Saturday, April 07, 2007

Seleção Natural de Darwin continua trabalhando em humanos

Darwin's Natural Selection Still at Work in Humans

By Ker Than

LiveScience Staff Writerposted: 02 November 200509:28 am ET


The evolutionary process that Charles Darwin discovered almost 150 years ago, responsible for transforming dinosaurs into birds and allowing the walking ancestors of whales to take to the seas, is still quietly at work in humans today.

Darwin's natural selection is the process by which nature rewards those individuals better adapted to their environments with survival and reproductive success. It works at the level of genes, sections of DNA that encode for proteins serve as the software of life.

In one of the most detailed human DNA studies ever conducted, researchers analyzed nearly 12,000 genes from 39 people and a chimpanzee, our closest living relative.

The findings suggest that about 9 percent of the human genes examined are undergoing rapid evolution.

"Our study suggests that natural selection has played an important role in patterning the human genome," said Carlos Bustamante, a biologist at Cornell University.

A separate study announced last month indicated the human brain is still evolving, too.

Compared to chimps ...

Bustamante's team found that the genes most affected were those involved in immunity, sperm and egg production and sensory perception. A comparison between human and chimpanzee genomes found that these genes have undergone more changes in humans than in chimps, despite the fact that the two species shared a common ancestor some 5 million years ago.

The genes for a group of proteins important for switching other genes on and off, known as "transcription factors," were found to vary significantly in humans and chimps. One reason for this could be that turning a gene on or off is easier than changing the gene itself.

"We believe that if you want to evolve a system, it's usually easier to tweak when the protein gets turns on or the total amount of a protein as opposed to the amino acid itself," Bustamante said.

Negative selection

The validity of Darwin's natural selection has been attacked lately by a small but vocal group who argue that it cannot explain all the complexity seen in nature. They advocate a concept called "intelligent design," in which a higher being is responsible for the variety of life. Scientists dismiss intelligent design as cloaked creationism and say that there are no significant problems with the widely accepted theory of evolution.

While mainstream scientists do not need further evidence that natural selection occurs, Bustamante's work provides examples of its pace and extent and offers the promise of medical advances down the road.

Another 13 percent of the genes examined in the study showed evidence for negative selection, whereby harmful mutations are weeded out of the population. These included some genes implicated in hereditary diseases, such as muscular dystrophy and Usher syndrome. The latter is the most common cause of congenital blindness and deafness in developed countries.

Medical geneticists are interested in finding genes sensitive to negative selection because they might one day be useful for predicting an individual's likelihood of developing a disease if the types of mutation to a gene and the environmental conditions are known.

Being able to determine which classes of genes are particularly vulnerable to negative selections is a first step, Bustamante said.

The findings were detailed in the Oct. 20 issue of the journal Nature.

Fonte: http://www.livescience.com/humanbiology/051102_natural_selection.html

COMENTÁRIOS:

Esse texto é esclarecedor.Um erro muito comum sobre a evolução adaptativa é postular que ela é algo que "demora a acontecer".Isso é errado.A evolução adaptativa pode ocorrer em curtos períodos de tempo geológico e até mesmo em frente aos nossos olhos.Ela continua ocorrendo em populações que possuem variação quanto ao sucesso reprodutivo e herdabilidade.

Obviamente, como houve fluxo genético em todo mundo, também não surgiu as condições apropriadas para que ocorresse a cladogênese (transformação de uma espécie em outra) nos últimos milhares de anos, mas a anagênese (transformação dentro da espécie) continuou a ocorrer.Só lembrando que mudança morfológica significativa pode ocorrer sem especiação.Isso que significa que mesmo que a população humana nunca mais se especie, nada garante que futuramente ela será muito semelhante a de hoje.

Segue-se nesse site um resumo de quatro adaptações humanas principais:

1) Tolerância à lactose
2) Melanismo
3) Adaptação à climas extremos
4) Adaptação à altas altitudes

Uma boa resposta de Mark Isaac do site Talk Origins Archive sobre a alegação que seres humanos não evoluíram nos últimos milhares de anos.


"There is evidence that humans have evolved in the last several thousand years and continue to evolve.

* Analysis of variation in the human genome indicates that genes associated with brain size have evolved over approximately the last 37,000 years and 5800 years (Evans et al. 2005; Mekel-Bobrov et al. 2005).

* Sickle-cell resistance has evolved to be more prevalent in areas where malaria is more common.

* Lactose tolerance has evolved in conjunction with cultural changes in dairy consumption (Durham 1992). * Some humans have recently acquired mutations which confer resistance to AIDS (Dean et al. 1996; Sullivan et al. 2001) and to heart disease (Long 1994; Weisgraber et al. 1983). "

Link: http://www.talkorigins.org/indexcc/CB/CB928_2.html

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