Evolution and Natural Selection
description
Transcript of Evolution and Natural Selection
Introduction to Natural Selection
History of Evolutionary Thought
Theory of Natural Selection
Examples of Natural Selection
Introduction to Natural Selection
History of Evolutionary Thought
Theory of Natural Selection
Examples of natural selection
Paving the way for Darwin
Charles Darwin’s theory relied upon the findings of other scientists– Casting doubt on Divine Creation
– Cuvier, Georges – Lyell, Charles– Darwin, Erasmus
– Contributing to the theory itself– Lamarck, Jean-Baptiste– Malthus, Thomas Robert– Wallace, Alfred Russell
http://goldberg.history.ohio-state.edu/naturalselection/
Georges Cuvier (1769-1832)
leading palaeontologist of his time
found that many species have gone extinct
Charles Lyell (1797-1875)
geologist
Earth was way older than the 5000 years or so allowed according to Biblical chronology
Erasmus Darwin (1731-1802)
Charles Darwin’s grandfather
proponent of the theory that species change over time
Jean-Baptiste Lamarck (1744-1829)
Believed scientists like Erasmus Darwin that life forms could change over time
Lamarckism: acquired traits can be inherited– e.g. a giraffe with a short neck stretches to get at
vegetation high up a tree and manages to make its neck longer. This giraffe passes its long neck to its offspring
got Darwin thinking about inheritance
Thomas Malthus (1766-1834)
found that all species have the potential to create far more offspring than there are resources to support
Alfred Russell Wallace (1823-1913)
came up with the theory of natural selection independently of Darwin
spurred Darwin to publish his own work on the subject
Charles Darwin (1802-1882)
“I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.”
(The Origin of Species)
http://www.interaktv.com/Darwin/Darwin.html
Introduction to Natural Selection
History of Evolutionary Thought
Theory of Natural Selection
Examples of natural selection
The “Theory” of Natural Selection
Natural Selection is a “Theory” in the same way that we consider gravity or Einstein’s relativity to be a theory
Theory of Natural Selection
Three conditions for Natural Selection:1) Variation in traits2) Heritability3) Survivorship/Competition
Natural selection “Survival of the fittest”
Variation and Heritability
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)
Heritability in Diploids
Two copies of each gene (diploid)– Humans have 23 chromosomes, 2 copies
of each, for a total of 46 chromosomes)
Each egg or sperm has only one copy of each chromosome
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)
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
Fitness = 1.0 means that individuals of this phenotype are successfully passing on 100% of their genes, on average
How is fitness calculated
Fitness = the number of genes passed on to the next generation
Because diploid organisms (i.e., most organisms) only pass on half of their genes to each child, they must have two offspring living to reproductive age to have Fitness = 1
Fitness = 1 does not exactly mean that you have passed on 100% of your genes to the next generation (Remember: sometimes you send two copies of the same gene and zero copies of the other)
Outcome
Some phenotypes will be better represented in the next generation than they are in the present generation
Could be extended: some entire lineages may be more successful than others as well resulting in some lineages going extinct (as Cuvier had found)
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
Gregor Mendel – father of genetics
conducted experiments on pea plants
discovered that most organisms have two copies of their genes, one from each parent.
Introduction to Natural Selection
History of Evolutionary Thought
Theory of Natural Selection
Examples of natural selection
Ground Finch (Geospiza fortis)
beak size has a lot to do with how well a finch feeds on certain seeds
seeds of Tribulus have the toughest seed coat that requires a large beak to break
Natural selection in finches
Drought causes collapse of food supply, survival plummets
High mortality in smaller individuals, strong selection for large birds that can crack large, tough seeds
Human-induced selection
1) Natural pop’n with variation for insecticide resistance
2) Insecticide appl’n kills all but those with resistance
3) Surviving insects breed new generation of insecticide resistance population
Summary
Darwin put together a number of ideas from different disciplines to come up with the Theory of Natural Selection
Natural selection states that heritable phenotypes that are well-suited to their environment will have more offspring and so will be better represented in the next generation.
Natural selection can operate so quickly that we can observe it in a single generation
Natural Selection – continued
Characteristics of natural selection
Types of natural selection
Natural selection Evolution
Natural Selection – continued
Characteristics of natural selection
Types of natural selection
Natural selection Evolution
Characteristics of Natural Selection
Natural Selection:
– dependent on the variation present in the population
– Short-sighted – acts only present selection pressures
Immigration leads to new variation
Immigration provides new genetic material for selection to act upon
Mutation at the Phenotype Level
Mutations can be:– beneficial– detrimental– neutral
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
Natural Selection – continued
Characteristics of natural selection
Types of natural selection
Natural selection Evolution
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.
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
Disruptive Selection
In the finch, Pyrenestes ostrinusboth very large and very small bills are beneficial for eating large and smallseeds, respectively
Natural Selection – continued
Characteristics of natural selection
Types of natural selection
Natural selection Evolution
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
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