EvolutionMr. Wright, 2011

The Work of Charles DarwinSections 15.1 & 15.2

Evolutionary Theory

•Evolution – change over time▫How modern organisms descended from their ancestors.

•Theory – a well-supported testable explanation for something

Evolutionary Theory• The main person

behind evolutionary theory was Charles Darwin.

• Darwin sailed around the world, recording his observations as he travelled.

• These observations led to him proposing the idea of evolution.

Darwin’s Observations

•Darwin was most influence by what he saw on the Galapagos Islands.

•Each island had a unique climate:▫Some were dry and barren▫Some were moist and diverse▫Others were inbetween

Galapagos Tortoises

•Darwin found tortoises on each island, but they looked different from those on other islands.

•How are they different, and what might this mean?

Galapagos Finches

•Darwin also studied finches on each island.

•How are they different? What might this mean?

Adaptations

•An adaptation is any inherited characteristic that increases an organism’s chance of survival.

•These are key for outcompeting other organisms.

•Let’s look at a few examples and figure out what purpose they serve.

Adaptation - Porcupine

Adaptation - Hare

Adaptation – Walking Stick

Adaptation - Giraffe

Adaptation - Whale

Natural SelectionSection 15.3

Variation is heritable.

•The variations that evolve over time can be passed on to offspring if an individual manages to reproduce.

Artificial Selection

•Artificial Selection is when breeders select for certain desirable traits over time.

The Struggle for Existence

•All organisms compete with each other to survive.

•Evolution selects for “better” organisms that have a greater chance at making it.

Fitness

•Fitness – the ability of an organism to survive and reproduce in its environment

Survival of the Fittest

•The organisms that are the most fit are the ones who survive. The weak will perish.

Relation to Natural Selection

•Nature selects for the most fit individuals over time – only the fit live to reproduce and pass on their traits!

Common Descent

Homologous Structures

•Structures with different forms that develop from the same embryonic tissue.

•Typically, the more similar two structures are, the closer they are related.

Embryology

•Embryos are very similar to one another from species to species – suggests a common ancestor?

Darwin’s Theory - Summary

•Individuals differ, and variation is heritable.

•Organisms produce more offspring than will survive, and not all will reproduce.

•Organisms compete for limited resources.•Survival of the fittest!•All species share a common ancestor they

descended from.

Genes and VariationSection 16.1

Genes and Variation

•In the 1930’s, biologists began to connect Darwin and Mendel’s work to each other.

•Evolution is controlled by gene frequencies.

Variation and Gene Pools

•Genetic variation is studied in whole populations.▫Population – group of individuals that can

interbreed.•Gene pool – ALL the different alleles that

occur in a population

Variation and Gene Pools

•Relative Frequency – the number of times that an allele occurs in a gene pool compared to the total number of alleles

•Evolution is any change in the relative frequencies of alleles over time.

Variation and Gene Pools•Relative Frequency

of the Green Allele = 12/15 = 4/5

•Relative Frequency of the Purple Allele = 2/15

•Relative Frequency of the Red Allele = 1/15

Sources of Genetic Variation

•Mutations▫Changes to the genetic sequence of an

organism•“Gene Shuffling”

▫Remember that meiosis produces gametes that are all genetically unique

▫Produces many different combinations of alleles but DOES NOT change their relative frequencies in a population.

Single-Gene Traits

•Variation for a trait can be caused by a single gene – you either have the trait, or you don’t.

Polygenic Traits

•Many traits are controlled by several genes together – polygenic traits.

•Depending on how the genes combine, you can have several different phenotypes.

•Generally forms a bell-curve.

Evolution as Genetic ChangeSection 16.2

Review Concepts

•Fitness – an organism’s success at passing on its genes

•Adaptation – a genetically controlled trait that increases an individual’s fitness

•Natural selection determines what individuals survive and reproduces – an organism either passes on ALL genes, or none at all.

Natural Selection on Single-Gene Traits•A forest lizard can appear in 3 colors:

brown, red (mutant), or black (mutant)

•What do you think might happen to the red allele over time?

•What do you think might happen to the black allele over time?

Natural Selection on Single-Gene Traits•Due to natural selection, over time…

▫If an allele increases fitness, it’s relative frequency will increase.

▫If an allele decreases fitness, it’s relative frequency will decrease.

▫If an allele has no effect on fitness, it will not be affected by natural selection.

Natural Selection on Polygenic Traits

•Remember that polygenic traits often form a bell curve.

•The fitness of individuals close to each other on the curve is similar, but can vary over great distances.

•Can be affected in three ways:

Directional Selection

•When individuals at one sight of the curve are more fit than the other side, directional selection occurs.

Stabilizing Selection

•When individuals at the center of the curve have the highest fitness stabilizing selection occurs.

Disruptive Selection

•When the extremes have more fitness than the individuals in the middle, disruptive selection occurs.

Genetic Drift

•Sometimes allele frequency changes simply due to chance – this is called genetic drift.

•Most common in small populations.

Founder Effect

•Genetic drift can also occur when a small group goes off and colonizes a new habitat.

•This is known as the founder effect.

The Hardy-Weinberg PrincipleSection 16.2

Overview

•In order to study how evolution takes place, it can be useful to study a situation where NO CHANGE takes place.

•When allele frequencies remain constant it is called genetic equilibrium. No evolution occurs.

•This is called the Hardy-Weinberg Principle, and is made up of the following five rules.

Random Mating

•All members of the population must have an equal opportunity to produce offspring.

•All alleles have an equal chance of being passed on.

Large Population

•Genetic drift has less of an affect on a large population.

No Immigration

•Immigrating individuals can either bring genes into or out of the population.

No Mutations

•Mutations can affect the allele frequencies in a population.

No Natural Selection

•All genotypes must have an equal chance of surviving and reproducing.

The Process of SpeciationSection 16.3

Speciation

•Speciation – the formation of a new species▫Species – group of organisms that can

breed and produce fertile offspring•Speciation is a product of evolution.•As species evolve, populations become

reproductively isolated and can no longer interbreed.

•3 ways that this can happen.

Behavioral Isolation

•When two populations are capable of interbreeding but do not due to different reproductive behaviors.

•Example: Bird songs

Geographic Isolation

•When a species is split into two new species by geological events it is geographic isolation

•Example: separation by mountains

Temporal Isolation

•When two species do not reproduce due to time differences it is called temporal isolation.

•Example: rain forest orchids

Example of Speciation - Assignment

•You will create a diagram showing the process of speciation in an organism of your choosing. The diagram should be at least 6 steps long.

•There is a good example of what I am looking for on pages 408-410 in your book.

•The book uses finches as their example – you must use something different. Tigers, turtles, dung beetles, aardvarks, whatever floats your boat.