Introduction

• An adaptation is a characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states.

• There is no goal to evolution. Evolutionary changes don’t have a goal it is working towards. There is no conscious anticipation of the future when a change occurs.

Adaptive Evolution Observed

• Although it was once thought that evolution occurs too slowly to be observed, we now have several examples where we can see evolution taking place

  • Bacteria can evolve to resist antibiotics very rapidly

  • Several species of insects have adapted to new food plants and sources, as well as adapted to chemical pesticides

  • Commercial overexploitation has forced some animals to sexually mature at younger ages and grow smaller to adapt to the changing environment

• These evolutionary changes can be so rapid because populations in altered environments, especially those altered by human activities, can experience stringent natural selection, and because they contain genetic variation in many characteristics- a necessary ingredient of evolution.

Natural Selection

The meaning of natural selection

• Natural selection is any consistent difference in fitness among different classes of biological entities.

  • Fitness is the number of offspring an individual leaves in the next generation; components of fitness are survival and reproduction

  • Reproductive success is sometimes used as another name for fitness; includes survival because organisms do not reproduce when they are dead

• If evolution by natural selection is to occur, there must be a change in the population across generations, and this requires that the phenotypic differences among the entities be inherited.

  • Thus, evolution by natural selection only occurs if:

    • (1) there is a correlation between an individual’s phenotype and their fitness

    • (2) variation in the phenotype is correlated between parents and their offspring

• Frequency usually refers to a proportion and is measured in decimals and changes from 0 to 1.

  • For example, the frequency of allele A may increase while allele B decreases.

• Sexual selection is differential reproduction as a result of variation in the ability to obtain mates.

• Individual selection is a form of natural selection consisting of nonrandom differences in fitness among different phenotypes (or genotypes) within a population.

• Natural selection is a name for statistical differences in reproductive success among genes, organisms, or populations – and nothing more.

Natural selection and chance

• “Natural selection” is not synonymous with “evolution”.

• Natural selection can occur without any evolutionary change, as when natural selection maintains the status quo by eliminating deviants from the optimal phenotype. Other processes can also drive evolution:

  • Genetic drift is the random fluctuations in the frequencies of genotypes within a population.

  • Neutral alleles are alleles that do not alter fitness. Neutral alleles may fluctuate in a population due to genetic drift.

The effective environment depends on the organism

• The environmental factors that impose natural selection on a species are greatly influenced by the characteristics of the species itself: the evolutionary history of a species affects its relationship to the environment.

Levels of Selection

• By “natural selection”, both Darwin and present-day evolutionary biologists usually mean consistent differences in fitness among phenotypically and genetically different individual organisms within populations

  • Our definition of natural selection applies to many classes of variable entities that can change in number.

  • Selection can occur among genes, cell types, individual organisms, populations, or a species, a hierarchy of levels of selection.

• Gene selection is the natural selection at the level of the gene.

  • This can be illustrated by transposable elements, which replicate and proliferate within the genome, irrespective of whether their proliferation affects the organism for good or ill.

  • Transposable elements are among the many kinds of selfish genetic elements, which are transmitted at a higher rate than the rest of an individual’s genome and may be detrimental to the organism.

  • Some selfish alleles exhibit segregation distortion and are passed to a heterozygous individual’s gametes more than 50 percent of the time.

    • Segregation distortion is any of several biological processes that alter the rules of Mendelian inheritance such that some alleles when heterozygous have a greater than 50-person chance of transmission to the offspring

      • This can result from the meiotic drive (meiosis does not follow Mendel’s laws).

Selfish genes and unselfish behaviors

• Evolutionary geneticists have long recognized that natural selection will cause an allele to increase in frequency if it consistently leaves more copies of itself to subsequent generations, no matter how it causes its greater success.

• The key issue is that it is often useful to think of selection among genes, based on the effects that change their frequencies- whether these effects are on the number of pollen grains, behavior that enhances the survival of relatives that share the same gene, or many other biological features.

• An important example of this approach is the behavior of worker bees: “socially valuable but individually disadvantageous characters”

  • Altruistic traits are traits that are selfless to the individual.

  • Kin selection is a form of selection whereby alleles differ in their rate of propagation by influencing the impact of their bearers on the reproductive success of individuals (kin) who carry the same alleles by common descent.

  • An allele for altruistic behavior can increase in frequency in a population if the beneficiaries of the behavior are usually related to the individual who performs it.

Selection of organisms and groups

• If traits evolve by individual selection – by the replacement of less fit by more fit individuals, generation by generation- the possibility of future extinction cannot possibly affect the course of evolution

• It would seem impossible that a trait could evolve that benefits the population at a cost to the individual. However, there is one way:

  • Group selection is the differential rate of origination or extinction of whole populations (or species) because of differences among them in one or more characteristics, as hypothesized by Wynne-Edwards.

  • Hypothesis criticized by George Williams; argued that these adaptations can only be explained by the benefit to the individual or individual’s genes or may not be adaptations at all.

• It is the majority view that few characteristics have evolved because they benefit the population or species, and the cooperation and seeming altruism are most likely to have evolved by other causes, especially kin selection.

Species selection

• Species selection is a form of group selection in which species with different characteristics increase (by speciation) or decrease (by extinction) in number at different rates because of a difference in their characteristics

  • This does not shape the adaptions of organisms, but it does affect the disparity (the diversity of biological characteristics) of the world’s organisms.

  • The consequence of this is the proportion of species that have one character state, rather than another, changes over time.

Nature of Adaptations

• There are two related meanings of “adaptation”:

  • (1) Evolutionary process by which, over the course of generations, organisms are altered to become improved with respect to features that affect survival or reproduction

  • (2) a characteristic of an organism that evolved by natural selection

• A preadaptation is the possession of the necessary properties to permit a shift to a new niche, habitat, or function. A structure is preadapted for a new function if it can assume that function without evolutionary modification; a feature that can be used for a different purpose, without meaning to or being evolved for that purpose

  • Ex: Parrots have strong, sharp beaks for feeding on fruits and seeds. When sheep were introduced to the environment, the parrots then used their beaks to attack the sheep and feed on their fat. They already had the sharp beaks, but just used them for a different purpose

• An exaptation is the evolution of a function of a gene, tissue, or structure other than the one it was originally adapted for.

Selection of and selection for

• We must distinguish the selection of objects from the selection for properties.

  • In natural selection, some features are selected for, but other features that relate to the one selected will also continue, solely because of their association with the selected feature.

  • For example, if all large balls are purple and all small balls are yellow, and small balls are selected, the yellow feature will also continue. The purple balls are not selected because of their color; they aren’t chosen because of their size, and the color just happens to be related to the size.

Recognizing adaptations

• There are at least four other possibilities of organisms’ characteristics other than adaptations:

  • (1) A trait may be a necessary consequence of physics or chemistry.

  • (2) A trait may have evolved by other mechanisms (such as genetic drift) rather than by natural selection.

  • (3) The feature may have evolved not because it conferred an adaptive advantage, but because it was correlated with another feature that did.

  • (4) A character state may be a consequence of phylogenetic history.

• Several methods for inferring if a feature is an adaption for some function:

  • Complexity

    • A function is often adaptive if it is complex because complexity cannot evolve except by natural selection.

  • Design

    • The function of a character is often inferred from its correspondence with the design and engineer might use to accomplish some tasks or with the predictions of a model about its function.

  • Experiment

    • Experiments may show that a feature enhances survival or reproduction or enhances performance in a way that is likely to increase fitness, relative to individuals with other features.

  • Comparative Method

    • The comparative method is the method for comparing sets of species to pose or test hypotheses on adaption and other evolutionary phenomena.

    • It takes advantage of “natural evolutionary experiments”.

    • If a feature evolves independently in many lineages because of similar selection pressure, we can often infer the function of that feature by determining the ecological or other selective factors with which it is correlated.

    • The hypothetico-deductive method is the method for making a prediction deduced from a hypothesis.

Imperfections and Constraints

• Darwin said, “Natural selection will not produce absolute perfection, nor do we always meet, as far as we can judge, with this high standard in nature”.

• Selection can fix only those genetic variations with higher fitness than other genetic variants in a particular population at a particular time.

• Trade-offs are the existence of both a fitness benefit and a fitness cost of a mutation or character state, relative to another.

Natural Selection and the Evolution of Diversity

• Any characteristic of an organism is likely to be advantageous under some circumstances but not others.

• Character displacement is the divergence of species as a consequence of their interaction.

What Not to Expect from Natural Selection

• Selection at the level of genes and individual organisms is inherently “selfish”: the gene or genotype with the highest rate of increase spreads at the expense of others.

• Darwin said, “Natural selection cannot possibly produce any modification in a species exclusively for the good of another species”.