How Do the Fittest Survive?
Two male rabbits, Peter and Robert live in a warren. A fox chases Robert but Robert runs faster than the fox and gets away. The next day the fox chases Peter. Peter is slower than the fox, so the fox kills and eats Peter. Peter is no longer alive to breed and pass on his genes, but Robert has a 'run-faster' gene that has enabled him to escape from the fox. Robert is still alive and, if Robert breeds, there is a chance that his 'run-faster' gene will be passed on to the next generation of rabbits.
Do all the Next Generation Have the Advantageous Gene?
No. Only half of Robert's sperm will carry the 'run-faster' gene, so half of Robert's progeny will have a chance of inheriting that gene. The female rabbit Robert mates with produces egg cells that also contain genes that affect running speed.
Every baby rabbit will have two genes for running speed. The rabbit's actual running speed depends on which gene is dominant, which depends on the random way that the sperm and egg cell combined.
Half of the sperm contained the run-faster gene. None of the female's egg cells contained the run-faster gene.
When the sperm and egg cell combine half of the eggs will be fertilised by run-faster sperm and half by normal sperm.
Only half of the baby rabbits will even have cells that contain the run-faster gene at all.
In half of the baby rabbits that contain the run-faster gene that gene will be dominant, in the other half of the baby rabbits the rabbit's running speed will be determined by the mother's running speed gene.
2 baby rabbits out of 4 have the run-faster gene.
1 baby rabbit out of those 2 will have a dominant run-faster gene. One out of the four baby rabbits fathered by Robert Rabbit will have Robert's running speed.
The young rabbit with the run-faster gene escapes being eaten by foxes and breeds successfully. The three slower rabbits were caught, eaten and did not breed.
The rabbit with the fast running gene survives and passes this gene on to the next generation.
Over many generations the run-faster gene becomes more common in the rabbit population, as slower running rabbits are caught and eaten by hungry foxes.
Rabbits have evolved to run-faster.
More breeding success in the fittest to survive would be a better way to paraphrase Darwin's Theory of Evolution than the usual Survival of the Fittest.
The fittest survive, but they must breed successfully if the characteristic that made them the fittest is to be passed on and to spread through the general population.
Humans have evolved into the most intelligent species on Earth.
This has happened because intelligence has allowed humans to make tools and to manipulate our environment. Humans with greater intelligence survived longer to have children because their intelligence allowed them to grow food, to make weapons to kill predators. Less intelligent human ancestors did not learn to grow crops or to make weapons, so they died from starvation or were killed by predators before they could have children.
The gene for increased intelligence spread through the human population as intelligent humans live longer and had more children as a result. More children had parents with the gene for increased intelligence and they in turn lived longer and had more children than humans without the increased intelligence gene.
If a tiger was born with big purple splotches on its skin would it survive and breed? Survival would be unlikely, even if its parents did not kill it. Its prey would always see it coming, so it would die of starvation at an early age, it would not reproduce and the gene for purple splotches would not pass on to the next generation of tigers.
Cats have evolved lightning fast reflexes in order to catch their prey. A cat that was faster than average would be better fed, live longer and have more breeding success than a slower cat. This would lead to its faster reflexes genes spreading more widely through the cat population.
In a herd of wildebeest the lions always attack animals that trail behind the main herd. These animals are the slower ones. The lions get an easier meal and the wildebeest without the fast running gene do not survive long enough to breed and pass on their slow running gene.
Is Evolution Still Happening?
Yes. In most cases the changes are very small and the gap between successive generations too large to enable easy observation of evolutionary changes within a species.
There was a study done in a highly polluted city in England of Peppered Moths. During the day these moths rest on tree trunks. Peppered Moths are normally a pale grey in color. Soot from vehicle exhaust fumes coated the trees and made them darker. Pale grey moths stood out against the pollution-darkened trees, so they were eaten by predators during the day. The few darker moths were better camouflaged against the darkened tree trunks, so they survived predation, lived longer and had better breeding success than the paler ones.
Pollution has now been reduced, tree trunks are paler and the dark moths stand out against the tree trunks. The dark colored moths are eaten before they can breed and pale colored peppered moths are becoming more common again. The gene for dark coloration is becoming less common in the peppered moth population because it is a hindrance to survival and consequent breeding success.
Are Humans Still Evolving?
Evolution only happens where a particular gene leads to a longer lifespan and more reproductive success. Reproductive success in humans is affected by many factors such as access to medical treatment that are not related to their genes. Modern medicine's success stories largely prevent further human evolution. Those individuals without the genes necessary for survival and breeding success now survive and have children because of medical intervention.