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UNIT 4: EVOLUTIONARY MECHANISMS AND
PROCESSES
Darwinian Theory of Evolution has six different hypotheses. Two of them are generally accepted (the existence of evolution and the common ancestor), but the rest have provoked a lot of controversy.
In this unit we will see the different orthodox and unorthodox views of evolutionary change. However, it is very common to find mixed versions between both types of views when taking about different characteristics:
- Tempo and mode of evolution
- Speciation
- Natural selection and other possible agents
- Adaptive nature of an organism
- Relation between an agent and its environment
- Modes of inheritance
4.1. MODE AND TEMPO OF EVOLUTIONARY CHANGE
4.4.1. Mode of evolutionary change
It refers to the way in which a new species appear. There are two different modes:
- Anagenesis (phyletic evolution): process of evolution within a lineage (no branches out). This theory is supported by the orthodox view of evolution.
- Cladogenesis (speciation): the process of evolution that results from the splitting of a linage creating new species. It is supported by both orthodox and heterodox views of evolution.
4.1.2. Tempo of evolutionary change The rhythm or tempo of evolutionary change can be gradual or intermittent:
- Gradualism: the evolutionary change is constant and takes place at roughly the same pace during anagenesis as during cladogenesis. It is supported by the orthodox view of evolution. They are known as gradualists.
- Punctualism: the evolutionary change is episodic-like and far faster during cladogenesis than during anagenesis. It is supported by the heterodox view of evolution and they are known as punctualists.
- Premating barriers: features that impede transfer of gametes to member of other species. We have two: a. Ecological isolation. Populations breed at different seasons or times of the day. b. Ethological isolation. Potential mates meet but do not mate.
- Postmating, prezygotic barriers: mating or gamete transfer occurs, but zygotes are not formed. a. Mechanical isolation. Copulation occurs, but no transfer of male gametes takes place because of failure of mechanical fit of reproductive structures. b. Copulatory behavioral isolation. Failure of fertilization because of behavior during copulation or because genitalia fail to stimulate properly. c. Gametic isolation. Failure of proper transfer of gametes; individuals do come into contact but the gametes are not compatible.
- Postzygotic barriers: hybrid zygotes are formed but have reduced fitness a. Extrinsic. Hybrids are less successful than parent species with ecological or ethological competences b. Intrinsic. Hybrids fitness is low because of problems that are independent of environmental context.
4.2.3. Modes of speciation Speciation is the evolutionary process that leads to the appearance of a new species. There are three different modes of speciation:
- Allopatric speciation: populations are geographically isolated. There are two types: vicariance (symmetric subpopulations) and peripatric (asymmetric subpopulations.
- Parapatric speciation: populations are geographically continuous but there is an area that difficult and reduce the gene flow between members of the species.
- Sympatric speciation: within the geographic range of a population. Individuals tend to mate with those with same preferences and genetic and physical similarities.
4.3. AGENTS OF EVOLUTION
There are several processes that can act as evolutionary change agents.
- (^) Migration (gene flow): increases inter-population genetic and phenotypic similarity
- Non-random mating: changes the frequency of different genotypes, but does not alter allele frequencies.
- Mutation: creates new alleles, increases both genetic and phenotypic variability.
- Genetic drift: changes the frequency of alleles randomly; can eliminate alleles
- Natural selection: increases the frequency of adaptive alleles (eliminates maladaptive alleles), produces adaptations.
NATURAL SELECTION: -It is differential survival or
4.4. FEATURES’ NATURE
4.4.1. Adaptation An adaptation is any trait that enhances an individual fitness and that is variable within a population. It has been designed by natural selection to accomplish such a fitness- enhancing function.
4.4.2. Aptation, exaptation, pre-aptation and spandrel An aptation is any trait that currently increases the carrier’s fitness. There are two types of aptatios: ▲ Adaptation: trait that has evolved by natural selection, that has all along been functional (confers a selective fitness advantage to its carrier), and whose current function coincides with the original function that drove its evolution in the first place. ▲ Exaptation: trait whose current function has been co-opted later in evolution. ✓ Pre-aptation: trait that has evolved by natural selection and that has all along been functional (confers a selective fitness advantage to its carrier). ✓ Spandrel: trait that did NOT evolved by natural selection, but has been co-opted later in evolution to fulfill an adaptive function. The trait was originally non-adaptive, then it was co-opted to be used, in a new context.
4.4.3. Why a lot of traits are not adaptations? The genetic drift plays an important role with those numerous traits that are not adaptations (they are neutral or even harmful). The causes that have produced these traits can be classified in the following categories:
- Coevolution (arms race [unit 1])
- Structural constraints: those that predicts an asymmetrical relation between the size of an individual and the size of its organs. Examples: brain, chin…
- Developmental constraints: those traits related with genetics. Here we can find the pleiotropy (when a gene has more than one phenotypic effect, that can be positive, neutral or negative) and the epistasis (several genes (and their phenotypic effects) tend to be inherited together, even when some of them are neutral or harmful).
- 1ST^ law: the general features of a higher taxon appear earlier than do the specialized features of a lower taxon. Example: vertebrate embryos are very similar between them just after the gastrulation
- 2nd^ law: less general characters develop from the more general; until finally the most specialized appear. Example: at first, all vertebrates have the same skin; later, they develop scales, feathers, hair…
- 3 rd^ law: instead of passing through the adult stages of lower animals, the embryo of a given species departs more and more from them. Example: branchial cleft are different in birds/mammals and adult fish.
- 4 th^ law: the early embryo of a higher animal is never like a lower animal, but only like its early embryo. Example: human embryo doesn’t go through an equivalent state to the fish or the bird ones.
de Beer said that a change in the developing rhythm in some parts of our organism can lead to phenotypic innovations (precursor of evolutionary change). Goldschmidt said that evolutionary change affects specifically to the early stages of development. 4.5.3. Macro-evolutionary change’s mechanisms Francois Jacob said that evolution doesn’t create new genes; it just “play” with those that we already have. This play (tinkering) can happen throughout four different mechanisms:
- Heterotopy: change in location where the gene is expressed. Example: presence or absence of membranes in ducks and hens’ legs.
- Heterometry: change in amount expressed by a gene that affects the size of a structure. Example: different sizes of the birds’ beak.
- Heterotypy: change in kind of the gene (a change in the functional properties of the protein that is going to be synthetized). Example: within arthropods, only the insects have three pair of legs.
- Heterochrony: change in time in the development of two or more characters. Example: somatic development and sexual development.
