Earth System HistoryGEOL 1020

[16]

An introduction to evolution and the fossil record

October 1, 2013

And now…

For something completely different…

Evolution and the fossil record

• Introduction to concepts of evolution and the fossil record.

• Natural selection

• Understanding life’s pathway

• Life history is not “predictable”

• Life history is not necessarily “progressive”

Fossils and evolutionPrinciple of fossil succession

(or biologic succession)

William Smith (late 1700's) certain rock units could be identified by the assemblages of fossils they contained.

Led to the "Principle of biologic succession"

(or fossil succession):

“where present, fossils occur in a consistent vertical order in sedimentary rocks all over the world.”

This SET THE STAGE for exploring BIOLOGICAL EVOLUTION

Key Concepts

• Fossil species appear and disappear throughout the stratigraphic record.

• The Geologic Time Scale was originally based on these fossil appearances and disappearances.

• Each of the Geologic Eras ends with a mass extinction.

• Period boundaries coincide with smaller extinction events, followed by appearances of new species.

Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time. What could bring about biological changes under natural conditions?

Natural selectionIn any population, there will be individuals that are slightly more tolerant of changes in the limiting factors of the environment (temperature, salinity, water depth, amount of food, etc.).

Charles DarwinBritish Naturalist1809 –1882

One of the greatest thinkers of all time.

Chuck D. was to biology what Newton was to physics!

When environmental conditions change, bringing individual organisms near their limits of tolerance, there will always be a few which are at least slightly more tolerant or BETTER ADAPTED than the population as a whole.  The BETTER ADAPTED individuals will survive to reproduce. This passes along favorable traits to their offspring.

The concept is familiar to us.

Think about SELECTION. Breeds of dogs and cats. Strains of Wheat and Corn. Selection by Breeding for the most desirable traits!

THE SAME THING HAPPENS IN NATURE OVER GEOLOGIC TIME SCALES.

Less-well-adapted individuals die off before reproducing. More tolerant, better adapted individuals live on to reproduce. Favorable traits will become dominant in the population. There will therefore be a shift in the characteristics of the population.

The environment "selects" the best adapted individuals, hence the term “Natural Selection".

“I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.”—Charles Darwin from The Origin of Species

In his time, Darwin and others did not know HOW traits were passed from ancestor to descendent

• Yet, since the concept of Natural Selection is a scientific one, that is, it is TESTABLE, it made some predictions.

1. There is a mechanism for biological inheritance

2. This mechanism has built-in capacity for change with time

3. The mechanism must be common to all life forms on Earth

4. The mechanism must be independently verifiable (e.g. with the fossil record).

5. New species must be arising now, and others are going extinct

Common misconceptionsabout natural

selection/biological evolution

• Evolution is a theory about the “origin of life”

• NOPE. Evolution deals mainly with how life changed AFTER its origin.

• However life started, it changed over time. It branched out. Life DIVERSIFIED.

Common misconceptionsabout natural

selection/biological evolution• Evolution is like a climb up a ladder of

progress; organisms are always getting better.

• NOPE. Life that survives to reproduce is life that’s successful.

• No organism has to be perfect! Many life forms have changed little over time (sharks, mosses, fungi, possums etc.). They are not “advancing” or “progressing”.

• Others changed a lot, but that doesn’t mean they’re getting better!

Common misconceptionsabout natural

selection/biological evolution

• Evolution means that life changed “by chance”

• NOPE. Natural selection is not random. Random mutations are random.

• Individuals that survive and reproduce better in their environment will have more offspring displaying the same traits!

Common misconceptionsabout natural

selection/biological evolution

• Natural selection involves organisms “trying to adapt”

• NOPE. Natural selection LEADS to adaptation, but the process isn’t about “trying”.

• Genetic variation and selection is a population is key. You can’t get the right genes by “trying”

Common misconceptionsabout natural

selection/biological evolution

• Natural selection gives organisms “What they need.”

• NOPE. Natural selection has no means, senses or intentions.

• If a population happens to have the genetic variation that allows some to survive more than others, their offspring will carry those traits.

Common misconceptionsabout natural

selection/biological evolution

• Evolution is “just a theory”.

• YES!, But you need to understand that A Theory is a very powerful thing. It means:

“In science, a well-substantiated explanation of some aspect of the natural world.”

• It’s not a “guess” or a “hunch”.

Common misconceptionsabout natural

selection/biological evolution

• Evolution is a “theory in crisis”

• NOPE. Scientists do not debate WHETHER evolution happens, but argue over HOW it takes place!

• The theory of evolution is on firmer ground than that of many physical sciences.

Common misconceptionsabout natural

selection/biological evolution

• Gaps in the fossil record disprove evolution.

• NOPE. Evolutionary biologists do not expect ALL life will get fossilized!

• Scientists have nevertheless found MANY transitional forms in the fossil record (as we will see)…

A few basic termsPopulation - a group of interbreeding organisms. Gene pool - the sum of all of the genetic components in a population. 

Species - The fundamental unit of biological classification. A group of individuals that are similar in structure, function, and development, with the potential to interbreed and produce fertile offspring. Reproductive barriers between species prevent interbreeding. Closely related (but different) species, such as the horse and the donkey CAN interbreed, but do not produce FERTILE offspring (mule).

How do organisms pass along favorable traits?

Consider young grasshoppers…

…the parts of a ladder

Watson and Crick discovered the structure of the DNA molecule in 1953!! This is a very recent science. It is equivalent, at the very least, to any of the other great scientific achievements, EVER.

Genetics is the branch of biology that deals with the study of heredity or inheritance. Darwin did not know about this, but now we do! 

Within the nucleus of each of our cells are chromosomes. In a human cell there are 23 pairs of chromosomes. (One of these pairs determines the sex.) Chromosomes consist of long DNA molecules, highly folded and coiled and combined with a variety of protein molecules. DNA stands for deoxyribonucleic acid.  The general form of the DNA molecule is described as a "double helix", which resembles a twisted ladder.

The long part of the ladder is made of phosphate and sugar compounds, and the rungs on the ladder are made of nitrogenous bases (adenine, thymine, guanine, and cytosine).

The part of the DNA molecule responsible for the transmission of inheritable traits is called a GENE.

Chromosomes are composed of genes.  

There are approximately 100,000 genes in a human cell, only a small number of which have been identified and roughly located on the various chromosomes. 

DNA carries chemically coded information from generation to generation, providing instructions for growth, development, and functioning. 

Sexual reproduction = new combinations of chromosomes. One member of each pair of chromosomes is inherited from each parent.

This sexual genetic recombination leads to variability within species of plants, animals and fungi.

Here is the complete set of human chromosomes. One member of each pair comes from each parent. Chr#23 determines sex.

Variability of a population

Genetic mutations produce alterations in genes and DNA.

Mutations are simply chemical changes to the DNA molecule. They are not necessarily bad!!! Mutations can be caused by a chemical substance, or by exposure to radiation (which includes cosmic radiation and ultraviolet light). Mutations produce much of the variability on which natural selection operates. Variability is the raw material for natural selection.

Low variability “mono culture” High variability “multi culture”

Chickens and bird flu

Low variability “mono culture”

High variability “multi culture”

Population “B”

P

T

W

F

Functional morphology The morphology (shape, body plan) of plants and animals reflects the way that they live. Specialized features of organisms which allow them to perform useful functions are called adaptations. Morphology can be examined in terms of the functions it performs - "functional morphology". When studying the form and function of morphological features, two categories can be considered:

Homology - body parts with similar origin, history and structure, without reference to function. (Homologous body parts.) Similar body plans suggest common origins. Examples?  Analogy - body parts with similar function, without regard to origin. (Analogous body parts.) Similar functions but different structures suggest different origins. Examples?

Example of homology:  Human and other nonaquatic embryos exhibit gill slits even though they never breathe through gills. These slits are found in the embryos of all vertebrates because they share as common ancestors the fish in which these structures first evolved. Human embryos also exhibit by the fourth week of development a well-defined tail, which reaches maximum length when the embryo is six weeks old. Similar embryonic tails are found in other mammals, such as dogs, horses, and monkeys; in humans, however, the tail eventually shortens, persisting only as a rudiment in the adult coccyx (the tail-bone!).

Embryonic rudiments that never fully develop, such as the gill slits in humans, are common in all sorts of animals.  Some, however, like the tail rudiment in humans, persist as adult vestiges reflecting evolutionary ancestry.  The most familiar rudimentary organ in humans is the vermiform appendix. This wormlike structure attaches to a short section of intestine called the cecum, which is located at the point where the large and small intestines join. The human vermiform appendix is a functionless vestige of a fully developed organ present in other mammals, such as the rabbit and other herbivores, where a large cecum and appendix store vegetable cellulose to enable its digestion with the help of bacteria.

Vestiges are instances of imperfections that argue against creation by design but are fully understandable as a result of evolution.

1. Human appendix useless yet in other mammals, including primates, it is necessary to aid in digestion of high cellulose diet

2. Human external ear muscles still present but useless

3. Humans have tailbones and some babies occasionally have tails

4. Human wisdom teeth vestigial compared to other primates

5. Some snakes have skeletal limbs 6. Cave dwelling crayfish have eyestalks yet no eyes 7. Sometimes vestigial organs may be adapted for

new uses, e.g. penguin wings can't be used for flight yet adapted for swimming

NEXT TIME: Midterm after that, more about evolution

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