Course Icon

Biology

Chapter 13: Early explanations of evolution

SO Icon

Lecture

Evolution: Natural Selection

WebLecture Topics


The origins of the theory of evolution

We usually associate the theory of evolution with Charles Darwin, but he was not the first person to propose the idea that animals and plants changed over time, developing different characteristics. The idea of progress from simple to complex in many things, including life forms, goes back to some of the ancient Greek philosophers, like Anaxagoras, who lived around 500 B. However, their ideas were countered both by Aristotle (who did not believe in change within a given species, but did believe that living organisms could be ordered), and by creation stories in which God made all species of plants and animals simultaneously during the six days of Creation described in Genesis.

Early Evolution Theories

Lamarck Statue

Any theory that deals with the diversity and similarity among living organisms also needs to account for the ability of individuals to inherit characteristics from their parents. In 1809, the French philosopher Lamarck proposed a theory in which an organism could inherit acquired characteristics, thus improving its chances for survival. For example, a giraffe stretches its neck over its lifetime, gradually lengthening the neck slightly. Lamarck believed this characteristic, which was learned or acquired by life experience, could be passed on to the giraffe's offspring, so that those with longer necks could survive periods of drought, when the only leaves available would be higher on the trees. Larmarck assumed that all evolutionary change was directed toward improving the organisms involved, but he was unable to show how such inheritance could occur.

Another theory of how populations change was proposed by Thomas Malthus, who observed that most species produce more than two offspring per mating couple. Populations grow the Galapogos Islands, near the equator off the west coast of South America, he had already observed many species of plants and animals unknown to him in England, including those of the Azores Islands in the mid-Atlantic, and those of the Faulkland Islands near the south eastern tip of South America. He expected the populations of the Galapagos to resemble the populations of other island groups; instead, he found a group of species that closely resembled the nearby coastal animals and plants of Ecuador, but which also differed within species from island to island in the Galapagos. He was faced with four major observations:

To account for these observations, Darwin proposed the theory of natural selection, which states that since there are limited resources, only those individuals within a species that possess traits which help them survive to maturity will reproduce offspring. If the traits are inheritable, then the offspring will also possess those traits. Over time, the number of individuals that lack the trait will decrease, and more and more of the population will possess the trait.

Darwin could not explain precisely how such a mechanism for inheritance worked, since he was not aware of the studies of inheritance performed by his contemporary, Mendel, at the time he published his Origin of the Species. Now we explain such a shift in terms of genetics. Each gene with more than one allele has a range of expression in the general population. When a particular allele programs for a trait that increases survivability (such as protective coloration), then the possessors are more likely to survive to productive maturity and pass that trait on to their offspring as part of the genetic pattern. Over time, the number of individuals in the population with the "survival" allele will increase, since fewer of those with the "non-survival" allele won't live to have kids. At its most basic definition in biology, evolution is simply a shift in the frequency of an allele in a given population.

Darwin's Book'

Note that this theory of natural selection does not explain the emergence of a new species. Modern evolution theory combines Darwin's natural selection idea with the concept of mutation in the DNA sequence to account for both allele shifts and the emergence of new alleles and even new genes that may in turn produce sufficient differences between parents and offspring that we consider them different species.

Evidence for evolution

Fossil

Biologists draw on evidence from many different sources in order to establish the relationships of different species to common ancestors according to evolutionary theories.

Fossils are found all over the world, in all habitats and at most depths. Because organic material usually disintegrates, most fossil conversions are of the "hard" parts of animals. Geologists use both radioactive mineral dating (and usually try to test for several different ones in a given sample, in order to establish the range of time periods more actively) as well as the location of the sample in a particular starta or type of material. By comparing the information for many different fossil remains, geologists have producd a set of standards for interpreting fossil evidence that is consistent across most fossils.

Populations and evolution

Notice that we are talking about populations changing, not individuals. One of the most common populations to consider is the species, which is defined in biology as any group of organisms that will breed and produce fertile offspring under natural conditions. In most cases, interbreeding is what produces the gene pool (the range of different alleles for each inherited gene) available to each generation of the population. If the species is isolated by its mating practices and opportunities, no new alleles will be introduced from outside organisms, although new alleles may be created by mutation. As the population becomes adapted to its environment, the frequency of a given allele for a particular gene in the pool may shift. If the shift continues over several generations with a measurable or significant change in the frequency, biologists say that microevolution is occuring. Note that microevolutionary changes are litmited to within species; a single microevolutionary event does not result in the emergence of new species.

To determine whether microevolution is occuring, we look at the frequencies of alleles in a particular population for a given gene and apply the Hardy-Weinberg principle. This principle states that in a large population, the process of inheritance by itself will not cause a change in allele frequency. If no microevolutionary factors are present (no mutations, no natural selection), then the relationship between the frequencies p2 and q2 for two alleles of the same gene are related by the equations

p + q = 1

and

p2 + 2pq + q2 = 1

For example, let's look at 1000 fruitflies, of which 90 are black and have the recessive homologous genotype bb, while the rest are white and are either BB or Bb.

By using the Hardy-Weinberg principle, we can predict the normal distribution of allele frequencies within a population by looking at only the number of homozygous recessives that occur, providing the population satisfies the following criteria:

  1. All matings are random.
  2. No mutations are occuring.
  3. The population is large, so that statistical deviations are muted.
  4. No migrations are occuring, so no genetic exchange from outside the gene pool takes place.
  5. No mechanisms for natural selection are occuring.

Obviously, in most natural populations, at least one of the above factors is operating, so the H-W principle works only on ideal, not on real world cases. While it won't predict a real-world frequency distribution over several generations, it will, however, give us a measure of how much change (or "evolution") is occuring. All we have to do is see how the actual distribution differs from the ideal one.