CHANGE OVER TIME

How does evolution happen?

This section introduce you to the concept of evolution as change over time in populationss of organisms. 

Did you know that the cockroach originated on Earth more than 250 millions years ago? Well, and whether we like it or not it is still thriving today all over the world. 

In contrast, the giant deer (more than 2 m tall and which had lived on the ice age) evolved less than 1 million years ago and became extinct  around 11,000 years ago. 

Why is that? Cockroach is succesfully adapted and the deer was not. We have to think that temperature fluctuations due to ice ages and other climate changes would have affected which plants and animals could survive. 

Before we start talking about evolution and Charles Darwin, there are some things you should know: 

• The species that live on Earth today range from a bacteria: unicellular organism without nucleus (prokaryote), to multicellular eukaryotic fungi, plants and animals.  Have these especies always existed on Earth?

• Well, the answer is no. Earth has changed a great deal during its history and living things has changed too. Since life first appeared on Earth, many spieces have died out and many new species have appeared. 

How do we know about all the species that died out long ago when we were not around? We know about them because of the fossil record: a historical sequence of life indicated by fossils found on Earth´s crust

Studing fossils Scientists have made a time line, organizing fossils by their estimated ages and phisical similarities. 

For example, whales seem similar to fish. But unlike fish they breath air, give birth to live young and produce milk. These traits show that whales are mammals. Thus, scientists think that whales evolved from ancient mammals as you can see in the figure shown below:

  

Now, you may be wondering what all this changing that is leading even to new species has to do with Charles Darwin. 

Well it may be a bit confusing, but by the time Darwing lived around  1850, there were many theories about evolution. The most famous of them all was Jean B Lamark´s theory of evolutionism: 

Lamark believed that the long neck of giraffes evolves voluntarily as giraffes need to reach the ever higher leaves of the trees, and that this trait was inherited to the offspring. Of course this is not possible. Traits appeared involuntarily and last if they are given an advantage to survive to the organism.

Charles Darwin a British Naturalist who sailed all around the world make important observations giving evidence for the adpatation of the organisms to their environment. 

For example, lets see some of his observations on the finches:

 Darwin notice that finches on each of the Galapagos Islands were different form the finches onthe other islands. as shown below.

He observed that the beak of each finch is adapted to the way the bird usually gets food. 

1. The large ground finge has a wide, strong beak that it uses to crack open big, hard seeds, working as a nutcracker
2. The cactus finch has a though beak that it uses for eating cactus parts and insects. This beak works like pair of needle-nose pliers.
3. The waarbler finch has a small narow beak that it uses to catch small insects. This beak works like a pair of tweezers. 

From all his observations Charles Darwin make the Natural Selection theory and published in his book The Origin of Species:

DARWIN PROPOSSED THAT THE EVOLUTION HAPPENS THROUGH THE NATURAL SELECTION

NATURAL SELECTION: is the process by which organisms that are best adapted to their environment survive and reporduce more successfuly than less well adapted organisms do. The process has four parts:

• Overpopulation: means that every organism can produce more offspring that will likely survive

• Genetic variation: means that all offspring will have some differences

• Struggle to survive: means the offspring have to complete with each other and with other organisms around them

• Successful reproduction: means those that are best adapted will probably have more offspring like themselves.