Natural Selection

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수업 노트

Natural Selection & Evolution

Evolution is defined as changes in the heritable characteristics of organisms over generations.
Natural selection is the process by which organisms better adapted to their environment survive, reproduce, and pass on their advantageous alleles, causing advantageous characteristics to increase in frequency within a population.
Charles Darwin proposed the theory of evolution by natural selection after a five-year expedition; he published×On the Origin of Species×in 1859.
Darwin's theory replaced Lamarckism, the idea that organisms could pass on physical characteristics acquired during their lifetime to their offspring.
Darwin's theory caused a paradigm shift because it contradicted previous assumptions; nearly 200 years of genetic research supports it.
Example: in a rabbit population, brown fur provides better camouflage against predators, so brown rabbits are more likely to survive and reproduce, increasing the frequency of brown fur alleles over generations.

Natural selection in peppered moths

Generating Genetic Variation

Variation exists between organisms of the same species and is essential for natural selection; without variation, no individual is favoured and the population cannot adapt.
Variation results from small differences in DNA base sequences between individuals.
Mutation is the original source of genetic variation; it is a change in the DNA base sequence resulting from a copying error during DNA replication, generating new alleles.
Mutations in sex organs can be passed to gametes; a mutation in a somatic (body) cell is not passed on and has no impact on natural selection.
Meiosis generates variation through crossing over (exchange of alleles between homologous chromosomes) and random orientation (independent assortment of homologous pairs during metaphase I).
Random fertilisation during sexual reproduction creates genetic variation because any male gamete can fuse with any female gamete, producing a unique combination of alleles.
The number of possible chromosome combinations from random orientation is 2ⁿ, where n is the haploid number; in humans this is 2²³ = 8,324,608 combinations.

Overproduction, Competition, and Heritable Traits

Darwin noted the overproduction of offspring: more offspring are produced than can be supported by the environment.
Overproduction leads to competition for limited resources such as food, space, and light, which limits the carrying capacity of a population.
Intraspecific competition (between individuals of the same species) plays a greater role in evolution because individuals share the same niche and are affected by the same abiotic and biotic factors.
Individuals with characteristics better adapted for survival are more likely to survive into adulthood, find a mate, and reproduce, passing on their advantageous alleles.
Heritable characteristics are determined by alleles and can be physical (e.g. giraffe neck length) or behavioural (e.g. woodlouse moving towards dark).
Non-heritable characteristics (e.g. weight gained during lifetime or injuries) are not passed on to offspring.
Better-adapted organisms are more likely to survive and reproduce, not guaranteed; less-suited organisms may still survive and reproduce but with lower probability.

Selection Pressures

Abiotic factors are non-living factors in an ecosystem (e.g. temperature, rainfall) that can act as selection pressures, causing population size to fluctuate.
Sexual selection is a form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex.
Sexual selection requires variation within the population; traits visible to the opposite sex can indicate overall fitness and affect mating success.
Sexual selection can cause reproductive isolation (changes preventing successful breeding with others) and sexual dimorphism (distinct differences in size or appearance between sexes).
Example: male Birds of Paradise are brightly coloured and perform courtship displays, while females have grey/brown plumage.
Natural selection occurs due to competition for resources, whereas sexual selection occurs due to competition for mates.
Sexual selection can produce traits that reduce survival (e.g. peacock's long tail makes it more prone to predation) but increase mating success.

Selection Pressures: Skills (Endler's Guppy Experiments)

Guppies in Trinidad and Tobago show variation in colour and pattern; males are brightly coloured and females are dull (sexual dimorphism).
Spot colouration provides camouflage by mimicking the gravel of streambeds; some streambeds are finer and sandier, others coarser and more pebble-like.
John Endler hypothesized that when predators are present, substrate type affects survival and spot brightness; low predation increases spot brightness due to sexual selection; high predation decreases spot brightness.
In Endler's laboratory experiment, guppies were placed in ponds with coarse or fine gravel and exposed to no, weak, or dangerous predators.
Results: high predation decreased mean number of spots; low predation increased spot number; coarse gravel favoured larger spots and fine gravel favoured smaller spots (mimicking gravel size).
In ponds with no predation, the opposite was observed: fine gravel favoured large spots and coarse gravel favoured small spots, possibly because not matching the background makes males more conspicuous to females.
In the field experiment, dull male guppies transferred from high to low predation areas developed more colourful patterning over 15 generations, likely due to sexual selection.
Endler concluded that guppy evolution is a dynamic trade-off between natural selection (predator avoidance) and sexual selection (attracting mates).

Gene Pools and Allele Frequencies

A gene pool consists of all the genes and their different alleles present in an interbreeding population.
Multiple gene pools can exist for a species if populations are geographically isolated.
A stable gene pool occurs when the population is large, all individuals have an equal chance of mating, matings are random, and there are no selection pressures acting on phenotype; a stable gene pool means the population is not evolving.
Allele frequency is the relative frequency of an allele at a particular gene locus; if more than one allele exists, their frequencies must add up to 1.
Changes in allele frequency occur due to natural selection, sexual selection, and genetic drift.
Modern understanding of evolution integrates genetics with Darwin's theory, often called neo-Darwinism.
Comparing allele frequencies can help identify genetic associations with diseases, estimate disease susceptibility or drug resistance, and perform evolutionary and anthropological studies.

Natural selection in bacteria

Allele Frequencies: Skills

For any gene locus, each individual carries two alleles (one from each parent), except for alleles on X or Y chromosomes.
In a population, there are twice as many total alleles as there are individuals.
Homozygous individuals contribute two of the same allele to the total; heterozygous individuals contribute one of each allele.
Allele frequencies can be calculated from phenotype frequencies using the Hardy-Weinberg formula: p + q = 1, where p is the dominant allele frequency and q is the recessive allele frequency.
For a recessive phenotype, the phenotype frequency equals q²; thus q is the square root of the recessive phenotype frequency.
Worked example: if 4/13 lizards are white (yy), then q² = 0.3077, q = 0.5547, and p = 1 - 0.5547 = 0.4453.
Geographic isolation can cause differences in allele frequencies between populations; human allele frequencies vary by geography and ethnicity, but truly isolated populations are rare due to travel and interbreeding.

Types of Natural Selection

Directional selection changes the population towards one extreme of a range of variation; it tends to occur when environmental conditions change.
Example of directional selection: a fall in average temperature selects for frost-resistant plants if an allele provides frost protection; fish body size decreases with warming ocean temperatures.
Stabilising selection favours the average individual and occurs when environmental conditions are stable; it is the most common form of natural selection.
Stabilising selection discards extreme phenotypes, decreases diversity, works mostly on polygenic traits, and is characterised by a normal distribution (bell-shaped curve).
Examples of stabilising selection: coat colour in mice (brown fur camouflaged against forest floor) and human birth mass (too low or too high reduces survival).
Disruptive selection selects against the average individual and is the rarest form; it occurs when habitats or resources change and can lead to speciation (diversifying selection).
Example of disruptive selection: Darwin's finches in the Galápagos, where different beak types adapted to different food sources, allowing species to occupy different niches.

Natural selection in snail shell colour

Hardy-Weinberg Principle

The Hardy-Weinberg principle states that if certain conditions are met, allele frequencies in a population will not change from one generation to the next.
The Hardy-Weinberg equation allows calculation of allele and genotype frequencies: p + q = 1 and p² + 2pq + q² = 1.
p represents the frequency of the dominant allele, q the recessive allele; p² is the frequency of homozygous dominant, 2pq of heterozygous, and q² of homozygous recessive genotypes.
Conditions for Hardy-Weinberg equilibrium: organisms are diploid, reproduce sexually, no overlap between generations, random mating, large population, no migration, mutation, or selection, and allele frequencies equal in both sexes.
If genotype frequencies do not fit the Hardy-Weinberg equation, one or more conditions are not being met.
The principle is useful for building models and making predictions, but its assumptions are rarely all present in nature.
When solving problems, start by determining the proportion of individuals with the recessive phenotype, as this is the only phenotype that directly gives the genotype (homozygous recessive).

Artificial Selection

Artificial selection (selective breeding) is the process by which humans choose organisms with desirable traits and breed them together to enhance those traits over many generations.
Humans have practiced selective breeding for thousands of years; individuals are selected by phenotype, not genotype, so knowledge of alleles is not required.
Breeders can accidentally enhance other traits genetically linked to the desired trait, sometimes negatively affecting health.
Steps: population shows phenotypic variation; breeder selects two individuals with desired phenotype (not closely related); they are bred; offspring are tested and the best are selected for further breeding; process continues for many generations.
Animals are selectively bred for traits such as higher milk or meat yield, large eggs, gentle nature, quality wool, and speed; all domestic dog breeds descend from wolves.
Plants are selectively bred for disease resistance, increased yield, hardiness, better taste, and large or unusual flowers; wild brassica has given rise to cauliflower, cabbage, broccoli, Brussels sprouts, kale, and kohlrabi.
Difference from natural selection: artificial selection involves deliberate choice by humans, whereas natural selection does not; artificial selection can result in animals poorly suited to their environment, while natural selection results in better adaptation.

Selective breeding in dogs

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1.What is the definition of the term 'gene pool'?
Easy
- AAll of the genes and their different alleles that exist within a single population
- BAll of the genes that exist within a population
- CAll of the genes and their different alleles that exist within a species
- DThe different alleles that exist for one gene within a species
2.An example of a trait that evolved by natural selection is body size in fish, which has declined with warming ocean temperatures. State the type of selection that this change represents.
Easy
- ADirectional
- BStabilizing
- CDisruptive
- DAllopatric
3.Which process does not generate heritable variation in a population?
Easy
- AMitosis
- BMutation
- CMetaphase I of meiosis
- DFertilisation
4.Which of the following statements correctly explains the process of natural selection? I. Some species produce small numbers of young and provide higher levels of parental care II. Advantageous alleles increase an individual's chance of surviving and reproducing III. Individuals that change to suit their environment pass their alleles on to offspring IV. Advantageous alleles increase in frequency in the population
Medium
- AII and IV only
- BI and II only
- CI, II, III, and IV
- DII, III, and IV only
5.A game farm in South Africa has a population of about 5 000 large antelope called Kudu. Kudu bulls have large, spiralled horns which draw the attention of trophy hunters that frequently visit the game farm. The antelope are all kept in a large, fenced-off area consisting of open grassland habitat. Could this be considered an example of a stable gene pool?
Medium
- ANo, since there will be a selective pressure for antelope with smaller horns
- BNo, since there will be a selective pressure to increase the allele frequency for antelope with large horns
- CYes, it is a large population of antelope with an equal chance to mate with each other
- DYes, it is a large population located in a habitat that enables random matings between antelope of different phenotypes
6.Which of the following applies to the process of evolution by natural selection? I. Changes in the phenotype of organisms in a population II. Selection pressures favouring certain alleles within a population III. Individuals with a certain genetic makeup will not pass on their genes IV. Changes in the allele frequencies within a population over time
Medium
- AII and IV only
- BII only
- CI, II and III
- DI, II, III and IV
7.What would be the most accurate description of the following graphs? I: A graph showing a shift in the distribution of giraffe neck lengths towards longer necks. II: A graph showing a bimodal distribution of turtle size with both larger and smaller turtles favoured. III: A graph showing a narrow distribution of cactus spine density with medium spine density favoured.
Hard
- ADirectional selection as giraffes with longer necks are selected for; Disruptive selection as larger and smaller turtles are selected for; Stabilising selection as cacti with a medium spine density are selected for
- BStabilising selection as giraffes with longer necks are selected for; Directional selection as larger and smaller turtles are selected for; Disruptive selection as cacti with a medium spine density are selected for
- CStabilising selection as giraffes with medium neck lengths are selected for; Disruptive selection as larger and smaller turtles are selected against; Directional selection as cacti with medium spine density are selected for
- DDirectional selection as giraffes with medium neck lengths are selected for; Stabilising selection as larger and smaller turtles are selected against; Disruptive selection as cacti with medium spine density are selected for
8.Which of the following examples would be a useful application of comparing allele frequencies between populations? I. Estimating the number of individuals in a population that may be susceptible to certain diseases II. Studying the evolutionary history of populations within a specific species III. Investigating the effect of environmental factors on the phenotype within different populations
Medium
- AI and II only
- BI only
- CIII only
- DI and III only

더 많은 문제

9.Which of the following processes generate genetic variation? I. Random assortment II. Mitosis III. DNA replication IV. Random fertilisation
Medium
- AI and IV only
- BI and III only
- CI, III, and IV only
- DI, II, III, and IV
10.Which of the following must occur for natural selection to take place?
Medium
- AGenetic variation.
- BSexual reproduction.
- CThe presence of predators.
- DAdvantageous characteristics.
11.Which of the following does not describe the process of natural selection?
Hard
- AWhen a bacteria is exposed to a new type of antibiotic the exposure causes the bacteria to become resistant
- BA species of antelope where the population has stronger muscles after the individuals with weak muscles are eaten by predators
- CSharks adapting to store more oxygen in their blood because the sharks with less oxygen were more likely to die
- DDark peppered moths increasing in number during the industrial revolution in response to increased predation of the light colour moths
12.The three graphs below show how the allele frequencies of two alleles in three different populations changed over several generations. I: Allele frequencies remain constant over generations. II: One allele increases in frequency while the other decreases. III: Allele frequencies fluctuate but show no overall change. Which of the graphs show changes in allele frequency that suggest that evolution is taking place?
Hard
- AII. and III. only
- BII. only
- CIII. only
- DI., II. and III.
13.Which of the following are sources of genetic variation in a population? (select all that apply)
Medium
- AMutation
- BCrossing over during meiosis
- CRandom fertilisation
- DMitosis
- EBinary fission
14.Which of the following are conditions required for a stable gene pool? (select all that apply)
Medium
- AThe population is large
- BMating is random
- CThere are no selection pressures acting on phenotype
- DThere is frequent migration
- EThere is strong sexual selection
15.Natural selection can only take place when there is variation in a population.
Easy
True or false?
16.Stabilising selection selects against the average individual in a population.
Easy
True or false?
17.Disruptive selection is the most common form of natural selection.
Medium
True or false?
18.Artificial selection involves a deliberate choice of traits by humans.
Easy
True or false?
19.Match each term with its correct definition.
Medium
- Gene pool
- Allele frequency
- Directional selection
- Stabilising selection
- All the genes and their different alleles in an interbreeding population
- The relative frequency of an allele at a particular gene locus
- Selection that favours one extreme of a range of variation
- Selection that favours the average individual in a population
20.Place the steps of natural selection in the correct order.
Medium
- Variation exists within a population
- Some individuals have advantageous characteristics
- These individuals are more likely to survive and reproduce
- Advantageous alleles increase in frequency over generations
21.Which of the following are conditions required for the Hardy-Weinberg principle to hold true? (select all that apply)
Hard
- AMating is random
- BThe population is large
- CThere is no migration, mutation, or selection
- DOrganisms reproduce asexually
- EThere is overlap between generations
22.Match each type of natural selection with its correct description.
Medium
- Directional selection
- Stabilising selection
- Disruptive selection
- Selects in favour of the average individual in a population
- Selects against the average individual in a population
- The population changes towards one extreme of a range of variation
23.Place the steps of artificial selection in the correct order.
Medium
- The population shows phenotypic variation
- A breeder selects an individual with the desired phenotype
- Another individual with the desired phenotype is selected
- The two selected individuals are bred together
- Offspring that display the desired phenotype to the greatest degree are selected for further breeding
24.Which of the following best defines evolution?
Medium
- AChanges in the heritable characteristics of organisms over generations
- BChanges in the phenotype of an individual during its lifetime
- CThe process by which organisms become better adapted to their environment
- DThe formation of new species from a common ancestor
25.Stabilising selection is the most common form of natural selection.
Easy
True or false?
26.In a population of 200 individuals, there are 400 alleles for a particular gene. If 240 of these alleles are dominant, what is the frequency of the dominant allele?
Medium
- A0.6
- B0.4
- C0.24
- D0.48
27.Which of the following is an example of a non-heritable characteristic that would not affect natural selection?
Medium
- AA rabbit gaining weight after eating lots of food in autumn
- BA giraffe having a longer neck due to inherited alleles
- CA moth having a darker wing colour due to a mutation
- DA bird having a larger beak due to genetic variation
28.In a population of island finches (Geospiza fortis), seeds are small and soft in normal years but become larger and tougher in drought years. The graph of mean beak size shows a rise from 9.5 mm in 1976 to 9.9 mm in 1977, followed by a slow fall to 9.7 mm by 1979. Which of the following statements explains the results in the graph?
Medium
- AFinches adapt to drought years by developing larger beaks, passing on the characteristic to their offspring, and leading to an increase in average beak size
- BAverage finch beak size increases during years of drought and decreases during wet years
- CFinches with larger beaks have an advantage when competing for food in drought years, and are therefore more likely to survive and pass on their alleles, leading to an increase in average beak size
- DBeak size is not heritable, so the change in average beak size must be caused by the drought itself
29.A population of 500 rabbits is in Hardy-Weinberg equilibrium. 4% of the rabbits have white fur, which is the recessive phenotype. How many rabbits in the population would be expected to be heterozygous?
Hard
- A80
- B160
- C200
- D320
30.In a population of 250 individuals there are 500 alleles for a particular gene. If 150 of these alleles are recessive, what is the frequency of the dominant allele?
Medium
- A0.30
- B0.60
- C0.70
- D0.85
31.A species of moth shows three colour forms. On soot-darkened trees, pale moths are eaten by birds far more often than dark moths, while on unpolluted pale-barked trees the reverse is true. Over many generations in a polluted forest, the frequency of pale moths falls sharply. Which type of selection does this represent?
Medium
- AStabilising selection
- BDisruptive selection
- CDirectional selection
- DArtificial selection
32.A bird population contains individuals with small, medium and large beaks. During a drought only very small seeds and very large hard seeds remain; birds with medium beaks cannot feed efficiently and are selected against, while small- and large-beaked birds both survive well. Which type of selection is acting, and what is its likely long-term consequence?
Hard
- AStabilising selection; the average beak size becomes more common
- BDisruptive selection; the population may split into two distinct groups and speciate
- CDirectional selection; beak size steadily increases in the whole population
- DDisruptive selection; the average beak size becomes more common
33.In Endler's guppy experiment, male guppies were kept in ponds with either coarse or fine gravel and with no predators, weak predators or strong predators. In ponds with strong predation the mean number of spots on males decreased, and in ponds with no predation spot numbers increased. Which conclusion is best supported by these results?
Hard
- APredation causes individual guppies to lose their spots during their lifetime
- BBright spots are always disadvantageous, so they are lost whenever predators are present
- CThere is a trade-off between natural selection (camouflage from predators) and sexual selection (attracting mates)
- DSpot number is not heritable and changes only with the type of gravel
34.Endler found that in ponds with coarse gravel, male guppies tended to have larger spots, whereas in ponds with fine gravel they had smaller spots; this was true at both high and low predation rates. What is the most likely explanation?
Hard
- ASpot size mimics the size of the gravel, providing camouflage from predators
- BCoarse gravel causes mutations that produce larger spots
- CLarger spots are always preferred by females regardless of the background
- DSpot size is determined by the amount of food available in each pond
35.In a population where every individual is genetically identical, natural selection cannot cause the population's characteristics to change over time.
Medium
True or false?
36.A mutation that arises in a skin cell of an adult mammal can be passed on to its offspring and so can contribute to natural selection.
Medium
True or false?
37.A small population of lizards is isolated on an island. Which of the following would prevent this population from having a stable gene pool? (select all that apply)
Hard
- AThe population is very small
- BFemales consistently choose to mate only with the largest males
- CMating occurs at random with respect to phenotype
- DSome alleles make individuals more likely to survive and reproduce
- EAll individuals have an equal chance of mating
38.Which of the following statements about the Hardy-Weinberg principle are correct? (select all that apply)
Medium
- AIt predicts that allele frequencies stay constant from one generation to the next if its conditions are met
- BIt requires that mating within the population is random
- CIt requires a large population size
- DIt requires that migration and mutation occur frequently
- EIt can be applied to any population, since its conditions are always met in nature
39.A farmer treats a herd of cattle with an antibiotic. A few bacteria in the herd already carry an allele giving resistance. Which statements correctly describe what happens next? (select all that apply)
Hard
- AThe antibiotic acts as a selection pressure on the bacteria
- BResistant bacteria are more likely to survive and reproduce
- CThe antibiotic causes resistant bacteria to mutate new resistance alleles
- DThe frequency of the resistance allele increases in the bacterial population
- EAll the bacteria in the herd become resistant during their lifetime
40.Place the following events in the order in which they occur when a population adapts to a changing environment by natural selection.
Medium
- A selection pressure acts on the population
- Individuals with advantageous alleles survive and reproduce more often
- Variation exists within the population
- The frequency of the advantageous allele increases over generations
41.In a population of 100 individuals, there are 200 alleles for a particular gene. If 120 of these alleles are dominant, what is the frequency of the dominant allele? (Give your answer as a decimal to two decimal places.)
Medium
42.In a population of birds, 10% of individuals show the recessive phenotype of white feathers. Assuming Hardy-Weinberg equilibrium, what is the frequency of the recessive allele? (Give your answer as a decimal to two decimal places.)
Hard
43.Complete the definition of evolution.
Medium
Evolution is defined as changes in the ____ characteristics of organisms over ____.
44.Name the three main types of natural selection.
Medium
The three main types of natural selection are ____, ____, and ____.
45.What is the term for the process by which humans choose organisms with desirable traits and selectively breed them together?
Medium
46.What is the term for the form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex?
Easy
47.Calculate the frequency of the recessive allele (b). Give your answer as a decimal to two decimal places.
Hard
48.Calculate the frequency of the dominant allele (B). Give your answer as a decimal to two decimal places.
Hard
49.Calculate the expected number of heterozygous rabbits in the population. Give your answer as a whole number.
Hard
50.In a population at Hardy-Weinberg equilibrium, 16% of individuals show the recessive phenotype. Calculate the frequency of the recessive allele (q). Give your answer as a decimal to two decimal places.
Medium
51.Complete the sentence about allele frequencies.
Easy
If there is more than one allele for a particular gene, the sum of their allele frequencies must equal ____.
52.In a population of 1000 people in Hardy-Weinberg equilibrium, 9% show a recessive condition. Calculate the frequency of the recessive allele. Give your answer as a decimal.
Medium
53.Explain how natural selection can lead to an increase in the frequency of a favourable characteristic in a population.
Hard
54.Describe the difference between natural selection and sexual selection.
Hard
55.Explain how a change in the environment can lead to directional selection in a population of fish.
Hard
56.Explain why sexual selection can produce traits, such as the peacock's long tail, that reduce an individual's chance of survival.
Hard
57.In a population of 100 individuals, there are 200 alleles for a particular gene. If 120 of these alleles are dominant, what is the frequency of the dominant allele? (Give your answer as a decimal to two decimal places.)
Medium
- A1.6
- B-0.4
- C0.6
- D2.6
58.In a population of birds, 10% of individuals show the recessive phenotype of white feathers. Assuming Hardy-Weinberg equilibrium, what is the frequency of the recessive allele? (Give your answer as a decimal to two decimal places.)
Hard
- A1.32
- B0.32
- C2.32
- D-0.68
59.Calculate the frequency of the recessive allele (b). Give your answer as a decimal to two decimal places.
Hard
- A2.4
- B-0.6
- C1.4
- D0.4
60.Calculate the frequency of the dominant allele (B). Give your answer as a decimal to two decimal places.
Hard
- A0.6
- B1.6
- C2.6
- D-0.4
61.Calculate the expected number of heterozygous rabbits in the population. Give your answer as a whole number.
Hard
- A58
- B43
- C53
- D48
62.In a population at Hardy-Weinberg equilibrium, 16% of individuals show the recessive phenotype. Calculate the frequency of the recessive allele (q). Give your answer as a decimal to two decimal places.
Medium
- A0.4
- B1.4
- C2.4
- D-0.6
63.In a population of 1000 people in Hardy-Weinberg equilibrium, 9% show a recessive condition. Calculate the frequency of the recessive allele. Give your answer as a decimal.
Medium
- A1.3
- B2.3
- C-0.7
- D0.3

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