Neo-Darwinism
As was stated earlier, “New” or Neo-Darwinism is a restatement of the concepts of evolution by natural selection in terms of Mendelian and post-Mendelian genetics.
Neo-Darwinism looks at:
1. Mutations as changes that are due to chance, but occur with predictable frequency.
2. Variations in populations are due to recombination of alleles.
3. Adaptations (or micro-evolutionary steps) may occur as a result of an allele frequency in a population’s gene pool.
- Evolution of one species into another species involves the accumulation of the advantageous alleles in a gene pool.
- The process of speciation
4. The pace of evolution is controlled by gradualism and punctuated equilibrium
1. Gene Mutations
Mutations are changes to genes or chromosomes due to chance, but with predictable frequencies. Because they happen, it is believed they play a role in evolution.
Some examples we have already mentioned, such as Down’s Syndrome and Klinefelter Syndrome. As a result, we get some variation, due to the mutation.
Another example is the Peppered Moth, which was mentioned earlier. The colour is determined by the alleles present for one gene. Originally there was a balanced polymorphism, or having multiple alleles for a gene in a population, which usually expresses different genotypes. In the case of the moth, the dark allele was rarely present, as it was selected against. As was mentioned previously, the lichens were growing in fewer numbers, and as a result the bark on trees became darker. The selection then favoured the dark species, and the dark allele in the species increased (transient polymorphism). Since the air pollution decreased, the light coloured morph and the allele for the light colour now increases in the population.
Another example is PKU or phenylketouria. It is a genetic disease caused by the presence of a homozygous recessive allele. A PKU individual cannot produce a certain enzyme to break down phenylalanine to tyrosine. Phenylalanine levels build up which are harmful to the brain. This can lead to brain damage. Once they eat a diet with little phenylalanine, they can eat normally.
The above examples show that, at some point in time, the normal allele mutated and a new allele was created. The new allele was not favourable but some individuals passed on the allele.
The moth example can be summarized below:
A contrasting case of polymorphism is the sickle cell trait, the product of a gene mutation. Here, the gene that codes for the amino acid sequence of one of the components ofthe haemoglobin molecule is prone to a base substitution which triggers the substitution of one amino acid in the protein chain. The effect on the haemoglobin molecule is to cause clumping of the molecules in the red cell, producing sickle-shaped red cells. In this condition, the cells transport little oxygen and may even block smaller vessels.
People who are heterozygous for the condition have less than 50% sickle haemoglobin. The person is said to have sickle cell trait, and they are only mildly anaemic. There is an advantage in having sickle cell trait where malaria is prevalent. The malarial parasite completes its life cycle in red cells, but it cannot do so in sickle cells. People with sickle cell trait are protected to a significant extent. Where malaria is endemic in Africa, possession of one mutant gene (the person is heterozygous and has sickle cell trait, not full anaemia) is advantageous. Under conditions of ‘survival of the fittest’, this allele is consequently selected for.
Fewer of the alleles for normal haemoglobin are carried into the next generation. Because of this selective advantage, the sickle cell condition is an example of balanced polymorphism – the stable co-existence of two (or more) distinct types of individual in a species (or population). The proportion of both alleles is maintained by natural selection.
2. Variation
We know from our Genetics unit that homologous chromosomes pair up during meiosis and then cross over. With 3 chromosomes, the possible gametes are 8 (23 = 8). This allows for variation.
The changes caused by variation are said to be non-directional, because every change has an equal chance of occurring. If and when the change is made, the environment determines if the change is beneficial or not. If it is beneficial, the individual will live to pass on its genes, thus increasing the percentage of the allele in the population or gene pool.
3. Allele Frequency and the Gene Pool (Microevolution)
Present-day flora and fauna have arisen by change from pre-existing forms of life. Most biologists believe this. This process has been variously called ‘descent with modification’, ‘organic evolution’, and ‘microevolution’, but perhaps speciation is appropriate here because it emphasizes that species change.
A species is a group of organisms of common ancestry that closely resemble each other structurally and biochemically, and which are members of natural populations that are actually or potentially capable of breeding with each other to produce fertile offspring, and which do not interbreed with members of other species.
The last part of this definition cannot be applied to self-fertilizing populations or to organisms that reproduce only asexually. Such groups are species because they look very similar (morphologically similar). They behave and respond in similar ways, with bodies that function similarly (they are physiologically similar).
But however we define the term, since species may change with time (mostly a slow process), there is a time when the differences between members of a species become significant enough to identify separate varieties or subspecies. Eventually these may become new species. All these points are a matter of judgement.
In nature, organisms occur in local populations. Therefore, we can look to local populations as the venue for evolution.
A population is a group of individuals of a species, living close together, and able to interbreed.
So a population of garden snails might occupy a small part of a garden, say around a compost heap. A population of thrushes (snail-eating birds) might occupy several gardens and surrounding fields. In other words, the area occupied by a population depends on the size of the organism and on how mobile it is, for example, as well as on environmental factors (e.g. food supply, predation, etc.).
The boundaries of a population may be hard to define. Some populations are fully open, with individuals moving in or out, from nearby populations. Alternatively, some populations are more or less closed – that is, isolated communities almost completely cut off from neighbours of the same species. Obviously, the fish found in small lakes are a good example of the latter.
Population genetics is the study of genes in populations. In any population, the total of the alleles of the genes located in the reproductive cells of the individuals make up a gene pool.
A gene pool consists of all the genes and their different alleles, present in an interbreeding population.
When breeding between members of a population occurs, a sample of the alleles of the gene pool will contribute to the genomes (gene sets of individuals) of the next generation, and so on, from generation to generation. Remember, an allele is one of a number of alternative forms of a gene that can occupy a given locus on a chromosome. The frequency with which any particular allele occurs in a given population will vary.
Allele frequency is the commonness of the occurrence of any particular allele in a population.
When allele frequencies of a particular population are investigated they may turn out to be static and unchanging. Alternatively, we may find allele frequencies changing. They might do so quite rapidly with succeeding generations, for example.
When the allele frequencies of a gene pool remain more or less unchanged, then we know that population is static as regards its inherited characteristics. We can say that the population is not evolving.
However, if the allele frequencies of the gene pool of a population are changing (the proportions of particular alleles are altered – we say they are disturbed), then we may assume that evolution is going on. For example, some alleles may be increasing in frequency because of an advantage they confer to the individuals carrying them. With possession of those alleles, the organism is more successful. It may produce more offspring, for example. If we can detect change in a gene pool we may detect evolution happening, possibly even well before a new species is observed. The Hardy–Weinberg formula may be used to detect change in allele frequency, in practical situations. We will be going over this next.
Speciation
Speciation, the evolution of new species, requires that allele frequencies change with time in populations.
Some of the processes known to bring about significant change, leading to the eventual appearance of a local population of organisms that are a new species, unable to breed successfully with members of the population from which they originated are due to isolation.
Speciation by isolation
A step towards speciation may be when a local population becomes isolated from the main bulk of the population, so the local gene pool is completely cut off and permanently isolated. The result is reproductive isolation within the original population. Even when reproductive isolation has occurred, many generations may elapse before the composition of the gene pool has changed sufficiently to allow us to call the new individuals a different species.
However it does happen, and isolation that is effective in leading to genetic change can occur in space (geographical isolation), time (temporal isolation) and as a product of behaviour (behavioural isolation).
A. Geographical isolation
This is the consequence of the development of a barrier within a local population. Today, both natural and human-imposed barriers can occur abruptly, sharply restricting movement of individuals (or their spores and gametes, in the case of plants) between divided populations.
Before separation, individuals shared a common gene pool, but after isolation, ‘disturbing processes’ like natural selection, mutation and random genetic drift may trigger change.
Genetic drift is random change in gene frequency in small isolated populations.
For example, a new population may form from a tiny sample that became isolated and separated from a much larger population. While numbers in the new population may rapidly increase, the gene pool from which they formed might have been totally unrepresentative of the original, with many alleles lost altogether.
The outcome of these processes may be marked divergence between populations, leading to their having distinctly different characteristics.
Geographic isolation also arises when motile or mobile species are dispersed to isolated habitats – as, for example, when organisms are accidentally rafted from mainland territories to distant islands. The 2004 tsunami generated examples of this in Southeast Asia. Violent events of this type have surprisingly frequently punctuated world geological history.
Another example would be in the Galapagos Islands. The iguana lizard here had no mammal competition when it arrived on the Galapagos. It became the dominant form of vertebrate life, and was extremely abundant when Darwin visited.
By then two species were present, one terrestrial and the other fully adapted to marine life. The latter is assumed to have evolved locally as a result of pressure from overcrowding and competition for food on the islands (both species are vegetarian) driving some members of the population out of the terrestrial habitat.
B. Temporal isolation
This is illustrated when two very closely related species occupy the same habitat and differ only in the time of year that they complete their life cycles. Reproductive isolation may develop in this situation within a local population so that some members produce gametes at distinctly different times of the year from others; thus, two distinctive gene pools start to evolve.
Examples of the outcome of temporal isolation include two members of the genus Pinus found in Californian forests.
C. Behavioural isolation
This type of isolation results when members of a population acquire distinctive behaviour routines in their growth and development, courtship or mating process that are not matched by all individuals of the same species.
An example occurs in the imprinting behaviour of the young of geese, swans and other birds. When chicks of these species hatch out of the egg, the adult birds are in the vicinity, caring for them. The young imprint the image of their parents as they relate to and learn from them. They associate socially only with their own species (or variety), and as adults, they will eventually only bond with and breed with their own species.
Imprinting became apparent when a goose chick, on hatching, was placed with swan adults as parents. The goose, when an adult, bred with a swan, and the offspring was an infertile ‘Gwan’. Clearly, the swan and goose are related species that have evolved apart for long enough for their progeny to be infertile, but not long enough to exclude the formation of a hybrid. (Konrad Lorenz)
Other examples of behavioural isolation are demonstrated by closely related species of fish, including in guppies (Poecilia spp.) with different, distinctive body markings by which pairs select their mates, and in four species of gull of the Canadian arctic (Larus spp.) with distinctive plumage by which they are identified during breeding periods.
In summary, species do not evolve in a simple or rapid way. The process is usually gradual, taking place over a long period of time. In fact, in many cases speciation may occur over several thousand years. Complex though it is, we can recognize that all cases of speciation require ‘isolation’.
A deme is the name we give to a small, isolated population.
The individuals of a deme are not exactly alike, but they resemble one another more closely than they resemble members of other demes. This similarity is to be expected, partly because the members are closely related genetically (similar genotypes), and partly because they experience
the same environmental conditions (which affect their phenotype).
The ways demes become isolated have been discussed already. Reviewing these, we see they fall into two groups, depending on the way isolation is brought about.
· Isolating mechanisms that involve special separation are known as allopatric speciation (literally ‘different country’).