The definition
Biological evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.

The explanation
Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't involve descent through genetic inheritance.

The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother.

Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we're all distant cousins: humans and oak trees, hummingbirds and whales.

a.Leaves on trees change color and fall over several weeks.

b.Mountain ranges erode over millions of years.

c.A genealogy illustrates change with inheritance over a small number of years..

d.Over a large number of years, evolution produces tremendous diversity in forms of life.

The history of life: looking at the patterns

The central ideas of evolution are that life has a history — it has changed over time — and that different species share common ancestors.

Here, you can explore how evolutionary change and evolutionary relationships are represented in "family trees," how these trees are constructed, and how this knowledge affects biological classification. You will also find a timeline of evolutionary history and information on some specific events in the history of life: human evolution and the origin of life.

The family tree

The process of evolution produces a pattern of relationships between species. As lineages evolve and split and modifications are inherited, their evolutionary paths diverge. This produces a branching pattern of evolutionary relationships.

By studying inherited species' characteristics and other historical evidence, we can reconstruct evolutionary relationships and represent them on a "family tree," called a phylogeny*. The phylogeny you see below represents the basic relationships that tie all life on Earth together.

*The evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered. Many of the phylogenies you encounter are the "family trees" of groups of closely related species, but we can also use a phylogeny to depict the relationships between all life forms.

The three domains
This tree, like all phylogenetic trees, is a hypothesis about the relationships among organisms. It illustrates the idea that all of life is related and can be divided into three major clades*, often referred to as the three domains: Archaea, Bacteria, and Eukaryota. We can zoom in on particular branches of the tree to explore the phylogeny of particular lineages, such as Animalia (outlined in red). And then we can zoom in even further to examine some of the major lineages within Vertebrata. Just click the button below.

A group of organisms that includes all the descendents of a common ancestor and that ancestor. For example, birds, dinosaurs, crocodiles and their extinct relatives form a clade.

The tree is supported by many lines of evidence, but it is probably not flawless. Scientists constantly reevaluate hypotheses and compare them to new evidence. As scientists gather even more data, they may revise these particular hypotheses, rearranging some of the branches on the tree. For example, evidence discovered in the last 50 years suggests that birds are dinosaurs, which required adjustment to several "vertebrate twigs."

You can see the rest of the article on www.evolution.berkeley.edu

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