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Topics for February 13

Natural selection and evolution together.

Hardy-Weinberg assumptions (again)

Processes which produce microevolution:

(1) genetic drift,

(2) gene flow,

(3) mutation,

(4) non-random mating,

(5) natural selection.

What does the study of Biston betularia reveal?

All the ingredients for natural selection are present:

variation (light or dark phenotype)

fitness difference (camouflage matters!)

inheritance (2 gene loci control color)

Evolution occurred when:

the environment changed so that the gene frequencies were not appropriate for new environment.

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. 45398-->

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!

Assumptions of the Hardy-Weinberg principle

The Hardy-Weinberg principle requires that there be:

No migration

No mutation

No selection

Large population

Mating is random

Usefulness of the Hardy-Weinberg principle

Hardy-Weinberg provides a theoretical benchmark against which real populations may be compared.

Departures from the assumptions occur: Hardy-Weinberg provides a point of reference for evaluating the causes and consequences of the departures.

Genetic drift: random changes in gene frequencies

Genetic drift means the random change of gene frequencies in a population.

Some such changes are "neutral": changes in allele frequencies when the alleles have no immediate consequence to the biology of the population. Example: synonym codons code for the same amino acids and thus make exactly the same protein.

Examples of genetic drift

Population bottleneck. Species temporarily reduced to very low number lose genetic diversity. Examples: cheetahs--low population during Pleistocene; elephant seals--hunted to near extinction during 19th century.

Founder effect. Populations founded by just a few individuals have unusual gene frequencies.

Significance of genetic drift

Founder effect may start a new population with unusual gene frequencies which become the basis of new adaptations.

Bottleneck causes reduced genetic diversity.

For neutral alleles, genetic drift occurs in all populations and species. As a consequence, separated populations (and species) accumulate genetic differences.

Gene flow

Gene flow means the movement of individual organisms from one population to another, or simply the movement of gametes (e.g. pollen).

Gene flow brings the gene frequencies of adjacent populations closer together. Gene flow has the opposite effect of the founder effect: if it occurs, it prevents the accumulation of genetic differences.

Significance of gene flow

If it occurs, gene flow keeps adjacent populations tied together.

If populations are to separate enough to be considered separate species, there must be barriers to prevent any significant gene flow.

Mutation

Mutations are spontaneous changes in the genetic material. These changes include:

Point mutations: changes in a single base pair in the DNA

Frame shift mutations: deletion or insertion of a single extra base pair (codon=3 bases).

Chromosomal changes: duplication, deletion, inversion, translocation.

Significance of mutation

Mutations introduce new alleles. Usually, the new alleles are deleterious. Some few, in a new environmental context, turn out to be beneficial.

Some chromosomal mutations (e.g. inversion) produce barriers to reproduction between a new chromosomal arrangement and the ancestral arrangement.

Non-random mating

The Hardy-Weinberg principle assumes random mating: mate selection without regard to genotype.

Non-random mating means that mate selection is influenced by phenotypic differences based on underlying genotypic differences.

Examples of non-random mating

In some species, males acquire harems and monopolize females. (Elk, elephant seals, horses, lions, etc.) Commonly, the males of such species are much larger than the females.

In some species, females choose more attractive mates. (Peacocks, Woodducks, Picture-wing fruit flies, etc.)

Significance of non-random mating.

Sexual dimorphism (conspicuous differences between the two sexes) result from non-random mating. The process is a special case of natural selection known as sexual selection.

Sexual selection may serve as a barrier to reproduction between closely similar species. Example: courtship rituals.

Summary of exceptions to H/W assumptions.

Genetic drift--random changes (founder effect, bottleneck, and neutral genetic drift).

Gene flow--movement of alleles.

Mutation-- new genetic material.

Non-random mating--sexual selection, etc.

Natural selection--adaptive changes in the gene pool.

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