How Organisms Evolve

Preface

• Natural selection is a mindless, mechanical process• some individuals reproduce more successfully than

others and that is it• therefore, their traits are more prevalent in the next

generation• Darwin understood this but he had NO knowledge

of the mechanics of heredity• today, we do

How Are Populations, Genes, and Evolution Related?

• Understand that the changes observed as an individual grows and develops are not evolutionary changes

• Evolutionary changes are those that occur from generation to generation; the changes that cause descendants to be different from their ancestors

• Furthermore, these changes cannot be detected by looking at individuals

• You must look at many individuals across many generations

• Evolution is, clearly then, a property of populations

• But, populations are comprised of individuals

• And, it is the actions and fates of them that determine what traits will get passed on

• Therefore, inheritance provides the link between the lives of individuals and the evolution of populations

A Quick Review of the Principles of Genetics (unit 2 starting on page 141)

• 1. Genes, influenced by the environment, determine the traits of each individual– there are usually more than one

allele(nucleotide sequence) for any given trait. Different alleles generate slightly different forms of the same enzyme which leads to slightly different phenotypes such as, blue eyes, green eyes, and brown eyes

• 2. The gene pool is the sum of all the genes in a population

• In other words, the gene pool consists of ALL the alleles of ALL the genes in ALL the individuals of that population

Example

• In a population of 100 pea plants(sorry), the gene for flower color would have 200 alleles (peas are diploid so there are 2 color alleles per plant)

• All those alleles can produce only 2 phenotypes: purple or white

• Analysis of the population shows that some individuals have alleles for white flowers some for purple and some have both

continued

• If we add up the color alleles of all the plants in the population, we could determine the relative proportions of the different alleles. This is called the allele frequency.

• So, if the gene pool for flower color consists of 140 alleles for purple and 60 for white the allele frequencies would be…

• correct! 0.7 or 70% for purple and 0.3 or 30% for white

• 3. Evolution is a change of gene frequencies within a population

• many people define evolution as a change in outward appearances or behaviors of members of a population

• But changes in phenotypes are outward expressions of changes in the gene pool

• So, evolution is nothing more or less than a change in the genetic makeup of populations over generations.

Hardy-Weinberg Equations

The Hardy-Weinberg Principle

• States that both allele and genotype frequencies in a population remain constant

• that is to say, they are in equilibrium from generation to generation unless disturbing influences are introduced

Outside of the lab...

• …these disturbing influences are always in effect

• Therefore, Hardy-Weinberg equilibrium is impossible in nature

In the simplest case...

• …of a single locus with two alleles

• the dominant allele is denoted by T and recessive by t (we’ll use “t’s” because of taste)

• Their frequencies are denoted by p an q

• frequency (T) = p and frequency (t) = q

• p + q =1

If the population is in equilibrium...

• then we will have frequency (TT) = p2 for the TT homozygotes in the populations

• frequency (tt) = q2 for the tt homozygotes

• frequency(Tt) = 2pq for the heterozygotes

Let’s Look At It This Way

• Alleles for any given individual are chosen randomly and independent of each other

• Consider two alleles A and a, with frequencies p and q respectively

• The different ways to form new genotypes can be derived using Punnet Squares

A (p) a(q)A(p) AA(p2) Aa(pq)a(q) Aa(pq) aa(q2)

• so the formula is p2 + 2pq +q2 = 1

The Equilibrium Population

• Lets look at it from a different angle

• What if there was a population that did not evolve?

• What would the characteristics of this population have to be?

• The equilibrium population is a hypothetical population that models this

The Characteristics of This Hypothetical Population

• 1. There must be NO mutation

• 2. There must be NO gene flow between populations (that is, there must be no net migration of alleles into the population or out of the population)

• 3. The population must be very large

• 4. ALL mating must be random, with NO tendency for certain genotypes to mate with specific other genotypes

• 5. There must be NO natural selection; that is, ALL genotypes must be equally adaptive and reproduce equally

• Under these conditions, allele frequencies within a population will remain the same indefinitely

• If one or more of these conditions are violated, allele frequencies will change and EVOLUTION WILL OCCUR.

What Causes Evolution?

• Based on the Hardy-Weinburg equilibrium, we can state that evolution occurs due to five major causes– mutation– gene flow– small population size– non-random mating– natural selection

Mutations Are the Ultimate Source of Genetic Variability

• Mutations are inevitable

• Cells have a pretty efficient system for protecting the integrity of their genes

• Some changes in the nucleotide sequence slip past the check and repair systems

• When these changes occur in the cells that produce gametes, the changes can enter the gene pool and be passed on to the next generation

Unfortunately, wedon’t get

superpowersfrom this.

I don’t have this issue.

I do have this one though.

Mutations are rare

• They occur once in every 100,000-1,000,000 genes per individual in each generation

• Therefore mutation alone is not a big factor in evolution, but...

• ...mutations are the source of new alleles, new heritable variations on which other evolutionary factors can act

• as such, they are the foundation of evolutionary change

Without mutations, there would be no evolution and no diversity

among life-forms.

MUTANTSRULE!!!

Mutations Are NOT Goal-directed

• They do not arise as a result of or in anticipation of environmental necessities

• They simply happen

• This may result in some change in structure and/or function of the organism

• Whether they are helpful, harmful or neutral depends on the environment over which the organism has little control

• The mutation provides potential

Other forces..

• Such as migration and natural selection that act on the potential may favor the spread of the mutation through the population or eliminate it.

Gene Flow Between Populations Changes Allele Frequencies

• Gene flow: When individuals move from one population to another, and interbreed at the new location, alleles are transferred from one gene pool to another.

• This alters the distributions of alleles among populations

Example: Baboons

• Baboons live in social groups called troops

• Within each troop the females mate with a handful of dominant males

• Young males usually leave the troop and join another troop and perhaps become dominant

• With this, the males carry genes from one troop to another (gene flow)

By the way, baboons are big and strong and scary.

See what I mean?

What the hell is this?

Where is your lip? What is going on here?!

Gene Flow Can Have Very Significant Effects

• Gene flow spreads advantageous alleles throughout the species

• Gene flow helps maintain all the organisms over a large area as one species

Genetic Drift

• Random events eliminate disproportionate numbers of individuals

• This can change the allele frequency in the population

• Most organisms DO NOT do this

• Only certain members of the population will mate

• Therefore the next generation will only be offspring of this select group, whose allele frequencies may differ from the population as a whole

Random Mating

• A form of genetic drift in which a population becomes extremely small

• This leads to differences in allelic frequencies and a loss of genetic variability

Population bottleneck

The Founder Effect

• A type of genetic drift in which an isolated population founded by a small group of individuals may develop allele frequencies that differ from the parent population as a result of chance inclusion of disproportionate numbers of certain alleles in the founders

Neanderthal Skull: Check out that nose....FOUNDER EFFECT!!!!!!!

Survival and Reproduction Influenced by Phenotype

• Phenotype depends, at least, partly on genotype

• Natural selection tends to favor the reproduction of certain alleles at the expense of others

Extinction

Natural Selection Can Cause This Too

• It is estimated that 99% of all the species that ever existed are extinct

• Extinction is almost always caused by environmental change

• the 3 major enviromental causes of extinction are– competition among species

– introduction of new predators or parasites

– habitat destruction