Thursday, April 30, 2015

The v>c world

Albert Einstein
In 1967, Gerald Feinberg game the name tachyon (from the Greek tacus, fast) to hypothetical particles whose possible existence had been proposed five years before by other researchers. Tachyons would have a unique property: they always move at speeds greater than the speed of light. Their mathematical behavior would not infringe the limitations of the special theory of relativity, which prohibits bodies with mass reaching the speed of light. Unfortunately this would cause other problems.
The idea of ​​the possible existence of tachyons was embraced with joy by science fiction writers, for they seemed to make interstellar travel possible in a reasonable time. For this, the following procedure would be effective:

Thursday, April 23, 2015

Sex and species, two related concepts

Taxonomic categories
The species is the basic taxonomic category used by biologists to classify living things. The other categories (genus, family, order, class and phylum) are considered artificial and arbitrary. On the other hand, we tend to regard the species as natural, obvious, similar to a concept when the represented objects are living beings. But we will not enter here into the famous problem of universals, nor wonder on whether concepts (and species) really exist or are mere constructs of the human mind.
The classic definition of a species is: a set of living beings that share common characteristics and can interbreed, giving rise to fertile offspring. Notice that the use of the word interbreed implies that the living things in question use sexual reproduction. This leads us to ask whether the concept of a species should be restricted to living beings with this type of reproduction, or it can be extended to those that reproduce otherwise, such as prokaryotes and some eukaryotes. This question can be answered in several ways:

Thursday, April 16, 2015

The origin of sexual reproduction

Cromosomas X e Y
Sexual reproduction is the most common form of reproduction among eukaryotes, including multi-cellular living beings. After billions of years of asexual reproduction among prokaryotes, who share genetic information by exchanging plasmids (small DNA fragments), a new type of reproduction suddenly appeared. As it was successful, we must assume that it must have provided some advantage over the alternative procedure.
Sexual reproduction can be defined as the alternation between two life cycles for the same type of organism:
  • Haploid cycle: each cell has a single copy of every chromosome.
  • Diploid cycle: each cell has two copies of every chromosome.
In eukaryotes, the haploid cycle is always unicellular; the diploid cycle may be unicellular (in unicellular eukaryotes) or multi-cellular (in multi-cellular eukaryotes). The individuals in the haploid cycle are called gametes.
How was this alternation established? It could have been caused by an alternation between two different environments. Haploid and diploid cells do not have the same properties. For example, diploid cells are more voluminous, having many duplicate organelles, so that the ratio of surface to volume is usually larger than in haploid cells (about 1.25 times). As the absorption of nutrients by the cells depends on their surface, haploids tend to grow faster than diploids.

Thursday, April 9, 2015

Outstanding problems in the history of life

Gregor Mendel
In a previous article I wrote about the origin of life and related problems. That is only the first of the outstanding issues regarding evolution. There are many more, for we are far from having an explanation for everything that happened during the history of life.
The theory of evolution through natural selection was first proposed by Darwin and refined by his followers when new discovered biological phenomena solved some of the problems posed since the beginning of the theory:
1.      The laws of heredity (Mendel, 1865).
2.      Mutations (Hugo de Vries, 1900).
3.      The laws of genetics (Thomas Hunt Morgan, early twentieth century).
4.      The synthetic theory of evolution (Simpson, Dobzhansky and others, around 1930)
5.      The transmission of inheritance through DNA (Oswald Avery, 1944).
6.      The structure of DNA and the deciphering of the genetic code (Watson, Crick, Rosalind Franklin and others).
7.      The neutral theory of evolution (Motoo Kimura, 1968).
8.      Punctuated equilibrium (Stephen Jay Gould, 1972).
9.      Epigenetics (early twenty first century).
Rosalind Franklin

Thursday, April 2, 2015

The slaughter of the innocents

This image shows a possible result of the tests
described in the post. Black figures represent
pregnant women who tested negative.
Red figures are false positives.
Only the white figures are true positives.
Above-left are the results
for the Down syndrome
A study performed a few years ago [1] describes a delicate situation regarding certain widely used medical tests. The triple test has been performed on many pregnant women in order to detect whether the fetus will have the Down syndrome and other deficiencies. When the test is positive, gynecologists often recommend an amniocentesis or a chorionic villus sampling (biopsy), but as these tests involve some risk, many pregnant women do not want to do it, and some may decide to have an abortion based only on the results of the triple test.
The Down syndrome affects approximately 0.13% of fetuses. The problem is that the triple test used to detect it has a 70% sensitivity (or what is the same, the probability of a false negative is 30%) and a 91% specificity, which means that the probability of a false positive is 9%.
How should we interpret these numbers?
Suppose the triple test is performed on 1000 pregnant women. By applying the above data we conclude that:
         The likely number of fetuses affected by Down syndrome will be 1 or 2.
         The probability of detecting them with the test is 70%.
         90 women (9% in 1000) will test positive, even if their child won’t be affected by the Down syndrome.