Hi, my name is Hary, I stumbled upon this forum a few days ago. I am a Christian and layman, with very limited understanding of the sciences, so please be patient with me. I want to learn more about evolution, and there is one issue that I have yet to understand:
Does inbreeding always result in a loss of genetic diversity and decrease a population’s survival fitness? If so, in order for the population to develop new favorable traits that can increase its survival rate, does it always have to depend on genetic mutation? Are there other mechanisms at play that can increase a population’s genetic diversity and fitness?
I apologize if this has been asked before, and thank you for your time.
I’m not the most qualified person here to answer these questions, but I’ll take a stab at it …
I think inbreeding is probably synonymous with loss of diversity, and least in the setting of captive breeding. In the wild that will be generally true, but limited inbreeding might not decrease diversity of the population.
Fitness is relative to the environment. Loss of an unfavorable trait could increase fitness. We can’t make blanket statements about fitness, because it always depends on other factors.
Strictly speaking, no. Sexual mixing can introduce new traits without new mutations.
For fitness a definite ‘yes’ (environment, competition), but keep in mind that fitness is relative. An increase in one environment might be a decrease in another environment. For diversity also ‘yes’, but it’s more complicated, and I’m not a biologist.
There have been many experiments on this question. The general expectation is that fitness will decrease (it’s called inbreeding depression). Then again, it isn’t rare for an experiment to be unable to find inbreedng depression. The answer, then, is “it depends”.
All things being equal, yes, simply because “inbreeding” by definition means that the breeding involves individuals that are closely related. If there were other options, in other words if the inbreeding happens in a population that offered other opportunities, then inbreeding will lead to loss of diversity. That’s just a simple result of sampling.
No, inbreeding does not automatically reduce fitness. As already noted, fitness is relative. Maybe more centrally, loss of genetic diversity does not necessarily reduce fitness of a population. In fact, purifying selection (worth looking up, it’s pretty straightforward) regularly reduces genetic diversity while (typically) increasing fitness.
No. Gene flow and recombination are other ways to introduce new genetic diversity.
Depending on the rate at which inbreeding occurs, its negative effect on fitness can be ameliorated by the fact that it removes deleterious mutations from the population. This purging effect is especially pronounced for strongly deleterious recessive mutations, less so for weakly deleterious mutations.
Here are a couple nice (freely-available) reviews on the genetics of inbreeding:
Thank you for the responses. I would like to request your opinions on whether I have understood the process correctly, and I will give 1 specific case below.
In order for a non-avian dinosaur to evolve into a flying one would these be the required steps:
dinosaur has mutations that allows it to develop wings and feather-like structures
said dinosaur has to mate with other dinosaur that allows the mutations and new features to be retained in their offspring
the offspring would in turn mate with one another for generations, in order to ‘develop’ the wings and feathers further
the population survives in spite of inbreeding depression and loss of genetic diversity
No, from the outset. As far as can be determined from fossils, feathers long predated wings. You’re talking about a series of mutations, adaptations, and behavioral changes that happened over millions of years. Evolution happens to populations, not individuals. New features gradually spread through a population. There doesn’t have to be any inbreeding at all.
Great question @haryonob. Seems that everyone agrees that this is an implausible theory. Good news right? That is not what evolutionary science supposes.
Re inbreeding and fitness generally, I recently stumbled across an episode about Hybrids from the BBC podcast “In Our Time” that I enjoyed as an interested layman. One point I had never thought about was that “hybrid” is difficult to define in about the same way that “species” is.
Thanks for the corrections. Does anyone have a link to articles that explain the process of non-avian to avian evolution of dinosaurs? Preferably one suitable for general public understanding.
It just seems confusing to me that if we want to breed hairy dogs, we just keep breeding the dogs that are the most hairy to get what we want right? But it seems that this is not what is happening on a large evolutionary scale?
That’s right. Artificial selection is an imperfect model of natural selection. First, in nature nobody is working toward a goal. In some particular environment, some morphotypes (and thus some genotypes) leave more offspring than others. The genotypes that leave more offspring thus become more common than the others, eventually making up the entire population. It takes longer than selective breeding, but it happens. A complex adaptation such as feathers can take many rounds of mutation and selection, not to mention the still further steps leading from feathers to flight.
Google is a great way to find many. One of my favorite parts of the story, because it is so surprising, is that one of the intermediate steps seems to have been four-winged birds:
It is also important to remember that feathers did not suddenly pop into existence, and are not merely useful for flight, nor are feathers required for flight. There is a progression in the fossil record on feathers, which is now more wide known because there is strong evidence that many dinos were feathered.
There are two competing hypotheses for the evolution of flight in feathered dinosaurs (feathers evolved first for thermal regulation and/or display). The first is the Tree Down idea - small dinos began by gliding down from tree limbs to catch small prey, eventually added flapping to the gliding. The second is the Ground Up hypothesis - small dinos used their feathered forelimbs for balance while running, eventually developed the ability to glide for longer leaps and then flap for flying. Both hypotheses have some supporting evidence and it’s quite possible both methods occurred in different dino lineages.
Here is a neat video on the ground up running hypothesis
My personal hypothesis is that feathers originated as sensory vibrissae, became recruited for thermoregulation, and only then became available for flight. Evolution is seldom a straight line progression.
Would it not also make sense to say that as they were recruited for thermoregulation, eventually they also increased in size due to sexual selection, and this had the final byproduct of giving the capacity for lift?
Sexual selection could indeed be another part of it. So we have four possible functions of feathers: sensory organs, thermoregulation, display, flight. All those are found in modern birds.
I have not heard of the sensory perception function before, but since hairs (and similar structures) can serve the same purpose in other animals that makes a lot of sense.
