Patterns of evolution

Evolution & life on Earth Earth – 4.6 billion years old. Life first appeared 3bya Cyanobacteria fossils indicate photosynthesis began 3-2.5bya. Cyanobacteria are prokaryotes. Oldest know eukaryotes – 1.8bya. First animals 1 billion to 600 mya. Cambrian explosion = most of today’s animals 550 mya. Mammals – 200 mya. Homo appearing 2.5 mya

The fossil record Most fossils are found in sedimentary rock layers – strata. Dating and analysis of strata and fossils gives a record of diversity of past life forms over millions of years & show that organisms appeared in historical sequence. These forms were ancestral to present-day forms. Transitional forms occur in fossil record – eg Archaeopteryx 150 – 145 mya is transitional between reptiles and birds. As more data is collected, more transitional forms are discovered ‘ filling in the gaps ‘.

Genetic analysis & evolutionary patterns Genetic analysis suggests that the last universal common ancestor (LUCA) of all existing life forms lived about 3.5bya (at least 100 genes found in existing life forms can be traced back to this LUCA). EG – Coelacanth retains primitive structures of ancestral fish. Scientists have decoded the DNA and are gaining insights into how today’s vertebrates evolved from fish.

Biogeography The study of geographic distribution of species. Continental drift – groups of life forms living on them became separate. EG – Plants of the genus Protea are found only in South Africa, South America and Australia. This indicates a common ancestor from Gondwana.

Vicariance Large-scale allopatric speciation of a taxonomic group by a significant geophysical event such as uplift of a mountain range, formation of a new river or ocean. Eg – splitting of Gondwana.

Rates of evolution - Gradualism Thought that the only way new species could evolve was over a very long time with a gradual transition from one form to another. This gradualism sees transitional forms in the fossil record in response to natural selection.

Rates of evolution – Punctuated equilibrium For many groups, the rate of evolution was not steady. Long periods of time elapsed where there was no change = stasis followed by sudden bursts of rapid selection (thousands of years rather than millions). This punctuated equilibrium formed many different species that are no longer closely related. There are few transitional fossils because of the rapid pace of change. EG – Horse evol. Gradualism followed by PE. Initially involves Mesohippus (37-32 mya) gradually evolving into Miohippus (32-25mya) – gradualism. Then from Miohippus, there is a burst of diversity into numerous distinct branches – PE.

Rates of evolution – Punctuated equilibrium In a large population extending over a wide range, evolution is likely to be slow – constant gene flow between areas will prevent change. However when a small population separates off, it is cut from the gene flow and its gene pool can evolve rapidly, enabling a new species to form. If the new species invades the area occupied by the original large population, it may replace it. The result (in the fossil record) would be an apparent jump from one species to another.