d 00 Ch+24+Origin+of+Species+Sp11 BIOL221

8/6/2019 d 00 Ch+24+Origin+of+Species+Sp11 BIOL221

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Chapter 24

Origin of Species


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Darwin referred to the first

appearance of a new organismon Earth as the mystery of mysteries.

Today, the process by which one species splits into two

or more species is called speciation.

Speciation


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Speciation forms a

conceptual bridge

between

microevolution ,

changes over time in

allele frequencies in a

population, and

macroevolution, the

broad pattern of evolution

over time.


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Biological Species Concept

Simple definition = a species is one kind of organism

As described by Ernst Mayr in 1942, species is a group of

populations whose members have the potential to interbreed

in nature, and produce viable, fertile offspring but do not

produce viable, fertile offspring with members of other suchgroups.


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The formation of a new species hinges on

reproductive isolation

Reproductive Isolation is barriers that prevent members

of two species from producing viable, fertile offspring.


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Reproductive Isolation

Barriers block gene flow between the species

and limit the formation ofhybrids,

offspring that result from interspecific mating


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What holds the gene pool of a species together even

when they are divided up into separate populations?

What if the populations are long distances apart?The answer is gene flow, the transfer of alleles between

populations.


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If researcher, Scott Edwards found

one population had an allele that

shared a recent common ancestor with

alleles found in a different population,

he reasoned that gene flow must

have occurred.

Do the data indicate gene flow

occurred as the transfer of an allele

from population A to B or the reverse?


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Reproductive Barriers

Prezygotic barriers= block fertilization

= no zygote


10/37Water vs terrestrial

Mate in winter

vs summer

high step courtship

display

Genitals not well

matched


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Postzygotic barriers = after fertilization occurs and the

zygote is formed.


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Sperm cannot

fertilize egg

Incomplete

development of

hybrids

Donkey + horse

= sterile mule

Viable & fertile

hybrids produce

small, sterile

offspring in next

generation


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Limitations of the Biological Species Concept

Can not be applied to Fossils

Can not be applied to organisms that reproduce asexually

Species = populations whosemembers have the potential to

interbreed in nature, and produce

viable, fertile offspring


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Other Definitions of Species

Morphological Species concept = species have common

body shape and other structural features

Actually the way most species are distinguished

Ecological Species concept = views a species in terms of

its ecological niche, the sum of how members of the speciesinteract with the nonliving and living parts of their

environment

Phylogenetic Species concept = defines a species as thesmallest group of individuals that share a common ancestor,

forming one branch on the tree of life.

More than 20 other species definitions have been proposed


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Speciation can occur in two main ways;

Allopatric (other country) Speciation

Sympatric (same country) Speciation


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Allopatric Speciation

Allopatric Speciation occurs when gene flow is interrupted

when a population is divided into geographically isolatedsubpopulations

Once geographic isolation has occurred, the separated gene

pools may diverge through normal mechanisms;

Different mutations arise

Natural selection acts on the separated organisms

Genetic drift alters allele frequency


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Geographic isolation leads to reproductive isolation.

Reproductive isolation may arise as a byproduct of

selection or drift having caused the populations to diverge

genetically.

This isolation must result from barriers that would

prevent reproduction even if the different populationscome in contact with each other.

Allopatric Speciation


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Allopatric speciation of antelope squirrels on opposite rims of the

Grand Canyon.

Harriss antelope squirrel inhabits the south rim and white-tailed antelope squirrels

live on the north rim.

Would birds that live in this area be likely to diverge into new species?


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After 40 generations

natural selection resulted

in divergent evolution

Flies raised on starch

digested it better and flies

raised on maltose digested

maltose better


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Sympatric Speciation

Sympatric Speciation occurs in populations that live inthe same geographic area.

Gene flow can be reduced by;

Polyploidy

Habitat differentiation

Sexual selection


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Polyploidy

Polyploidy is a condition of having extra sets of

chromosomes due to a mistake during cell division.


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Second type of polyploidy can occur when two different species

interbreed and produce hybrid offspring. Usually sterile because

chromosomes cant pair during meiosis.

Sometimes a fertile polyploid may develop and since these polyploidsare only fertile when mating with each other, they represent a new

species.


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Polyploid speciation rarely occurs in animals,

but it is common in plants.

Estimates are that about 80% of Earths plant species

are the result of polyploid speciation.


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Sexual selection

Sympatric speciation may be driven by sexual selection.

How did so many different cichlid fish (up to 600 species)

originate in Lake Victoria in East Africa over a period of only

about 100,000 years?


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Does sexual selection in cichlids result in reproductive isolation?

Under normal light female cichlids strongly preferred males of their own

species.

Under monochromatic orange light the two species are very similar incolor. Under orange light females showed no preference in mating. The

resulting hybrids were viable and fertile


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Hybrid zones

Hybrid zones are regions where members of different

species meet and mate, producing at least some offspring

of mixed ancestry.

Some hybrid zones are narrow bands, like between fire-bellied and yellow-bellied toad ranges.


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Yellow-bellied

toad alleles


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Three possible outcomes for hybrid zone over time


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1) Reinforcement:

Strengthening Reproductive

Barriers

Example: Two species of European

flycatcher

In sympatric populations (potential for

hybrid zone) females are super good

at telling males of each species apart

In allopatric populations ??


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2) Fusion: Weakening


Fusion occurs when reproductivebarriers are so weak that gene flow is

common between gene pools and

eventually may lead to a new species

forming.

May be happening in Lake Victoria,

where mate choice has kept many of

the similar species of cichlids

reproductively isolated.


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3) Stability: Continued Formation of Hybrid Individuals

Remember the Bombina toads, since the hybrids were at a disadvantage why

hasnt reinforcement occurred, with strengthened barriers?

In over 20 years of study no evidence of reinforcement.

Perhaps the narrowness allows more gene flow from outside the zone,

overwhelming selection for isolation within the zone.


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Speciation can occur rapidly or slowly and can result

from changes in few or many genes.


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Time between speciation events = time from formation of

a new species until it begins to diverge into more than one

species, plus the time it takes for speciation to be completeonce divergence has begun.

= Time elapsed between New species #1 created

until New species #2 created

Survey of 84 plants and animals showed the interval

ranged from 4,000 to 40 million years.

The average was 6.5 million and rarely took less than500,000 years.


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Genetics of Speciation

Evolution of as little as one gene can cause reproductive isolation.

In some Japanese snails a single gene controls the direction the shells spiral.

When their shells spiral in opposite directions the snails genitals are oriented in

a manner that prevents mating.


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Studies have shown that a very few genes

or many genes can influence the

evolution of reproductive isolation

and the emergence of a new species.

Bornean clouded leopard

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