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Evolutionary Ecology > Natural Variation (DONE) > Flashcards

Natural Variation (DONE) Flashcards

1
Q

What is the raw material of evolution + why?

A

Variation:
- no natual selection without phenotypic variation
- No evolution without genetic variation

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2
Q

What is the breeders equation and what does each component mean?

A

R = h²S
R = response to selection = how does a trait change from one generation to the next
h = heritability = proportion of variation that is genetic- the squared part is irrelevant
S = selection differential = measures strength of selection

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3
Q

Where does the breeders equation come from?

A

selecting for animals so they are favorable for humans e.g. beef cattle, faster race horse

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4
Q

What does h² range from?

A

From 0-1
- 0 = no genetic variability

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5
Q

In terms of the breeders equation, when do you get a change in the next generation?

A

variation in some selection (= S not 0) + some variation has genetic component (=h² not 0) = non 0 value of R = change in next generation

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6
Q

Once you have calculated R, what can be calculated next?

A

The trait mean in the next generation

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7
Q

How do you calculate the trait mean in the next generation?

A

Z next gen = Z before + R
- Z before = trait mean before selection in present generation
- R = response to selection

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8
Q

Name + define the 2 things that affect how traits change in a population

A
  1. Selection itself determines which individuals will survive and reproduce = selection differential = initial step
  2. Transmission = heretibility = how much variation in a trait is due to genetic factors that can be inherited
    - almost always less than 0 as most traits are not purely genetic
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9
Q

In terms of traits, what does selection change?

A

mean of traits
Variation of traits
= these are both true for most kind of selection

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10
Q

Name and define the 2 most common forms of selection

A

Directional = linear selection of a trait = individuals with a trait at one end of spectrum have a better chance of surviving than those with intermediate traits
Stabalising = favours individuals with average traits and selects against those with extreme traits

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11
Q

How do directional and stabalising selection affect variation?

A

Less variation in population after episode of selection
- Stronger selection is = the more rapidly it uses up variation

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12
Q

Name a type of selection that generates variation

A

disruptive selection

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13
Q

If Stronger selection = the more rapidly it uses up variation, what does this mean for traits closely related to fitness?

A

Traits closely related to fitness should be less variable- as variation is used up

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14
Q

Scientists concluded that there was minimal genetic variation in populations due to natural selection eliminating most diversity- is this true + example?

A

Electrophoresis- looking at allozyme variation in drosophila
= more genetic variation existed than expected- less and less of this is considered to be neutral

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15
Q

Define: polymorphism

A

discrete morphs present in populations – may correspond to discrete niches
- become noticeable when they become common enough in a population

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16
Q

What are polymorphisms controlled by?

A

Usually arise from a small number of genetic loci (often less than 3)- often a single gene with multiple alleles

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17
Q

Are polymorphisms common?

A

They are not very common in most populations but do exist

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18
Q

What is a non-genetic polymorphism?

A

the ability of an organism to change its phenotype in response to environmental conditions- no change to genetic makeup

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19
Q

What is the difference between continuous and discontinuous variation + example of each?

A

continuous = traits are influenced by multiple genes (often environment too) that show a range of values between 2 extremes e.g. human height
Discontinuous = traits controlled by one or few genes that fall into a distinct category e.g. blood type

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20
Q

Name a method that is used to analyse traits that show continuous variation + why is it useful?

A

Quantitative genetics- useful when you can’t study individual genes at the molecular level, especially when there are many genes involved

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21
Q

What factors is the total phenotypic variation (Vp) influenced by?

A

Vp = addition of different bits of variation:
- additive genetic variance = caused by genes inherited from parents
- Non-additive genetic variance = more complex and includes things like epistasis and dominance
- environmental + developmental variance e.g. temperature, food eaten

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22
Q

what is epistasis?

A

Interactions between different genes

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23
Q

What is dominance (non-additive genetic variance)?

A

Interactions between alleles at the same gene locus (e.g. one allele is dominant over another)

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24
Q

What is h²?

A

= heritibility- the ratio of additive genetic variance to the total phenotypic variance
i.e. how much of the variation in a trait is due to genetics as opposed to environmental factors

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25
Q

How is h² estimated + example?

A

By looking at the similarity between relatives
e.g. parent-offspring regression- human height, the heritability is around 0.7, meaning 70% of the height variation is due to genetic factors

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26
Q

What is the underlying assumption of the parent offspring regression example + why is this a problem?

A

Assume the environment doesnt affect the correlation between parents and offspring
- in reality- parents and offspring often share the same environment, which can influence traits

27
Q

How can you address the problem of parent-offspring regression?

A

To separate genetic and environmental effects, scientists can use cross-fostering experiments. In these experiments:
- Birds’ eggs or chicks are swapped between different nests, so the baby birds are raised by foster parents, not their biological ones.
= helps to break the connection between genetics (from biological parents) and environment (from foster parents).

28
Q

Why is the mean not a good way to measure variation in biological data?

A

The mean can be larger for populations with greater variation, but it doesn’t give a clear picture of how spread out the data is. It only tells you the average, not how much the values differ from it.

29
Q

What is the Coefficient of Variation (CV) and why is it useful?

A

The Coefficient of Variation (CV) is a measure of variation that allows you to compare variability across different traits of very different sizes.

30
Q

How do you calculate the coefficient of variation (CV)?

A

It’s calculated as the standard deviation divided by the mean:
Mean/ StandardDeviation ×100

31
Q

What does the Coefficient of Variation (CV) tell us?

A

expresses the relative variability of a trait, allowing comparison of variation across traits with different scales
- A higher CV indicates more relative variability in a trait, regardless of its size.

32
Q

What is the typical Coefficient of Variation (CV) for birds’ traits + why?

A

Birds typically have a CV of 2-4% for most traits, as they generally exhibit determinate growth (growth stops at a certain size).

33
Q

How does the Coefficient of Variation (CV) differ in organisms with indeterminate growth?

A

Organisms with indeterminate growth (like fish) may have larger CVs because they continue growing throughout their life, leading to more variation in traits over time.

34
Q

What is the paradox of variation?

A

Suggests that traits that are closely related to fitness (traits under strong natural selection) should have low variation because selection should reduce variability by favoring individuals with the most advantageous traits. However, we often observe some variation even in traits strongly tied to fitness.

35
Q

Why should traits related to fitness have low variation?

A

Traits that strongly influence fitness are under strong natural selection, which tends to reduce variation by favoring individuals with the most advantageous traits. Over time, this should lead to less variation in those traits.

36
Q

Why might we still observe variation in fitness-related traits?

A

Genetic drift, mutation, and environmental factors can introduce new sources of variation, even in traits that are strongly selected for. This can maintain some level of variation in fitness-related traits despite strong selection.

37
Q

What did Mouseau and Roff conclude about life history and morphological traits?

A

concluded that life history traits have less genetic variation and lower heritability (h²) compared to morphological traits.
= traits closely related to fitness (like life history traits) tend to have lower heritability (h²) because selection works to reduce variation more quickly in these traits.

38
Q

What do the following notations mean: VA, VP, VR, VG and VE

A

VA = additive genetic variance = variance in traits due to the additive effects of genes passed from parents to offspring.

VP = phenotypic variance = total variance observed in a trait, combining genetic and environmental effects.

VR = residual variation = variance in traits not explained by additive genetic effects, including environmental and interaction effects.

VG = genetic interaction variance = variance due to interactions among genes (e.g., dominance and epistasis).

VE = environmental variance = variance in traits caused by environmental factors rather than genetic factors.

39
Q

What did Houle find about genetic variance in life history traits compared to morphological traits?

A

Houle found that life history traits have more additive genetic variance (VA) compared to morphological traits. He also showed that the Coefficient of Variation for Additive genetic variance (CVA) is not lower for life history traits, despite their h2 (heritability) appearing lower.

40
Q

What is the formula for heritability
(ℎ2) and why does it appear lower for fitness-related traits?

A

h2 = VA / VP = VA / VA + VR

Fitness-related traits have more additive genetic variance (VA) but because residual variance (VR) is also higher, the total phenotypic variance (VP) increases. As h2 is the proportion of VA in VP, the larger the VR reduces this proportion, making h2 appear lower.

41
Q

What is the formula for residual variation (VR) + why do life history traits exhibit higher residual variation ?

A

VR = VG + VE
Life history traits often involve many genes and environmental factors, leading to higher residual variation beyond genetic factors.

42
Q

How can variation in traits be explained in evolutionary ecology?

A
  • Neutral variation: Maintained by genetic drift, but does not contribute to evolutionary change in traits of ecological relevance.
  • Mutation-selection balance: New mutations replace variation eroded by selection, but is not enough to generate heritability at ecological timescales.
  • Balance of selective forces: Various selective mechanisms such as heterosis, frequency dependence, antagonistic pleiotropy, and genotype-by-environment interaction maintain variation in populations
43
Q

What is the Mutation-Selection Balance, and how does it relate to variation?

A

balance between the introduction of new mutations and the selective elimination of deleterious mutations. Mutation introduces genetic variation, while selection works to remove harmful mutations, maintaining variation in populations. This process is slow and often has little effect on variation in life history traits over ecological timescales.

44
Q

Name and define the selective forces that maintain genetic variation in populations

A
  • Heterosis (heterozygote advantage): Advantage of being heterozygous in certain traits.
  • Frequency dependence: Fitness changes depending on the frequency of a trait in the population.
  • Antagonistic pleiotropy: Genes affecting multiple traits, where the effects are beneficial in one context but detrimental in another.
  • Genotype-by-environment interaction: The effect of genetic variation depends on environmental conditions.
45
Q

Why is mutation considered the ultimate source of genetic variation?

A

the process through which new alleles and genetic diversity arise. However, most mutations are deleterious and are typically removed by natural selection, but they still contribute to the genetic diversity in populations.

46
Q

How do ecological mechanisms compare to mutation-selection balance in maintaining variation?

A

Ecological mechanisms are more likely to maintain genetic variation in populations than mutation-selection balance, which is slow and hard to measure in practice. Ecological factors like environmental pressures, interactions between organisms, and genotype-by-environment interactions can help maintain variation more effectively and quickly.

47
Q

What is the difference between variation and evolution in response to selection and why?

A

Variation exists in populations, but evolution may not occur even if there is genetic variation and selection (i.e. S and h² appear to be non-zero but there is still no R)- because
- Selection acts on a trait, but the expected evolutionary change does not occur.
- Factors such as environmental changes (e.g., climate change) can alter patterns in traits, preventing evolution despite selection pressures.

48
Q

What is the general pattern observed in long-term studies of selection and evolution?

A
  • Selection is observed, and heritability is non-zero.
  • However, evolutionary changes either do not occur or evolve in the opposite direction than expected.
  • This suggests that despite selection, evolutionary change may be absent or counterproductive over extended periods.
49
Q

Why might we fail to see a response to selection despite selection and heritability?

A
  • Biased heritability estimates
  • Countervailing selection
  • Fluctuating selection
  • Genetic constraints: Correlations between traits may prevent selection from acting on a single trait.
  • Selection on environmental component: Traits influenced by environment rather than genetics.
  • Environmental deterioration: Declining environments counteract genetic improvements.
50
Q

What is biased heritability estimates? + brief example?

A

Environmental similarity between relatives inflates heritability- relatives share not just genes but also their environments

e.g. Great tit- birds nesting closer to parents have higher apparent heritability for laying date- as dispersal distance increases, environmental similarity decreases and heritability estimates decline

51
Q

What is countervailing selection? + brief example?

A

Opposing effects of a gene on different traits or life stages cancel out.

e.g. larger song bird have higher reproductive success but also less likely to survive = cancels out overall fitness benefits

52
Q

What is fluctuating selection? + examples?

A

Environmental change lead to alternating selective pressures- Traits that are advantageous in one period might become disadvantageous in another = Over long periods, there may be no consistent evolutionary trend because selection “fluctuates” back and forth.

e.g. Galápagos finches showed strong selection for larger beaks during droughts but reversed selection in wet years.
e.g. Soay sheep, body size selection shifts with population density due to resource availability- at high pop density smaller body sizes favoured + vice versa for low pop density

53
Q

What are genetic constraints?

A

Correlations between traits that may prevent selection from acting on a single trait and can cause undesirable changes in one trait whilst selection acts on another

54
Q

How can the response to selection be rewritten?

A

R = selection gradient x additive genetic variance

55
Q

What does the additive genetic covariance matrix (G matrix) represent?

A

captures variances and covariances of traits, showing how traits vary independently or are correlated genetically.

Variances = additive genetic variance for each trait individually = lengths of axes
Covariances = additive genetic covariance between 2 traits = tilt of ellipse

56
Q

What problem can arise from correlated characters when using multiple regression analysis?

A

It may misidentify the target of selection, as the apparent correlation with one trait might be driven by another underlying relationship.

57
Q

Why are genetic constraints significant in evolution?

A

They can prevent populations from adapting optimally, even when heritable variation and strong selection pressures exist.

58
Q

What is the additive model in genetics?

A

It is a population-level model that partitions phenotype into components:

μ = Population mean
b = Fixed effects (e.g., birth year)
e = Error (random effects, often ignored)
a = Additive genetic component (breeding value, Va at the individual level).

59
Q

What is the animal model used for?

A

It estimates genetic parameters for individuals using a matrix form of the additive model. It incorporates breeding values to measure heritability from pedigree data.

60
Q

How can pedigrees help estimate heritability?

A

By analyzing traits across generations, such as offspring traits compared to parents or siblings, allowing calculation of additive genetic variance (Va).

61
Q

Why might selection fail to produce evolutionary change?

A

Selection may act on environmental components rather than genetic ones, leading to phenotypic variation that isn’t heritable.

62
Q

What is environmental deterioration, and how does it affect phenotypic traits?

A

Environmental deterioration is a decline in environmental quality over time, reducing phenotypic expression despite underlying genetic improvement.

63
Q

Why might selection appear to act in the wrong direction over time?

A

Environmental decline can offset genetic improvements, leading to an apparent decline in trait value despite evolutionary changes.

Evolutionary Ecology (13 decks)

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