Spring 2004
Lecture 3: February 3
Reading:
Web resources for this lecture:
Lecture notes: Species and Speciation:
What is a species? (also look at notes lecture 2)
Biological species concept: 'A species is an array of populations which are actually or potentially interbreeding, and which are reproductively isolated from other such arrays under natural conditions'. (Ernest Mayr)
Sympatric: live in same place (have opportunity to breed); they may not breed because of behavioral differences.
Allopatric: live in different places. How do we know whether allopatric species could reproduce if they lived in the same place?
Sibling species: look the same. They may be reproductively isolated because of behavioral differences, e.g., mate in different seasons; song birds: song.
All life on Earth has descended from universal ancestor, as inidctaed by the fact that all life shares complex properties:.
- Plants (multicellular, photosynthetic autotrophs)
- Animals (multicellular, heterotrophs)
- Fungi (multicellular, heterotrophs)
- Many unicellular groups (photosynthetic autotrophs/ heterotrophs)
How many species are there on Earth?
Presently described: ~1.8 million. There are far more species on land than in the sea (about 15% of the 1.8 million live in sea, 85% on land). Estimates of total numbers of species on Earth (using various techniques in estimating) range widely, between 5 and 120 million. Marine diversity estimates range between 180,000 and10 million species. Main unkowns: species in tropical rainforests (especially insects and other arthropods), species in the oceans. Note that these estimates do not include unicellular organisms (prokaryotes and eukaryotes). Prokaryote species are not well defined: no sexual reproduction.
Marine Estimate # of
species Continental Estimate # of
species Porifera
(Sponges) 9,000 Fungi
(mushrooms) 1,000,000 Cnidaria
(jellyfish, corals) 9,000 Plants 300,000 Nematoda
(worms) 35,000 Nematoda
(worms) 1,000,000 Annelida
(worms) 15,000 Arthropods
(mites) 750,000 Arthropoda
(lobsters, shrimp) 200,000 Arthropods
(spiders) 170,000 Mollusca (snails,
clams, octopus) 190,000 Arthropods
(insects) 9,000,000 Bryozoa 15,000 Mollusca
(snails) 20,000 Chordata (include
Fish) 15,000 Chordates 25,000 Others 12,000 Other 75,000 Total 500,000 Total 12,340,000
Total estimated number of species: 12,8340,000. Data after M. J. Benton, 2001, Biodiversity on land and in the sea. Geological Journal, 36, 211-230. This paper can be downloaded from the web as a pdf file.
There are, however, many more Phyla in sea than on land: 32 animal Phyla in the sea, 12 on land. We could say that on land we see a larger diversity, in the sea a larger disparity (more fundamentally different building plans in animals).
Over geological time, the number of species must have increased. How? How does a new species arise? (speciation). Since the total number of species must have increased, at least in some cases new species must have originated by 'branching off' from another species, thus becoming reproductively isolated. Evolution thus can not have consisted of anagenesis only.
Populations share a gene pool (interbreeding); gene flow occurs. Problem: new mutation in large population may not become established, interbreeding possible with many individuals without the mutation. In small populations, new mutations may pervade the population much easier.
Founder principle (small populations). Most species on islands, for example, descend from few individuals. These few individuals do not share ALL possible genetic information present in the much larger 'mother' population. The descendant population thus differs from main population; is smaller, new mutation may become established.
Geographic isolation then may result in group on island diverging from mainland group; becoming new species (reproductively isolated): no gene flow between island population and mainland population. Process: allopatric speciation.
Sympatric speciation: different sub-environment (insects on leaf, bark of tree).
Species with wide geographic distribution: inhabit areas with e.g., varying climate. Local populations become differentiated: colder climates have larger individuals (e.g., polar bars; volume/surface area), with shorter ears, legs, etc. (preserve body heat). Insects at higher latitudes: shorter wings. Drosophila (fruitfly) showed such a wing-length variability with latitude. Introduced in US 20 years ago; only one wing-length. Within 20 years, latitudinal wing-variation developed. Note that in this case 'evolution' appears driven by environment, predictable (good engineering).
Species that show morphological variation over geographic range: clines. Maybe split into 'subspecies'. Example; ring species (gulls in text book; Asian greenish warblers).
Subspecies may become split by many causes; changing climates (during warming cool-adapted species must move up mountains, loose contact; during warming, sea levels rise and what was one land mass becomes a group of islands).
Paleontological problem: the time dimension. Only morphological
species can be recognized. How do such species change over time?? IF
species change gradually over time, paleontologist can not
distinguish species: they grade imperceptibly into each other. One
could only separate species if the record is really incomplete: a
'new species' seems to occur at each gap in the record. How does one
split such lineages (populations over time) into species? And what do
we see when a species 'branches off'??
In fact, paleontologists DO in most cases easily recognize species
over time. Over long times, species do not appear to change
significantly, then there is rapid change, possibly splitting into
more species. Long periods of little change are called stasis, short
period of rapid change are called punctuations. Theory that calls for
alternation of periods of stasis and punctuations is called
punctuated equilibria (Eldredge & Gould, mid 1970s).
Punctuated Equilibrium:
Ontogenetic growth: how do size and shape change,
jointly?
Allometric growth is commonly necessary as a result of the requirements of increased size; allometric growth is the rule, isometric growth is very rare. Allometry: figures in d'Arcy Wentworth Thompson
Principle of similitude: the mass of an organism increases by a power of 3, while its cross-sectional area increases by the power of 2, its length by power of one: as animals grow larger, their limbs most become more robust, rather than just being larger versions of those of smaller animals. Gallileo (1638).
Also: convoluted surfaces in lungs, intestines.
Thermoregulation: surface areas with regard to volume. Small animals lose heat very quickly (large surface area for volume); small animals with fixed body temperature must eat the whole day. Big animals risk heat stress.
Evolution and ontogenetic change:
Organisms change shape while growing up.
Speeding up or slowing down growth: evolution. Example: axolotl remains larval in shape while it become sexually mature.
Evolution by changing developmental timing: heterochrony. Small genetic changes (mutations) can cause major morphological changes. Chimpanzee-human: 97% similarity genetically.
Paedomorphosis: retention of juvenile features into sexual maturity:
Peramorphosis: addition of extra ontogenetic stages beyond the adult reproductive stage of the ancestor (e.g., jaw to jawless fish; earbones to mammalian ear)