Enlighten Biology Note on Speciation and Artificial Selection.
Speciation
Speciation is the formation of new species through the process of evolution.
The organisms belonging to the new species will no longer be able to interbreed to produce fertile offspring with organisms belonging to the original species.
Several events that happen leading to speciation:
- Members of a population become isolated and no longer interbreed with the rest of the population resulting in no gene flow between the two groups.
- Alleles within the groups continue to undergo random mutations.
- The environment of each group may be different or change (resulting in different selection pressures) so different characteristics will be selected for and against.
- The accumulation of mutations and changes in allele frequencies over many generations eventually lead to large changes in phenotype.
- The members of the different populations become so different that they are no longer able to interbreed (to produce fertile offspring).
- They are now reproductively isolated and are different species.
Allopatric speciation
Allopatric speciation is the more common form of speciation and happens when some members of a population are separated from the rest of the group by a physical barrier such as a river or the sea they are geographically isolated.
The environments of the different groups will often be different and so will the selection pressures result in different physical adaptations.
Separation of a small group will often result in the founder effect leading to genetic drift further enhancing
A famous example of allopatric speciation is the finches inhabiting the Galapagos Islands located in the Pacific Ocean off the coast of South America.
For about two million years, small groups of finches, from an original population on the mainland, have flown to, and been stranded on, different islands.
The finches, separated from finches on other islands and the mainland by the sea, have formed new colonies on the different islands.
The finches have evolved and adapted to the different environments, particularly food sources, present on the islands and are an example of adaptive radiation where rapid organism diversification takes place.
As the finches are unable to breed with each other, new species have evolved with unique beaks adapted to the type of food available.
Some species have large, blunt beaks that can crack nuts, some have long, thin beaks for getting to the nectar in flowers, and some have medium-sized beaks that are ideal for catching insects.
The honeycreepers (family Drepanidinae) of the islands of Hawaii are birds that are an even larger example of adaptive radiation.
A single ancestor species has led to the evolution of at least 54 species that have filled every available niche in the different islands.
Panama is a narrow strip of land (isthmus) that joins North and South America and separates the Atlantic and Pacific oceans, and was formed about three million years ago.
This was due to the movement of tectonic plates and resulted in the separation of the organisms that had originally occupied the same habitat when the two oceans were joined.
There were originally about 15 species of snapping shrimp present, now there are 15 species present on one side of the isthmus and 15 different species present on the other side.
Although the shrimp from either side appear to be identical if males and females are mixed, they will snap at each other rather than mate.
In 1995 15 iguanas, Iguana iguana, survived a hurricane in the Caribbean on a raft of uprooted trees.
They eventually reached the Caribbean Island of Anguilla.
These iguanas were the first of their species to reach the island.
If these iguanas are successful in colonizing the island it could be the start of an allopatric speciation.
Of course, it could be thousands, if not millions, of years before this is known.
Sympatric speciation
Sympatric speciation occurs within populations that share the same habitat.
It happens less frequently than allopatric speciation and is more common in plants than animals.
It can occur when members of two different species interbreed and form fertile offspring this often happens in plants.
The hybrid formed, which is a new species, will have a different number of chromosomes to either parent and may no longer be able to interbreed with members of either parent population.
This slops gene flow and reproductively isolates the hybrid organisms.
Examples of sympatric speciation include fungus-farming ants and blind mole rats.
Fungus-farming ants cultivate the growth of fungi, which is their source of nutrition, by supplying organic material to keep the fungi growing.
Parasitic ants have been found in one colony of these industrious ants.
Instead of helping in the growth of this fungus these parasitic ants spend their time eating the fungi and reproducing.
The parasitic ants are sometimes ignored and at other times attacked and killed.
Genetic analysis has shown that although genetically different from the fungus-farming ants the parasitic ants are, in fact, their descendants.
They are not a species of ant that has evolved in geographic isolation but within the same habitat as a result of a behavior change.
It is believed that the genetic division of the original species of ant only happened 37,000 years ago, not long in evolutionary terms.
Blind mole rats live in a small area of northern Israel that is part igneous basalt rock and part chalk bedrock.
The different types of soils formed above the bedrock support a different range of plants.
Blind mole rats found in both types of soil are sometimes only separated by a few meters of loose soil.
Mole rats will only interbreed with mole rats living in the same type of soil.
DNA analysis has shown that the lack of gene flow between the two species is already resulting in genetic differences even though members of the different groups often come into contact with each other as there is no physical barrier.
Over time the genetic differences could accumulate to the point that the mole rats from different soil types will no longer be able to interbreed and they will be separate species.
Plants cross with plants of different species forming hybrids much more frequently than animals.
The indiscriminate release of large numbers of pollen grains by plants is one reason for this.
The hybrids arc reproductively isolated from each parent species but could still be present in the same habitat.
The evolution of modern wheat has involved at least two hybridization events and the formation of new species along the way.
Disruptive selection, mating preferences, and other behavioral differences can all result in individuals or small groups becoming reproductively isolated.
They will, however, still be living in the same habitat so gene flow, even if reduced, often interferes with the process of speciation.
Reproductive barriers.
Barriers to successful interbreeding can form within populations before or after fertilization has occurred.
Prezygotic reproductive barriers prevent fertilization and the formation of a zygote.
Postzygotic reproductive barriers, often produced as a result of hybridization, reduce the viability or reproductive potential of offspring.
Artificial selection
Populations are usually polymorphic (display more than one distinct phenotype) for most characteristics.
The allele coding for the most common, or normal, characteristic is called the wild-type allele.
Other forms of that allele, resulting from mutations, are called mutants.
Artificial selection (or selective breeding) is fundamentally the same as natural selection except for the nature of the selection pressure applied.
Instead of changes in the environment leading to survival of the fittest, it is the selection for breeding of plants or animals with desirable characteristics by farmers or breeders.
Farmers have been selectively breeding plants and animals since before genes were discovered or the theory of evolution was proposed.
Individuals with the desired characteristics are selected and interbred.
Offspring from this cross showing the best examples of the desired traits arc then selected to breed.
This breeding of closely related individuals is called inbreeding.
The process is repeated over many generations resulting in changes to the frequency of alleles within the population and eventually speciation.
Brassica oleracea is a wild mustard that has been selectively bred for many centuries producing several common vegetables.
Problems caused by inbreeding
Limiting the gene pool and so decreasing genetic diversity reduces the chances of a population of inbred organisms evolving and adapting to changes in their environment.
Many genetic disorders are caused by recessive alleles, for example, cystic fibrosis, a condition where the digestive system and lungs become clogged with mucus.
Recessive alleles are not uncommon in most populations but two recessive alleles are needed before they are expressed and most individuals will be heterozygous.
Organisms that are closely related are genetically similar and are likely to have the same recessive alleles.
The breeding of closely related organisms therefore results in offspring that have a greater chance of being homozygous for these recessive traits and being affected by genetic disorders.
Over time this reduces the ability of these organisms to survive and reproduce.
This results in the organisms being less biologically fit in other words, less likely to survive and produce two surviving offspring to replace themselves.
Gene banks
Seed banks keep samples of seeds from both wild-type and domesticated varieties.
They are an important genetic resource.
Gene banks store biological samples, other than seeds, such as sperm or eggs.
They are usually frozen.
Owing to the problems caused by inbreeding, alleles from gene banks are used to increase genetic diversity in a process called outbreeding.
Breeding unrelated or distantly related varieties is also a form of outbreeding.
This reduces the occurrence of homozygous recessives and increases the potential to adapt to environmental change.
Pedigree dogs and the ethics of artificial selection.
Domesticated dogs are all members of the same species, Canisfamilioris.
They are another, sometimes controversial, example of how selective breeding has created a variety of different-looking individuals from one wild species.
The wild species was the grey wolf and the process began between 18 000 and 32 000 years ago when humans were still leading a hunter-gatherer lifestyle.
It is thought that wolves started ‘hanging around’ human hunting parties because of the availability of scraps of leftover food.
Over time, the more social wolves would have become integrated into the human groups and so the process of selective breeding began.
The wolves that became integrated were eventually used to help catch animals during the hunts or served a protective role like modern-day guard dogs.
Hunter-gatherers eventually started forming settled communities and began the practice of farming.
The evolving wolves would have had new roles such as herding animals.
Different traits were selected depending on whether they were used for hunting, fighting, herding, or even as status symbols, and so a range of dogs with different characteristics evolved.
Many of those characteristics have been exaggerated by continued selective breeding and are most obvious in pedigree dogs seen today.
Rather than being selected for the role they performed which was not dissimilar to natural selection, they began to be selected for their looks which took no account of any impact on their health.
Interbreeding with wild wolves would have been common which can make tracing the evolution of dogs difficult.
Dachshunds were selected for small size and short legs so that they could follow prey such as foxes and badgers into burrows.
Great Danes were selected for large size and strength for hunting and fighting.
The breeding of pedigree dogs is restricted to the descendants of dogs that were registered by the Kennel Club in 18. 3 after the different types or breeds of dog had been developed by breeders and the standard characteristics of each breed identified.
With the limited gene pool and lack of outbreeding it is inevitable that unwanted traits are selected for also.
- Big dogs often have hip or heart problems.
- The skull of the King Charles spaniel is too small to accommodate the brain comfortably leading to pain and discomfort.
- Bulldogs usually have breathing difficulties due to the shape of their noses.
- Dachshunds often have back problems and suffer from epilepsy.
- Diseased dogs have effectively been deliberately interbred.
There are now moves to change some of the breed descriptions to prevent some of the worst examples of this practice.
Selective breeding has been used for centuries in farming to improve the quality and yield of crops and animal produce.
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