Concept 8: Evidence of Evolution

Success Criteria

I can explain how fossil records are evidence for evolution

I can discuss how bibliography explains the origin & distribution of modern species.

I can discuss how comparative anatomy (structures) can explain evolutionary patterns & processes.

I can discuss molecular biology regarding the evolutionary relationship between species and their ancestry.

I can discuss evolutionary processes and patterns using New Zealand examples.

Key Vocabulary

Fossil Records, Biogeography, Homologous Structures, Mitochondrial DNA, Y Chromosome, Vestigial Structures

Overview of Evolutionary Patterns

Evidence of Evolutionary Patterns

Fossils

Fossils are the preserved remains of previously living organisms or their traces, dating from the distant past. The fossil record is not, alas, complete or unbroken: most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. Still, the fossils we have been lucky enough to find offer unique insights into evolution over long timescales.

To interpret fossils accurately, we need to know how old they are. Fossils are often contained in rocks that build up in layers called strata, and the strata provide a sort of timeline, with layers near the top being newer and layers near the bottom being older. Fossils found in different strata at the same site can be ordered by their positions, and "reference" strata with unique features can be used to compare the ages of fossils across locations. In addition, scientists can roughly date fossils using radiometric dating, a process that measures the radioactive decay of certain elements.

Fossils document the existence of now-extinct species, showing that different organisms have lived on Earth during different periods of the planet's history. They can also help scientists reconstruct the evolutionary histories of present-day species. For instance, some of the best-studied fossils are of the horse lineage. Using these fossils, scientists have been able to reconstruct a large, branching "family tree" for horses and their now-extinct relatives. Changes in the lineage leading to modern-day horses, such as the reduction of toed feet to hooves, may reflect adaptation to changes in the environment.

Biogeography

The geographic distribution of organisms on Earth follows patterns that are best explained by evolution, in combination with the movement of tectonic plates over geological time. For example, broad groupings of organisms that had already evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) tend to be distributed worldwide.

The evolution of unique species on islands is another example of how evolution and geography intersect. For instance, most of the mammal species in Australia are marsupials (carry young in a pouch), while most mammal species elsewhere in the world are placental (nourish young through a placenta). Australia’s marsupial species are very diverse and fill a wide range of ecological roles. Because Australia was isolated by water for millions of years, these species were able to evolve without competition from (or exchange with) mammal species elsewhere in the world.

The marsupials of Australia, Darwin's finches in the Galápagos, and many species on the Hawaiian Islands are unique to their island settings, but have distant relationships to ancestral species on mainland's.

Geographical isolation can happen when populations become geographically separated due to environmental events such as mountain uplift / tectonic plate movement / sea level changes/ ice ages so gene flow cannot happen between populations so becomes reproductively isolated. Physical barriers may change the environment / habitat / niche, which alters the selection pressures. New phenotypes/ adaptations lead to new species.

New Zealand's Geology

Mountain Building

NZ Mountains are very young when compared with many mountains around the world. The main ranges are still being formed by collisions between the Pacific and Australian plates. Mountain barriers tend to isolate animals populations from each other. This produces selection pressures for new species to evolve.

Volcanic Activity

Volcanoes have a large effect on the species present both before and after an eruption. Many species may be completely wiped out and may not re-colonise an effected area again. Plants may re-colonise areas buried in ash and new species evolve to occupy the barren lands. This can be linked to the bottleneck and founder effects.

Changing Sea Levels

Several times in the past few million years, ices ages have caused sea level changes. Each time the polar ice caps increase in size and the sea level drops 80-100 metres. Shallow sea floors become dry land and some islands become joined. During interglacial periods sea levels rise again and some mountains become islands. These islands cause isolation and new species evolve. E.g. Kaka and Kea and the Weta species.

Climate Change

Ice ages also have a more direct effect on life – It gets colder. 20 000 years ago NZ was covered mostly by snowfields and cold-climate grasslands. The climate caused forests to shrink to a few coastal areas in the north containing mostly Totara and Rimu.

All this changed when the Earth’s climate began to warm up again 15 000 years ago. Forest spread and many warm climate species arrived and new species evolved.

Comparitive Antomy

Comparative Anatomy is the study of the similarities and differences in the anatomy of difference species. It has long served as one of the main evidences for evolution, due to the fact that it is very concrete, and does not require extensive technology.

Homologous features

If two or more species share a unique physical feature, such as a complex bone structure or a body plan, they may all have inherited this feature from a common ancestor. Physical features shared due to evolutionary history (a common ancestor) are said to be homologous.

To give one classic example, the forelimbs of whales, humans, and birds look quite different on the outside. That's because they're adapted to function in different environments. However, if you look at the bone structure of the forelimbs, you'll find that the organization of the bones is remarkably similar across species. It's unlikely that such similar structures would have evolved independently in each species, and more likely that the basic layout of bones was already present in a common ancestor of whales, humans, and birds.

The similar bone arrangement of the human, bird, and whale forelimb is a structural homology. Structural homologies indicate a shared common ancestor. Some homologous structures can be seen only in embryos. For instance, did you know that you once had a tail and gill slits? All vertebrate embryos, from humans to chickens to fish, share these features during early development. Of course, the developmental patterns of these species become increasingly different later on (which is why your embryonic tail is now your tailbone, and your gill slits have turned into your jaw and inner ear)

However, the shared embryonic features are still homologous structures, and they reflect that the developmental patterns of vertebrates are variations on an ancestral program

Vestigial structures are reduced or non-functional versions of features, ones that serve little or no present purpose for an organism. The human tail, which is reduced to the tailbone during development, is one example. Vestigial structures are homologous to useful structures found in other organisms, and they can provide insights an organism's ancestry. For instance, the tiny vestigial legs found in some snakes, like the boa constrictor at right, reflect that snakes had a four-legged ancestor.

Molecular Biology

Like structural homologies, similarities between biological molecules can reflect shared evolutionary ancestry. At the most basic level, all living organisms share:

The same genetic material (DNA)
The same, or highly similar, genetic codes
The same basic process of gene expression (transcription and translation)

These shared features suggest that all living things are descended from a common ancestor, and that this ancestor had DNA as its genetic material, used the genetic code, and expressed its genes by transcription and translation. Present-day organisms all share these features because they were "inherited" from the ancestor (and because any big changes in this basic machinery would have broken the basic functionality of cells).

Although they're great for establishing the common origins of life, features like having DNA or carrying out transcription and translation are not so useful for figuring out how related particular organisms are. If we want to determine which organisms in a group are most closely related, we need to use different types of molecular features, such as the nucleotide sequences of genes.

Homologous genes

Biologists often compare the sequences of related genes found in different species (often called homologous or orthologous genes) to figure out how those species are evolutionarily related to one another.

The basic idea behind this approach is that two species have the "same" gene because they inherited it from a common ancestor. For instance, humans, cows, chickens, and chimpanzees all have a gene that encodes the hormone insulin, because this gene was already present in their last common ancestor.

In general, the more DNA differences in homologous genes between two species, the more distantly the species are related. For instance, human and chimpanzee insulin genes are much more similar (about 98% identical) than human and chicken insulin genes (about 64% identical), reflecting that humans and chimpanzees are more closely related than humans and chickens.

mtDNA & Y Chromosome Analysis

mtDNA analysis provides evidence for how closely related the organisms / groups are and their times of divergence mtDNA is found only in the mitochondria of cells (and not the nucleus of cells). Therefore, it is not subject to the processes of meiosis and crossing over.

Mitochondria remain in the cytoplasm of egg cells so are passed on when the egg is fertilized. They are therefore passed down the female / maternal lineage from generation to generation.

Changes in mtDNA result from mutation only and these are not subject to natural selection. Mutations in mtDNA typically occur at a steady rate and this typically is more rapid than in nuclear DNA, this allows scientists to use mtDNA as a molecular clock.

Therefore, by comparing presence of mutations in mtDNA from different individuals / groups, scientists can determine not only how closely related they are but the likely times of divergence of individuals / groups. These comparisons are now the main source of evidence used by scientists to produce phylogenetic trees.