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| Drosophila melanogaster |
Despite what I said in the last paragraph of my previous post, there are still well-intentioned people who oppose the theory of evolution (although this is very rare among biologists), and sometimes offer arguments to defend their way of thinking. I will consider some of those arguments here and offer my answers.
Answer: The claim that evolution has not been successfully reproduced artificially is mistaken. We have been doing this for thousands of years through artificial selection, which gave Darwin the idea of natural selection. We have been doing it for decades in the laboratory in a controlled way, as this Wikipedia article explains: Experimental evolution. And this has been done not just with bacteria, which have a very short life cycle, but also with higher animals.
Despite this, it is true that new species have not
been created, because there has not been enough time to do so. The minimum
number of accumulated mutations for a new species to emerge is unknown, but it
is almost certainly large, perhaps several million. The usual time considered
necessary for one species to evolve and transform naturally into another species,
in the case of animals, is one or two million years. For microorganisms, it may
be shorter, but there we have the additional problem that the concept of
species is not well-defined. Some even argue that this concept makes no sense
for prokaryotes (bacteria and archaea), as I explained in another
post.
Question: Assuming the previous answer is correct, and one
or two million years are necessary for a new species to emerge, in 65 million
years there was not enough time for mammals to progress from the level of
insectivores to humans.
Answer: Let’s do the computation, assuming a worst-case
scenario. Suppose that 65 million years ago there was only one species of
mammal that could serve as a starting point for evolution (in fact there were
many). Suppose it takes two million years for a species to transform into two
new evolved species (it doesn’t have to be two; it could be three or more, and
the time required could be shorter). How many different mammal species would
there be after 65 million years? The calculation is easy: more than 232,
or about 4.3 billion different species. How many mammal species are there
today? 5,486. So, in 65 million years, there is more than enough time for all
current mammal species to have evolved. Remember the legend about the inventor
of chess and the reward he requested. This is a very similar case of
exponential growth.
Question: Are there mutations that lead from one species to another?
Answer: This is a typical mistake of anti-evolutionists.
There are no such mutations. A species does not change as a result of a single
mutation, but of many, one after the other.
Let’s consider a case close to our own. The genomes
of a chimpanzee and a human differ in approximately 1.5% of their DNA. (This is
usually expressed by saying that we have 98.5% of our genomes in common). Since
our genome contains about 6 billion nucleotides, a 1.5% difference means that
approximately 90 million mutations were necessary to separate the two genomes. If we
assume that the common ancestor of both species lived about 13 million years
ago, it follows that 3 or 4 mutations per year in each of the two branches (the
chimpanzee’s and ours) would have been sufficient to achieve the current
separation. But there was no specific mutation that can be said to have led to
the separation of the two species.
We have been experimenting with fruit flies (Drosophila melanogaster) for less than 120 years. In that time, between 300 and 500 mutations would have occurred naturally. In the 10,000 years that we have been carrying out artificial selection of animals, at most 50,000 mutations will have occurred. Compare these figures with the 90 million differences between the genomes of chimpanzees and humans, and with the two or three million mutations that typically separate two very similar species. For two species to separate, it usually takes at least a million years: a hundred times longer than the time we have been carrying out artificial selection. It is reasonable that no new species have arisen among the flies in our laboratories and among our domestic animals.
Thematic Thread on Evolution: Previous Next
Manuel Alfonseca
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