Sunday, June 26, 2011


Humans come in different shapes, sizes, colours, temperaments, propensities to disease and the like.  Although the human population continues to increase, we seem to never run out of variations on the human ‘type’.  All of the dog breeds of the world ultimately descend from the same wolf ancestor.  The bulldog, the terrier, the great Dane, the German shepherd, were all produced when humans took existing varieties and made some choice selections.  The same goes for our agricultural products: kale, cabbage, broccoli and cauliflower all descend from the same wild cabbage species.  The fruit fly Drosophila pseudoobscura, according to Dobzhansky, has seven different types of Y chromosomes; there are strains of wheat with 7, 14 or 21 chromosomes.

Without variation, there can be no evolution.  Dobzhansky spends the first four chapters of his book driving this point home, because it is that essential.  If variation was not continually being produced, there would be no differences between individuals; without differences between individuals, there would be no selection; and without selection, there would be no adaptation.
As we have seen over the past few weeks, mutations are the ultimate source of all of the variation that surrounds us.  There are several types of mutations:

1. Genic mutations, which involve small-scale changes to the chemical makeup of genes

2. Chromosomal mutations, which involve large-scale changes to regions of genes or to chromosome sets.

These mutations, in turn, have two primary causes:

1. Environment à Radiation and chemical mutagens can induce both genic and chromosomal mutations.  We saw this with X-ray subjected fruit flies; we experience this every time we get a tan (the sun’s rays damage or ‘mutate’ our skin cells’ DNA, which our body must repair.  People with malfunctioning repair mechanisms suffer from sunlight-induced skin tumours).

2. Mutations are commonly caused by normal everyday bodily processes.  Errors during meiosis, crossing-over, and DNA replication are large sources of new mutations.

These mutations, as we have seen, can be good, bad, or neutral.  Neutral mutations and recessive mutations can hide from selection, providing a large source of raw genetic material that is at the ready for environmental change.  Chromosomal mutations, likewise, can free a gene from selection, allowing it to accumulate mutations and conceivably produce something with a new function.  

There is a fish known as the Antarctic eelpout, for example, that survives in highly saline waters that can have temperatures below freezing.  For most fish, this is an inhospitable habitat, as water molecules in the body form ice crystals that essentially freeze the fish.  These eelpout are able to survive because they produce a suite of antifreeze proteins that bind to ice crystals and inhibit ice formation.  A December 2010 paper in PNAS reported the discovery that one of these antifreeze proteins evolved via gene duplication.  A gene for sialic acid synthase was duplicated, freed from the constraints of selection, mutated, and produced a useful protein.  In this case, then, gene duplication was partly responsible for this fish invading a new habitat.

The Antarctic eelpout can survive in remarkably cold waters, due to a gene duplication

Mutations are the basis for all variation, but there are lower-level ways in which existing mutations can be rearranged to form new varieties:

1.  Sex à First, sex obviously brings together two different individuals with potentially different traits, and combines these traits in their offspring.  But there is more to it than that.  During meiosis, eggs/sperm/pollen are formed with only one copy of each chromosome type, but whether a particular gamete receives your mother’s or your father’s version of the chromosome is completely random.  This is known as independent assortment, and is believed to be one of the evolutionary significances of sex.  You can see why through a hypothetical scenario.  Imagine you are an organism with only two chromosome types, four chromosomes in all.  From your mother you inherited an allele for purple eyes and an allele for body hair.  From your father you inherited an allele for yellow eyes and for excessive body hair.  Let’s imagine that these two traits, eye colour and body hair, are found on different chromosomes.  Now, there were two original varieties: the purple eye/normal hair variety, and the yellow eye/hairy variety.  During meiosis, the purple eye and yellow eye chromosomes pair up, and the normal hair and excessively hairy chromosomes pair up, and then the pairs are split.  Now, they could be split in such a way that the mom’s chromosomes go to one gamete and the dad’s to the other, thereby reforming the original two varieties.  But it does not have to be this way.  Which chromosome goes to which gamete is random.  Thus purple eye/excessively hairy and yellow eye/normal hair will also be produced.  These are two new varieties.  Now imagine that we are dealing with 23 chromosome pairs, and you can see how readily new combinations of pre-existing traits can be formed.  In the same way, then, new mutations that arise in sexually-reproducing organisms can be constantly tested against new genetic backgrounds, without the need for more mutations to arise.  Let’s say a mutation arises that produces no hair at all, and let’s say it occurs in a purple-eyed individual.  It is possible that the combination purple eyes/no hair is deleterious.  But if this individual is able to produce offspring, new combinations may be formed through independent assortment, testing the no-hair mutation against other eye colours.  Perhaps one combination will end up being neutral or beneficial. 

The basics of independent assortment during meiosis

2. Recombination/crossing-over à Before homologous chromosomes split during meiosis, they are temporarily brought together, and sometimes they undergo crossing-over (which I described in my last post).  Alleles from your mom swap places with alleles from your dad, producing a maternal chromosome with some of dad’s genetic information, and a paternal chromosome with some of mom’s.  This too produces new genetic combinations, increasing the amount of variation that is possible.  In the above example, imagine that eye colour and ear size are found on the same chromosome, with the maternal chromosome being purple eyes/large ears and the paternal being yellow eyes/small ears.  During crossing over, the result could be a purple eyes/small ears chromosome and a yellow eyes/large ears chromosome.  

3. Inbreeding à Recessive mutations can easily hide in a population and rarely produce a phenotype.  Inbreeding unmasks these mutations.  Phenotypes that are remarkably rare can be expressed at higher rates within inbred lines, giving these mutations a chance to ‘prove’ their worth.   Most mutations fail miserably, which is why inbred dog breeds tend to have so many problems, but the unmasking of recessive mutations along different inbred lines can test these mutations against different genetic backgrounds.

4. Migration à Migration allows a mutation that arose in one population to work its way into another population.  A mutation that might have been negative in population A, when brought by migrants to population B, may find itself exposed to a new environment and/or a new genetic background in which it is favourable.  Or vice versa. 

5. Hybridization à Although successful hybridization breaks down species’ distinctions, it increases variability, potentially in a more dramatic way than migration does, by combining the genetic differences of two populations into one.  Never-before-seen combinations of traits are the result.  Think of the dramatic example of the cabbage/radish hybrid, for instance, which has the head of a radish and the roots of a cabbage.

Example of a new variety: horse/zebra hybrid

6. Horizontal gene transfer à This is an important but under recognized source of genetic variation.  Normally we think of vertical gene transfer, in which parents pass on their traits to their offspring through sexual or asexual means.  This is usually visualized as a tree of life, with descendants branching from ancestors.  But sometimes members of completely different species can exchange genetic information without needing to reproduce.  This is a broad and interesting category, and I would like to give four examples:
                a. DNA repair in rotifers à Rotifers are small, nearly microscopic animals that live in water.  Whenever their environment dries up, one group of asexual rotifers can remain dormant until the water returns.  In the process, however, their DNA gets badly damaged; when they awaken from dormancy, they have proteins that can put their broken DNA back together.  But if, in the surrounding water, there happens to be other bits of DNA floating around, the proteins grab these and stick them into the rotifer’s genome as well.  Rotifers have been found that have genes from E. coli, fungi and other organisms.  This is a remarkable way of making large evolutionary leaps.
                b. Bacterial plasmids à Bacteria have small bits of circular DNA that they can pass on to other bacteria, but they are often not species-specific.  Thus one bacterium can inherit DNA from a completely different bacterial species.  This is of practical importance if the plasmid confers resistance to antibiotics; such resistance can quickly spread among different bacterial species.

Bacterial plasmids can be transferred between species

                c. Endogenous retroviruses à Certain viruses splice their genetic material into the genome of the host cell.  Usually they end up in body cells, but sometimes they end up in gametes, getting passed down to the host's offspring.  Normally these viruses are infective, but in these rare instances they rapidly accumulate mutations during DNA replication, inactivating the virus.  And thus they remain, in some cases for millions of years, as a part of the host’s genome.  Not all of our DNA is our own – approximately 8% of the human genome is composed of such fossil viruses.  One type, which entered our ancestors after they split from chimpanzees, is currently still active, and has been potentially implicated in schizophrenia.
                d. Endosymbiosis à Many organisms live inside of other organisms without being parasitic.  In fact, they may mutually benefit one another.  There is a good deal of evidence to suggest that chloroplasts in plants and mitochondria in plants and animals were, at one point, bacteria.  Mitochondria are the power-generators of our cells; chloroplasts are the photosynthetic factories of a plant.  Both of these organelles contain their own circular DNA, much like the DNA of a bacterium, and they divide independently of the cell through a process similar to bacterial division.  The story goes that in our ancient past, when our ancestors were single-celled, one such ancestor swallowed but did not digest a bacterium.  That bacterium produced energy for the cell, while using the cell’s waste products for nutrients.  Over long spans of time, this endosymbiotic relationship became so important that the bacterium’s progeny lost their independence and became a part of our ancestor.  Today, then, we all have the remnants of a bacterial species living inside of our cells, producing the energy we need to survive.  This story might sound far-fetched, but the evidence is compelling.  We can even see examples of it occurring today, such as in algal-coral relationships, or in Canadian salamanders that get some of their energy from algal cells that live in their embryos.

Algae were recently found living inside of developing salamander embryos in Canada
6. Phenotypic plasticity à A change in the environment may produce a new, never-before-seen phenotype, which may or may not be beneficial.

A classic example of phenotypic plasticity.  These two water fleas are genetically identical; the one on the left was raised with predators, the one on the right in the absence of predators.  The predatory environment triggered the production of longer spines and body armour.

7.  Alternative splicing à Mutations may cause the same RNA molecule to get processed in different ways, such that a single gene can produce several related but different proteins.

All of these are ways of producing new varieties, but really you can think of them as different ways of rearranging mutations.  Without mutations, none of these could be possible. 

So, clearly there is plenty of variation in nature.  What next?  Says Dobzhansky:

‘These variations may be compared with building materials, but the presence of an unlimited supply of materials does not in itself give assurance that a building is going to be constructed.  The impact of mutation tends to increase variability.  Mutations and chromosomal changes are constantly arising at a finite rate, presumably in all organisms.  But in nature we do not find a single greatly variable population of living beings which becomes more and more variable as time goes on; instead, the organic world is segregated into more than a million separate species, each of which possesses its own limited supply of variability which it does not share with the others.  A change of the species from one state to the other, or a differentiation of a single variable population into separate ones, the origin of species in the strict sense of the word, constitutes a problem which is logically distinct from that of the origin of hereditary variation.’

Variation alone does not a species make.  Next week we will start talking about selection.

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