BIOSCI101 Evolution - Lecture 5

NATURAL SELECTION Natural selection is the process in which [blank_start]individuals[blank_end] with certain [blank_start]heritable[blank_end] traits survive and reproduce at a [blank_start]higher[blank_end] rate than those without said traits.

Which of these statements is FALSE with regards to natural selection?

If an individual is physically strong and lives a long life but has no offspring, it is still sit in a Darwinian sense.

FITNESS Darwinian fitness refers to the ability to [blank_start]survive[blank_end] and reproduce, it considers both [blank_start]viability[blank_end] and [blank_start]fecundity[blank_end]. Fitness depends on the [blank_start]environment[blank_end], the fittest individual during an ice age is probably not the fittest individual once the ice age is over. [blank_start]Absolute fitness (W)[blank_end] is a measure of the total number of offspring an individual produces. [blank_start]Relative fitness (w)[blank_end] is the contribution an individual makes to the gene pool of the next generation relative to the contribution of other individuals. Although it is often referred to as the relative fitness of a genotype, it is the [blank_start]whole organism[blank_end] that is subject to selection not the [blank_start]genotype[blank_end].

CALCULATING RELATIVE FITNESS Relative fitness is the absolute fitness of an individual relative to the absolute fitness of the fittest individual in the population (w), w can range from 0 to 1.

MAJOR TYPES OF NATURAL SELECTION Natural selection can change the frequency distribution of a heritable trait in three ways: 1. [blank_start]Directional selection[blank_end] occurs when one of the extremes is favoured, driving the population in a particular direction. Directional selection is common when an environment changes or when individuals move to a new environment. 2. [blank_start]Disruptive selection[blank_end] occurs when the extreme phenotypes are selected at the expense of intermediate forms. 3. [blank_start]Stabilising selection[blank_end] acts against both extreme phenotypes and favours intermediates.

Match the type of natural selection with the corresponding graph.

A selective sweep occurs when a favourable allele reaches fixation.

SEXUAL SELECTION Darwin described a second form of selection - [blank_start]sexual[blank_end] selection. This form of selection is the result of competition between individuals for [blank_start]mates[blank_end]. Sexual selection is argued to result in sexual di[blank_start]morph[blank_end]ism in some species.

Sexual selection can operate in two ways: 1. [blank_start]Intrasexual[blank_end] selection: individuals of the same sex competing directly, and is thought to occur between males in many species. 2. [blank_start]Intersexual[blank_end] selection: or mate choice, occurs between different sexes, and is typically argued to occur when females select males.

HETEROZYGOTE ADVANTAGE Heterozygote advantage occurs when individuals that are [blank_start]heterozygous[blank_end] have a greater fitness than do either kind of [blank_start]homozygote[blank_end].

One example of heterozygote advantage is sickle-cell anaemia, in which heterozygotes are protected against the severe effects of malaria. Although sickle-cell disease is lethal if not treated in some areas the sickle-cell allele has a frequency of 15-20%.

FREQUENCY DEPENDENT SELECTION In frequency-dependent selection, the fitness of a phenotype depends on how [blank_start]common[blank_end] it is in a population.

WHY NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS Natural selection can lead to adaptation but there are many non-adaptive traits. 1. Selection can only act on [blank_start]existing variation[blank_end]. New advantageous alleles [blank_start]do not arise on demand[blank_end]. 2. Evolution is limited by [blank_start]historical constraints[blank_end]. Evolution co-opts existing structures and adapts them to [blank_start]new situations[blank_end] e.g. birds and bats have not grown new limbs to serve as wings but have modified the forelimb. 3. Adaptations are often [blank_start]compromises[blank_end]. Each organism must do many things, that is, there can be many competing selective forces operating at the same time [blank_start]e.g. sexual verses natural selection[blank_end]. 4. Not all evolutionary change is [blank_start]adaptive[blank_end]. There are forces acting in evolution that do not result in adaptation [blank_start]e.g. genetic drift[blank_end]. 5. Chance, natural selection and the environment interact. Chance events can affect the evolution of a population. For example, not all the alleles in a [blank_start]founder population[blank_end] are better suited to the new environment than the alleles ‘left behind’. Environments may [blank_start]change rapidly and unpredictably[blank_end] limiting the extent to which natural selection can result in adaptation.

ADAPTATION AND EXAPTATION [blank_start]Adaptations[blank_end] are characteristics or traits that have evolved by natural selection for their current function. [blank_start]Exaptations[blank_end] are the complement of adaptations, as they are traits that have been co-opted or enlisted for a new use.

New characteristics or traits cannot arise when structures that originally had one function are used for a different function.

Exaptations demonstrate that a characteristic or traits current function does not always explain its origin.

Some biologists believe that exaptation may play a much more important role in evolution than has generally been thought.