17.1 Understanding evolution  (Page 5/21)

In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution     , where similar traits evolve independently in species that do not share a recent common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous spans of time and help explain how evolution occurs. Natural selection acts on individual organisms, which in turn can shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for the genotype of an entire species to evolve. It is over these large time spans that life on earth has changed and continues to change.

Natural selection can take a long time to occur, but artificial selection     can occur much faster. Artificial selection is like natural selection, but humans are the ones selecting which traits are passed on to the next generation. For example, all dog breeds are the same species, but they can look dramatically different and can have different behaviors based on the selective breeding that humans have controlled. Humans have selectively bred many plants to produce more fruit, or withstand the elements, or fight pests. Humans have selectively bred many different animals: sheep, cattle, chickens, dogs, cats, pigeons, etc. Darwin's theory of evolution by natural selection was strongly influenced by his observation of the success of selective breeding.

17.1c evidence of evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species , to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.

Fossils

Fossils are mineralized structures or imprints indicating ancient life. Fossils form when sediment or volcanic ash accumulates quickly over an organism or its traces (for example, footprints). Over time, that sediment becomes rock, with the mineralized remains left inside. It can take millions of years to rock to form, and therefore scientists need a much larger timescale than people are used to. The geologic timescale is a timeline of Earth's history, see [link] ), and [link] ).