Evolution Unit 5A

 

Introduction

 

We hear many things when discussing evolution.

“Struggle for existence”  “Survival of the fittest”  “Descent with modification”  “Just a theory”

Evolution is a fact. It is the unifying principle of biology. Natural selection is a theory.

 

            First, we should clarify what "evolution" means. Like so many other words, it has more than one meaning.

            Its strict biological definition is "a change in allele frequencies over time." By that definition, evolution

            is an indisputable fact. Most people seem to associate the word "evolution" mainly with common descent,

            the theory that all life arose from one common ancestor. Many people believe that there is enough

            evidence to call this a fact, too. However, common descent is still not the theory of evolution, but just a

            fraction of it (and a part of several quite different theories as well). The theory of evolution not only

            says that life evolved, it also includes mechanisms, like mutations, natural selection, and genetic drift,

            which go a long way towards explaining how life evolved.

 

            Calling the theory of evolution "only a theory" is, strictly speaking, true, but the idea it tries to convey is

            completely wrong. The argument rests on confusion between what "theory" means in informal usage and

            in a scientific context. A theory, in the scientific sense, is "a coherent group of general propositions

            used as principles of explanation for a class of phenomena" [Random House American College

            Dictionary]. The term does not imply tentativeness or lack of certainty. Generally speaking, scientific

            theories differ from scientific laws only in that laws can be expressed more tersely. Being a theory

            implies self-consistency, agreement with observations, and usefulness.

 

            http://www.talkorigins.org/

 

Recommended reading “Five misconceptions about evolution”

http://www.talkorigins.org/faqs/faq-misconceptions.html

 

Further discussions may be found at

http://www.talkorigins.org/origins/faqs-mustread.html

 

Must you “believe” in evolution? Not at all, just know the arguments. 

 

 

The world view before Darwin

 

The scala natura - the scale of nature.  Each species has a rung on this ladder of life, once a spot is taken, it does not change.  Humans, of course, are highest among the animals (it’s OUR ladder!) – Plato and Aristotle 4^th century BCE.

 

 “Natural theology”- Adaptations and variations were seen as evidence of a Creator’s perfect plan. All species were designed for a particular purpose and those designs could not change. This view agrees and supports the Biblical account of creation found in Genesis.

 

Carolus Linneaus (1707-1778) was the father of taxonomy. He developed the system of binomial nomenclature still used today. He stated about his work, “Deus creavit, Linneaus disposuit – God creates, Linneaus arranges.”

 

catastrophism – species do not change but may be wiped out by catastrophic events (the Biblical flood, etc). New species appeared through migration – George Cuvier (1769 – 1832, founder of the science of paleontology)

 

gradualism- large-scale changes arise from the accumulation of slow but continuous processes -James Hutton (1726-1797).

 

uniformatarianism – forces that build and destroy mountains are operating today as they did in the past – Charles Lyell (1797-1875). This leads to the speculation that the Earth may be more than 6 000 years old. His book “Principles of Geology” had a profound influence on Darwin.

 

inheritance of acquired characteristics – The year Darwin was born (1809), a theory of evolution was published by Jean-Baptiste Lamarck (1744-1829).  Lamarck, like many of his contemporaries, believed that life could arise spontaneously, e.g. maggots and hornets from horseflesh. Evolution, Lamarck believed, was driven by a tendency towards complexity (perfection). The long necks of giraffes were the gradual accumulation of changes brought about by generations of stretched necks and the inheritance of this characteristic.  Although Lamarck’s mechanism of “use and disuse” was incorrect, he was the first to emphasize the importance of adaptation to the environment. He also acknowledged the great age of the Earth.

 

Artificial selection: Darwin was aware of the slight changes occurring in domestic plants and animals through selective breeding. Darwin reasoned that if substantial changes could be brought about in the relatively short period of time since the dawn of agriculture, then natural selection should be capable of considerable modification over hundreds and thousands of generations.

 

Take a look at these True/False questions: from http://gsoft.smu.edu/IntroBio/StudyQ/WkOneAns.html

 

1. Darwin was unable to convince biologists that evolution had occurred because he could not provide a mechanism for heredity.

FALSE. He did convince biologists that evolution had occurred, what he couldn't convince them of was that natural selection was its mechanism.

 

2. Lamarck's model of evolution differs from Darwin's in that it does not include common descent.

TRUE. In Lamarck's model, organisms belong to different species because they had ancestors spontaneously generated at different times. There is no common ancestor.

 

3. Linnaeus proposed that his Systema Naturae represented evolutionary kinship among species.

FALSE. Linnaeus never had any evolutionary ideas that we know of; he simply found a hierarchy to be a better organization than the old Scala Naturae.

 

 

Charles Darwin and the HMS Beagle

 

In the years 1831-1836, Charles Darwin travelled around the world as a naturalist on the HMS Beagle. In his travels, he made many observations regarding the diversity and unity of life. He read books written by the leading geologists of the time including Lyell and Cuvier, and an essay on population by Malthus. In the 1840s, Darwin began to piece together his theory but he did not publish until 1859. The impetus to publish finally came when a researcher named Alfred Wallace wrote a letter to Darwin explaining his theory of descent, which closely resembled Darwin’s thoughts. Darwin published and presented the theory first, but Wallace is given his due credit. The book “On the Origin of Species by Means of Natural Selection” was a best seller and went through many reprints. It’s available online in its entirety at

 

 http://www.talkorigins.org/faqs/origin.html

 

Darwin’s arguments

 

Ernst Mayr condensed Darwin’s theory into the following observations and inferences (adapted somewhat).

 

Observation #1: All species have such great reproductive capacity that their population would increase exponentially if all offspring born lived to maturity and reproduced successfully.

 

Observation #2: Populations tend to remain stable through time (with the exception of seasonal fluctuations)

 

Observation #3: Resources in the environment are limited.

 

Inference #1: A struggle for existence arises as individuals compete for limited resources. Only a fraction of offspring will survive.

 

Observation #4: Individuals in a population exhibit variation in their characteristics.

 

Observation #5: Much of this variation is heritable.

 

Inference #2: Surviving individuals whose inherited characteristics best fit to their environment are likely to leave more offspring than less fit individuals.

 

Inference #3: This difference in reproductive success will lead to a gradual change in a population, with favourable characteristics accumulating over generations.

 

In a nutshell:

Natural Selection is the differential reproduction of individuals in the same population based on genetic differences among them. It acts on phenotypes of individuals, not directly on genotypes.  If a gene does not produce any phenotypic expression or difference, it cannot be “selected”.  Whole populations may rise and fall.  When a new one has phenotypic advantages that help it prevail over its rivals, it persists longer.

All existing (extant) species represent unbroken descent from the first dividing cell.  Millions of others have vanished along the way.

Diversity has arisen through descent with modification, and the mechanism of modification is natural selection working continuously over immense time periods.