older chick kills younger chick without intervention from parents.
What is behavior?
- A behavior is an internally generated response to an external stimulus. Like other phenotypic traits, behavior is capable of evolving through natural selection.
- Costs and benefits of behaviors may not be the same for all individuals. Factors such as sex, status, or body condition may shift the relative value of specific behaviors.
Study of ultimate causes of behavior, including the relationship between variation in the expression of a behavior and fitness in a particular ecological setting as well as phylogenetic patterns of behavioral evolution.
- A decline with age in per capita reproductive performance, physiological function, or the probability of survival.
- As organisms age, they decline in components of fitness, reproduction and survival.
Describes selection arising from variation in fitness among individuals.
Group selection and its problems?
- Selection arising from variation in fitness among groups.
- This was the prevailing theory explaining altruism until the 1960s (until Hamilton's rule).
- Group selection occurs much more slowly than individual level selection.
- Natural selection favors cheating phenotypes.
Explain relationship between individual selection and group selection
- Selective pressures may operate in opposite directions. For example, a selfish behavior that increases the fitness of an individual is likely to spread in the population despite being harmful to the group as a whole.
- Under a breadth of natural conditions, individual selection is stronger than group selection. In practice, this means that most organisms are selfish. Thus, group selection is a rare occurrence.
- 3, However, under kin selection, group selection may be stronger than individual selection.
If individual selection can undermine group selection so easily, why be a group at all?
Natural selection can favor group formation if the benefits outweigh the costs.
What is the dilution effect?
Safety in numbers that arises through swamping the foraging capacity of predators.
- Quantifies whether an altruistic behavior is likely to spread in a population.
- rB > C
- R = coefficient of relatedness between the individual that’s donating the altruistic behavior and the person that’s receiving the altruistic behavior.
- C = How much the altruistic behavior costs you in terms of your own direct fitness.
- B = Benefit to the direct fitness of your relative
R = relatedness coefficient
- coefficient of relatedness between the individual that’s donating the altruistic behavior and the person that’s receiving the altruistic behavior.
- Probability that the altruist shares identical descent copies of a particular allele with a recipient.
- Offspring: 0.5
- Full siblings: 0.5
- Grandchildren: 0.25
- Half siblings: 0.25
- Nieces and Nephews: 0.25
- Greatgrandchildren: 0.125
- Cousins: 0.125
C = cost to donor
How much the altruistic behavior costs you in terms of your own direct fitness. How many less children you have.
B = benefit to recipient.
Benefit to the direct fitness of your relative. How many more children they have.
- This is when an organism sacrifices its own direct fitness in order to increase its indirect fitness by benefiting a relative and helping them increase their own direct fitness.
- While hamilton and kin selection R U L E, keep in mind that altruism can evolve among non-relatives, as well. (reciprocal altruism)
What are the benefits of group-living?
- Avoiding predators (vigilance, dilution, defense)
- Defending resources
- Cooperative foraging/ hunting
What are the costs of group-living?
- Conspicuousness to predators
- Competition for food/mates
- Decreased certainty to maternity/paternity
- Increased transmission of pathogens and parasites
probability that two alleles at a given locus are identical by descent in different individuals