The Evidence and Mechanisms of Evolution
Darwin had finally come up with a unified theory on life…an evolutionary family tree.
Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process could be found in the fossil record, the geographical distribution of living species, homologous structures of living organisms, and similarities in early development.
The Fossil Record…
At the time of Darwin, it was finally well known that fossils were the remains of ancient life and that different layers of rock formed at different times in the Earth’s history.
However, very few people linked these two concepts together. Darwin proposed that the Earth was millions, not just thousands of years old. And in that time, many species lived and died. It was in those rock layers that gave evidence of what lived in the past.
The deeper the rock layer, the older (usually) the organism was. And the farther to the surface the rock layer was, the newer the species.
By examining these fossils among the rock layers, one could see the gradual change in time among the creatures that once lived and the ones that exist today.
Geographic Distribution…
This was best shown in Darwin’s finches. Each finch was slightly different from one another depending on which island you visited. Then, if you looked as the finches on mainland South America, you would see yet another slight variation from the island species. This showed how animals adapted to their environments, in order to survive and live in their own unique environments.
This was also shown in Darwin’s observation that although parts of South America and Australia are similar in climate, they have very different species, but all are somehow adapted similarly to that environment. This showed that no matter where an organism lives, it will find a way to change and adapt itself to that environment.
Homologous Structures…
Further evidence of evolution and adaptation was shown in animals that were still alive today. It was noticed that among animals with a backbone, arms has been adapted for many uses…flying swimming, running, grasping, etc. The limbs greatly differed in function, but all were constructed from the same basic bones.
Structures that have different mature forms but develop from the same embryo tissues are called HOMOLOGUS STRUCTURES. These structures form hard evidence that in this case, all living creatures with a backbone evolved and adapted from a common ancestor.
This can also be useful in determining the relationships between organisms. Useful comparisons can be made in bone structure showing common relationships between similar birds and dinosaurs. But these same bones could distinguish a bird from something in the bat family.
Not all homologus structures can be useful. Sometimes the similar bones or structure may be not used over long periods of time. If this is the case, these organisms may lose their ability and often times, disappear. These are called VESTIGIAL STRUCTURES.
Similarities in Early Development…
The early stages of some animals, such as those with backbones, can be so similar, that one may not be able to tell them apart.
These common cell developments further point towards a common relationship between, at least in this case, all backboned organisms.
When Darwin developed his theory of natural selection in the 1800’s, he did so without knowing about genes, DNA, etc. Since then, we have learned much about genes and modified Darwin’s views to include this new information. Soon, genetic information was used to explain the variations among a population…
Populations, not Individuals, Evolve…
Can individuals evolve? Can an organism respond to natural selection by getting new, or losing characteristics? Well, yes, and no. Individuals cannot evolve a new trait within their own lifetime in order to respond to their environment. However, it is the overall population that will change slowly, over generations.
The Evolution of Species
Things like mutations and natural selection can change a population’s overall gene pool over time. But how do these changes actually lead to the evolution of a new species?
The evolution of new species, known as SPECIATION, occurs when members of a similar population no longer breed with each other, therefore no longer producing offspring in their natural environment. But what can lead to a species no longer breeding?
- PHYSICAL BARRIES
o In nature, physical barriers can break large populations into smaller, separate ones.
§ Lava flows
§ Changes in water levels
§ Formation of a river
o This separation by physical barriers is known as GEOGRAPHIC ISOLATION.
o Tree Frog Example:
- REPRODUCTIVE ISOLATION
o As populations become increasingly distinct, reproductive isolation can occur.
o REPRODUCTIVE ISOLATION occurs when formerly interbreeding organisms can no longer mate to produce offspring.
o There are two main types of reproductive isolation…
§ Differences between genetic material
· Over time, two populations separated by geographic isolation will fail to reproduce, therefore separating them.
§ Differences between behavior
· Some populations, even though they may live in the same geographic area, may be genetically separated due to when the mating season is.
· If one population mates in the spring and one in the fall, they will never mate, therefore isolating them.
- CHANGE IN THE CHROMOSOME NUMBERS
o Many new species of plants and animals have evolved in the same geographic area due to POLYPLOIDY.
§ Polyploidy is when an individual or a species has more than one normal set of chromosomes.
o Mistakes during mitosis or meiosis can lead to chromosomes not separating correctly, therefore creating flaws in a population which cannot reproduce correctly.
- SPECIATION RATES
o Most speciation does not occur quickly, but instead occurs at a slow, steady rate, with small, adaptive changes, gradually accumulating over time.
o GRADUALISM is the idea that species originate through a gradual change of adaptations over many generations.
Patterns of Evolution
Biologists have observed different patterns of evolution that occur throughout the world in different environments.
Diversity in New Environments
- Much like the finches of the Galapagos Islands, as a species is separated by geographic isolation, the species will adapt over time to their new environments, adapting their size, body shape and even how they live.
- This separation, leading to new species diverged from a common ancestor is know as ADAPTIVE RADIATION.
- Adaptive radiation is an example of what is known as DIVERGENT EVOLUTION.
o As a species, which once was similar, is separated the species diverges, or separates becoming distinct.
- A pattern of evolution in which different organisms evolve similar traits is called CONVERGENT EVOLUTION.
o Convergent evolution occurs when unrelated species occupy similar environments in different parts of the world.
§ Because they have similar environmental pressures, they share the same pressures of natural selection.
§ Cactus Example