Thanks for the great question.

You are right! Populations of living things change over time because of evolution by natural selection.

A scientist named Charles Darwin observed that living things
(1) have to compete to survive,
(2) exhibit variation (differences in size, coloration, behavior, etc.), and
(3) pass down their traits to their offspring.

It was those living things that fit best in their environment that were able to survive and reproduce the most. The traits of the fittest individuals then spread throughout a species, changing the population.

Over time, natural selection results in the evolution of adaptations, or functional features of a living thing that promote its survival and reproduction. An elephant’s trunk or giraffe’s long neck are examples of adaptations.

Populations can change due to a variety of factors. Some of the major causes are
1) changes in the environment that require the population to change and adapt to survive, specifically, changes that alter the availability of resources such as food, water, and shelter, such as the climate changing from hot to cold (can decrease amount of food and water available and require certain types of shelters);
2) spontaneous mutations, which can change a fraction of a population such that this fraction can grow faster and stronger under existing conditions and eventually out-compete the rest of the population;
3) introduction of another species/population, which can compete with, coexist with, or benefit the existing population, and the effects from these three different modes are very different in terms of changes to the existing population.

This is, of course, not an exhaustive list; 1) and 3) are examples of selection (though 3) can be seen as a subset of 1)), and 2) is an example of variation (assuming no outside forces). Adaptation is the word for the population responding to changes, so the concept of adaptation would apply to any changes in the population to better survive a given environment.

All of the terms used in the second question are interrelated in the context of population change and evolution, which makes it difficult to answer the first question. Variation can be thought of as the differences in phenotype (i.e. differences in outwardly expressed traits) between individuals in a population. The variations may make an individual more or less "fit" in the evolutionary sense (or have no effect on fitness). For those variations which do affect fitness, natural selection would lead to more/fewer of the individuals which express that trait surviving.

Adaptation is the term for the process by which the population changes as a result to natural selection increasing the number of individuals expressing a particular variant of a trait.

Yes, to all those. I just read in Science magazine about sea nomads - people who have lived for hundreds of years on houseboats on the ocean between Malaysia, the Philippines, and Indonesia [Apr 20 issue, pg 245].

Researchers say they have 'evolved adaptations' for fishing underwater for long periods of time, where oxygen is low.

'Evolved' would be natural selection of people who have beneficial 'adaptations' that arise through 'variation' in genes among individuals. These people have big spleens that seem to be due to about 25 changes in their genes, compared with people in the same region who live on land.

Evolution can be caused by a number of processes. These include natural selection, artificial selection, genetic drift, mutation, and gene flow.

Except for mutation, all of these require there to be existing variation in a population, and furthermore that variation must be inherited from parent to offspring. Mutation is different because it creates variation. All other mechanisms of evolution destroy variation, which means that without mutation, evolution would eventually grind to a halt as all of the variation got used up.

Natural selection takes place when individuals that possess traits that enable them to survive and/or reproduce do so, in the process passing on any heritable variation they may possess. If there is no variation, then natural selection will not occur, because all of the individuals are the same and have the same chances of surviving and reproducing. If there is variation but that variation cannot be inherited, then natural selection will occur, but evolution will not: the offspring will be no different from the previous generation of offspring, even if the parents have been filtered by selection. Only if there is heritable variation that affects survival and reproduction can natural selection result in evolution.

Artificial selection works just like natural selection, except that it is humans who decide who gets to survive and reproduce rather than natural forces like finding food, avoiding becoming prey, fighting off parasites, surviving dangerous weather, etc. Like natural selection, artificial selection also requires heritable variation, for the same reason.

Genetic drift is what happens when some individuals just happen to have more offspring than others , and who also happen to have different heritable traits. Evolution happens as a result of some traits being passed on while others do not, but those traits do not affect survival or reproduction in any way. They winners were just lucky, and the losers unlucky.

Gene flow takes place where individuals with heritable traits move to another population with different heritable traits. This still causes evolution because the traits in a population are changing, but there are no winners or losers, just individuals moving around.

Notice that genetic drift, mutation, and gene flow are all completely random. This means that, while they cause evolution, they do not cause adaptation; the direction of evolution may help offspring cope with their environment, or it may harm them, or it may have no effect. Of the natural evolutionary forces, the only one that can cause adaptation is natural selection. Artificial selection can also cause adaptation, if what the humans doing the selecting want is adaptive, but it can also be mal-adaptive (the opposite of adaptive) as well, if the humans want to create something mal-adaptive instead.

There may be other forces of evolution besides these, as well, but these are the ones that we know about.

Finally, there are ways to make a population look different without it evolving. For example, limber pine (Pinus flexilis) grows tall at lower altitudes and is stunted at higher altitudes. Warming the climate will make it grow taller and cooling the climate will make it grow shorter. This is not evolution because it's not acting on heritable variation. If the climate changes, the trees will grow differently. If the climate changes back to what it was before, and the trees change back to how they were before.