EEB 2244: Evolution and Adaptation

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Evolution and Adaptation

Overview:

Evolution depends on phenotypic and genotypic variation

Different processes contribute to evolution

Speciation

Artificial Selection

Evolution:

Evolution: change in genetic composition of a population over time

Allele frequencies

Phenotypic variation that is inherited (genetic) is the basis for evolution

Can result in beneficial, detrimental, or neutral traits

Phenotype:

The observable traits of an individual organism:

morphological, physiological, behavioral, or phenological characteristics

Genes, environmental effects and their interaction (plasticity)

Traits may be discrete or continuous.

Phenotypic Variation in Corn (maize), peppers (capsicum)

Variation in phenotype doesn't need to be visible (ex. flavor! Nutrient! Hormones!)

Variation has biological meaning: for example, flavor variation often comes from defense chemicals

Genotype

Genes: regions of DNA coding for particular proteins

phenotypes can be determined by one or many genes (polygenic)

polygenic control ---> continuous variation

Primary Source of Genetic Variation ?

Mutation: a random permanent, change in nucleotide sequence of DNA

may be neutral, beneficial, or harmful

most are neutral (silent/synonymous)

One gene can affect more than one trait: pleiotropy

Other Random Sources of Genotypic Variation

Independent assortment: during meiosis homologous chromosomes separate randomly

Recombination through crossing over: homologous chromosomes overlap during metaphase, exchanging nucleotides.

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Evolution:Evidence and Processes

Multiple Scales and Processes:

Macroevolution: species level or higher

Microevolution: populations

Different processes

selection: natural and artificial

random processes: mutations, genetic drift, founder effects, bottlenecks

Evidence for Evolution:

Fossil record

Recent speciation in plants and insects

Evolution of resistance

antibiotic resistance in bacteria

pesticides resistance in crop and human pests

Natural Selection:

Not all offspring survive to reproduce

Population has heritable variation

Change in the environment causes a change in selection pressures

A phenotype determined by a certain allele(s) becomes favorable

Organisms with that allele are more likely to survive and reproduce

Next generation offspring are more likely to have that allele-->more common in the population

Selection Pressures:

Can be "man-made" or influenced by human activity

Ex: Industrial melanism: industrial activities cause habitats to become darker due to pollution; individuals possessing darker phenotypes are favored by selection

still natural selection

Directional Selection:

Individuals at one extreme are disadvantage

Frequency curve shifts on direction of favored phenotype

Mean (average) value of the phenotype changes

Stabilizing Selection:

Intermediate phenotype favored

Curve becomes thinner, taller

decreased variation

Removes harmful genetic variation

Also when environment is relatively unchanging

Disruptive Selection:

Individuals at both extremes favored

Relatively uncommon

Increased phenotypic variation

Can lead to biological speciation

Evolution by Natural Selection is not an Intelligent Process:

3.8 billion years of life

~99% of all species ever to exist have gone extinct

~10 million success stories

Evolution by Natural Selection does not...

Predict

Environmental changes are unpredictable

Plan or Design

Random chance (genetic drift) and natural selection interact

Selection edits existing variation, doesn't invent solutions

Perfect

Selection is limited by historical constraints of biological, physical, and chemical characteristics--"evolutionary baggage"

Multiple selective factors operate simultaneously to influence adaptations--trade offs

Random Processes:

Mutations: neutral, harmful or beneficial

Genetic drift: random fluctuations in a gene pool over time can cause variation to be lost

important in small populations

Bottleneck effect: reduction in genetic diversity due to large reduction in population size

Founders effects: small number of colonizers start new population with small amount of genetic variation

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Speciation

Species come from a common ancestor

Phylogenetic tree represent patterns of relatedness among different groups

Premise: more similarities--> more closely related

What is a species?:

group of organisms that naturally interbreed with each other and produce fertile offspring

No single definition works for all organisms

Some organisms don't require interbreeding

Sometimes separate species can interbreed

Allopatric Speciation:

The evolution of new species due to geographic isolation

Most common mechanism of speciation

Many (not all) of Darwin's finches

Sympatric Speciation:

Evolution of new species without geographic isolation.

Can be caused by small-scale habitat variation, resource diversity.

Polyploid:

Polyploid: an organism that has 3+ sets of chromosomes (homologous chromosomes fail to separate during meiosis producing a diploid gamete)

Genetically distinct from parents

Unable to breed with a diploid

Can cause sympatric speciation

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Artificial Selection

What is Artificial Selection?:

Artificial Selection: selection in which humans decide which individuals will breed; done with a preconceived goal for the traits in the population

Same genetic process as natural selection, but

human preferences affect the evolution of particular populations or species

Selective breeding means changes in gene frequencies can happen quickly