THE ORIGIN OF SPECIES

Microevolution explains evolutionary changes within a

population.Macroevolution considers the

origin of new taxonomic groups

SPECIATION

• The origin of a new species

• Anagenesis (phyletic evolution)- describes the transformation of one species into a new species

• Cladogenesis (branching evolution)- new species arise from parent species that continues to exist

An evolutionary lineage, illustrating cladogenesis (lineage splitting at nodes A, B, C, D & E), anagensis (evolutionary change between these points), and extinction (termination of secondary lineages arising from nodes A, C, & D). The hatched and shaded portions represent a lineage that splits at node B by cladogenesis and diverges therafter by anagenesis as two separate lineages. (after Minkoff 1983)

SPECIES

• Biological species concept was developed by Ernst Mayr in 1942.

• It states that a species consist of individuals that can interbreed in nature and produce viable, fertile offspring.

Ecological species concept

• Defines species on their ecological niche, the role they play and the resources they use in the specific environments in which they live.

Pluralistic species concept

• Includes reproductive barriers and niche

Morphological species concept

• Based on physical characteristics

Genealogical species concept

• Based on evolutionary lineages

Please note:

•Biological species concept does not apply to asexual species

•Extinct species cannot be grouped on the basis of interbreeding

MODES OF SPECIATION

• SPECIATION REQUIRES THE INTERRUPTION OF GENE FLOW BETWEEN POPULATIONS

Reproductive barrier

• A Mechanism that prevents two species from producing viable, fertile offspring; thereby preserving the genetic integrity of a biological species

There are two types of reproductive barriers-

Prezygotic and Postzygotic

• Prezygotic barriers function before the zygote is formed

• Postzygotic barriers prevent zygote from developing into a fertile offspring

Prezygotic barriers include:

• Habitat isolation-

• Temporal isolation- breed at different times

• Behavioral isolation- courtship rituals and behavioral signals are species specific

• Mechanical isolation- anatomical incompatibility

• Gametic isolation gametes fail to fuse

Lions and tigers are a good example of habitat isolation. They once covered the same territory, however they became isolated as lions remained on the open savanna and tigers kept to the jungles. Eventually, they became distinct enough from one another to be unable to produce fertile offspring.

Rana aurora - breeds January - March

Rana boylii - breeds late March - May

e.g. - Rana aurora (Red-legged frog) breeds in fast-moving, ephemeral streams, whereas its relative Rana catesbiana (Bullfrog) breeds in permanent ponds. (The metamorphosis times of the tadpoles are correspondingly different.)

Postzygotic barriers

• Reduced hybrid viability- zygote fails to survive

• Reduced hybrid fertility- hybrid individual is sterile

• Hybrid breakdown- hybrids are viable and fertile, but their offspring are feeble or sterile

Allopatric speciation

• Two populations are geographically separated

Conditions for Allopatric speciation:

• Isolation of populations may occur due to a geographic barrier

• Or colonization of new areas• Or a “Ring Species” may develop- meaning

members of a population spread around a geographic barrier and may evolve enough that they can no longer interbreed with the original pop.

• Or Adaptive radiation my occur on island chains when small founding populations evolve in isolation and under different environmental conditions

• Adaptive Radiation- evolution of numerous, variously adapted species from a common ancestor.

Sympatric speciation

• Biological barriers prevent gene flow between overlapping populations

Conditions for Sympatric speciation

• Sympatric speciation may occur due to polyploidy

• An Autopolyploid has more than two sets of chromosomes that have all come for the same species (tetraploids 4n)

• Tetraploids can mate with themselves or other tetraploids but no longer with diploids from the parent population

• An Allopolyploid is an interspecific hybrid

Auto- And Allopolyploidy of Cultured Plants

species basic

number (x)

number of chromosomes

(2n)

AUTOPOLYPLOIDY

potato (Solanum tuberosum)

12 48

coffee (Coffea arabica)

11 22, 44, 66, 88

banana (Musa sapientum)

11 22, 33

alfalfa (Medicago sativa)

8 32

peanut (Arachis hypogaea)

10 40

sweet potato (Ipomoea batata)

15 90

ALLOPOLYPLOIDY

tobacco (Nicotiana tabacum)

12 48

cotton (Gossypium hirsutum)

13 52

wheat (Triticum aestivum)

7 42

oats (Avena sativa) 7 42sugar-cane (Saccharum officinarum)

10 80

plum (Prunus domesticus)

8 16, 24, 32, 48strawberry (Fragaria

grandiflora)7 56

apple (Malus sylvestris)

17 34, 51

pear (Pyrus communis)

17 34, 51

• Polyploidy has been very important in the evolution of new plants. Many of our agricultural plants are polyploids and plant geneticists now hybridize plants and induce meitoic and mitotic errors to create new species

Sympatric speciation in animals

• May evolve from different resource usage

• Non random mating (sexual selection)

Punctuated Equilibrium

• Long periods of stasis are punctuated by episodes of relatively rapid speciation and change. ( A few thousand years)

Gradualism

• Species continuously evolve over long periods of time

EXAPTATION

• STRUCTURES THAT HAD EVOLVED AND FUNCTIONED IN ONE SETTING AND WERE THEN CO-OPTED FOR A NEW FUNCTION

Now what is this exaptation stuff? Exaptation is the evolution of new adaptations from adaptations that evolved in a different context. For example, leaves which evolved mainly as organs that carry out photosynthesis, become thorns in cacti and become adaptations that protect the cacti from grazing animals.

Trap jaw ants

Evo-devo

• Combination of the fields of evolution and developmental biology

• Explores how slight changes in developmental genes can result in major morphological differences between species

Allometric growth

• The differing rates of growth of various parts of the body lead to the final shape of the organism. A small genetic change that affects allometric growth can produce a very different proportioned adult form

This study examines differences in fighting strategies between small and large male crayfish, Orconectes rusticus. Due to allometric growth of claws, fighting weapons are of disproportionate size in large crayfish compared to those in smaller individuals. Presumably, such differences in the prominence of claws are reflected in differences in the likelihood of injuries, and we thus explored fighting in size-matched pairs of small or large crayfish and assessed associated strategies in situations of conflict. Although fighting reached the highest intensities in a similar proportion of instances in small and large pairs, differences in fighting strategies were evident. Small crayfish escalated more rapidly, fights were settled more quickly, and were resolved overall at lower intensities. This may be explained by lower risks of injury compared to encounters among larger males due to proportionally smaller claws. Larger males thus appear to spend considerably more time in assessing their opponent's fighting ability before each escalation event.

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Heterochrony

• An evolutionary change in the rate or timing of development

Several heterochronies have been described in humans, relative to the chimpanzee. Chimpanzee brain and head growth in the fetus starts at about the same developmental stage and have a growth rate similar to humans. However chimp brain and head growth stops soon after birth. Humans continue their brain and head growth several years after birth. This particular type of heterochrony is called neoteny and involves a delay in the offset of a developmental process in later stages of development. Human are known to have about 30 different neotonies in comparison to the chimp.

Paedomorphosis

• The retention in the adult of juvenile traits of the ancestral organism. Can occur when genetic changes speed up the development of reproductive organs relative to the development of body form.

The Mexican axolotl opposite is a famous example of paedomorphosis, retaining in maturity the feathery gills that related species lose in infancy.

Find the meaning of these roots

• Allo• Ana• Auto• Clado• hetero

• Macro• Paedo• Post• sym