Here is the question posed by the commentator:
Have any evolution researchers investigated the possibility that organisms have built-in mechanisms which facilitate evolution. An obvious example of this is the male/female reproductive system for many organisms. I am wondering if something more subtle or sophisticated exists. For example, could some creatures have the ability to generate new traits in offspring which are likely to be beneficial? This is what I meant by questioning Darwin's theory.
I want to start by defining some terms : natural selection and evolution. In evolutionary theory, evolution is a change in traits, frequencies of alleles (different forms of genes) in a population through successive generations. The key elements for evolution to occur are that it occurs at a population level. There must be a heritable change in a particular trait (behavioral, morphological, etc) or allele frequency over multiple generations. This change then is an outcome of a mechanism or process.
How traits or genes change over time (generations) depends on many aspects of an organism’s ecology. Darwin believed that one mechanism that can effect this type of evolutionary change was natural selection. Natural selection happens when individuals in a population that have particular phenotypes produce survive to reproductive ages at a higher rate than individuals with other phenotypes or if they produce more babies than individuals with other phenotypes. There are many excellent examples of evolution by natural selection - antibiotic resistance, Galapagos finches, peppered moths during the Industrial Revolution in England, and mine-tailing resistance in plants. Although Darwin never believed that all change occurred that way, he did believe that natural selection was the most important. Here is a quote from Darwin,
"I am inclined to suspect that we see, at least in some [cases], variations which are of no service to the species, and which consequently have not been seized on and rendered definite by natural selection.… Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed."
The Origin of Species, 6th ed.
The fluctuating element that Darwin was referring to is genetic drift. As with any good scientist Darwin was aware of his limitations. But you know he was able to describe so many things in such detail without knowing exactly how things were inherited and frankly, that in and of itself is quite amazing. He didn’t know the names or terms but he knew that other mechanisms could result in evolution. Darwin said something intelligent about a lot of what we currently study in evolutionary biology and that’s why evolutionary biologists get all hot and bothered around his birthday.
Today we know there are other processes that can cause a change in trait values or allele frequencies in a population over many generations – genetic drift, mutation, migration. Most biologists today are pluralists because they incorporate all of these processes/mechanisms when thinking about evolution.
To answer the question, I need to provide a little history. What the commentator is describing is known to most of us as Lamarckian inheritance. Jean Baptiste Lamarck (1744-1829) was a biologist who was trying to figure out how things were inherited. He thought traits could be acquired. Environmental change will affect the behavior of an organism, which in turn will lead to a change in the use of a organ or structure. The oft-cited example is the giraffe that finds itself in an environment where the food it wants is high in the tree and so during its lifespan, it stretches out its neck and as a consequence of repeated stretching this one giraffe gets a long neck. This long neck is passed on to the giraffe’s offspring.
We now know, because of the modern evolutionary synthesis by R.A. Fisher, Sewall Wright, Theodosius Dobzhansky, Ernst Mayr, Simpson, Stebbins, J.B.S. Haldane and many others, that discrete units called genes underly inheritance, evolution is gradual, populations experience changes in traits/genes when forces like natural selection, drift, etc act upon them across several generations.
That said, what is similar to the commentators description is what is known as epigenetics. The term, epigenetic inheritance, was originally coined by Conrad Waddington in the 1940s as a descriptor of protein by gene development interaction, but is now used to describe changes in a phenotype or gene expression caused by non-DNA sequence variation. These non-DNA sequence changes are things like DNA hypermethylation, istone deactylation, microRNAs or small interfering RNAs and typically affect spatial, temporal and abundance patterns of gene expression.
What’s interesting is that epigenetic alterations are observed after allopolyploidization, extreme stress or extreme selection in plants and can affect traits like flowering time, floral symmetry, flower initiation, organ development. These epigenetic tags can, in fact, be passed down to subsequent generations. How far into the future is still unknown.
This is the best I could do off the top of my head and I hope it answers some of the question. There are two more questions that will have to wait until a different post. The questions deserve answers, however, I think that they deserve more detailed and thoughtful responses. Right now I'm really busy at work so I may not get to them for a while.
Disclaimer: In theory, I have a PhD in Genetics with a specialization in Evolutionary Genetics, but that doesn't make me an expert everything evolutionary. This goes for anyone with a PhD, as far as I'm concerned. The thing I realized after I got my PhD is that oh yes indeed, I am an expert but only in one very very small corner of evolutionary biology. As I progress through my academic career, I realize that the more I learn about something, the less I know about everything. It's in your best interest to read everything with a critical eye and follow up on the references I provide.
For more on Lamarck see:
http://www.ucmp.berkeley.edu/history/lamarck.html
For more on natural selection see Douglas Futuyma's web resource
http://www.actionbioscience.org/evolution/futuyma.html
For more on epigenetics
http://learn.genetics.utah.edu/content/epigenetics/
Knapp and Wendel (2005). Epigenetics and plant evolution. New Phytologist 168:81-91.
Jablonka and Raz (2009). Transgenerational epigenetic inheritance: prevalence, mechanisms, and implications for the study of heredity and evolution. Quarterly Review of Biology 84:131-176.
7 comments:
Thanks-- I learned something.
Have biologists actually observed organisms evolve into completely new creatures, possibly with a different number of chromosomes, as opposed to simply acquiring new traits?
Have biologists determined if it is statistically possible for plain old natural selection to produce the organisms we have today in just a billion years?
How are epigenetic traits passed from parent to child?
@Ed
Yes biologists like Richard Lenski are performing what are called "long term experimental evolution." This is witnessing organisms evolve through time both at the genetic and phenotypic level.
http://myxo.css.msu.edu/ecoli/celebrate50K.html
To answer your second question, I recommend you read two people: Stephen Jay Gould and Richard Dawkins. Start with the Selfish Gene.
The altered form of DNA is transmitted through the germline. In a simplistic sense, the cellular memory of environmental stress of mom is passed on to her offspring.
Thanks again for the explanation. It sounds like there are interesting developments in your field.
@Girlpostdoc
In looking at the wikipedia article on the selfish gene, I came across an article on non-coding DNA which included this:
http://en.wikipedia.org/wiki/Junk_DNA#Noncoding_DNA_and_evolution
Pseudogene sequences appear to accumulate mutations more rapidly than coding sequences due to a loss of selective pressure.[11] This allows for the creation of mutant alleles that incorporate new functions that may be favored by natural selection; thus, pseudogenes can serve as raw material for evolution and can be considered "protogenes".[26]
This sounds like another mechanism that facilitates adaptation.
The fact that the function of non-coding DNA is not understood means that it could facilitate adaptation in ways that are not known.
Ed,
Mutation occurs randomly in different parts of the genome. Mutation is considered the only source of novel variation.
Pseudogenes are exactly what the term says. They were once genes but for whatever reason (and there are many) are no longer genes. What this means is selection is no longer present in these regions to remove those genetic changes that accrue.
Girlpostdoc,
The article cited by the wikipedia article argues that the pseudogenes can play a role in adaptation. I have copied the citation and abstract for the article below.
Balakirev ES, Ayala FJ (2003). "Pseudogenes: are they "junk" or functional DNA?". Annu. Rev. Genet. 37: 123–51. doi:10.1146/annurev.genet.37.040103.103949. PMID 14616058.
Pseudogenes have been defined as nonfunctional sequences of genomic DNA originally derived from functional genes. It is therefore assumed that all pseudogene mutations are selectively neutral and have equal probability to become fixed in the population. Rather, pseudogenes that have been suitably investigated often exhibit functional roles, such as gene expression, gene regulation, generation of genetic (antibody, antigenic, and other) diversity. Pseudogenes are involved in gene conversion or recombination with functional genes. Pseudogenes exhibit evolutionary conservation of gene sequence, reduced nucleotide variability, excess synonymous over nonsynonymous nucleotide polymorphism, and other features that are expected in genes or DNA sequences that have functional roles. We first review the Drosophila literature and then extend the discussion to the various functional features identified in the pseudogenes of other organisms. A pseudogene that has arisen by duplication or retroposition may, at first, not be subject to natural selection if the source gene remains functional. Mutant alleles that incorporate new functions may, nevertheless, be favored by natural selection and will have enhanced probability of becoming fixed in the population. We agree with the proposal that pseudogenes be considered as potogenes, i.e., DNA sequences with a potentiality for becoming new genes.
Ed
They're not really saying anything different than what I've said. The debate here is whether these parts of the genome are actually functional or adaptive.
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