EVOLUTION AND NATURAL SELECTION

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SUMMARY OF THE HISTORY OF EVOLUTIONARY THOUGHT

CONDITIONS FOR NATURAL SELECTION

1) LOTS OF OFFSPRING

2) VARIATION IN TRAITS

3) HERITABILITY

4) SURVIVORSHIP/COMPETITION

VARIATION• OBSERVATIONS FROM LAMARCK AND

ERASMUS DARWIN THAT OFFSPRING ARE NOT EXACTLY LIKE PARENTS (CHANGE CAN OCCUR IN A SINGLE GENERATION)

• OBSERVED THE COMMONLY KNOWN FACTS THAT:

• ALL INDIVIDUALS ARE NOT ALIKE (I.E., THERE ARE DIFFERENT PHENOTYPES)

• OFFSPRING INHERIT THE MAJORITY OF THEIR TRAITS FROM THEIR PARENTS.

VARIATION WITHIN A SPECIES

• VARIATION CAN BE:

• CONTINUOUS: HAVING A MULTITUDE OF VARIANTS (E.G., COLOUR BANDS IN THE SNAIL)

• DISCRETE: LIMITED # OF TYPES (SUCH AS BLOOD TYPES)

PASSING ON GENES IS LIKE TOSSING COINS

• TWO COPIES EXIST FOR EACH GENE

• WHETHER YOU PASS ON A CERTAIN COPY OF A GENE IS AN INDEPENDENT EVENT FOR EACH CHILD

• IF YOU HAVE TWO CHILDREN, SOMETIMES YOU WILL PASS ON THE SAME COPY TO BOTH CHILDREN (LEAVING THE SECOND COPY PASSED ON TO NEITHER CHILD)

HERITABILITY OF SIMPLE TRAITS

COMPETITION

• FROM MALTHUS: MORE OFFSPRING ARE PRODUCED THAN THERE ARE RESOURCES TO SUPPORT

• CREATES A “STRUGGLE FOR EXISTENCE”

• SOME OFFSPRING WILL BE BETTER AT SURVIVING AND REPRODUCING THAN OTHERS (I.E., HAVE HIGHER FITNESS)

FITNESS

FITNESS:

• THE NUMBER OF OFFSPRING AN INDIVIDUAL PRODUCES THAT SURVIVE TO REPRODUCE THEMSELVES

NATURAL SELECTION WILL NOT TAKE PLACE IF:

• THERE IS NO VARIATION• E.G., NO HUMANS HAVE GILLS, SO WE CANNOT

SELECT FOR THEM, REGARDLESS OF HOW BENEFICIAL THEY MIGHT BE

• IF THE GENE IS NOT HERITABLE• E.G., WORKING OUT AND GETTING A STRONG

HEART MIGHT MAKE YOU LIVE LONGER AND HAVE MORE CHILDREN BUT SELECTION CAN NOT ACT UPON IT IF IS NOT A GENETIC TRAIT

• IF THERE IS NO DIFFERENCE IN SURVIVORSHIP OR REPRODUCTIVE ABILITY BETWEEN VARIANTS

• E.G., HAVING ATTACHED OR FREE EARLOBES DOESN’T REALLY MATTER

“SURVIVAL OF THE FITTEST”

• THIS SAYING IS A BIT MISLEADING AND DOESN’T QUITE CAPTURE THE ESSENCE OF WHAT IS NATURAL SELECTION

• YOU CAN BE AS “FIT” AN INDIVIDUAL AS CAN BE BUT IT IS THE ABILITY TO REPRODUCE THAT IS THE KEY FEATURE FOR AN INCREASE IN REPRESENTATION IN THE NEXT GENERATION

SOURCES OF VARIATION

• GENE FLOW: IMMIGRATION

• RECOMBINATION

• ULTIMATELY, FROM MUTATION

IMMIGRATION LEADS TO NEW VARIATION

• IMMIGRATION PROVIDES NEW GENETIC MATERIAL FOR SELECTION TO ACT UPON

RECOMBINATION CREATES VARIATION IN OFFSPRING

MUTATION AT THE PHENOTYPE LEVEL

• MUTATIONS CAN BE:

• BENEFICIAL

• DETRIMENTAL

• NEUTRAL

                                                                                             

MUTATION AT THE DNA LEVEL

• A MUTATION IS CAUSED WHEN THE CHROMOSOMAL MACHINERY MAKES A MISTAKE

MUTAGENS

• MANY THINGS MAY INCREASE THE MUTATION RATE:

• RADIATION

• CERTAIN CHEMICALS (E.G. CARCINOGENS)

VARIATION IS RANDOM

• WHEN A NEW RECOMBINANT OR MUTANT GENOTYPE ARISES, THERE IS NO TENDENCY FOR IT TO ARISE IN THE DIRECTION OF IMPROVED ADAPTATION

• NATURAL SELECTION IMPOSES DIRECTION ON EVOLUTION, USING UNDIRECTED VARIATION

TYPES OF NATURAL SELECTION

• THREE KINDS OF NATURAL SELECTION:

• DIRECTIONAL SELECTION

• STABILIZING SELECTION

• DISRUPTIVE SELECTION

DIRECTIONAL SELECTION

Larger individuals may have higher fitness (i.e., produce more offspring) than smallerindividuals.

DIRECTIONAL SELECTION

FISHING INDUSTRY PRODUCES SELECTION THAT FAVOURS SMALLER COD AND CAN PRODUCE A DECREASE IN AVERAGE BODY SIZE.

DIRECTIONAL SELECTION

• COMMON DURING TIMES OF ENVIRONMENTAL CHANGE OR MIGRATION TO A NEW HABITAT

• EXAMPLES INCLUDE THE PEPPERED MOTH

• GALAPAGOS FINCHES (LACK OF INSECTS TO EAT FAVOURED LARGE BEAKS FOR CRACKING SEEDS)

STABILIZING SELECTION

The average members of the population may

have higher fitness than the extremes.

STABILIZING SELECTION

Babies of intermediatebirth weight have highersurvivorship than very small and very large babies

STABILIZING SELECTION

• GENETIC DIVERSITY DECREASES

• THOUGHT TO BE THE MOST COMMON MECHANISM FOR NATURAL SELECTION

• TRAITS DO NOT CHANGE DRASTICALLY OVER TIME

DISRUPTIVE SELECTION

Natural selection could favour both extremes over the intermediate types

DISRUPTIVE SELECTION

In the finch, Pyrenestes ostrinusboth very large and very small bills are beneficial for eating large and smallseeds, respectively

DISRUPTIVE SELECTION

• INTERMEDIATE PHENOTYPES ARE ELIMINATED

• EXAMPLE: EXTREME SIZE DIFFERENCES IN COHO SALMON; MATURE MALES WEIGH EITHER ABOUT 500G OR 4500 G OR MORE (EXPLANATION?)

SELECTION PRESSURES MAY CONFLICT

OTHER FACTORS IN EVOLUTION

• IF THERE IS NO RELATION BETWEEN FITNESS AND THE CHARACTER IN QUESTION, THEN NATURAL SELECTION IS NOT ACTING ON IT

• CHANCE EVENTS CAN STILL MAKE THESE TRAITS SHOW CHANGE OVER TIME = RANDOM DRIFT

MICRO VS. MACRO EVOLUTION• EVOLUTION OCCURS AT DIFFERENT SCALES

CHANGE IN FREQUENCY OF GENE FOR DARK WINGS IN A BEETLE FROM ONE GENERATION TO THE NEXT

OR

EVOLUTION AND

RADIATION OF THE

DINOSAUR LINEAGE

MICROEVOLUTION

• SMALL SCALE IN SINGLE POPULATIONS

• CHANGES IN ALLELE (ALTERNATIVE VERSIONS OF A GENE) FREQUENCIES

E.G. BACTERIAL STRAINS DEVELOPING ANTIBIOTIC RESISTANCE

MACROEVOLUTION

• EVOLUTION THAT OCCURS AT OR ABOVE THE SPECIES LEVEL (DIFFERENT GENE POOLS)

E.G. APPEARANCE OF FEATHERS DURING THE EVOLUTION OF BIRDS FROM THEROPOD DINOSAURS

THE EVOLUTION OF EQUIDAE (HORSE FAMILY)

MICRO AND MACRO EVOLUTION

DIFFERENT SCALES BUT BOTH RELY ON THE SAME MECHANISMS OF CHANGE, NAMELY:

•MUTATION

•MIGRATION/GENE FLOW

•GENETIC DRIFT

•NATURAL SELECTION

CHANCE EVENTS INFLUENCE EVOLUTION

SUMMARY

• NATURAL SELECTION ACTS ON WHATEVER VARIATION IS PRESENT AT THE TIME. THIS VARIATION IS GENERATED RANDOMLY WITH RESPECT TO SELECTION PRESSURES

• SELECTION CAN BE DIRECTIONAL, STABILIZING OR DISRUPTIVE

• RANDOM FACTORS CAN ALSO PLAY A PART IN EVOLUTION

"nothing in biology makes sense except in the light of evolution"-Theodosius Dobzhansky (1900-1975)