Wednesday, December 19, 2007

The End of Darwinian Evolution?

It’s hard to make predictions, especially about the future- American baseball player Yogi Berra, or was it Mark Twain who made this comment? Either way, it’s hard to disagree. Maybe that’s why I chose to study geology. So much easier to figure out what happened in the past! Darwin’s Last Gasp was the provocative title of an editorial in the Times of India a few days ago. Mukul Sharma writes:

With globalization in progress the different races are no longer going to be evolving away from each other. Unlike what happened earlier when humans dispersed to separate regions and developed unique gene pools that made them less alike, the trend now is being reversed with a tendency of merging into a single mixed humanity.

The other reason why Darwinian evolution may finally have to give up on its mechanisms is because natural selection is about to be bypassed altogether by genetic manipulation technology. Ultimately - say in about another 50 years or so - artificial selection using genetic engineering techniques will make evolution forever irrelevant, at least as far as our species is concerned.

The research that prompted this article suggested that human adaptive evolution fueled by the advent of agriculture and a population explosion has accelerated over the last ten thousand years or so. That may well be so says Mukul Sharma, but not anymore. Take his first assertion. Humans are migrating in large numbers and mixing up the gene pool and that’s the end of Darwinian evolution for humans. Is it the end of evolution or the end of evolution through natural selection commonly known as Darwinian evolution and what’s the difference between the two? Most people take evolution to mean adaptive evolution through natural selection i.e. a change over generations to a better adapted state. It is this kind of evolution that the research on accelerated evolution concentrated on. Biologists have a more inclusive definition which is changes in the frequency of alleles in a population from one generation to the next. Evolution is a process that causes heritable changes in the properties of populations over generations. Individuals don’t evolve. Modern population migrations cause great demographic changes especially given the declining fertility rates in many western nations. So, say with Indians immigrating to England in large numbers, northern European genes will decline in frequency relative to south Asian ones. The genetic structure of the population will change over generations i.e. it will evolve. Even without migration, populations can evolve without natural selection taking place. This is by a process known as random genetic drift. Biologists acknowledge that the vast majority of mutations may not be advantageous but neutral or almost neutral, meaning they do not affect reproductive fitness. Natural selection is blind to such neutral changes and the frequency of such genes can fluctuation randomly over generations until just by chance one allele completely replaces the alternate version i.e. the mutation becomes fixed in the population. So mixing of populations won’t stop humans from evolving even in the absence of natural selection. And it certainly has not stopped acting in the poorer countries where enormous differences in mortality rates still exert strong directional selection pressures for change.

The notion that natural selection has stopped working on human populations is mainly related to advances in medical technology and the overall improvements in standards of living. This undoubtedly has led to a relaxation of mortality selection, the most familiar form of natural selection. People who used to die before reproducing due to disease and poor health linked to bad genes now don’t die in the same numbers. Such faulty genes therefore don’t get eliminated from the gene pool. For example the frequency of genetically determined red-green color blindness is about 5-10% higher in western urban males than in traditional hunter gatherer or non-industrialized farming societies living today. This is an indication that selection has been relaxed in western populations. But natural selection works on humans (and other organisms) in several other ways at various stages of our life cycle, and these have not been affected by medical advances so far. These other forms of natural selection are 1) selection at the gamete stage, 2) selection at the embryo stage and 3) sexual selection at the reproductive stage. There is some evidence that selection can weed out defective sperm mostly based on the extreme gymnastic contortions sperm cell have to perform within the female reproductive system. It is possible this is selection for the quality of sperm. And there is also some evidence that there are more mutations in early-stage cells than in the final fertilized egg, suggesting that mutations are being weeded in the female gamete as well. Out of several hundreds of million sperm cells, only a few taste success, and out of 5-7 million oocycts only a few hundred or so become eggs, so selection most probably is playing a role eliminating faulty cells at this stage. How would selection work at an embryonic stage? About 60% of our genes are expressed at this stage, ones involved in basic cellular physiological functions and those involved in development. Again bad genes which are expressed at an early stage in the embryo will cause faulty development of the embryo and likely death. The estimate is that about 80% of conceptions that are lost before birth are likely the result of natural selection. Finally there is sexual selection. If individuals have the ability to screen potential mates for bad genes, then such choice may lead to elimination of these genes as bearers of these genes remain without a partner. Do humans have such abilities? As usual Darwin got there first by referring to a French and Scottish study which showed that single men had twice the natural death rate as married men at years 20 to 30. Similar patterns exist in Asia, Europe and America and being single is apparently one the biggest risk factors for human beings. This may off course mean that marriage improves health, but the other explanation that health improves your chances of marriage is also highly probable. It is likely we make conscious choices or get unconsciously influenced regarding the genetic quality of potential mates.These forms of natural selection will continue to affect us humans even during demographic changes, population mixing and the current medical technologies.

Mukul Sharma’s second assertion is that “artificial selection using genetic engineering techniques will make evolution forever irrelevant, at least as far as our species is concerned”. In making this assertion he misses the altogether important point that artificial selection is also a mechanism of Darwinian evolution and contrary to making evolution irrelevant it has the potential of transforming it into an even more potent force. Darwin himself began his one long argument using artificial selection carried out by pigeon breeders and domestication of cattle to ram home the point that populations can change very rapidly through such means. Darwin thought of natural selection as acting through the differential survival and reproduction of individuals. Artificial selection if society goes in for it will change human populations not by eliminating individuals or banning some from reproducing but by preferential representation of certain genes by making changes at the early gamete stage of the life cycle. Initially such selection will focus on disease genes but there could be other types of changes possible. Recently I read an article about the super rich in Newsweek magazine. The tendency is to form exclusive clubs and networks and isolate oneself socially and culturally from the rest of the plebeian population. If this trend of cultural isolation continues then how long until certain groups opt for genetic isolation through artificial selection? Will we see evolution of enhanced features or abilities in certain groups? All this is speculation. Most predictions about the future including the ones I am making will be wrong. Humans may decide to regulate the use of genetic engineering for certain medical applications only, or it may go the other way, where cultures may decide to isolate themselves from other groups and go in for strong directional evolution in many traits through artificial means. An as yet undiscovered pathogen may evolve to which our medical knowledge have no answer to, restoring strong mortality selection.

Is there a possibility of even more fundamental changes to our genome? Evolutionary biologist Mark Ridley has speculated that our current level of biological complexity is limited by mutational load mainly through copying errors when our DNA gets replicated. Nearly 2 billion years ago repair enzymes evolved that reduced copying errors from about 1 in 10,000 to about 1 in 10,000 million. This enabled an increase in biological complexity. There is a hint that our mutation rates are very high. For humans it takes on average about five estrus cycles for a fertile male and a fertile female to produce a baby. This indicates, although is not proof, that the four unsuccessful attempts are related to genetic errors mostly copying errors. Could we fashion an even more efficient repair enzyme, one that reduces the error rate even further multi-fold fueling evolution of humans towards even greater complexity? The only certainty is change. I would wager (to be settled by my potential descendants :-) ) that 5,000 years from now our descendants will be as different from us as we are from our ancestors who lived 10-15,000 years ago.

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