Evolution and Natural Selection Introduction to Natural Selection

v History of Evolutionary Thought

v Theory of Natural Selection

v Examples of Natural Selection

Chapters 1.4-1.6, Bush

Introduction to Natural Selection Paving the way for Darwin

v Charles Darwin’s theory relied upon the v History of Evolutionary Thought findings of other scientists – Casting doubt on Divine Creation v Theory of Natural Selection – Cuvier, Georges – Lyell, Charles – Darwin, Erasmus v Examples of natural selection – 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) Charles Lyell (1797-1875)

v leading palaeontologist of his time v geologist

v found that many species have gone v Earth was way older than the 5000 extinct years or so allowed according to Biblical chronology Erasmus Darwin (1731-1802) Jean-Baptiste Lamarck (1744-1829)

v Believed scientists like Erasmus Darwin that life forms could change over time v Charles Darwin’s grandfather

v Lamarckism: acquired traits can be inherited – e.g. a giraffe with a short neck stretches to get at v proponent of the theory that species vegetation high up a tree and manages to make its neck longer. This giraffe passes its long neck to its change over time offspring

v got Darwin thinking about inheritance

Thomas Malthus (1766-1834) Alfred Russell Wallace (1823-1913)

v found that all species have the potential v came up with the theory of natural to create far more offspring than there selection independently of Darwin are resources to support

v spurred Darwin to publish his own work on the subject

Charles Darwin (1802-1882) Summary of the history of evolutionary thought

“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 The “Theory” of Natural Selection

v History of Evolutionary Thought Natural Selection is a “Theory” in the same way that we consider v Theory of Natural Selection gravity or Einstein’s relativity to be a theory

v Examples of natural selection

Theory of Natural Selection Variation and Heritability

v Observations from Lamarck and Erasmus Darwin that offspring are not exactly like v Three conditions for Natural Selection: parents (change can occur in a single 1) Variation in traits generation) 2) Heritability 3) Survivorship/Competition v Observed the commonly known facts that: – all individuals are not alike (i.e., there are different phenotypes) v Natural selection ¹ “Survival of the fittest” – Offspring inherit the majority of their traits from their parents.

Variation within a species Heritability in Diploids

v Variation can be: v Two copies of each gene (diploid) – Humans have 23 chromosomes, 2 copies – CONTINUOUS: having a of each, for a total of 46 chromosomes) multitude of variants (e.g., colour bands in the snail) v Each egg or sperm has only one copy – DISCRETE: limited # of of each chromosome types (such as blood types) Passing on genes is like tossing coins Heritability of simple traits

v Two copies exist for each gene

v Whether you pass on a certain copy of a gene is an independent event for each child

v 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 Fitness

v From Malthus: more offspring are produced FITNESS: than there are resources to support – the number of offspring an individual produces that survive to reproduce v Creates a “struggle for existence” themselves

v Some offspring will be better at surviving and v Fitness = 1.0 means that individuals of reproducing than others (i.e., have higher this phenotype are successfully passing fitness) on 100% of their genes, on average

How is fitness calculated Outcome

v Fitness = the number of genes passed on to the v Some phenotypes will be better represented next generation in the next generation than they are in the

v Because diploid organisms (i.e., most organisms) present generation only pass on half of their genes to each child, they must have two offspring living to reproductive age to have Fitness = 1 v Could be extended: some entire lineages may be more successful than others as well v Fitness = 1 does not exactly mean that you have resulting in some lineages going extinct (as passed on 100% of your genes to the next Cuvier had found) generation (Remember: sometimes you send two copies of the same gene and zero copies of the other) Natural selection will not take place if: “Survival of the fittest”

v there is no variation v This saying is a bit misleading and – E.g., No humans have gills, so we cannot select for them, regardless of how beneficial they might be doesn’t quite capture the essence of what is natural selection

v 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 v You can be as “fit” an individual as can selection can not act upon it if is not a genetic trait be but it is the ability to reproduce that is the key feature for an increase in v If there is no difference in survivorship or representation in the next generation reproductive ability between variants – E.g., Having attached or free earlobes doesn’t really matter

Aside: Darwin’s nemesis was genetics! Gregor Mendel – father of genetics

v conducted experiments on pea plants v discovered that most organisms have two copies of their genes, one from each parent.

Darwin never read Mendel’s Paper Introduction to Natural Selection

v History of Evolutionary Thought

v Theory of Natural Selection

v Examples of natural selection Ground Finch (Geospiza fortis) Natural selection in finches

v beak size has a lot Drought causes to do with how well collapse of food supply, a finch feeds on certain seeds survival plummets

v seeds of Tribulus High mortality in have the toughest smaller individuals, seed coat that requires a large strong selection for beak to break large birds that can crack large, tough seeds

Human-induced selection Natural selection can occur rapidly

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

Rock plants Summary

v Darwin put together a number of ideas from different disciplines to come up with the Theory of Natural Selection

v 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.

v Natural selection can operate so quickly that we can observe it in a single generation Natural Selection reviewed

Natural Selection – continued Natural Selection – continued

v Characteristics of natural selection v Characteristics of natural selection

v Types of natural selection v Types of natural selection

v Natural selection ¹ Evolution v Natural selection ¹ Evolution

Characteristics of Natural Selection Sources of variation

v Natural Selection: v Gene flow: immigration

– dependent on the variation present in the v recombination population

– Short-sighted – acts only present selection v ultimately, from mutation pressures Immigration leads to new variation Recombination creates variation in offspring

v Immigration provides new genetic material for selection to act upon

Mutation at the Phenotype Level Mutation at the DNA Level

v A mutation is v Mutations can be: caused when the – beneficial chromosomal – detrimental machinery makes a – neutral mistake

Mutagens Variation is random

v Many things may increase the v When a new recombinant or mutant mutation rate: genotype arises, there is no tendency for it to arise in the direction of – radiation improved adaptation – certain chemicals (e.g. carcinogens) v Natural selection imposes direction on evolution, using undirected variation Natural Selection – continued Types of Natural Selection

v Three kinds of natural selection: v Characteristics of natural selection – Directional selection

v Types of natural selection – Stabilizing selection – Disruptive selection

v Natural selection ¹ Evolution

Directional Selection Directional Selection

Fishing industry produces selection that favours smaller cod and can produce a decrease in Larger individuals may have higher fitness average body size. (i.e., produce more offspring) than smaller individuals.

Stabilizing selection Stabilizing Selection

Babies of intermediate birth weight have higher survivorship than very small and very The average members of the population may large babies have higher fitness than the extremes. Disruptive selection Disruptive Selection

In the finch, Pyrenestes ostrinus both very large and very small bills are beneficial for eating large and small seeds, respectively Natural selection could favour both extremes over the intermediate types

Natural Selection – continued Selection pressures may conflict

v Characteristics of natural selection

v Types of natural selection

v Natural selection ¹ Evolution

Other factors in evolution Chance events influence evolution

v If there is no relation between fitness and the character in question, then natural selection is not acting on it

v Chance events can still make these traits show change over time = RANDOM DRIFT Summary "nothing in biology makes sense except in the light of evolution" -Theodosius Dobzhansky (1900-1975) v Natural Selection acts on whatever variation is present at the time. This variation is generated randomly with respect to selection pressures v Selection can be directional, stabilizing or disruptive v Random factors can also play a part in evolution