5b:Evolution

5b:Evolution

DarwinDarwin observed several things on his voyage on the Beagle:

Geological oddities: marine shells in the mountains, for example

Fossils: differed from modern examples

Animals: similar to some in England but different as well- the Patagonian hare, for example

DarwinDarwin’s observations:

Geographic change in species: Variation due to environmental differences

Galapagos finches:many species with different beaks, living on different islands, eating different things

Galapagos tortoises: One type of tortoise per island

Adaptation: a feature that allows the organism to survive better in its environment

Natural SelectionDarwin’s observations led him to propose a method by which adaptations might arise: Natural Selection

The 4 steps, revisited:

Heritable variations passed to offspring

More offspring produced than environment can support

Favorable traits in some offspring lead to those having a higher survival rate

Over time, more and more of the population possesses the favorable traits

Natural Selection

IMPORTANT:

Variation arises from RANDOM changes that happen to arise from genetic mutations-- there is no directedness or ability to predict future needs

Some mutations are detrimental, and some are neutral

Natural Selection

IMPORTANT:

Natural selection is ongoing continuously because the environment is constantly changing

Natural Selection

Because resources are always limited, some individuals will fail to survive and reproduce-- thus removing their genes from the population

Failing to reproduce is functionally the same as failing to survive

Natural SelectionFitness varies among individuals

It is the reproductive success of one individual compared to others in the population

More fit animal:

Uses more resources

Avoids death

Leaves more offspring

Natural vs. Artificial SelectionHumans practice artificial selection on domestic animals and plants

Individuals with desired traits are bred, while those with undesired traits are not

Eventually, offspring change to resemble the predetermined idea the humans had

Ex. Any domestic animal, vegetables

Natural vs. Artificial Selection

In artificial selection, the result is predesired, and the breeding is directed

In natural selection, the result is determined by the environment with no direction or desire involved

Natural Selection- TermsSelection pressure: What acts on an animal to either increase or decrease its chances to survive and/ or reproduce

Selected for: the trait conveys advantage

Selected against: the trait conveys disadvantage

Adaptations

Adaptations may take many generations to evolve

Explain why animals are suited to their environment and ‘lifestyle’

The more ‘adaptive’ a trait is, the greater the advantage that the individual has

Convergent Evolution

Convergent Evolution: when unrelated species share similar characteristics

Ex: Flippers on manatees, penguins and sea turtles

Ex: The horny toad of the US, and the thorny devil of Australia

Wallace

Important to realize that Darwin was not the only guy to come up with Natural Selection- Alfred Wallace came to the same conclusions studying different organisms, and they first published at the same time

Evidence for Evolution

Fossils:

Fossils frequently show the pattern of a succession of species from simple to more complex, though this is not universal

Transitional fossils: Archaeopteryx

Whales: see p. 226

Evidence for EvolutionBiogeography: the study of the distribution of organisms

Different mix of organisms when geography separates areas

Ex: Cacti and Spurges in deserts

Ex: Marsupials in Australia

Americas and Australia connected at one time, but marsupials able to evolve separately from placental mammals in Australia

Evidence for EvolutionAnatomy:

Vestigial structures: fully developed in one group, but reduced and nonfunctional in others

Ex: Appendix, tailbone in humans

Ex: Hipbones in snakes

Ex: Wings in flightless birds

These occur because organisms inherit structures from their ancestors

Evidence for EvolutionAnatomy:

Homologous structures: Anatomically similar structures, frequently used for different purposes, explained by a common ancestor

Ex: Horse legs vs. bird wings. vs. bat wings vs. whale flippers vs. human arms

Homologous StructuresExample from humans:

Pharyngeal pouches- seen in all vertebrate embryos

Develop into gills in fish and amphibians

Develop into tonsils, inner ear canal, thymus and glands in humans

It is easier (therefore more likely to arise randomly) to modify an existing structure than create an entirely new one

Evidence for Evolution

Molecular Evidence:

All living organisms use the same biological molecules-- DNA, RNA, ATP, etc.

DNA code is the same for all living organisms

Many of the same genes have been modified to result in the wide variety we see

Gene sequences are more different the further apart organisms are evolutionarily

Evolution on a Small Scale

Remember: individual organisms can not evolve, populations/ species evolve

Microevolution: small measurable changes in a population from generation to generation

Hardy-Weinberg EquilibriumHardy-Weinberg Equilibrium allows us to measure small changes in the frequency of alleles in a population

This is how we know microevolution is occurring

Population genetics is the study of the occurrence and flow of genes in populations

We will look at the example of peppered moths

Peppered MothsPeppered moths can be either light or dark, and this is controlled by a single set of alleles:

D= dark color

d= light color

We know the frequency of genotypes in the population:

4% DD

32% Dd

64% dd

Peppered MothsFrom genotypes, we can figure out the frequency of each allele in the population:

.04 + .16 =.2 D

.16 + .64 =.8 d

Frequency of each gamete type will be the same as the frequency of occurrence of each allele

We can use a Punnett square to figure out the gene frequencies in the next generation

Peppered Moths

.2D.2D .8d.8d

.2D.2D .04DD .16Dd

.8d.8d .16Dd .64dd

** This is a punnett square for freqencies, not individuals

Peppered MothsNotice: the allele frequencies in the next generation are EXACTLY THE SAME

This means that sexual reproduction alone can not change the frequency of gene/ allele frequencies in a population, provided some assumptions are met

Notice the dominant allele does not increase in frequency

This is Hardy-Weinberg equilibrium

The Assumptions

1. No mutations: allelic changes do not occur

2. No gene flow into or out of the population

3. Random mating

4. No genetic drift- large population, changes in frequency due to chance are insignificant

5. No selection: one genotype is not favored over another

HW Equilibrium

However, those assumptions are basically never met

Changes in allele frequency do occur, and we can see those by comparing RL to the numbers predicted by HW equilibrium

Also, we know that deviation from the assumptions is what causes evolution

Back to the moths

After the industrial revolution, peppered moths that were light did not blend into trees with lots of soot on their bark, and they were eaten by birds

The frequency of D then increased in the population to around 80%

Causes of microevolution

Genetic mutations

The ultimate source for allele differences

Mutations can be harmful in one environment and helpful in another

Causes of microevolutionGene flow:

Movement of alleles among populations by migrating animals

Can increase variation in a population by introducing new mutations

Can prevent speciation by making the gene pool the same across populations

Causes of microevolution

Nonrandom mating:

Assortive mating: individuals mate with those that are similar

Ex. tall people mating with each other

Causes two groups of homozygotes to become more common, and hets to become less common

Sexual selection: Favors characteristics that increase the chance of mating

Ex. Male birds with bright colors or crazy feathers

Animals that compete for mates

Causes of microevolutionGenetic drift: changes due to chance

Allele frequencies ‘drift’ over time

More common in small populations, like on islands or other isolated areas

Can result in the loss of rare alleles completely, and fixation of others as they are the only ones left

Causes of microevolution

Bottleneck Effect: Catastrophic loss of most of a population, only a few individuals survive by chance

Those few are all that is left to pass on to later generations

Ex. cheetas and poor sperm

Founder Effect: Rare alleles are more common in a population isolated from the main population

Only a few individuals founded the new population, so their alleles are the ones that are represented

Ex. Amish have higher rate of two limb mutations

Causes: Natural SelectionThere are several types of selection, so it gets its own category

Directional Selection: an extreme phenotype is more successful and population shifts in that direction

Ex. the peppered moths

Ex. drug resistance in bacteria

Ex. Anacondas

Natural SelectionStabilizing Selection: an intermediate phenotype is best, extremes are selected against

Ex. male anacondas

Disruptive Selection: Two or more extremes are favored over the intermediate

Favors polymorphism: the occurrence of different forms in the same species

Ex. Snail with two different shell colorings, in two different habitats

Perfection?NO!! Animals do NOT tend to be perfectly adapted

Evolution does not start from scratch, it is modifying what is already there

Compromises: there may be costs to the adaptive benefit

Sexual selection may not result in adaptive traits- giant feathers may be energetically expensive and make it difficult to fly, for example

Maintenance of Variation

Populations will always show variation

New mutations always arising

Gene flow may be occurring

Diploidy and heterozygotes:

Only alleles that are expressed in phenotype can feel selective pressures

Heterozygous animals can protect alleles that might otherwise be selected against

The homozygous recessive will still occur occasionally

Example of Sickle CellsIndividuals with sickle cell are homozygous for S and typically die very young due to change in shape of RBCs

Individuals that are heterozygous are OK, because cells are normal shape until in low O2 environment

Individuals that are homozygous A are usually the most fit (most of us fall into this group)

Sickle Cell However, individuals with African descent have higher frequency of S- more sickle cell disease, but also:

Heterozygote is immune to malaria

So, in parts of Africa w/ malaria, heterozygote is selected for, but homozygotes continue to exist to do statistical frequency of each genotype in the next generation