Reactions to Evolution and refinements

EPS 106 Sept. 26^th, 2000

“How extremely stupid not to have thought of that!” was Thomas Huxley’s reaction in 1859 when he first read The Origin of Species Huxley was a leading proponent of Hutton and Lyell’s concept of Uniformitarianism and a defender of the old earth hypothesis. Huxley and Lyell were the minority in the acceptance of Darwin’s hypothesis. Many were hostile. Among these was Adam Sedgewick, Darwin’s old teacher!

Several scientists clung to the idea that divine inspiration made each organism perfectly for its environment. Charles Lyell argued against this, citing the fact that introduction of non-native species could wipe out the natural inhabitants, proving that there was nothing ‘perfect’ about a particular organism living in a particular place.

Wallace, in a study of geographic distributions of animals recognized that certain regions seemed to have a divide -- that is, the animals on one side of the divide were quite different from those on the other. From the distribution of birds, Wallace proposed a divide between the islands of Bali and Lombok in the Australian Archipelago. Although they were separated by only a few miles, he concluded

“facts such as these, can only be explained by a bold acceptance of vast changes in the surface of the Earth”

If there were no barriers now, there must have been some in the past. Wallace knew, from the geology of the continental shelf below the sea, that the two faunas were connected with two distinct continental areas. He supposed that they must have risen and fallen in the past. It remained a discovery of the 20^th century that they had actually moved. Wallace had therefore, a vague premonition for continental drift.

Wallace had other examples. Camels existed in the deserts of Africa and Asia, but South America had llamas and Alpacas. Pliocene rocks of N. America had fossils of camel ancestors “in a region where they do not now exist, but which is situated so that the now widely separated living forms could easily be derived from it”.

People were still concerned over not seeing direct fossil evidence for evolution. Where were the ancestors? A big discovery was made in the 1860’s in the Solenhofen Limestone in Bavaria, where a feathered fossil, Archaeopteryx (Greek for ‘ancient wing’) was found. (It is now known that this deposit is about 150 million years old). The Archaeopteryx clearly had feathers, but also lots of characteristics of reptiles. It had teeth and a bony tail.

A still more classic example was to follow. This was for horses, from various paleontologists around the world. American Othniel Marsh had seen horse fossils in Europe, and his quest in the United States was richly rewarded. He had found a complete sequence of fossil horses that bridged primitive non-horselike mammals to the modern horse.

The early horse is ‘eohippus’ (dawn horse) about 65 million years ago. It was small (25 to 50 cm) with three toes one its hind feet and four on its front feet, with doglike footprints.

With time, these three-toes grew to become one, and the very small molars became massive, high-crowned teeth. Why? The answer is the change in vegetation. It went from forest to open grassland. Bigger teeth were needed to eat the tough, gritty grass, and horses got bigger and were able to run on the rough surface of the grassy landscape instead of ‘wriggle’ through the jungle or forest.

Another example is the polar bear, which is closely related to the regular brown bear. But after being separated from its ancestor during the massive Pleistocene glaciation, it changed somewhat. It became a carnivore, developed carnivore-like teeth, it stopped hibernating, spends most of its time in water, and has stiff bristly fur on the soles of its feet. Of course, it became white as well.

But still, there was no living proof of slow and gradual evolution, until Henry Bates presented some lovely evidence.

He had shared Alfred Wallace’s adventure in S. America and noticed mimicry in butterflies. Some had a distinctive and unpleasant odor. Others mimicked them. Even the different varieties of the same species had mimics of the same ‘look’. This was clearly to ward off predators, who would find them distasteful. But if one species changed its look, the mimic had to change with it. Even in different regions, they changed together, strongly supporting the idea of slow and gradual change. Those ‘sports’ that didn’t have the appropriate camouflage, would be eaten and not reproduce.

Some critics accepted minor changes in evolution, but not major. How could an insect resemble a leaf? Darwin had an answer for that. There would be one or two that more closely resembled a leaf than all others. They would have a slightly less chance of being eaten, and their offspring would survive. With time, the characteristic would be inherited. What about wings for flying? What about eyes? There are amazing things that have indeed happened.

Mendel’s contributions were huge, but there was a paradox. If he has white peas and red peas, but nothing in between, then how could things change? Several scientists came to the rescue. The variation in the size of ‘large beans’ changed. Or another example comes from Nilsson-Ehle, on wheat. There is red wheat and white wheat, with the red always dominant. But there are different intensities of red, some brighter than others. But the changes in wheat were small, and these couldn’t be considered the large ‘mutations’ that others foresaw as necessary.

The change came in 1909 by Thomas Morgan, working with fruit flies, Drosophila. He found that breeding true red-eyed fruit flies, he got a single white-eyed male fly. When he bred it with red-eyed females, he got only red eyed offspring, but those produced white-eyed male offspring in generation F2, indicating a recessive allele. With further work he found that mutations were common in domestic fruit flies.

Another classic example is the Peppered Moth Biston betularia. It lives in England. Until 1845, all were lightly colored to match the trees. After industrialization, a black moth was found around Manchester. As the trunks of trees became more blackened by soot, more and more black moths showed up. Eventually 99% were black. Eventually, it was discovered that the black moth was the result of a rare mutation.

H.B.D. Kettlewell hypothesized that the color was for protection – camouflage. He marked the underside of the wings and released them around Manchester and in an upolluted area. Lo and behold. He recollected them at night. In the unpolluted area, he had 12.5% of gray moths retrieved and 6% of the black moths. Around Birmingham (another big industrial center), he retrieved 40% black and 19% gray. Clearly, the mutation to black, would not have been beneficial had the environment not changed. As it turned out, the darker trees favored the mutation.

Ernst Mayr made an important discovery in 1942. Mayr said that in trying to characterize species, “90 percent of his work consists in the study of variations”. In other words, variation is the norm. As you look throughout the range of a species, you invariable see variation within the range. Local populations tend to differ from the norms. It is the exception rather than the rule for species to be uniform within their range.

When a subspecies becomes quite isolated, it becomes a moot point as to when it becomes a new species. BUT AT ANY ONE LOCALITY, THE DISTINCTION BETWEEN SPECIES WERE NEARLY ALWAYS CLEAR CUT. At left you can see a species separating, and then remixing, either creating two different species, or a hybrid between the two.

Mayr’s claim was that new species arose from geographic isolation, which leads to reproductive isolation when the geographic barriers break down.

The most recent modification of the theory of evolution is related to the ideas put forward by Niles Eldredge and Stephen J Gould on ‘punctuated equilibria’. In their theory, they propose that the rate of evolutionary change is rapid during the formation of a new species and virtually zero at other times. There are many examples of this from the fossil record.

An example is snails and clams from some continuous deposits in East Africa. During the late Pliocene and Early Pleistocene, a large lake subsided, leaving lakes Turkana behind. Rapid evolution occurred in this isolated basin. After 100’s of thousands of years of stasis, distinguishable species evolved over a period of 5,000 to 15,000 years. When the lake levels rose, the surrounding species invaded, wiping out these short-lived evolutionary taxa.

Changes in climate – Great extinctions controlling evolution. Creating a void and filling it; the dinosaurs and mammals.

Up until this point we have learned that things change slowly. Uniformitarianism is the norm. The catastrophic floods of the past (as accounted for in the Bible) are false. And yet, the idea of catastrophism has been reexamined in the face of new evidence and appears to be a far more important mechanism for causing major changes in fauna than previously thought.

The K/T (Cretaceous – Tertiary) boundary occurred 65 Ma. (65 million years before present). It is the third most significant extinction after the end of Ordovician and the catastrophic Permian-Triassic extinctions. Most notable is the end of the

dinosaurs.

Dinosaurs has a connotation of failure. We talk about people or things that are dinosaurs. They are relics of an age past. Something that was once O.K., but is now out of date, and clearly inferior to the modern equivalent. When we were teenagers, our parents were dinosaurs.

Were they that bad? Could they compare with Man? Dinosaurs were around for over 100 million years. Man, less than 3 Ma.

Timeline:

Permian-Triassic: 245

Triassic-Jurassic 208

Jurassic-Cretaceous 146

K-T 65

Paleogene-Neogene 23

There were small mammals before the Cenozoic (65 Ma). Fossil evidence extends all the way back to the late Triassic, more than 200 Ma. But they were dominated by reptiles and dinosaurs, playing a waiting game. Small insectivores. But when the dinosaurs went extinct, they filled the niches.

They underwent extreme adaptive radiation. They took over the lands, growing to large size, and even took over the water (whales and later seals).

Two major groups, carnivores and ungulates, with others including primative insectivores, primates, the subungulates (elephants and relatives), rodents and lagamorphs (rabbits and hares), and edentates (sloths, armadillos, and anteaters).

Mammals can be divided into three groups on the basis of age. The pre-Tertiary, the archaic or Paleogene mammalian fauna and the Neogene mammalian fauna or modern fauna.

Most of the archaic fauna have become extinct, with some remaining vestiges.

Most of our information comes from teeth (molars). Without going into detail, the late Triassic mammals were the size of a mouse, small brain and short legs – very primative. Jurassic mammals were larger, up to rat sized. Complete skeletons are unknown. Subordinate to reptiles, the made up all groups (carnivores, insectivores, etc.).

How did they reproduce? Today, there are two monotremes, echidnas and duck-billed platypus. They lay eggs, but have primative mammary glands. Were the Jurassic mammals monotremes? Cannot tell, because, while their teeth are distinct, those in the modern have degenerate molars – stumpy pads only.

Marsupials are those that essentially give birth to an embryo. They spend long periods of time attached to a teat in the pouch. Marsupials do not appear until the late Cretaceous.

Placentals also appear in the late Cretaceous. They have considerable development prior to birth and many exhibit extensive parental care. They tend to have larger brains, as well.

In the Cretaceous, the abundant mammals were marsupials. They had an amazing resemblance to the modern opossum. There are minor placentals, mostly primitive insectivores, like moles, shrews and hedgehogs. The difference can be ascertained by the molars.

Tertiary (or Cenozoic)

The dinosaurs are gone. Now the others, turtles, amphibians, birds, lizards and mammals have the place to themselves. Of these, mammals and birds diversified to dominate. The early archaic fauna would be unfamiliar to us. Carnivores and herbivores are distinguished by teeth. Teeth became more specialized for nipping, cutting, tearing, shreading, etc in the carnivores. Still short legged and probably relatively slow moving.

In the mid Paleocene, the early creodonts were replaced by the miacidids. The creodonts were flat footed, the miacidids were standing up on their toes. Eventually the maicidids did in the creodonts. Two groups are recognized today:

· the dogs, bears, raccons, weasels,

· the cats, hyaenas, Old world civet cats

The most abundant and diversified were the ungulates in the early Cenozoic. These started out as small insectivores, with long slender bodies, short legs, little specialization. They did walk up on their toes. With time, their teeth became more specialized.

Dividing off from the ungulates are the notoungulates. They are common in the very early Cenozoic, populating both N. and S. America. Soon after, the land bridge disappeared. They became extinct in N. America, but flourished in S. America along with marsupials. Some looked like camels, bears and cattle. Others were rodentlike, like giant beavers. This is an example of parallel evolution. The looked similar and filled the same niches, but were not related. Of the ungulates, there were two

branches, the odd toed (horse tapir, and rhinoceros) and even toed groups (pigs, camels, sheep gots, antelope, deer and cattle). A final group of the archaic mammals is the proboscideans (primitive elephants the size of babies today)

The Neogene, or modern mammals forced most of the earlier species into extinction. In N. America, 75% remain, worldwide, 50% because of S. America, where there was mass extinction. Dogs, and saber tooth cats abounded. The saber-toothed cats were huge, and probably ate things like mammoths and mastodons and elephants.

The ungulates continued to evolve, getting higher up on their toes. The extreme is the modern horses, which stand on their middle toenail. This is for greater speed and harsher, harder terrain.

Why? Changing from forest-dwelling to plains-dwelling horses. Teeth also became larger and more convoluted. (Tapirs and rhinos were conservative and continue to eat plants).

Early horses were forest browsers and small – 18”, simple teeth and three toes. The eruption of grassland (due to the rainshadow of the Rockies) created a new environment, and the horses evolved. Bigger, stronger teeth, single toes. Only in N. America. They did not exist elsewhere until the landbridge across the Bearing Strait. Horses became extinct in N. America 10,000 years ago.

S. America had a isolated evolution. There were even placental saber-toothed like carnivores. Others were like dogs and wolves. A 10 foot tall flightless bird with a 1 ½ foot long skull was a successful predator.

Then, 3 mya, the land bridge reappeared, and S. America fauna went through a catastrophic extinction. Most of the migration was southward. Opossums, armadillo and porcupine and the extinct giant ground sloth were the exception.

Australia is the modern example. Only some rodents made it on rafts.

The Pleistocene extinction 10,000 years ago, was a major event. The large mammals went extinct. Mastadons, Wooly Mammoths, elephants, camel, horse, giant beaver, ground sloths, giant bison, dire wolf, saber-toothed cats, large deer and large

moose. 95% went extinct! They did not affect Africa.

There is no driving force for evolution. It is generally rapid and then reaches a plateau. Lungfish evolved rapidly, and then remain unchanged since the Devonian times (OVER 350 MYA).