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Exactly what is natural selection?

Natural selection occurs whenever some individuals have more than their share of offspring because they possess certain genetically determined traits.

In this context, fitness is measured by successful reproduction. More offspring means better fitness. Fewer offspring means poorer fitness.

The ingredients for natural selection.

Natural selection occurs if:

(1) Variation: If there are phenotypic differences among individuals in a population, and

(2) Inheritance: If the phenotypic differences are based on genetics, and

(3) Fitness differences: If some individuals have more offspring because of (1) and (2).

Does natural selection cause evolution?

Evolution is change in the genetics of a population.

Natural selection can cause genetic change if the population is not in equilibrium with environmental conditions (cause evolution).

Natural selection can maintain current conditions if a population is in equilibrium with its environment (maintain status quo).

An example of natural selection and no change in the gene pool.

An example of a balance maintained by natural selection. The case of PKU.

mutation--->gene pool--->selection

Mutation repeatedly adds a deleterious allele to the gene pool and selection repeatedly subtracts it. The result for the population as a whole is a balance: gene frequency doesn’t change.

An example of natural selection and no change: phenylketonuria.

The allele which causes PKU in humans originates by mutation. During DNA replication (perhaps during meiosis) the gene which codes for a particular enzyme for processing phenylalanine may be copied incorrectly.

Although rare, the mutation occurs. As a result, mutation adds to the number of carriers for PKU in the population.

PKU: a balance between mutation and selection

PKU alleles accumulate in the gene pool because of mutation.

When two carriers have children, about 1/4 of the children will be afflicted with PKU. Because of the PKU, these children do not live long enough to reproduce. Therefore, compared to other parents, carriers for PKU will, on average, leave fewer offspring.

But is this natural selection?

Yes. All 3 ingredients are present.

(1) Variation: Some individuals suffer from PKU, others do not.

(2) Fitness differences: Individuals with PKU do not reproduce.

(3) Inheritance: The PKU is based on the defective allele (in this case, recessive).

The result is: balance, not change. Nevertheless, this is natural selection.

Example of evolution (change) caused by natural selection (p228).

Populations of peppered moth (Biston betularia) have changed in historical time. This change (i.e. evolution) has been produced by natural selection.

The ingredients: variation (some moths are peppered, some black); fitness differences (moths are likely to survive if they have good camouflage); inheritance (the variation has a basis in genetics).

Why does natural selection cause change in this case?

The genetics of this population have changed over time. Evolution has occurred because of a change in the environment: good camouflage depends on fitting in with the background, and the background changed because of air pollution. (see photos, p229). (In other respects, this case is very similar to the PKU example: the unusual allele occurred by mutation, etc.)

History of the peppered moth

Originally (before 19th century), nearly all moths were peppered and were well camouflaged against the lichen covered tree trunks.

Because of occasional mutation, a few black moths appeared in the population, but remained rare. We presume they were selected against at that time.

The effect of air pollution

Coal burning during the industrial revolution in England killed the lichens and stained the tree trunks black.

Subsequently, the relative abundance of black moths increased in the population. In areas with heavy air pollution, the black variety became predominant.

Studies to interpret this event (p229)

Scientists (G. Kettlewell and students) have conducted experiments to test the interpretation that camouflage is important to the moths.

Experiments: capture-recapture studies with both color types in areas with or without air pollution; genetic studies; direct observation of bird predators and moths.

Results of studies

More of the peppered variety were recaptured in areas free of air pollution; more of the black variety were captured in areas with air pollution.

Direct observation from blinds: birds found the more poorly camouflaged moths faster.

Genetic studies identified two genes that influence the color pattern of the moths.

Table of Kettlewell’s results

See table 14.1

Current experience

Because of air quality laws, air pollution has declined.

The shift from peppered to black variety of moth has been reversed: the peppered variety is now becoming more common again.

Is this natural selection? Is this evolution?

YES and YES

Natural selection: all 3 necessary ingredients are present: variation, fitness differences, and inheritance.

Evolution: the population genetics have changed in response to environmental change.

"A closer look at natural selection" (p227 of textbook).

1. Natural populations have excess capacity to reproduce.

2. Population size cannot increase indefinitely: competition for resources is unavoidable.

3. Inference: Sooner or later, individuals will compete for resources.

"A closer look at natural selection" (cont.)

4. Observation: All individuals have the same genes. Collectively, their genes represent a pool of information.

5. Observation: There are differences in alleles which give rise to differences in phenotypes.

6. Inference: Some phenotypes are better than others. Fitness is defined in terms of reproductive success.

"A closer look at natural selection" (cont.)

7. Conclusion: Natural selection is the outcome of differences in survival and reproduction of individuals that vary in heritable traits. Adaptation is one outcome of this process. (That is, the ability of organisms to cope with the demands of their environment is a consequence of natural selection operating on their ancestors.)

Additional examples of natural selection and its consequences.

Pesticide resistance (p228): The widespread use of DDT and other pesticides has killed many insects. Some individuals survive because they have means of excluding or detoxifying the pesticide. Surviving individuals pass on their genes to their offspring. As a result, resistance to common pesticides has increased with time.

Antibiotic resistance (p229).

Widespread use of antibiotics (such as penicillin and tetracycline) has killed many susceptible microorganisms. Surviving microorganisms possess traits which allow them to exclude or overcome antibiotic. Survivors pass along their genes to their "daughters". Over time, resistance has increased.

Types of selection.

Stabilizing selection: natural selection that maintains the status quo by removing extreme phenotypes.

Directional selection: natural selection against one extreme of a range of phenotypes.

Disruptive selection: natural selection against the "average": extremists survive.

An example of stabilizing selection: PKU

The gene for PKU is repeatedly introduced to the human gene pool by mutation.

Natural selection removes the rare individuals who are homozygous recessive for this trait.

The end result: the allele frequency for PKU gene stays low in spite of repeated introduction by mutation.

Examples of directional selection

Antibiotic resistance: after antibiotics were developed to assist human health, bacterial resistance became much more common.

Pesticide resistance: widespread use of synthetic pesticides has been followed by an increase in resistance in insects and other target species.

Examples of disruptive selection.

Disruptive selection means that the intermediate or "average" phenotypes have lower reproductive fitness, and that extreme phenotypes have higher reproductive fitness.

The example in the textbook: African finches (p231). Data (see graph) imply that intermediate phenotypes do not survive.

Natural selection and human biology: Sickle cell anemia.

Sickle cell anemia results from inheriting an unusual allele for hemoglobin: HbS instead of the more typical HbA.

The affects of the HbS allele are widespread (see page 147 for a description).

The affects are extreme for individuals that are homozygous HbS/HbS. Few homozygous individuals survive.

Sickle trait and ecology.

Sickle trait is very common in certain regions: where malaria is common.

The connection: Individuals who are heterozygous for HbS are resistant to malaria. (The resistance is because the parasite kills the red blood cells it infects and thereby kills itself, if some HbS is present in the cells.)

Sickle trait and malaria.

Each of the 3 genotypes produces a different phenotype:

Homozygous HbA/HbA: Normal physiology, and very susceptible to malaria.

Heterozygous HbA/HbS: Some tendency to develop anemia, but very resistant to malaria.

Homozygous HbS/HbS: pronounced anemia and poor survival

Sickle trait and malaria: the outcome.

The two maps presented in figure 14.17 present the coincidence of the geography of malaria and the distribution of the HbS allele in human populations.

Where malaria is widespread, HbS is common. Where malaria is nearly non-existent, HbS is rare.

HbS: Does natural selection operate?

Variation? Yes, there are 3 distinct phenotypes with respect to resistance to malaria and with respect to anemia.

Fitness differences? Yes, the heterozygotes survive best, and have the most offspring.

Heritability? Yes, the sickle trait is based on inheriting the HbS allele.

Natural selection? Yes, by definition!

Are these human populations evolving because of sickle trait?

No: malaria and the frequency of the HbS are not changing: they are in equilibrium.

I.e. Stabilizing selection without evolution.

What about populations in areas without malaria? The HbS allele is present, but...

In North America, the HbS allele is gradually decreasing. Directional selection and evolution are occurring!

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