Thursday, November 15, 2012

Survival of the Fittest

Our interest in health and fitness is huge, and our interest in sex and family even more so. In biology, fitness combines all three!

In biological terms, the fittest individuals must survive AND reproduce.

However, they are not necessarily the strongest, fastest, or biggest --- the fittest individuals are those that consistently leaves more offspring in the next generation.

Consider:
  • An undersized plant with a large seed pod may leave behind more offspring than a taller, heartier plant.
  • Artists and musicians are often known for less-than-healthy lifestyles (think insomnia, depression, drugs, stress) yet may have more offspring than a superbly trained decathlete or body builder.

playwright gets the girl:
Marilyn & Arthur Miller

Winning army bodybuilders (photo: C.Smith (USAG Wiesbaden), Wikimedia Commons)

So fitness has a very different meaning in biology and evolution than the one we use in everyday life – it doesn’t necessarily mean healthier, just more able to get its genes into the next generation.

How does this apply to lions, leopards, bears and beetles?



Passing down your genes into immortality involves either competition or avoidance.

You can outcompete other species or others of your own species, like a large male lion might fight and defeat a rival lion to access food or females and, thus, survive and breed.
On the other hand, while a leopard may outcompete other leopards, it avoids competition with the larger lion by hunting at night and dragging its kill up a tree to eat there.

(Which is one tough trick -- note admiring gaze from the young at left)

The leopard partitions the food resource (antelopes) temporally, leaving daytime hunting to the lions.
A snow leopard really avoids competition by living alone in the jagged, rocky terrain of the high Himalaya Mountains, where its heavy coat to stay warm, super long tail to balance on cliff edges, and enlarged nasal cavities to heat inhaled cold air all contribute to its ability to survive and breed in that harsh environment.

These traits – strength for the lion, climbing ability for the leopard, resistance to cold in the snow leopard – that lead to success in living and breeding are those that will continue to the next generation.

In other words, the traits we see today in a given plant or animal are those that have passed the test of time – they helped (or at least didn’t interfere with) individuals of the species to survive and/or reproduce and so were then passed on to subsequent generations.

To visualize natural selection -- how differences in reproductive success will, over time, make certain traits more or less common in a population -- imagine a population of beetles on the African plains:

Some of the beetles are green, and some are brown. (i.e. there is some variability in the trait of interest)

However, the environment can't support unlimited growth in the beetle population, so not all beetles get to reproduce to their full potential.
In this population, green beetles tend to get eaten by birds, so they survive and, thus, reproduce less often than brown beetles do. (i.e. there is differential reproduction)
The surviving brown beetles have brown baby beetles because this trait (color) has a genetic basis. (i.e. the trait is hereditary)

Green beetles have green babies, but there are increasingly fewer of them.
Brown coloration, which is the more advantageous trait because it allows the beetle to have more offspring, becomes more common in the population.

If this process continues, eventually all individuals in the population will be brown. (i.e. this is the end result)
Images from: University of California Museum of Paleontology's Understanding Evolution (http://evolution.berkeley.edu)

To see the original page with these images, click here.

What do you think might have happened if the cartoon beetle population had been living in a forest?


Importance of one’s surroundings

The success of a given trait often depends on the individual’s surroundings, so the fitness of a given trait may vary with time, as well as location.

For example, a population of people with dark or smaller eyes will probably do better in a sunny region than one with light eyes, yet might have no advantage or even a disadvantage in a place with a long dark winter.


polar bear on ice
polar bear people watching
photo: G. Powell

Similarly, bears living in the Arctic region (polar bears!) eat seals that they hunt on ice, so you might guess that, over time, bears with genes that produced a lighter fur have been consistently more successful and thus have become more common in that environment.

However, picture this white bear in a pine-spruce forest or a spring meadow – she or he would shine like a light bulb, making hunting much more difficult.


grizzly bear in the woods
photo: Geek Philosopher
Bears we see today in this environment are those with fur color genes coded for shades like brown and black.

Bears with light fur have a higher fitness in the Arctic, while those with dark fur have a higher fitness in temperate forests.


Survival of the “fit enough”

Biological fitness, as in “survival of the fittest,” might be better described as “survival of the fit enough".

The traits we see today are all a result of a long series of adaptations to a given environment over many generations. Species we see today are not perfect: lions or leopards don’t catch every antelope, and some may starve without ever reproducing. If you are a lion and a certain set of traits is “good enough” to allow you to produce some successful offspring, you are golden: those traits, coded in your genes, will have another chance for immortality in your kids.

The green cartoon beetles may have survived and reproduced with some success for generations before their greater tendency to get eaten by birds finally reduced their breeding success to zero.

birds eat green bugs, selection for brown coloration
Image: Univ. of California Museum of Paleontology's Understanding Evolution


This is natural selection in action: genetic traits get passed on to different extents in different environments by different combinations of parents, and the results include you, me, the lions and leopards, and all other species currently on Earth.

No comments:

Post a Comment