Evolution: How species have changed over time

First a Perspective of Time

Those who influenced Darwin

Charles DarwinWas a Naturalist – mostly observed

organisms in their natural habitats rather than conducting experiments.

Made most of his observations on the Galapagos Islands

Charles Darwin

Did much of his work in the Mid-1800’s** Keep in mind this is BEFORE

Mendel, Watson and Crick***

Charles Darwin

Introduced the idea of Natural Selection as a way for new species to form (speciation).

Published The origin of Species in 1859

The Theory of Natural SelectionAssumptions:

There are not enough resources for all to survive

genetic variation exits in all populations.

Results: 1. Competition2. Survival of the fittest3. Descent with modification

Assumption 1: Not enough resources

What resources are we talking about?

Are there enough for everyone?

Food Shelter Suitable Mates

Assumption 2: Genetic variation exists

Where do these differences come from?

Remember it doesn’t have to be a NEW gene, just a new combination of genes

Mutations Genetic Recombination

Sexual reproduction

Migration

Result 1. Competition

What are we competing over?

Who wins? What is the prize?

What happens to those that don’t win?

Result 2. Survival of the FittestIn nature are we all really equal?

What do we mean by “fittest”?

Is it enough to survive?

Result 3. Descent with ModificationBreak it up, what does it mean?

What happens to the frequency of fit genes and unfit genes?

What do we see in future generations?

3. Descent with ModificationNew generations will resemble

previous generations (descent) BUT

more individuals will have the “best” variation PLUS new mutations and combinations (with modification)

An Example

Example: What is the genetic variation? What is the selective pressure? Who has the advantage? What would we predict for the next

generation? Why might the “unfit” phenotype stick

around?

Rules of EvolutionMutations and their phenotypes are

random. Meaning?

Variation must exist in the population BEFORE selective pressure occurs

Rules of EvolutionIndividuals can not evolve, only

species

A fit trait in one environment might be eliminated as a weakness in another

Types of Selection

Natural SelectionWhat determines which variation gets passed on?What is the outcome?

Artificial Selection (a.k.a. selective breeding)What determines which variation gets passed on?What is the outcome?

Types of Selection

Directional Selection: One extreme or the other is “favored” and increases in frequency while midrange and other extreme decrease

Types of Selection

Stabilizing Selection: Midrange is favored and increases in frequency while both extremes decrease.

Types of Selection

Diversifying/disruptive Selection: Both extremes are favored and increase while midrange decreases.

At what point is a new species formed?

Evolution – change in allele frequency

Speciation – such change that new population is a different species

– two organisms that can successfully reproduce and produce viable, fertile offspring

Examples:

Cross between a Pug and a Beagle - different breeds but SAME species

Examples:

Offspring: Puggle! Both viable (obviously) and fertile

Examples:

Cross between a Horse and Donkey - different species

Examples:

Offspring: Mule! Viable but infertile

Gene Pool Isolation

Two populations become separated so their genes are no longer mixed

Mutations appear independently in each population

Selection happens independently in each population

Mechanisms of IsolationGeographic – Physical barrier separates two

populations

Behavioral – mating behaviors of some are not attractive to others.

Temporal – fertility occurs at different times

Mechanical – different physical means of reproduction

Principle of a Common Ancestor

Descent with Modification – over generations descendents can look quite different from ancestors.

Thus, organisms that seem very different might share a common ancestor

Suggests if you go far enough back, we are all related!

Phylogenetic tree: Family Tree of Life

Common ancestorHumans and chimps have a common ancestor.

THAT IS NOT THE SAME AS SAYING WE WERE ONCE CHIMPS!!!

Think about it: Do you and your cousin share a common ancestor? Does that mean you are your cousin? Does that mean that either of you are that ancestor?

Evidence of Common ancestryComparative AnatomyComparative EmbryologyComparative Biochemistry

See Determining evolutionary relationships assignment

Evidence of a Universal Common Ancestor

What do we ALL have in common

Additional Evidence of Evolution (but not necessarily common ancestry)

Fossil RecordPreserved

remains of ancient life in sedimentary rock

Even of species no longer in existence (most!)

Fossils

Fossils are often found in the layers of sedimentary rock.

See changes in fossils over time

Dating FossilsAbsolute Dating: Using radioactive

organic material in a sample we can get a more accurate age of a fossil

Dating FossilsRelative Dating: Fossils found in

lower levels are older than upper levels.

Can’t provide exact age, just which is older

Dating FossilsAbsolute Dating: Radioactive

organic material is used to get a more accurate age of a specimen.

Radioactive material decays into a non-radioactive decay product at a steady rate.

Half life = time it takes half a sample to decay.

Example: Some carbon is naturally radioactive – C14.

Half life of C14 – 5,730 years

Decay product is N14

If we look at the sample and determine the ratio of C14 to N14 we can get an idea of how much time has passed

C-14 remaining

C14:N14 Years from start date

100g 1:0 0

50g 1:1 5,730

25g 1:3 11,460

12.5 1:7 17,190

Assume we start with a sample that is 100g of C14

Geographic DistributionBiogeography and Convergent

Evolution:

See Determining evolutionary relationships assignment

Vestigial Organs

Structures that serve little to no purpose NOWSnake skeletons with leg bones and

pelvisBlind, cave-dwelling fish have eye-

sockets but no eyes.

Vestigial OrgansGives insight into PAST needs of

organism as well as where this organism has come from

What happens first: Need for organ disappears? Or mutated organ appears?

Genetics in EvolutionDarwin did his work before Mendel

and didn’t understand genes or how inheritance worked.

Thanks to Mendel we know how/why traits get passed from parent to offspring

Phenotypes NOT genotypesNatural selection acts on phenotypes

NOT genotypes

But in turn will influence allele frequency.

Why aren’t all bad alleles eliminated??

Mechanisms of Evolution

Remember, it is variation that proposes and selection that disposes

Mechanisms of Evolution

Genetic DriftEvolution without natural selection

Chance occurrences change allele frequency

More common in small populations

What if more of the “unfit” survive?

Genetic Drift Founder Effect

Founding Population B

DescendantsSample of

Original Population

Mechanisms of EvolutionEndosymbiotic theory• Mitochondria and chloroplasts evolved

from free living prokaryotic organisms

• A larger cell engulfed them

• A symbiotic relationship formed

Endosymbiotic theory

Evidence of endosymbiosisBoth have their own DNA and produce

their own proteins

Both reproduce independently from the cell through a process like binary fission (bacterial reproduction)

Double membranes of both are similar to prokaryotic membranes

Patterns of EvolutionMass Extinction

Periodic large-scale extinction events

Dramatically changes landscape eliminating or creating selective pressures

Patterns of EvolutionAdaptive Radiation

Single species evolves into several different species that live in different ways (adaptations)

Patterns of EvolutionCo-evolution

Due to close relationship two species share with each other, change in one organism results in a change with the other.

Patterns of Evolution

GradualismWhat Darwin subscribed toTiny changes accumulate over

huge period of time to yield large changes.Think Grand Canyon only organisms

Patterns of Evolution

Punctuated EquilibriumMore modern theory proposed by Gould

and Eldridge

Proposed change occurs in spurts followed by periods of stasis

More support in fossils!

Are organisms always evolving?Hardy Weinberg Equilibrium – suggests no!

Under certain conditions, populations won’t evolveConditions: 1. Large population2. No migration in or out3. No natural selection4. Random Mating5. No net mutations

How do we tell? Determine allele frequencies over different

generations and see if they changep = frequency of dominant alleleq = frequency of recessive allele

p + q = 1

p2 + 2pq + q2 = 1 p2 = frequency of homozygous dominant q2 = frequency of recessive genotype2pq = frequency of heterozygote

Example problem: A population of aphids can either be brown or

green. Green is recessive. In a population of 1000 aphids 250 are green. What are the allele frequencies for the green and brown alleles?

Then figure out the homozygous dominant and heterozygote populations too.