Which of the following is not an observation or inference on which natural selection is based?

A. There is heritable variation among individuals. B. Poorly adapted individuals never produce offspring. C. Species produce more offspring than the environment can support. D. Individuals whose characteristics are best suited to the environment generally leave more offspring than those whose characteristics are less suited. E. Only a fraction of the offspring produced by an individual may survive.

The correct answer is B. Poorly adapted individuals never produce offspring.

Diagram showing how natural selection can lead to evolution in a theoretical butterfly species
Diagram showing how natural selection can lead to evolution in a theoretical butterfly species (Lizethlionetti [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)])
The mechanism of natural selection was proposed by Darwin as a way to explain how the evolution of plants and animals occur. Natural selection relies on natural variation that is heritable.

Natural selection means that the traits of individuals that are best adapted for survival and that result in the most offspring being produced will become more frequent in a population over time, leading to evolution.

It is important to note that even poorly adapted individuals may produce progeny, but they may not produce as many or the progeny may be less fit than those produced by other fitter individuals.

Variation is introduced by meiosis in sexually reproducing organisms, while it is genetic mutations that bring variation into a population of asexually reproducing species. Genetic variation is important because it ensures that some individuals in a population will survive in a changing environment.

Darwin also derived the theory of sexual selection to explain some features of animals that appeared to be potentially deleterious for survival yet were maintained in a population.

These features increased reproductive success despite possibly increasing predation risk. Darwin proposed that even though a long tail of a bird or bright colors may make them more visible to predators that this risk was offset by the increased mating success that such birds obtained.

Humans have acted as selection agents to manipulate living organisms to produce offspring with specific traits. It is this artificial selection that has produced the domestic animals we have today.

Natural selection

Natural selection is an idea that was developed by Charles Darwin to explain how living organisms evolved. Evolution is a change over time in a population, which relies on the accumulation of genetic changes.

The theory of natural selection relies on certain observations and inferences that are made. The first observation is that there is variation among individuals within a given population.

This is something that Charles Darwin observed when he observed the fauna and flora of the Galapagos Islands.

The other tenants of natural selection are that only some organisms in a population will survive and only some of these individuals will successfully reproduce and leave offspring.

There are also more individuals produced than will survive to reproduce. Even among offspring that are produced, not all will be equally well-adapted or able to produce many offspring.

Variation

Variation is heritable and the genotypes of those individuals that leave the most offspring will be passed on into the next generation.

Genetic variation is essential for the survival of a population because it means that not all organisms are equally affected by a particular selection agent. Variation is introduced during the meiotic cell division that occurs to form the sex cells of sexually reproducing organisms.

Even asexually reproducing organisms have some variation that is introduced through mutation. However, scientists believe that sexual reproduction was an important advantage for living organisms because it ensures that there is sufficient variation for selection to occur without causing extinction.

Mutations do also occur in sexually reproducing organisms and sometimes these genetic changes can produce a trait that increases fitness. For instance, the sickle-cell trait is an example of a beneficial mutation, although it may not seem to be.

This is because when the trait is in the heterozygous form it protects against the malaria parasite Plasmodium. However, in the homozygous form, it can cause severe sickle-cell anemia in a person.

Genotype frequencies

The effects of natural selection are evident if one examines changes in the frequencies of genotypes in a population. Over time the frequencies of different genotypes are expected to vary if the population is evolving.

These frequencies can be compared to those that are predicted by the Hardy-Weinberg equation. This equation was developed for a theoretical population that is in genetic equilibrium over time.

The equation states that: p2 + 2pq + q2 = 1 in which q is the homozygote recessive allele, p is the homozygote dominant allele, and pq is the heterozygote genotype. A few of the conditions that have to be met for this model to be true are that there is no selection and no gene flow in a population.

In nature, these conditions are unlikely to be met, but scientists can compare how the frequencies of alleles differ from the model and thus can use this as a gauge as to how much a population is evolving over time.

Sexual selection

The theory of sexual selection states that animals may have certain characteristics that make them more attractive as mates and that the risk of increased visibility to predators is outweighed by the increased likelihood that they will have reproductive success.

Classic examples include the long tails of widow birds which make flight difficult and the fact that many male birds are brightly colored and thus more visible to predators.

The sexual selection theory was also proposed by Darwin to explain the presence of these features that are evident in many animals.

Mate choice is an important component of reproductive success in animals such as birds and mammals that have internal fertilization and a great deal of parental care of offspring.

Artificial selection

Humans have acted as selection agents to pair up organisms that have desirable traits. This means that people have been able to breed animals and plants to obtain specific types or breeds.

This is how different breeds of domestic animals have been produced. This type of selective breeding has to be done carefully because too much inbreeding actually produces weakened stock.

This is because living organisms that are closely related are likely to share undesirable mutations or problems.

References

  1. SCP Williams (2016). News Feature: Genetic mutations you want.  Proceedings of the National Academy of Sciences.
  2. The Editors of Encyclopedia Britannica (2019). Selection. Retrieved from Encyclopedia Britannica.
  3. FJ Ayala (2019). Sexual selection. Retrieved from Encyclopedia Britannica.
  4. R Hershberg (2015). Mutation—The Engine of Evolution: Studying Mutation and Its Role in the Evolution of Bacteria. Cold Spring Harbor perspectives in biology.
  5. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing.

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