Evolution of Populations:The Origin of Species

How do species change and new species come about?

Getting the terms right: What is a population?

What is a species? What is a gene pool?

• Population - same species, same place, same time• Species - members of the same population that can

interbreed and produce fertile offspring.. Reproductive compatibility is key.

• Gene pool - the collection of all of the genes within a population.

• Populations evolve. Individuals do not!

Natural selection works on individuals, but populations/species evolve

• Geospiza fortis (Galapagos ground finch of Daphne Major)

• In 1977, 180 of 1200 individuals survived a drought.

• Larger harder-shelled seeds survived the drought and so did the bird with the deepest and and larger beaks.

• Evolution occurred - allele frequencies change in a population

Figure 23.1

How do Populations and Species change?

2 Primary Sources of Genetic Variation

• Mutations – creates variation

• Sex - shuffles the deck and spreads mutations around…..

Three basic mechanisms of evolutionary change

• 1. Natural Selection

• 2. Genetic Drift

• 3. Migration

(Gene Flow or Gene Migration)

Evolution in Genetic Terms

In genetic terms, evolution is any change in the relative frequency of alleles in a population.

This can be measured. +++++

As in albino otters …..

“Fitness”Darwin called the ability of an organism to survive and reproduce in its specific environment “fitness.”

Sexual Selection – the other half of “fitness” is producing offspring!

• Darwin was stumped by some characteristics until he understood the big role of sexual selection in natural selection

• “The sight of a feather in a peacock’s tail…makes me sick!”

• Handicap…

Reproductive Strategies based on level of investment

• In vertebrates….

• Female makes larger investment in offspring ! ….Choosy about mates

• Males make relatively small investment in offspring. Larger investment not required (no pregnancy, lactation…) and paternity not always guaranteed, which discourages investment. Wants as many mates as possible, in evolutionary theory.

2 different types of sexual selection

• Intrasexual selection - competition for mates (usually between males)

• Intersexual selection – mate choice (usually females choose the best provider and protector)

Choosing a mate is complicated - female

sometimes has a tough choice• High testosterone

creates bright feathers and aggressive behavior.

• Helpful for protection – but may not make a caring father.

Natural Selection Trends Most traits are controlled by more than

one gene and are called polygenic. When we graph their distribution in a

population , it looks like the classic Bell Curve…

For example, human height is controlled by over 220 genes

and distributes as a Bell Curve:

3 Modes of Selection that skew the Bell Curve:

(a) Directional selection shifts the overallmakeup of the population by favoringvariants at one extreme of thedistribution. In this case, darker mice arefavored because they live among darkrocks and a darker fur color conceals themfrom predators.

(b) Disruptive selection favors variantsat both ends of the distribution. Thesemice have colonized a patchy habitatmade up of light and dark rocks, with theresult that mice of an intermediate color areat a disadvantage.

(c) Stabilizing selection removesextreme variants from the populationand preserves intermediate types. Ifthe environment consists of rocks ofan intermediate color, both light anddark mice will be selected against.

Phenotypes (fur color)

Original population

Originalpopulation

Evolvedpopulation

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There are other sources of evolutionary change besides

natural selection….

Genetic Drift and Gene Flow

Genetic DriftRandom change in allele frequency. Caused by a “freak” occurrence that

randomly (not selectively!) reduces the population number and therefore the

genetic variation

• Bottleneck Effect – when a disaster or other occurrence severely reduces the population in one generation

• Northern Elephant Seals – hunted extensively

• -loss of variation• -fixed harmful alleles

Genetic Drift – another cause • Founder Effect • Loss of genetic

variation that occurs when a few individuals from a larger population colonize an isolated new area

• Trista da Cunha – island colonized by 15 British in 1814 – high rate of inherited blindess

Gene flow

• Gene flow/ gene migration - movement of fertile individuals/gametes between populations

• Reduces differences/variation between populations – think of humans in the U.S. today….

• Makes populations more homogenous

• Cheetahs and pandas

• How does gene flow affect speciation?

Which is more likely to have a higher rate of gene flow – a

bird or a plant?

A human in the 21st Century or a human in the

16th Century?

Interrupting Gene Flow to form a new Species: Allopatric Speciation and Sympatric Speciation

• Allopatric - geographic isolation/barrier interrupts gene flow between populations (antelope squirrels and the Grand Canyon)

• Over time, the separate populations acquire more and more mutations until interbreeding impossible

Sympatric Speciation – Rare!

• New species arises in the midst of the parent population

• Usually due to disruptive selection (Lake Victoria Cichlids – caused by sexual selection? )

• More common in bacteria and plants than animals – faster genetic changes

MACROEVOLUTION: the big picture over the looong termBig Patterns and Ideas in Macroevolution:• Extinctions – some individual species due to natural selection

and some massive due to big environmental changes• Adaptive Radiation – a single species into many diverse

forms and species (Darwin’s finches; also dinosaurs then mammals….)

• Coevolution – interconnected species evolve in response to each other (like the crabs and snails)

• Evolution of Development - “Evo-Devo” - a few “master control genes” guide cell differentiation in embryo. One gene can have huge impact.

• PACE OF SPECIATION?

• Gradual or quick? No consensus – still debated

• Gradualism vs. Punctuated Equilibrium

• Although there exist some debate over how long the “punctuations” last, supporters of punctuated equilibrium generally place the figure between 50,000 and 100,000 years. In other words, its still pretty gradual!

How are differences between species preserved?

• Reproductive isolation – individuals of different species cannot interbreed

• But why?

• What are the barriers to gene flow?

Prezygotic Barriers• habitat isolation

(butterflies in canopies; hair lice and pubic lice!)

• behavioral isolation (bird song; firefly flash rhythms),

• temporal isolation (mating times, seasons)

• mechanical isolation (great danes and pugs; flower anatomy)

• gamete isolation (incompatibility)

Postzygotic barriers - problems after mating

Hybrid (offspring) viability – offspring don’t survive

Frogs in a pond Hybrid fertility – offspring

sterile

Ex. Horses (Chr.# 64) and Donkeys (Chr. # 62) produce sterile mules with 63 chromosomes.

Hybrid Breakdown – future generations weak or sterile

Exaptations

• New use for a preexisting structure - Ex. vestigial structures (penguin wings became flippers), reptilian vs. mammalian jaw

• Mechanism for novel features to arise gradually from a series of intermediates

Clines• Gradual change in a trait along a

geographic axis – like Moose –