Variation Chap 10

8/13/2019 Variation Chap 10

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CHAPTER 10

VARIATION


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Variation

Outline

The nature of species

Prezygotic isolating mechanisms

Postzygotic isolating mechanisms

The geography of speciation

Darwins finches

Problems with the biological species concept


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knowledge in species is the fundamentals importance

to evolutionary biology

any concept of species must account for 2 phenomena:

i. distinctiveness of species that occur together at a

single locality

ii. connections among population of the same

species that are geography separated


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The distinctiveness of sympatric species

species that occur together (sympatric) are distinctive

entities that are phenotypically different, utilize

different parts of the habitat and behave separately

sympatric species that appear to be nearly identical

are called sibling species.



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The biological species concept

if sympatric species exchanged genes, such species are

rapidly lose their distinctions

- gene pools of different species become homogenized

if the geographically distant populations share genes, it

will keep the populations integrated as members of the

same species


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Ernst Mayr coined the biological species concept

which define species as:

.. groups of actually or potentially interbreeding natural populations

which are reproductively isolated from other such groups.

species - individuals that are capable of interbreeding

and producing fertile offspring

member of different species that cannot produce fertile

offspring are said to be reproductively isolated


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hybridization - member of different species interbreed,

results in unhealthy and sterile

offspring or no offspring at all.- genes from one species unable to enter

gene pool of another species


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Problems with applying the biological species concept

effective way of understanding the existing of species

but difficult to apply to populations that do not occur

naturally together (allopatric)

many organisms undergo asexual reproduction,

reproductive isolation is not a matter.


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Factors involved in the formation of new species

reproductive isolating mechanisms fall in 2 categories:

i. prezygotic isolating

- prevent the formation of zygotes

ii. postzygotic isolating

- prevent the proper functioning of zygotes


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Ecological isolation

even two species occur together in the same area, they

may utilized different portions of environment and not

hybridize because they do not encounter each other because of differences in behavioral and ecological,

two species rarely came into direct contact

eg. : lions stay in open grassland, tigers in forestvalley oak can achieve 35 m tall, scrub oak only

1-3 m tall


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Behavioral isolation

related species of organisms often differ in their

mating rituals, make them distinct in nature even share

the same places eg. : mallard and pintail duck share the freshwater in

North America, but nest side-by-side


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Temporal isolation

different breeding seasons become the factor that

prevent hybridization

hybrids between two species can easily madeexperimentally, but in nature rarely occurs

eg. :L. graminifoliaflowers in early spring,L.

canadensisflowers in summer


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Mechanical isolation

structural differences prevent mating between related

organisms

eg. : flowers of related species often differ in theirstructures, prevent the proper pollination

Prevention of gamete fusion

fertilization never takes place because of the sperm of1 species may function poorly within the reproductive

tract of another


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Postzygotic isolating mechanisms

prezygotic prevent the formation of hybrid zygotes

if mating do occur and zygotes are produced, many

factors still prevent it to become fertile individuals

genetic complement of two species may be so differentmake they cannot function together normally

even if they can survive the embryo stage, if they are

weaker then their parents, they will be eliminate innature

if they strong, they may still be sterile


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Selection

natural selection favor allele in parent that prevent

hybridization until the two populations completely

reproductive isolated

natural selection is a process selecting a good genesand eliminated the bad genes (make them easier to

adapt to environment)

reinforcement process - initially incomplete isolating

mechanisms, are reinforced by natural selection until

completely effective

selection also act on mating behavior


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speciation is two part process

1. identical populations must diverge

2. reproduction isolation must evolve to maintain

these differences

the major problems is homogenizing effect of gene

flow between population


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Speciation in allopatry

geographically separated populations appear leading to

speciation

eg. : Papuan kingfisher, varies little in main island ofNew Guinea, but strikingly different from each

other for the same species at the nearby islands


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Speciation in sympatry

selection lead to formation of two difference species

before become two difference species, they must

evolve reproductive isolating mechanisms

most of the time, the two phenotypes would be

retained as polymorphisms (the existence of two

distinct hereditary types based on different alleles) in asingle population

sympatric speciation a rare event


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Genetic changes underlying speciation

studies show that there are very few genes involved in

separating two species

numbers of genes involved are different for eachspecies


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Role of polyploidy in species formation

polyploidy - doubles the chromosome number of the

original sterile individual

hybrid are sterile because of their chromosomes fromtwo species of parents do not pair with one another

results of doubling, they will have duplicate of each

chromosome, this chromosome will pair and fertilityof the polyploid hybrid may be restored


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Darwins finches

1835, Charles Darwin visited Galapagos Islands to

study the evolution in plant and animals

evolution led to the existence of groups of closely

related species from a common ancestor ancestor of Darwins finches was the first reached the

island, many spaces was unoccupied

other species of birds moved to these vacant ecologicalniches and adopted a new lifestyle


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Darwins finches

adaptation to their environment and aided by the

geographic isolation make the ancestral finches rapidly

split into a series of diverse populations

some evolved into different species, now occupyhabitats on the Galapagos Islands

13 species of Darwins finches comprise of four

groups:

i. ground finches

- six species, feed on seeds, bills is related to the

size of the seeds they eat


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Darwins finches

ii. tree finches

- five species of insect-eating, four of them have

bills suitable for feeding insects, the woodpecker

finch has a chisel-like beakiii. warbler finch

- same with warblers on the mainland, has a

slender, warbler-like beakiv. vegetarian finch

- bud-eating bird, very heavy bill used to wrench

buds from branches


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Darwins finches


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The extent of hybridization

biological species are reproductively isolated, so

hybridization should be rare

if hybridization is common, two quick outcomes:

i. reinforcement would occur

ii. two populations would merge into a single

homogeneous gene pool in recent years, hybridization is no longer a rarely

cases, either in plants or animals


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Alternatives to the biological species concept

distinctions among species are maintained by natural

selection

each species has adapted to its own environment

hybridization has little effects because alleles

introduced into the gene pool would be eliminated by

natural selection variety of ideas have been put forward to establish

criteria for defining species

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Variation Chap 10